My home's central A/C system is driving me nuts

[TheOtherGoose]

At night my central A/C takes several (like 5) hours to drop my home's temp from 75 degrees to 74. The highest temp today, for example, was 84.

I can't figure out what's wrong. The four-year-old HVAC system is the correct size for the house, the system/filter is clean and the refrigerant is properly charged. I have checked the outside condenser and it's clean. The obvious things are good: the registers are open and the windows/doors are closed. I've actually tried a few different thermostats just to eliminate it as the problem. I've got cool air coming from all the registers, although I can't say exactly what the temp of the outlet air is.

It's odd - during the day, the A/C will maintain 75 degrees on a hot day, cycling normally (about 10-15 minutes every so often). Turn down the temp a degree or two (74 or 73) for a cooler night's sleep, though, and the HVAC struggles to reach the new temp.

Any ideas of what to have checked?

I'm quite frustrated with the situation... especially since I'm not asking much from the A/C. Could you imagine this system trying to achieve 70 degrees?

 

[OccupantRJ]

I would check the evaporator coil to see if it is clean and to see if it is icing up and check input/output temps with a thermometer. Are you using one of the restrictive filters? Are the outlet registers the fancy designer restrictive style? No air flow, no air conditioning.

 

[MovingAlong]

I'd be looking at my insulation and duct work, right after reviewing historical trends: review your electric bills for the last 4 yrs, review local temps for the last 4 years, plug it into your favorite AI model - does one trend follow the other?

If the data doesn't track, then you're looking for something that changed..

 

[MovingAlong]

I would check the evaporator coil to see if it is clean and to see if it is icing up and check input/output temps with a thermometer.

Absolutely, need hard data to track trends. Real numbers..

 

[danski0224]

At night my central A/C takes several (like 5) hours to drop my home's temp from 75 degrees to 74. The highest temp today, for example, was 84.

Is this a change, or has it always performed in this manner?

I can't figure out what's wrong. The four-year-old HVAC system is the correct size for the house,

How do you know this? Load calculation?

the system/filter is clean

Observation only, or static pressure readings? A 1" pleated air filter absolutely kills airflow.

and the refrigerant is properly charged.

How do you know this? Subcooling? Superheat?

I have checked the outside condenser and it's clean.

Double row of coils? Did you look really closely- some have an extra "louver" in the fins that traps dirt.

The obvious things are good: the registers are open and the windows/doors are closed. I've actually tried a few different thermostats just to eliminate it as the problem. I've got cool air coming from all the registers,

Ok. But what is the actual delivered airflow at the fan and static pressure (TESP)?

although I can't say exactly what the temp of the outlet air is.

Observation only.

It's odd - during the day, the A/C will maintain 75 degrees on a hot day, cycling normally (about 10-15 minutes every so often). Turn down the temp a degree or two (74 or 73) for a cooler night's sleep, though, and the HVAC struggles to reach the new temp.

If it is sized correctly, it will run 24/7 at design temp. It can take hours to drop a degree on the AC side. May need to check enthalpy to see what actual capacity is under operating conditions.

Any ideas of what to have checked?

I'm quite frustrated with the situation... especially since I'm not asking much from the A/C.

Without knowing actual load calc and equipment specs, hard to say.

Could you imagine this system trying to achieve 70 degrees?

Air conditioning specs are figured for 85º outside and 77º inside. Anything lower indoors requires that the equipment capacity is derated. Higher outdoor temps will do the same thing to overall capacity.

Manufacturers provide actual cooling capacity specs at a specified airflow for wide range of indoor and outdoor design conditions.

It will not be easy to actually get to the cause of the issue, and almost no one that is in this forum will actually do the detail work, so good luck.

You can search for procedures to do anything mentioned above. There are no easy answers to specific and detailed comfort complaints.

 

[BillK]

At night my central A/C takes several (like 5) hours to drop my home's temp from 75 degrees to 74. The highest temp today, for example, was 84.

