Calling All A/C Experts For Advice
06-03-2013, 10:39 PM
#1
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Thread Starter
Calling All A/C Experts For Advice
OK. After much trial/error and confusion, I want to get this mess straightened out and make this the end all post to my AC troubles. Here is the deal.
My car has not had functioning AC in the 5 years I've owned it. Absolutely everything was taken from the car by a PO with the exception of the evap core that got destroyed so I replaced that too.
Everything in the system is 100% brand new with the exception of the evap core and body liquid line which were both taken from a 40k mile firebird.
I assembled the system with green o-rings, used a blue 0.67" ford orifice tube then added 4oz of PAG 150 oil to the drier and 4 to the compressor before sealing the system. Adjusted the compressor cycle switch to approx 21psi. Had a vacuum pulled and looked like everything was good to go. Now aside from some tweaking required to the bracket setup and some belt troubles, here is where the clusterbang begins.
I set the AC to max and began charging with the first can of R134a, making sure to purge the charging hose before connecting. I ended up using about 3.5 cans of r134a which put between 40 and 42oz of total charge. Everything seemed to be working fine, however there were a few things I misinterpreted while reading data and retrofit guides.
Mistake #1 was that I did not understand that the 21psi setting for the compressor cycle switch was ONLY to prevent freezing over of the evap in cold ambient temps or running of the compressor in a low charge situation. I was thinking that the low side was supposed to get down to this pressure ALL the time during normal operation before the compressor cycled off.
Mistake #2 was I did not understand that it is normal for the compressor to never cycle off with engine at speed on a hot day. I thought cycling was supposed to happen for any given scenario. And so within those two misconceptions is where things got a little mucked up. Read on...
I was at my friend's house yesterday and for the hell of it we decided to use his manifold gauge set to see where the system was at. Ambient temperature outside was in the high 80s. When we hooked it up, I set the HVAC controls to max AC and brought the engine to around a 1500rpm give or take idle speed with the radiator fan cranking. We noted a high side pressure of around 280psi and low side of 45psi. So maybe a HAIR bit overcharged by an oz or two, but nothing serious. However I was still under the two above misconceptions at the time and my friend seemed to think the same thing. So in a moment of stupendous automotive idiocy, I bled refrigerant out of the low side manifold valve until the compressor got down to around 21psi and began cycling. High side dropped to around 225. Very little oil came out while bleeding. Maybe .5 - 1oz at the most which still leaves at least 7oz in the system.
NOW I understand that my system in it's current state is undercharged. I only put maybe about 20 miles on it in this condition while going down the highway so I am not concerned about anything being damaged. However I am going to tackle this one more time and want to make sure it is done CORRECT.
So if I am understanding things correctly now, on an 85 - 90F ambient day with the engine at a high idle the system should be charged to around 40psi on the low side with the compressor running and a high side of 250 - 280psi would not be considered unusual. Also, duct temps should be taken with the fan on the lowest setting and the AC controls set to blow in outside air. Also on a hot day it is perfectly normal for the compressor to never cycle off.
Please correct anything I am still not understanding correctly. I haven't blown anything up yet or seized anything but I want to make sure the situation doesn't head in that direction.
Anyone feel free to add any information I may have missed. I want this to be done RIGHT.
My car has not had functioning AC in the 5 years I've owned it. Absolutely everything was taken from the car by a PO with the exception of the evap core that got destroyed so I replaced that too.
Everything in the system is 100% brand new with the exception of the evap core and body liquid line which were both taken from a 40k mile firebird.
I assembled the system with green o-rings, used a blue 0.67" ford orifice tube then added 4oz of PAG 150 oil to the drier and 4 to the compressor before sealing the system. Adjusted the compressor cycle switch to approx 21psi. Had a vacuum pulled and looked like everything was good to go. Now aside from some tweaking required to the bracket setup and some belt troubles, here is where the clusterbang begins.
I set the AC to max and began charging with the first can of R134a, making sure to purge the charging hose before connecting. I ended up using about 3.5 cans of r134a which put between 40 and 42oz of total charge. Everything seemed to be working fine, however there were a few things I misinterpreted while reading data and retrofit guides.
