Any home A/C experts here?

[Cougar281]

Have an issue going on with the central air in my house. I've noticed that recently, ti'll cycle in the through the morning, then around mid day, 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 coils outside weren't too bad, but per his recommendation, I degreased and cleaned the heck out of the outside coils, which made no difference in the temperature differential. After that, though, I noticed something odd. 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.

Does anyone have any thoughts or suggestions?

 

[gmduramax]

I would check the pressures first. Could be low, could be a blockage. Usually you should have about a 20-25° split.

 

[Cougar281]

But would being low cause the 'band' of hotter coil? The temperature difference not being where it should be could, by itself, be the result of a low charge, but the band of much hotter coil seems really odd. Unfortunately, my FLIR One died, so I'm waiting on a warranty replacement, so I can't put a picture to my description.

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[gmduramax]

But would being low cause the 'band' of hotter coil? The temperature difference not being where it should be could, by itself, be the result of a low charge, but the band of much hotter coil seems really odd. Unfortunately, my FLIR One died, so I'm waiting on a warranty replacement, so I can't put a picture to my description.

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that I don’t know, I would think it could be possible because there isn’t enough freon dissipating heat

 

[2004LB7]

temp differences between supply and return vents are typically closer to 20* when all is working well. I usually don't expect more then that for a properly working and sized unit in a house setting. 15* is not too uncommon when it is really hot out, like 90*+.

what you may want to do is get us the square footage and rough idea of the insulation quality of your house. get us the unit size in BTUs or Tons. and outdoor temps and what temp you are trying to maintain. I can check to see how it was sized. most home units are sized so that the unit is just big enough to maintain the temperature on the hottest days while running continuously. this is to prevent short cycling during other times of the year which is hard on the compressor.

the hot band on the condenser is likely the hot gas discharge from the compressor. depending on the design, there is usually about 1/3 to 2/3 of the condenser filled with liquid refrigerant which has been subcooled. the, normally but not always depending on the tubing layout, top one or two 3rds is the hot compressed refrigerant being cooled by the airflow. when it drops below the point in which it condenses in to a liquid, it sits at the bottom of the coil. this is where it moves in the the liquid line (smaller of the two and usually hot) and up to your indoor coil or evaporator.

you can also measure the temperature of the two copper lines that run into your house right next to the condenser with a contact thermometer and give me those too. make sure you get good contact and maybe even tape over the probe.

while it would be nice to get pressure readings too, so as to get a much better idea of of how your unit is running, this will have to do for now

 

[Cougar281]

temp differences between supply and return vents are typically closer to 20* when all is working well. I usually don't expect more then that for a properly working and sized unit in a house setting. 15* is not too uncommon when it is really hot out, like 90*+.

what you may want to do is get us the square footage and rough idea of the insulation quality of your house. get us the unit size in BTUs or Tons. and outdoor temps and what temp you are trying to maintain. I can check to see how it was sized. most home units are sized so that the unit is just big enough to maintain the temperature on the hottest days while running continuously. this is to prevent short cycling during other times of the year which is hard on the compressor.

the hot band on the condenser is likely the hot gas discharge from the compressor. depending on the design, there is usually about 1/3 to 2/3 of the condenser filled with liquid refrigerant which has been subcooled. the, normally but not always depending on the tubing layout, top one or two 3rds is the hot compressed refrigerant being cooled by the airflow. when it drops below the point in which it condenses in to a liquid, it sits at the bottom of the coil. this is where it moves in the the liquid line (smaller of the two and usually hot) and up to your indoor coil or evaporator.

you can also measure the temperature of the two copper lines that run into your house right next to the condenser with a contact thermometer and give me those too. make sure you get good contact and maybe even tape over the probe.

while it would be nice to get pressure readings too, so as to get a much better idea of of how your unit is running, this will have to do for now