1. Has this system always done this or is it a new development ?
2. If this system is 4 years old what was there before ? How well did it work ?
3. Before you go any further you really need to know what the temperatures are at the intake and at the outlet vents.

 

[TheOtherGoose]

I appreciate all of your thoughtful replies. Thanks.

I have only a basic understanding of home A/C. I had the HVAC unit replaced when I moved in (replacing the 15-year old unit was more cost-effective), so I don't know how the last system performed. The current unit is a Daikin 2 ton 13 SEER size for a ~1,400 sq ft single story house built in 2008. The old unit was about the same size. The contractor sized the current unit, which to my basic understanding (for what that's worth) seems right for the house.

For all I know, the previous HVAC system performed the same way.

One of the HVAC guys (whom I've gotten to know over the last four years) commented on the design of the duct work. Blunt-ended (rather than angled) transitions and dead ends have been pointed out. I assume such things probably hurt efficiency, but I also assume such things won't be that dramatic.

My contractor has been very reputable and has responded to several call-backs. I am starting to wonder how good his techs are, though, due to the various problems we've had:

During the first heating season, the intake/exhaust pipes had to be re-sized to correct the furnace ignition.
Only now have the original refrigerant lines been replaced. Up to this point, the A/C has had a slow leak that has required a seasonal top-up. The install crew purged/re-used some of the old system's line to avoid installation difficulty. I assumed this was okay, since I know that call-backs are expensive. The recent line replacement was free to me (I assume two guys for 4+ hours would cost a fair amount).

I know I should pay another company for their opinion of the situation, but I hate the idea in light of what I paid for the install.

At this point, I guess I should just pay for another pro's assessment of the system...

 

[Dakotadadv8]

Change the filter it may look clean but is not. Run the fan only to get rid of potential icing. Change filter every 30 days IMHO cheap insurance.

 

[BillK]

I know I should pay another company for their opinion of the situation, but I hate the idea in light of what I paid for the install.

At this point, I guess I should just pay for another pro's assessment of the system...

From all of your descriptions I would say it is absolutely time to have somebody else take a look at it.

 

[FullRaceMerc]

From all of your descriptions I would say it is absolutely time to have somebody else take a look at it.

x2.

2 ton seems a little undersized for 1,400 sq ft, at least around here. There are online calculators that factor in your region.

Other factors can affect performance. Is the attic vented properly? Ducting all in good shape? Return air duct & plenum in good condition inside? Loose sidewall insulation can be sucked into the path & hinder airflow. Good clearance & airflow around the condensor? Single pane windows? Good insulation in the house? As mentioned above is the evaporator icing up?

 

[mike93lx]

x2.

2 ton seems a little undersized for 1,400 sq ft, at least around here. There are online calculators that factor in your region.

Other factors can affect performance. Is the attic vented properly? Ducting all in good shape? Return air duct & plenum in good condition inside? Loose sidewall insulation can be sucked into the path & hinder airflow. Good clearance & airflow around the condensor? Single pane windows? Good insulation in the house? As mentioned above is the evaporator icing up?

2 tons could be anywhere between oversized to undersized. Too many factors to determine anything based on square footage. If it was a good way to measure, I'd say it feels about right based on the cooling needs my house has, but it's a different climate than Ohio and I have no idea on how our houses compare.

Also, most systems aren't sized based on data, so that makes it even harder to compare

 

[PoorUB]

x2.

2 ton seems a little undersized for 1,400 sq ft, at least around here. There are online calculators that factor in your region.

Other factors can affect performance. Is the attic vented properly? Ducting all in good shape? Return air duct & plenum in good condition inside? Loose sidewall insulation can be sucked into the path & hinder airflow. Good clearance & airflow around the condensor? Single pane windows? Good insulation in the house? As mentioned above is the evaporator icing up?

It depends. I am cooling 1800 sqft on 1-1/2 tons, and it works well.

I see split entry, 2400 sqft homes with 2 tons all the time.

My daughter's house is 3500 sqft with 3 tons.

Going by those numbers, 1 ton per 1000 sqft, but it depends!