Mistake #1 was that I did not understand that the 21psi setting for the compressor cycle switch was ONLY to prevent freezing over of the evap in cold ambient temps or running of the compressor in a low charge situation. I was thinking that the low side was supposed to get down to this pressure ALL the time during normal operation before the compressor cycled off.
Mistake #2 was I did not understand that it is normal for the compressor to never cycle off with engine at speed on a hot day. I thought cycling was supposed to happen for any given scenario. And so within those two misconceptions is where things got a little mucked up. Read on...
I was at my friend's house yesterday and for the hell of it we decided to use his manifold gauge set to see where the system was at. Ambient temperature outside was in the high 80s. When we hooked it up, I set the HVAC controls to max AC and brought the engine to around a 1500rpm give or take idle speed with the radiator fan cranking. We noted a high side pressure of around 280psi and low side of 45psi. So maybe a HAIR bit overcharged by an oz or two, but nothing serious. However I was still under the two above misconceptions at the time and my friend seemed to think the same thing. So in a moment of stupendous automotive idiocy, I bled refrigerant out of the low side manifold valve until the compressor got down to around 21psi and began cycling. High side dropped to around 225. Very little oil came out while bleeding. Maybe .5 - 1oz at the most which still leaves at least 7oz in the system.
NOW I understand that my system in it's current state is undercharged. I only put maybe about 20 miles on it in this condition while going down the highway so I am not concerned about anything being damaged. However I am going to tackle this one more time and want to make sure it is done CORRECT.
So if I am understanding things correctly now, on an 85 - 90F ambient day with the engine at a high idle the system should be charged to around 40psi on the low side with the compressor running and a high side of 250 - 280psi would not be considered unusual. Also, duct temps should be taken with the fan on the lowest setting and the AC controls set to blow in outside air. Also on a hot day it is perfectly normal for the compressor to never cycle off.
Please correct anything I am still not understanding correctly. I haven't blown anything up yet or seized anything but I want to make sure the situation doesn't head in that direction.
Anyone feel free to add any information I may have missed. I want this to be done RIGHT.
Last edited by FireDemonSiC; 06-03-2013 at 10:45 PM.
06-04-2013, 08:46 PM
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Re: Calling All A/C Experts For Advice
We noted a high side pressure of around 280psi and low side of 45psi. So maybe a HAIR bit overcharged by an oz or two, but nothing serious. However I was still under the two above misconceptions at the time and my friend seemed to think the same thing. So in a moment of stupendous automotive idiocy, I bled refrigerant out of the low side manifold valve until the compressor got down to around 21psi and began cycling. High side dropped to around 225. Very little oil came out while bleeding. Maybe .5 - 1oz at the most which still leaves at least 7oz in the system.
So if I am understanding things correctly now, on an 85 - 90F ambient day with the engine at a high idle the system should be charged to around 40psi on the low side with the compressor running and a high side of 250 - 280psi would not be considered unusual. Also, duct temps should be taken with the fan on the lowest setting and the AC controls set to blow in outside air. Also on a hot day it is perfectly normal for the compressor to never cycle off.
Please correct anything I am still not understanding correctly. I haven't blown anything up yet or seized anything but I want to make sure the situation doesn't head in that direction.
Anyone feel free to add any information I may have missed. I want this to be done RIGHT.
So if I am understanding things correctly now, on an 85 - 90F ambient day with the engine at a high idle the system should be charged to around 40psi on the low side with the compressor running and a high side of 250 - 280psi would not be considered unusual. Also, duct temps should be taken with the fan on the lowest setting and the AC controls set to blow in outside air. Also on a hot day it is perfectly normal for the compressor to never cycle off.
Please correct anything I am still not understanding correctly. I haven't blown anything up yet or seized anything but I want to make sure the situation doesn't head in that direction.
Anyone feel free to add any information I may have missed. I want this to be done RIGHT.
Refrigerant pressures can be a bit tricky because we equate low pressure with cold, thus low pressure is always better......Not quite.....You have to remember that refrigerant is the "capacity" of the air conditioning system. Too little refrigerant = less capacity to cool. So while it is easier to compress a smaller amount of refrigerant inside the lines. You're still getting less capacity to cool.
