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Old 07-13-2020, 11:20 PM   #1
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A/C Cooling problem


Have an issue going on with the central air in my house. I've noticed that recently, it'll cycle in the through the morning, then around mid afternoon, it'll come on and not shut off until 3-4am or so. Checking the temperatures of the return and supply, there's a 15* or so degree difference, but I'm told by my step-father, who knows more about heating, but knows a bit about A/C, that it should be more a 20-25* difference. The condenser coils weren't too bad (I had already cleaned the huge majority of the 'fuzz' that routinely builds up on the coils), but per his recommendation, I degreased and cleaned the heck out of them, which made no difference in the temperature differential. After that, though, I noticed something odd (I doubt it has anything to do with the cleaning, I just never noticed it before). As I was feeling different spots on the coil, I noticed that there's a 'band' about 2/3's of the way down that's MUCH hotter than above or below that area, which seems odd. It's about a 6 or so inch band that I wouldn't say is 'burn you' hot, but I think it's reasonable to say that it's uncomfortably hot. Above and below (below being the bottom 4-6" of the coil), it's FAR cooler. It seems almost as if all (or most) of the heat that it's trying to removed from the house is only being stuffed into that band of the coil (And I could be totally wrong on that). The temperature differential not being quite where it should be, by itself, could possibly be the result of a low charge, but the 'hot band' on the condenser coils seems really odd to me. Unfortunately, my FLIR One died, and I'm waiting on it to get to them and for them to send me a replacement, so I can't add the proverbial 'Picture that's worth 1,000 words' to show the temperature differential that I'm feeling in the coil.

That being said, does anyone have any 'off the cuff' thoughts? The main thing that has me scratching my head is the 'hot band' on the condenser. That seems really odd.
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Old 07-13-2020, 11:47 PM   #2
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Re: A/C Cooling problem


It's normal to get continuous operation in very hot weather. Whether your system is performing well or not, I don't know.

A 15F drop is a bit low if the return air humidity is around 50% - but, the measurement must be taken at the air handler to be accurate as there's duct heat gain.



Return air humidity affects the temperature drop, as does fan speed.

The system should be professionally checked:
The fan speed must be set to move the correct cubic feet per minute for the size of a/c on your duct system. There are different ways to determine airflow.

The refrigerant charge needs to be checked properly and adjusted if necessary using the superheat/subcooling method (no beer can cold, no pressures only). This should only be done after making sure the blower speed is correct.

Operating capacity can be calculated based on measured airflow and wetbulb return/supply temperatures.

If the a/c itself is working properly yet the house isn't cooling well, you could have a air duct problem.

Quote:
It's about a 6 or so inch band that I wouldn't say is 'burn you' hot, but I think it's reasonable to say that it's uncomfortably hot. Above and below (below being the bottom 4-6" of the coil), it's FAR cooler. It seems almost as if all (or most) of the heat that it's trying to removed from the house is only being stuffed into that band of the coil (And I could be totally wrong on that).
Normal.


The refrigerant is warmer than boiling point when it enters the outdoor coil, but quickly cools down and starts condensing into a liquid.

Do you know how you can't heat water above 212F on a stove unless you're using a pressure cooker?

After that point, the heat applied to the water doesn't raise the temp, it goes towards converting the water to steam. Only after the water is steam can it be heated above boiling point.

The same applies to refrigerant in the condenser coil, but backwards. It starts as superheated "steam", cools to the boiling point and starts condensing - only after it finishes condensing can the temperature drop below the boiling point.

Most of the coil is used to condense the refrigerant, not cool it off in a conventional sense.

That "band" is where the refrigerant enters the coil. It's very hot after being discharged by the compressor. The rest of the coil is at condensing temperature.
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Old 07-13-2020, 11:50 PM   #3
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Re: A/C Cooling problem


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Originally Posted by SubnetMask View Post
Have an issue going on with the central air in my house. I've noticed that recently, it'll cycle in the through the morning, then around mid afternoon, it'll come on and not shut off until 3-4am or so. Checking the temperatures of the return and supply, there's a 15* or so degree difference, but I'm told by my step-father, who knows more about heating, but knows a bit about A/C, that it should be more a 20-25* difference. The condenser coils weren't too bad (I had already cleaned the huge majority of the 'fuzz' that routinely builds up on the coils), but per his recommendation, I degreased and cleaned the heck out of them, which made no difference in the temperature differential. After that, though, I noticed something odd (I doubt it has anything to do with the cleaning, I just never noticed it before). As I was feeling different spots on the coil, I noticed that there's a 'band' about 2/3's of the way down that's MUCH hotter than above or below that area, which seems odd. It's about a 6 or so inch band that I wouldn't say is 'burn you' hot, but I think it's reasonable to say that it's uncomfortably hot. Above and below (below being the bottom 4-6" of the coil), it's FAR cooler. It seems almost as if all (or most) of the heat that it's trying to removed from the house is only being stuffed into that band of the coil (And I could be totally wrong on that). The temperature differential not being quite where it should be, by itself, could possibly be the result of a low charge, but the 'hot band' on the condenser coils seems really odd to me. Unfortunately, my FLIR One died, and I'm waiting on it to get to them and for them to send me a replacement, so I can't add the proverbial 'Picture that's worth 1,000 words' to show the temperature differential that I'm feeling in the coil.