 

[fitter30]

With a probe type thermometer not ir. Unit running for 20 minutes take temp at filter and 16-18" from ac coil. 18-23°. 20° is usually design. Now compare temps at return air grill and at filter. At the outside unit insulated line feel the bare copper is it ice cold and sweating? Electronic thermostat? Look at installer setup. Can download the install manual has tat info for setup. Cycle rate cooling is it set on 3? Any changes write them down. Ashrae design for Ohio ranges 90-93°. Have a digital meat thermometer can use that.

 

[jshillin]

If you haven't had it serviced recently, might be a good idea to have a tech out. 13 SEER is really inefficient, but unless something is wrong it should satisfy the thermostat at those temperature settings.

 

[TheOtherGoose]

Again, thank you for your replies. I'm having another HVAC company come by tomorrow afternoon to evaluate and test the system.

I'm sad to say that the recent total replacement of the refrigerant lines may also be a failure. I may be imagining it, but it sounds like the fitting at the A coil might not be fully tightened; there's a hissing sound that I don't remember hearing before the line replacement. It's possible that the hissing is simply air escaping the A coil enclosure, but I'm not sure.

I'll be very interested to find out the state of the refrigerant charge (among other things) tomorrow.

 

[jhelrey]

Hissing could be refrigerant moving through the lines.

Put an instant read thermometer in one of your vents and note the temp. Spray down your condenser extremely well, especially if it's HE. check your temp readings again.

Make sure your filter is good.

 

[Yankeefarmer]

If you want 74 at night, have you tried just setting it at 74 and leaving it there for a few days?

 

[fitter30]

Again, thank you for your replies. I'm having another HVAC company come by tomorrow afternoon to evaluate and test the system.

I'm sad to say that the recent total replacement of the refrigerant lines may also be a failure. I may be imagining it, but it sounds like the fitting at the A coil might not be fully tightened; there's a hissing sound that I don't remember hearing before the line replacement. It's possible that the hissing is simply air escaping the A coil enclosure, but I'm not sure.

I'll be very interested to find out the state of the refrigerant charge (among other things) tomorrow.

When unit shut off the hissing us cause by equalizing pressures since running there is a high pressure liquid and low pressure vapor.

 

[danski0224]

I'll be very interested to find out the state of the refrigerant charge (among other things) tomorrow.

You will want to know what the subcooling and superheat values are.

Without those, the refrigerant charge isn't being checked properly.

You don't want the guy that says it's 2 pounds low, before even hooking up the gauges. Lots of those guys out there.

 

[mike93lx]

You will want to know what the subcooling and superheat values are.

Without those, the refrigerant charge isn't being checked properly.

You don't want the guy that says it's 2 pounds low, before even hooking up the gauges. Lots of those guys out there.

I'll do it for a pound and a half

 

[danski0224]

I'll do it for a pound and a half

Sold...

 

[Aileron]

Until someone actaully puts a set of gauges on it, your beating your head against the wall. You can change a clean air filter all you want , if it has non condensables in it, low on ref, too much ref, resticted filter drier, hell a inefficent compressor itself,a plugged orfice, stuck txv , it isn't going to help. You will not know until a set of guages go on and most importantly and this is the hard one these days it seems, getting someone that knows how to use them and knows what the little numbers mean.

 

[danski0224]

knows what the little numbers mean.

Eighths, sixteenths or 32nd's?

 

[Aileron]

Eighths, sixteenths or 32nd's?

Those numbers are beyond repair!

 

[joebass3]

What filters do you use? Filters with a very high Merv rating will affect cooling.

 

[OccupantRJ]

As some say around here, “might need some freeze-on in it!

 

[TheOtherGoose]

Again, thank you for your advice, guys.

Today's visit will be from another HVAC contractor in town that has a good reputation. Unfortunately, the contractor that I've been using up to this point also has a "good reputation". My plan is to have this new guy take a look at my system with (I assume) far more understanding about A/C than I have.