The opposite occurs in an overcharged system. You have more capacity. But you have a harder time compressing it. Since it won't compress as far down (only getting to say 50 psi instead of 30 psi), you never get cold air blowing at you.
In the spring time, a properly charged system acts like an undercharged system. It cycles often. In the middle of summer hell, a properly charged system acts like an overcharged system. Never cycles off and runs higher pressures. This is why it's hard to charge a system by pressure alone because the ambient weather outside changes from month to month. Humidity makes a difference too. This is why charging by weight is always a good idea.
You listed two pressures:
BEFORE: 45/285
AFTER: 21/225
Your before pressures were actually really good. Those were taken with the car sitting in the driveway with the fans blowing. You were getting "x" amount of air cooling the condenser up front (from the fans alone). Now what happens when you drive the car? At 30 mph, you're still getting "x" amount of air from the fans plus 30 mph of wind blowing up through the air dam and onto the condenser. This means your 45/285 psi would change to say 30/250 psi.
Your after pressures are too low. The capacity of the system is now lower (due to less refrigerant). As such, you're exhausting less heat to the condenser. This is why the high side pressure dropped from 285 to 225 psi. And if you noticed it cycling sporadically in the driveway, you'll find that it cycles ALL THE TIME when driving down the road. An air conditioning system that cycles all the time won't cool you because it doesn't have much run time.
People who live in the humid south often make the mistake of buying an oversized air conditioner for their house. Because a) It's hot and b) It's humid......This creates expensive bills and uncomfortable high humidity homes. The oversized A/C runs for 5 minutes and shuts off for 10 minutes over and over. Thus it doesn't have much run time. And what little run time it does have is spent blowing warm-semi cool air at you. This won't dehumidify the air. And it's costing you money to blow warm-semi cool air at you...... A properly sized air conditioner should have very long run times. That way it spends the maximum amount of time at maximum cold (great for dehumidifying) as well as gives you maximum bang for the buck in terms of paying for electricity and getting COLD air blown at you.
Fortunately we don't have sizing issues to deal with in cars.
06-05-2013, 12:45 AM
#3
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Thread Starter
Re: Calling All A/C Experts For Advice
Check duct temps while driving with it on MAX (recirc). Fan speed can be anything. Lower fan speed = colder air coming out the ducts. But less air. Faster fan speed = warmer air coming out the ducts. But more air...... All the fan does is blow hot air from the cabin (MAX) or from outside (NORM) over a cold evap coil. Blowing hot air over the cold coil warms the coil. This is why running the fan speed slow = colder air coming out your ducts. I don't like to check duct temps while the car is stationary because there are too many factors going on. (Hot engine? Shade? How long has it been running? Are the ducts themselves hot from being parked in the sun all day etc)
Refrigerant pressures can be a bit tricky because we equate low pressure with cold, thus low pressure is always better......Not quite.....You have to remember that refrigerant is the "capacity" of the air conditioning system. Too little refrigerant = less capacity to cool. So while it is easier to compress a smaller amount of refrigerant inside the lines. You're still getting less capacity to cool.
The opposite occurs in an overcharged system. You have more capacity. But you have a harder time compressing it. Since it won't compress as far down (only getting to say 50 psi instead of 30 psi), you never get cold air blowing at you.
In the spring time, a properly charged system acts like an undercharged system. It cycles often. In the middle of summer hell, a properly charged system acts like an overcharged system. Never cycles off and runs higher pressures. This is why it's hard to charge a system by pressure alone because the ambient weather outside changes from month to month. Humidity makes a difference too. This is why charging by weight is always a good idea.
You listed two pressures:
BEFORE: 45/285
AFTER: 21/225
Your before pressures were actually really good. Those were taken with the car sitting in the driveway with the fans blowing. You were getting "x" amount of air cooling the condenser up front (from the fans alone). Now what happens when you drive the car? At 30 mph, you're still getting "x" amount of air from the fans plus 30 mph of wind blowing up through the air dam and onto the condenser. This means your 45/285 psi would change to say 30/250 psi.