That being said, does anyone have any 'off the cuff' thoughts? The main thing that has me scratching my head is the 'hot band' on the condenser. That seems really odd.
New filter. If you dont change your filter at the furnace once a month, and its more like once every time you can remember? then you have a dirty A-coil. And the air isnt able to pass through that A-Coil.

Causing all sorts of fits.
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Old 07-15-2020, 09:02 AM   #4
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Re: A/C Cooling problem


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Originally Posted by user_12345a View Post
It's normal to get continuous operation in very hot weather. Whether your system is performing well or not, I don't know.

A 15F drop is a bit low if the return air humidity is around 50% - but, the measurement must be taken at the air handler to be accurate as there's duct heat gain.



Return air humidity affects the temperature drop, as does fan speed.

The system should be professionally checked:
The fan speed must be set to move the correct cubic feet per minute for the size of a/c on your duct system. There are different ways to determine airflow.

The refrigerant charge needs to be checked properly and adjusted if necessary using the superheat/subcooling method (no beer can cold, no pressures only). This should only be done after making sure the blower speed is correct.

Operating capacity can be calculated based on measured airflow and wetbulb return/supply temperatures.

If the a/c itself is working properly yet the house isn't cooling well, you could have a air duct problem.



Normal.


The refrigerant is warmer than boiling point when it enters the outdoor coil, but quickly cools down and starts condensing into a liquid.

Do you know how you can't heat water above 212F on a stove unless you're using a pressure cooker?

After that point, the heat applied to the water doesn't raise the temp, it goes towards converting the water to steam. Only after the water is steam can it be heated above boiling point.

The same applies to refrigerant in the condenser coil, but backwards. It starts as superheated "steam", cools to the boiling point and starts condensing - only after it finishes condensing can the temperature drop below the boiling point.

Most of the coil is used to condense the refrigerant, not cool it off in a conventional sense.

That "band" is where the refrigerant enters the coil. It's very hot after being discharged by the compressor. The rest of the coil is at condensing temperature.
Thanks for the info. According to my thermostat, the inside air is 45-50% humidity. Temperature readings were taken at the air handler. Some other helpful info is the condenser is a Bryant 113RNA030-B, which I thought I read was a 14.5 SEER unit, but it may be 13 SEER.

A number of years ago the system was professionally checked at which time it was found that whoever had installed it had set the blower controller up wrong. When set up right, it knows if it's heating or cooling and can run the appropriate fan speed. It was set up to run on low all the time, so since heating has much lower airflow requirements than cooling, it was fine for heating but not enough airflow for cooling. This was changed and since then it runs on low for heat and high for cooling. It's had several 'checkups' since that time, and no one has ever found anything wrong with it.

I did check the pressures, and with an external temp of about 93* and air returning the the air handler at about 74, it was running at 69/320. This may not be the proper way to FULLY check the charge as you indicated, but none of the service techs that have ever been out have done anything more than that.

I do feel like it's ALWAYS had trouble properly cooling the house and have wondered for some time if the house isn't properly ducted or balanced.

Quote:
Originally Posted by Hackwork
New filter. If you dont change your filter at the furnace once a month, and its more like once every time you can remember? then you have a dirty A-coil. And the air isnt able to pass through that A-Coil.

Causing all sorts of fits.
It has a brand new filter.

Not changing the filter often enough won't cause the A coil to get dirty. What not changing the filter often enough will do is as the filter gets dirtier, airflow reduces. As the airflow reduces, pressures go up and the system becomes less efficient. If the filter gets restricted enough and the airflow low enough, then you can get into the A Coil icing up.

What WILL cause the A Coil to get dirty is running no filter at all.
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Old 07-15-2020, 03:29 PM   #5
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Re: A/C Cooling problem


For a home owner (without a gauge), 3 things can be checked if you think you have an AC problem. 1 temp drop, 2 airflow, 3. home insulation. your 15F drop is a little low at 45-50 hum, but should still be able to reach the target temp in the afternoon unless the house is not well insulated (or West sun) or has weak airflow. to check airflow without tool, just stand in front of one or two strong AC vents, you should feel the cool air from 10 feet away. Now If you do have an AC gauge, and know how to use it, you should get the superheat/subcool numbers to make sure your freon charge is correct. I can see your 320 highside pressure (assume you have R22) is out of the norm. For Temp drop, I like to see 18-19, but not above 20.