The condenser is a Bryant 113RNA030-B, which from what I gather is a 14.5 SEER unit that I THINK is in the ballpark of a 3 ton model. Could be a little more, could be a little less. I have no idea what the A coil is. Square footage of the house is something like 1,200. Upper insulation in about half of it I couldn't tell you as it's 2x6 vaulted ceiling (roof on one side, inside ceiling on the other). The other half of the house has attic insulation that's probably not 'awesome', but also not the worst I've seen (It's mostly 6" or so thick fiberglass). The main part of the house with the unknown insulation vaulted ceilings is the part of the house that always seems to be too hot, and where the thermostat is is roughly in the middle of the 'not awesome but not horrible' part of the house and usually feels cooler than the main part of the house. Wall insulation I have no idea. 2x4 wall construction, with one spot between the garage and main house that I opened up being reasonably stuffed with insulation, so I ASSUME most of the exterior walls are more or less the same. Most of the 'basement' walls have been re-insulated with what I think was R22 (what I could fit in the existing cavities). Measuring the cold line right now, it came in at about 40*, just outside the A frame. The hot side seems to be around 74, which is not that far above ambient in the house (current temp at the thermostat is 71). Current temperature trend for my area, from my logged data, shows the peaks to be in the 90's, the last few days being 93, 95, 94, 95, 93, 93, but generally, by midnight, it's down to about 85. The peaks seem to be around 5:15-5:30 and they pretty much tank from there. Tonight it was down to 85 at about 10:30. I'll see about getting my step-fathers R-22 gauges to get a read on what the pressures are. Proper spec for the unit seems to be 300 on the high side and 150 on the low side according to the spec label.

 

[2004LB7]

so would you say the insulation is "average"? most newer homes have good to great insulation while older homes could be poor.

off the top of my head, 3 tons seems appropriate for 1200 sf unless the insulation was pretty bad.

return line temp doesn't look out of line but discharge line is way too low for your outside temperatures. you should be seeing temps closer to outdoor temps or slightly warmer. makes me think the refrigerant is boiling in the high pressure line. only way that happens is the refrigerant is low from leaking out.

inspect the indoor evaporator coil for icing, dirt, clogged filters ect.

the 300/150 psi is in reference to the design test pressure and not the operating pressure. if the coils are clean, the fans are all working, etc. then the pressures for a properly operating system should be closer to 65-68 psi on the low side and 250-300* on the high. of course you need to take into account the indoor and outdoor temps when judging those pressures. take a look at the attached pressure temperature chart. you want the inside temp of the coil to be above freezing but not so warm that it doesn't cool well. 38-40* is a good temp to shoot for. if you look at the chart, 38-40* runs at 65-68 psi. but this all depends on the air temp running across the coil, or in other words, the indoor return temp. if it is warm in the house it may be as high as 85 psi. also, keep in mind, if the run between your condenser and evaporator is long there may be a few psi drop on top of that. for the high pressure side, it needs to run hot enough to be able to reject the heat into the outside air. this will normally run around 20* to 30* above outside temps. looking at the chart again, we can see that if it is 95* outside then we need to run about 115-125* which puts you at 243-278 psi. add a little dirt, sun, etc and that pressure can run a little higher.

just remember, it is easy to get hung up on exact numbers but that is the wrong way to do it. the proper way to measure charge is to weight in the specified amount. the other ways are measuring superheat or subcooling depending on the unit type. but pressure can get you close if you know what you are looking for

 

[02greysixer]

I wish mine held the house down to 74* at the warmest. I'm in central Fl so with the humidity factor it feels like the underside of satan's nutsack.
1400 sqft built in 06 w/ original 3 ton unit. House is pretty well insulated but 0 shade at all. Middle of a cleared 2 acre lot. On a sunny day it will run continuous by 10ish a.m. and not cycle again till midnight or so. With it set to 75* I'm lucky to maintain 80* at the thermostat if the wife is using the stove/oven.
I have a buddy with an hvac business that gives it a once over every spring and it's as good as it can get. He said given the fact that there is no shade he would have installed a larger unit.

I'm just riding it out till it takes a shit then we will upgrade. Luckily it rains 4 or 5 days a week here and that helps out a lot.

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[THEFERMANATOR]

The higher the SEER of the unit, the lower the difference you will see between intake and outlet duct temp. 15 degrees for a 14.5 SEER unit sounds pretty good. I know Rheem specs 12-14 degrees temp drop on there 15 SEER unit as my grandfather swore his wasn't working right when he had them put in. 11 SEER should see a 18-22 degree difference. Being a 14.5 SEER unit, I'm surprised it's not r410a.