Your after pressures are too low. The capacity of the system is now lower (due to less refrigerant). As such, you're exhausting less heat to the condenser. This is why the high side pressure dropped from 285 to 225 psi. And if you noticed it cycling sporadically in the driveway, you'll find that it cycles ALL THE TIME when driving down the road. An air conditioning system that cycles all the time won't cool you because it doesn't have much run time.
People who live in the humid south often make the mistake of buying an oversized air conditioner for their house. Because a) It's hot and b) It's humid......This creates expensive bills and uncomfortable high humidity homes. The oversized A/C runs for 5 minutes and shuts off for 10 minutes over and over. Thus it doesn't have much run time. And what little run time it does have is spent blowing warm-semi cool air at you. This won't dehumidify the air. And it's costing you money to blow warm-semi cool air at you...... A properly sized air conditioner should have very long run times. That way it spends the maximum amount of time at maximum cold (great for dehumidifying) as well as gives you maximum bang for the buck in terms of paying for electricity and getting COLD air blown at you.
Fortunately we don't have sizing issues to deal with in cars.
Refrigerant pressures can be a bit tricky because we equate low pressure with cold, thus low pressure is always better......Not quite.....You have to remember that refrigerant is the "capacity" of the air conditioning system. Too little refrigerant = less capacity to cool. So while it is easier to compress a smaller amount of refrigerant inside the lines. You're still getting less capacity to cool.
The opposite occurs in an overcharged system. You have more capacity. But you have a harder time compressing it. Since it won't compress as far down (only getting to say 50 psi instead of 30 psi), you never get cold air blowing at you.
In the spring time, a properly charged system acts like an undercharged system. It cycles often. In the middle of summer hell, a properly charged system acts like an overcharged system. Never cycles off and runs higher pressures. This is why it's hard to charge a system by pressure alone because the ambient weather outside changes from month to month. Humidity makes a difference too. This is why charging by weight is always a good idea.
You listed two pressures:
BEFORE: 45/285
AFTER: 21/225
Your before pressures were actually really good. Those were taken with the car sitting in the driveway with the fans blowing. You were getting "x" amount of air cooling the condenser up front (from the fans alone). Now what happens when you drive the car? At 30 mph, you're still getting "x" amount of air from the fans plus 30 mph of wind blowing up through the air dam and onto the condenser. This means your 45/285 psi would change to say 30/250 psi.
Your after pressures are too low. The capacity of the system is now lower (due to less refrigerant). As such, you're exhausting less heat to the condenser. This is why the high side pressure dropped from 285 to 225 psi. And if you noticed it cycling sporadically in the driveway, you'll find that it cycles ALL THE TIME when driving down the road. An air conditioning system that cycles all the time won't cool you because it doesn't have much run time.
People who live in the humid south often make the mistake of buying an oversized air conditioner for their house. Because a) It's hot and b) It's humid......This creates expensive bills and uncomfortable high humidity homes. The oversized A/C runs for 5 minutes and shuts off for 10 minutes over and over. Thus it doesn't have much run time. And what little run time it does have is spent blowing warm-semi cool air at you. This won't dehumidify the air. And it's costing you money to blow warm-semi cool air at you...... A properly sized air conditioner should have very long run times. That way it spends the maximum amount of time at maximum cold (great for dehumidifying) as well as gives you maximum bang for the buck in terms of paying for electricity and getting COLD air blown at you.
Fortunately we don't have sizing issues to deal with in cars.
) wrestling with the stupid compressor. Got the missing bolt into the rear bracket and stabilizer bar on the water pump.I also located a very very very small and slow pinhole leak in the liquid line where the high side fitting is welded on. I would keep finding a small drop of dye at the welding crease in the same spot no matter how times I wiped it off. I'd come back the next day and it would be there again. I am trying to patch it before replacing the pipe given how small it is. I cleaned the area with rubbing alcohol, wiped a small film of superglue over the area for a fast seal then sanded and slapped some JB weld on the offending area.