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Old 07-15-2020, 03:46 PM   #6
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Re: A/C Cooling problem


Quote:
I did check the pressures, and with an external temp of about 93* and air returning the the air handler at about 74, it was running at 69/320
Your head pressure is way too high, thoroughly clean your condenser coil and make sure the outdoor fan is working properly. Weak capacitor, failing bearings can slow down the condenser fan.

The condenser coil can look clean on the outside but be packed, especially the 13-14 seer builder's units with fins very close together.

You have to measure return air wetbulb to determine target supply temperature.

Lower than normal drop is caused by high humidity, blower speed being too high or reduced capacity or a combo of those.

With high humidity or blower speed too high, i would think the low side pressure would be a little higher than 69 under those conditions.

Does your evap coil have a txv or piston?
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Old 07-15-2020, 04:38 PM   #7
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Re: A/C Cooling problem


Also be aware that non-condensibles can and will accumulate in the top of the condenser coil and cause the symptoms you are experiencing. You'd maybe know if someone has worked on the unit with less than full knowledge and care and possibly let air into the system.
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Old 07-15-2020, 05:25 PM   #8
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Re: A/C Cooling problem


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Your head pressure is way too high, thoroughly clean your condenser coil and make sure the outdoor fan is working properly. Weak capacitor, failing bearings can slow down the condenser fan.

The condenser coil can look clean on the outside but be packed, especially the 13-14 seer builder's units with fins very close together.

You have to measure return air wetbulb to determine target supply temperature.

Lower than normal drop is caused by high humidity, blower speed being too high or reduced capacity or a combo of those.

With high humidity or blower speed too high, i would think the low side pressure would be a little higher than 69 under those conditions.

Does your evap coil have a txv or piston?
The condenser was very thoroughly cleaned. I pulled the top off, used diluted simple green on the inside and outside and thoroughly rinsed it off spraying from the inside to the outside to push any junk out rather than deeper in.

How would you test the condenser fan? The bearings are probably ok as it turned freely and smoothly with no odd noises when I had the top off. Only thing left might be the capacitor. It doesn't seem to be running any different than in the past and it is moving a ton of air.

I don't it has a txv or piston.

This may be a matter of the unit is working fine, but it simply isn't big enough or the ducting wasn't designed properly to allow it to keep up with the baking sun. In the mornings, the house is fully shaded, so the only heat it has to contend with is the 'warm bodies' in the house and ambient temperature, but come noon or so, the house starts getting baked by the sun because the westward exposure is in full sun with zero shade. Add to that the house was build in 1978 or so, the insulation in one half of the house I have no idea on, because it's encased in the 2x8 construction that is the roof and ceiling, and the insulation in the other half of the house, which of course is the end that is generally better and where the thermostat is, is I believe two layers of roll insulation (don't know the R value).

I may end up having to break down and call someone out, making sure they come on a hot, sunny afternoon, and see if they can come up with any more info.
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Old 07-15-2020, 05:31 PM   #9
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Re: A/C Cooling problem


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Also be aware that non-condensibles can and will accumulate in the top of the condenser coil and cause the symptoms you are experiencing. You'd maybe know if someone has worked on the unit with less than full knowledge and care and possibly let air into the system.
Not too many people have looked at it since we bought the house, but I also don't know anything about those that installed it. For all I know, maybe it wasn't fully and properly vacuumed down when it was installed.
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Old 07-15-2020, 05:43 PM   #10
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Re: A/C Cooling problem


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This may be a matter of the unit is working fine, but it simply isn't big enough or the ducting wasn't designed properl
No - it's not working fine with that head pressure at 93F outside and 74 inside.

The blower speed is set to match the tonnage on your duct system and being undersized won't cause a slightly low drop unless something else is out of whack.

It does take more info to know what's really going on.

It has either a piston or a txv and you have to find out which. Google how to tell if metering device is a txv.

Sometimes the txv is buried in the air handler.

Superheat and subcooling readings as well as wetbulb return air in addition to what you've already measured is required to tell what's going on.

Quote:
How would you test the condenser fan? The bearings are probably ok as it turned freely and smoothly with no odd noises when I had the top off. Only thing left might be the capacitor. It doesn't seem to be running any different than in the past and it is moving a ton of air.
Can check the capacitor and amp draw - tight bearings increase amp draw.

Is the air very hot? Is it

Based on your description, i don't think the condenser fan is the issue.

It could be a metering device or charge issue but again - need a complete set of readings.

Since you have gauges and thermometer, you should have no problem getting the readings.