 

[Chevy1925]

interesting info. didnt realize its good to have the units run more than less. we are still running the original roof top 1979 unit on our house. we keep the inside at 79-80* on 110+* days. it will cycle till about 3 or 4pm, then run continuous till about 8 or 9 but we drop the temp to 78/77 at 7pm as well.

i thought it died last year when my wife called that the a/c aint kicking on, was just an old ass fuse that blew. threw two new ones in for good measure and off it goes.

 

[2004LB7]

so your Bryant unit, based on model number, specs out as a 13 seer, 208/240 volt single phase, and what I believe to be 3 ton or perhaps 2.5 tone but its unclear as it doesn't specify if the numbers are in BTUs or tons. the last three digits (030) is supposed to specify the capacity, and that could be interpreted as 30,000 btus or 3 tons

 

[Cougar281]

so your Bryant unit, based on model number, specs out as a 13 seer, 208/240 volt single phase, and what I believe to be 3 ton or perhaps 2.5 tone but its unclear as it doesn't specify if the numbers are in BTUs or tons. the last three digits (030) is supposed to specify the capacity, and that could be interpreted as 30,000 btus or 3 tonsView attachment 104203

Interesting. I'd SWEAR I saw it referenced as 14.5 SEER somewhere. Anyway, I got my step father's guages and it's running at about 69/320. The condenser coils were just cleaned and the filter replaced. I can't see the underside of the A coil but I stuck an endoscope in through a crack and the top seems clean with no signs of icing that I can see. The filter is a pleated filter that is probably a tad more restrictive than the blue ones you csn read a book through that are basically only good for keeping mice and birds from being sucked through lol.

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[2004LB7]

what was the temperature of the return line when you took the pressure measurement? 320 psi is a little on the high side but not too unusual on a hot day. are you sure the condenser fan is operating correctly?

 

[Cougar281]

what was the temperature of the return line when you took the pressure measurement? 320 psi is a little on the high side but not too unusual on a hot day. are you sure the condenser fan is operating correctly?

I believe it was around 43 at that time. It cooled off some outside and a bit later it was down around 41.

 

[2004LB7]

well, if you are taking the measurements correctly then all it looks like is the load is high or you have a dirty condenser. since you said you recently cleaned it, that leaves high load. just a guess, the unit may be slightly undersized for your needs, especially on a hot 90+* outside temp.

looks like your three choices are, add insulation, add a second unit, or upgrade to a bigger one. but not being on site and taking the measurements and seeing the situation myself, it is hard to tell correctly if we are on the right track and haven't missed anything. if you have still have doubts or would like a second opinion, get a service technician out and have them go over everything to see if there is any issues

 

[Cougar281]

Thanks for the input. I did determine that the A coil is a 2.5 ton, so either the condenser is a 3 ton and the a coil is under sized, or Bryant uses BTUs instead of tonnage on that line.

If I go the route of having someone out, I may have to do a lot of research and find a different company. Of the last guys that were out, the only thing that was found wrong a long time ago was the blower was wired up to always run on low for both heating and cooling. Since cooling requires more airflow than heating, it wasn't getting enough originally.

 

[2004LB7]

I've seen missmatched coils a few times. nothing particularly wrong with it, as long as it isn't too far off. what you want is a condenser large enough to remove all the heat from the evaporator. but if the condenser is measured in BTUs then they are exactly matched.

remember, if there isn't any issues with the unit and it is just slightly undersized or it is excessively hot that day, the new service tech may say the same thing.

I just serviced a unit for a friend the other day. low cooling rate. tried cleaning the condenser and then it just stopped all together. called me out, I found a bad capacitor as the reason for quitting. and a dirty evap coil for low cooling efficiency. their unit uses the same A style coil. inspection of the coil by them was not very thorough as access was poor. but by removing some panels and covers, I was able to see about half of each coil on one side was blocked with dirt and pet hair. with an acid wash and a thorough cleaning, the unit preformed well again. I left then with some instructions on the proper filters to use as they were using those fiber mesh type and it was letting a ton of dirt and hair past it and over time the coil plugged up