I won't have access to the manifold gauge set again until this weekend however I have half a can of r134a left and will put that in tomorrow. Hopefully it will tide things over til the weekend. I am estimating that about a full can was evacuated
.I had one more question for you. How much can the heatercore actually impact A/C system performance? The heater diverter valve has been gone since a PO and I was curious about this. I bought a 3/4" shutoff valve from Lowes but haven't put it on yet.
06-05-2013, 06:32 AM
#4
Re: Calling All A/C Experts For Advice
I know that on my car, the heater diverter valve was my main problem. No matter what we did I couldnt get cold air to blow. I always assumed that the "hot" "cold" setting on the HVAC controls would full block air from one or the other. Well maybe that is the case, maybe not, I dont know, but a $5 3/4" ball valve fixed it. It also stopped the car being so dang hot in the summer when the A/C was turned off. Maybe something else is broke in my car and the ball valve is covering up for it, but it works.
Alittle annoying on a cold morning with a hot evening but takes all of 20 seconds to pop the hood, turn the lever, and close the hood back.
Alittle annoying on a cold morning with a hot evening but takes all of 20 seconds to pop the hood, turn the lever, and close the hood back.
06-05-2013, 11:17 PM
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Re: Calling All A/C Experts For Advice
Check duct temps while driving with it on MAX (recirc). Fan speed can be anything. Lower fan speed = colder air coming out the ducts. But less air. Faster fan speed = warmer air coming out the ducts. But more air...... All the fan does is blow hot air from the cabin (MAX) or from outside (NORM) over a cold evap coil. Blowing hot air over the cold coil warms the coil. This is why running the fan speed slow = colder air coming out your ducts. I don't like to check duct temps while the car is stationary because there are too many factors going on. (Hot engine? Shade? How long has it been running? Are the ducts themselves hot from being parked in the sun all day etc)
Refrigerant pressures can be a bit tricky because we equate low pressure with cold, thus low pressure is always better......Not quite.....You have to remember that refrigerant is the "capacity" of the air conditioning system. Too little refrigerant = less capacity to cool. So while it is easier to compress a smaller amount of refrigerant inside the lines. You're still getting less capacity to cool.
The opposite occurs in an overcharged system. You have more capacity. But you have a harder time compressing it. Since it won't compress as far down (only getting to say 50 psi instead of 30 psi), you never get cold air blowing at you.
In the spring time, a properly charged system acts like an undercharged system. It cycles often. In the middle of summer hell, a properly charged system acts like an overcharged system. Never cycles off and runs higher pressures. This is why it's hard to charge a system by pressure alone because the ambient weather outside changes from month to month. Humidity makes a difference too. This is why charging by weight is always a good idea.
You listed two pressures:
BEFORE: 45/285
AFTER: 21/225
Your before pressures were actually really good. Those were taken with the car sitting in the driveway with the fans blowing. You were getting "x" amount of air cooling the condenser up front (from the fans alone). Now what happens when you drive the car? At 30 mph, you're still getting "x" amount of air from the fans plus 30 mph of wind blowing up through the air dam and onto the condenser. This means your 45/285 psi would change to say 30/250 psi.
Your after pressures are too low. The capacity of the system is now lower (due to less refrigerant). As such, you're exhausting less heat to the condenser. This is why the high side pressure dropped from 285 to 225 psi. And if you noticed it cycling sporadically in the driveway, you'll find that it cycles ALL THE TIME when driving down the road. An air conditioning system that cycles all the time won't cool you because it doesn't have much run time.
People who live in the humid south often make the mistake of buying an oversized air conditioner for their house. Because a) It's hot and b) It's humid......This creates expensive bills and uncomfortable high humidity homes. The oversized A/C runs for 5 minutes and shuts off for 10 minutes over and over. Thus it doesn't have much run time. And what little run time it does have is spent blowing warm-semi cool air at you. This won't dehumidify the air. And it's costing you money to blow warm-semi cool air at you...... A properly sized air conditioner should have very long run times. That way it spends the maximum amount of time at maximum cold (great for dehumidifying) as well as gives you maximum bang for the buck in terms of paying for electricity and getting COLD air blown at you.