Wetbulb - wrap the end of the thermometer with thin moist fabric, insert into return at air handler and wait for it to stabilize with the system running.

Superheat - suction line temperature minus saturation low side temperature. (your gauge should have r22 saturation temps)

Subcooling - saturation high side temperature mine liquid line temperature.
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Old 07-15-2020, 05:45 PM   #11
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Re: A/C Cooling problem


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Also be aware that non-condensibles can and will accumulate in the top of the condenser coil and cause the symptoms you are experiencing. You'd maybe know if someone has worked on the unit with less than full knowledge and care and possibly let air into the system.
supporting info:

"ir doesn’t belong in a refrigeration system, and if it gets in, it will cause problems. Air can enter a refrigeration system in many ways, including through tube, gasket, or flange leaks; poor charging procedures; poor recovery or recycling procedures; or by forgetting to purge hoses when accessing systems.

If air gets into a system, it will collect in the top of the condenser and be trapped. Air is a non-condensable and cannot be condensed like refrigerant vapors. The liquid seal (subcooled liquid) at the bottom of the condenser will prevent air from leaving the condenser. Air will cause a reduction of condensing surface area and cause high condensing (head) pressures."

https://www.achrnews.com/articles/12...eration-system
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Old 07-15-2020, 05:54 PM   #12
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Re: A/C Cooling problem


Improper initial install is the usual reason for non condensables to show up in the system. Small amounts can be generated by the breakdown of the oil or from moisture reacting to the refrigerant to release gasses. It can be said that small amounts exist in all systems but with good practices they can be held to a low enough level to be negligable.

They always migrate to the condenser coil. To the extent that they occupy space there, that portion of the condenser coil is ineffective for condensing refrigerant. When NCG's reach a high enough level, the head pressure will increase dramatically and subcooling will be lowered or non existent. System performance is lower as a result.
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Old 07-15-2020, 06:13 PM   #13
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Re: A/C Cooling problem


^I'm thinking it could just be overcharged with a txv keeping the low side in check or metering device under-feeding and it was over-charged to compensate.

Without more data it's a guess.
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Old 07-15-2020, 06:50 PM   #14
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Re: A/C Cooling problem


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Without more data it's a guess.
I agree, it'll take a savvy tech measuring the temps and pressures to narrow it down and if it's indeed NCG's in the system, a recovery unit and a virgin gas will be needed.

I can't guarantee it's a NCG problem since doing so without being there is like hearing hoofbeats and expecting unicorns. It would be a bit unusual but I have seen it happen a few times and it came to mind when the OP mentioned the unusual temperature distribution on the condenser coil.
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Old 07-15-2020, 08:40 PM   #15
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Re: A/C Cooling problem


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No - it's not working fine with that head pressure at 93F outside and 74 inside.

The blower speed is set to match the tonnage on your duct system and being undersized won't cause a slightly low drop unless something else is out of whack.

It does take more info to know what's really going on.

It has either a piston or a txv and you have to find out which. Google how to tell if metering device is a txv.

Sometimes the txv is buried in the air handler.

Superheat and subcooling readings as well as wetbulb return air in addition to what you've already measured is required to tell what's going on.



Can check the capacitor and amp draw - tight bearings increase amp draw.

Is the air very hot? Is it

Based on your description, i don't think the condenser fan is the issue.

It could be a metering device or charge issue but again - need a complete set of readings.

Since you have gauges and thermometer, you should have no problem getting the readings.

Wetbulb - wrap the end of the thermometer with thin moist fabric, insert into return at air handler and wait for it to stabilize with the system running.

Superheat - suction line temperature minus saturation low side temperature. (your gauge should have r22 saturation temps)

Subcooling - saturation high side temperature mine liquid line temperature.
Thanks, I'll try to get the additional readings. I'll probably have to wait until the weekend when it's supposed to be hotter and sunnier. It'll probably be easier and more accurate to take them when it's running constant rather than running around trying to get readings during separate run cycles.

One question on the wet bulb - the 'how-to' for getting wet bulb that I read says to use muslin for the fabric. Does it have to be a specific fabric?

Also, one other likely useful piece of information - I was poking around the A coil with an endoscope trying to determine if it has a txv or piston. I couldn't find anything that looked like a txv, but I got the manufacturer and model number - ADP HA02230C175B160567, which from what I gather is a 2.5 ton with a piston, not a txv, and it looks like superheat is needed with a piston.

Edit: But I just noticed something else - On the condenser label, it says 'Metering Device: TXV Indoor'. I couldn't find much on the A Coil, but I did find a thread on justanswer, where it seems that coil has a piston metering valve..... So either this one got switched to a txv and it's buried where I can't find it, or it doesn't. If it doesn't have a txv, and the condenser specifies that the metering device should be a txv, what are the repercussions?

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