Fortunately we don't have sizing issues to deal with in cars.
Refrigerant pressures can be a bit tricky because we equate low pressure with cold, thus low pressure is always better......Not quite.....You have to remember that refrigerant is the "capacity" of the air conditioning system. Too little refrigerant = less capacity to cool. So while it is easier to compress a smaller amount of refrigerant inside the lines. You're still getting less capacity to cool.
The opposite occurs in an overcharged system. You have more capacity. But you have a harder time compressing it. Since it won't compress as far down (only getting to say 50 psi instead of 30 psi), you never get cold air blowing at you.
In the spring time, a properly charged system acts like an undercharged system. It cycles often. In the middle of summer hell, a properly charged system acts like an overcharged system. Never cycles off and runs higher pressures. This is why it's hard to charge a system by pressure alone because the ambient weather outside changes from month to month. Humidity makes a difference too. This is why charging by weight is always a good idea.
You listed two pressures:
BEFORE: 45/285
AFTER: 21/225
Your before pressures were actually really good. Those were taken with the car sitting in the driveway with the fans blowing. You were getting "x" amount of air cooling the condenser up front (from the fans alone). Now what happens when you drive the car? At 30 mph, you're still getting "x" amount of air from the fans plus 30 mph of wind blowing up through the air dam and onto the condenser. This means your 45/285 psi would change to say 30/250 psi.
Your after pressures are too low. The capacity of the system is now lower (due to less refrigerant). As such, you're exhausting less heat to the condenser. This is why the high side pressure dropped from 285 to 225 psi. And if you noticed it cycling sporadically in the driveway, you'll find that it cycles ALL THE TIME when driving down the road. An air conditioning system that cycles all the time won't cool you because it doesn't have much run time.
People who live in the humid south often make the mistake of buying an oversized air conditioner for their house. Because a) It's hot and b) It's humid......This creates expensive bills and uncomfortable high humidity homes. The oversized A/C runs for 5 minutes and shuts off for 10 minutes over and over. Thus it doesn't have much run time. And what little run time it does have is spent blowing warm-semi cool air at you. This won't dehumidify the air. And it's costing you money to blow warm-semi cool air at you...... A properly sized air conditioner should have very long run times. That way it spends the maximum amount of time at maximum cold (great for dehumidifying) as well as gives you maximum bang for the buck in terms of paying for electricity and getting COLD air blown at you.
Fortunately we don't have sizing issues to deal with in cars.
Cars also have sizing issues, especially their condensors. My Ram had a tiny condensor mounted to the side of the radiator with its own electric fan, that was undersize for the amount of heat it needed to dissapate and would cause the system to shut off on the 425 PSI high pressure cutout even properly charged around town. The original tube and fin design condensor in my 83 G20 Van was also undersize for use with dual a/c and R134a around town but cooled ok on the open road.
Where my Ram barely cooled with R134a on a hot day, my 97 Express would blow ice cubes, the difference is a/c condensor and airflow.
Last summer I switched my brand new 2012 Titan from R134a to R152a. With the R152a it has much quicker cooldown, stays cold at idle even in warmer weather with the blower cranked up, and provides better dehumidification. On an automotive a/c unit it is better for the unit to actually get cold enough to cause the evaporator to frost and then thaw repeatedly. It helps purge contaminants off the surface and dehumidify the air more quickly.
This is the pressure I saw on a Humid 95*F day, it has a huge mechanical clutch driven fan and a monster condensor along with a TXV.
That is moisture condensing on the outside of the window.
Last edited by Fast355; 06-05-2013 at 11:24 PM.
06-06-2013, 07:20 PM
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Re: Calling All A/C Experts For Advice
(My apologies for the thread jack. But this is good debate.)
Oh hell no. 
Ducting can make a huge difference. Undersize ducts, kinks, holes, leaky connectors, chances are the old unit had dirty coils, could be anything. Stuck TXV or no TXV at all.
Residential A/C isn't designed to cycle at frost points and maintain supply temps at frost + a few degrees like cars do. Instead they just barely keep the humidity at bay. Night time is the worst, when the temperature drops a tiny bit, the sun goes down, but the humidity is sky high. Something in the range of 74°/95%. You have little to no cooling going on, but you need to drop the humidity like a rock in that small window of time. An oversized unit just makes this worse. It doesn't run for long enough. I suffer from this during the winter cooling season.
There is little more frustrating than being at 72°/50% and wanting more humidity reduction, but the only answer is to freeze yourself out by going down to 70°F or perhaps even colder. This also increases your chances for mold in the walls of houses (in the humid south) when it's cold enough inside that you have exterior condensation on your windows.
My apartment is only 1300 sq ft. But it has 3.5 tons of A/C. Even for Houston, this is oversized. I have to run it at 71-73° to keep the humidity at bay. Running it at 75-78° as many people prefer just won't work in my home. The A/C just won't run long enough.
A general rule of thumb is that 1 ton of A/C per 500 sq ft in Houston works "good."
Throwing aside the real world aspect and concentrating just on theory, the most efficient electricity bill would be from an air conditioner that never shut off. It's better to have 5 tons that runs for 60 minutes straight than it is to have 10 tons that runs for 30 minutes out of the hour. That 10 ton unit would spent probably 15-20 minutes of the hour just getting down to maximum cold temperature......The 5 ton unit would spend less time getting down to temp and more time at cold temp. Cheaper A/C bill being the result.
Having owned a house with too small of an a/c unit because it was sized by someone with your mentality, I can tell you that an OVERSIZED A/C is where it is at in the hot, humid south. My energy bill went from over $300 a month, to $150 a month when I went from a 3 ton to a 5 ton unit and replaced the ducting.
Ducting can make a huge difference. Undersize ducts, kinks, holes, leaky connectors, chances are the old unit had dirty coils, could be anything. Stuck TXV or no TXV at all.
Residential A/C isn't designed to cycle at frost points and maintain supply temps at frost + a few degrees like cars do. Instead they just barely keep the humidity at bay. Night time is the worst, when the temperature drops a tiny bit, the sun goes down, but the humidity is sky high. Something in the range of 74°/95%. You have little to no cooling going on, but you need to drop the humidity like a rock in that small window of time. An oversized unit just makes this worse. It doesn't run for long enough. I suffer from this during the winter cooling season.
There is little more frustrating than being at 72°/50% and wanting more humidity reduction, but the only answer is to freeze yourself out by going down to 70°F or perhaps even colder. This also increases your chances for mold in the walls of houses (in the humid south) when it's cold enough inside that you have exterior condensation on your windows.
My apartment is only 1300 sq ft. But it has 3.5 tons of A/C. Even for Houston, this is oversized. I have to run it at 71-73° to keep the humidity at bay. Running it at 75-78° as many people prefer just won't work in my home. The A/C just won't run long enough.
A general rule of thumb is that 1 ton of A/C per 500 sq ft in Houston works "good."
Throwing aside the real world aspect and concentrating just on theory, the most efficient electricity bill would be from an air conditioner that never shut off. It's better to have 5 tons that runs for 60 minutes straight than it is to have 10 tons that runs for 30 minutes out of the hour. That 10 ton unit would spent probably 15-20 minutes of the hour just getting down to maximum cold temperature......The 5 ton unit would spend less time getting down to temp and more time at cold temp. Cheaper A/C bill being the result.
Last edited by Reid Fleming; 06-06-2013 at 07:30 PM.
06-06-2013, 08:13 PM
#7
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Re: Calling All A/C Experts For Advice
Just a quick update.
I patched the pinhole leak, Got the compressor properly mounted up with the waterpump stabilizer bar and missing rear bracket bolt and tossed in the half a can of r134a I had left. I also installed the heatercore bypass valve.
Performance seems to be better however I am pretty sure the system is still somewhat low.
Right now is not a good time to check things because #1 it's raining and I live in a townhouse
and #2 it's been rather cool lately. I want ti to be in the 80s before I take low side readings and charge accordingly.
I patched the pinhole leak, Got the compressor properly mounted up with the waterpump stabilizer bar and missing rear bracket bolt and tossed in the half a can of r134a I had left. I also installed the heatercore bypass valve.
Performance seems to be better however I am pretty sure the system is still somewhat low.
Right now is not a good time to check things because #1 it's raining and I live in a townhouse
and #2 it's been rather cool lately. I want ti to be in the 80s before I take low side readings and charge accordingly. Trending Topics
06-06-2013, 10:59 PM
#8
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Re: Calling All A/C Experts For Advice
(My apologies for the thread jack. But this is good debate.)
Oh hell no.
Ducting can make a huge difference. Undersize ducts, kinks, holes, leaky connectors, chances are the old unit had dirty coils, could be anything. Stuck TXV or no TXV at all.
Residential A/C isn't designed to cycle at frost points and maintain supply temps at frost + a few degrees like cars do. Instead they just barely keep the humidity at bay. Night time is the worst, when the temperature drops a tiny bit, the sun goes down, but the humidity is sky high. Something in the range of 74°/95%. You have little to no cooling going on, but you need to drop the humidity like a rock in that small window of time. An oversized unit just makes this worse. It doesn't run for long enough. I suffer from this during the winter cooling season.
There is little more frustrating than being at 72°/50% and wanting more humidity reduction, but the only answer is to freeze yourself out by going down to 70°F or perhaps even colder. This also increases your chances for mold in the walls of houses (in the humid south) when it's cold enough inside that you have exterior condensation on your windows.
My apartment is only 1300 sq ft. But it has 3.5 tons of A/C. Even for Houston, this is oversized. I have to run it at 71-73° to keep the humidity at bay. Running it at 75-78° as many people prefer just won't work in my home. The A/C just won't run long enough.
A general rule of thumb is that 1 ton of A/C per 500 sq ft in Houston works "good."
Throwing aside the real world aspect and concentrating just on theory, the most efficient electricity bill would be from an air conditioner that never shut off. It's better to have 5 tons that runs for 60 minutes straight than it is to have 10 tons that runs for 30 minutes out of the hour. That 10 ton unit would spent probably 15-20 minutes of the hour just getting down to maximum cold temperature......The 5 ton unit would spend less time getting down to temp and more time at cold temp. Cheaper A/C bill being the result.
Oh hell no.
Ducting can make a huge difference. Undersize ducts, kinks, holes, leaky connectors, chances are the old unit had dirty coils, could be anything. Stuck TXV or no TXV at all.
Residential A/C isn't designed to cycle at frost points and maintain supply temps at frost + a few degrees like cars do. Instead they just barely keep the humidity at bay. Night time is the worst, when the temperature drops a tiny bit, the sun goes down, but the humidity is sky high. Something in the range of 74°/95%. You have little to no cooling going on, but you need to drop the humidity like a rock in that small window of time. An oversized unit just makes this worse. It doesn't run for long enough. I suffer from this during the winter cooling season.
There is little more frustrating than being at 72°/50% and wanting more humidity reduction, but the only answer is to freeze yourself out by going down to 70°F or perhaps even colder. This also increases your chances for mold in the walls of houses (in the humid south) when it's cold enough inside that you have exterior condensation on your windows.
My apartment is only 1300 sq ft. But it has 3.5 tons of A/C. Even for Houston, this is oversized. I have to run it at 71-73° to keep the humidity at bay. Running it at 75-78° as many people prefer just won't work in my home. The A/C just won't run long enough.
A general rule of thumb is that 1 ton of A/C per 500 sq ft in Houston works "good."
Throwing aside the real world aspect and concentrating just on theory, the most efficient electricity bill would be from an air conditioner that never shut off. It's better to have 5 tons that runs for 60 minutes straight than it is to have 10 tons that runs for 30 minutes out of the hour. That 10 ton unit would spent probably 15-20 minutes of the hour just getting down to maximum cold temperature......The 5 ton unit would spend less time getting down to temp and more time at cold temp. Cheaper A/C bill being the result.
I am not talking about a unit double the size, but one that is just a little bigger than necessary works better than one that is a little too small for peak demand in the middle of the day on a hot day.
That being said, I keep my house 68*F year round.
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