I hope that we can establish what (if anything) may be wrong with my A/C. I will be sure to get, as requested, subcooling and superheat figures. I will also get input/output temps. Frankly, I'll be impressed if this new guy will know what anthalpy is (I certainly don't), but I'll be sure to mention it.

I noticed that the fan in furnace has low/med/high speed options. Since this fan moves A/C air as well, is this part of the overall equation for A/C performance? I ask about this when I think about the conditioned air coming from the registers -- temp is important, but what about the volume of air?

 

[BillK]

I will be sure to get, as requested, subcooling and superheat figures. I will also get input/output temps. Frankly, I'll be impressed if this new guy will know what anthalpy is (I certainly don't), but I'll be sure to mention it.

Just my opinion but . . . . . if I was an HVAC contractor and a customer started asking me if I knew what I was doing (which is what asking those questions amounts to) it would be a pretty big turnoff and might affect the results of his visit.

Just tell him what your concern is and let him do his job.

The same thing applies to any of the trades to be honest with you.

Just my opinion

 

[TheOtherGoose]

I agree with you, Bill.

I always approach these situations as a learning experience (as long as the tradesperson doesn't mind). I'm quick to understand if someone is uncomfortable with a customer "hovering" while they work.

 

[Aileron]

I was in heavy commercial , industrial. had guy I worked with come in and asked if I would go help him that evening to a another building another contractor took care off. We where the two largest in town. The other contractor told the cutomer they need a compressor but he said the coils just needed cleaning. I went and it didnt sound right when we got on the roof. It was a 4 cylinder Trane semi hermatic compressor. I put my gauges on it and yep it was running 425# head pressure around 6 in the evening, getting ready to trip the internal relief in the compressor (reason thyye said for needing a new compressor). I reached down and felt a cool liquid line. It should have been red hot. I told my buddy,clean the coils if you want to but that isn't the issue, this has to have non condensables in it. I'll grab a reclaim tank and take some off the high side and see what it does. The charge is screwed anyway. I grabbed a 40# reclaim tank and pulled out of the liquid line. Now these Tranes have sight on the liquid line, but you cannot charge by them. I filled the 40# tank up and never did get it to flash in the sight glass. it wasn't non condensables , it was overcharged. Unit took off and was working great. Gave the maintenace man the tank of R22 for their future use since it was still good. He called on the radio to his boss and mind you the unit is running like a finally tuned cadillac now and his boss calls back and says "hows does he know that was it"? I just shook my head and walked away. They let the other company come in and change that compressor to save face. That compressor was 5 storys up and had to been a hell of a change out. We actually replaced all the rooftops about a year later. I wound up taking the maintenace over a few years after that.
So I say again, its hard to find someone who knows what the gauges are actually saying.

 

[TheOtherGoose]

Welp, I guess the situation is still up in the air and I'm not impressed. An hour after the appointed time, no HVAC guy. I call the guy. My "well-regarded" local HVAC contractor couldn't make the appointment and wasn't going to call. I'm supposed to get a call on Monday to set up another visit, but I'm not holding my breath.

People can be very disappointing...

 

[danski0224]

Maybe the other guy is reading this thread and doesn't want an ambush...

 

[mm08822]

Maybe the other guy is reading this thread and doesn't want an ambush...

Here he is>>>>>>>>

 

[TheOtherGoose]

Maybe the other guy is reading this thread and doesn't want an ambush...

You may be right on that one. I've told this "new guy" that I'm looking for his experienced, professional assessment; if we solve the problem, that would be great. The "new guy" might not understand the situation.

While I know my filter is clean, it is an upper mid-level merv "allergy" filter. As joebass3 suggested, I'm going to temporarily pull the filter just to see if performance improves.

Last night it took 2.5 hours running constantly to go from 74 to 73 degrees which just doesn't seem right.

 

[danski0224]

While I know my filter is clean, it is an upper mid-level merv "allergy" filter. As joebass3 suggested, I'm going to temporarily pull the filter just to see if performance improves.

Plain fiberglass cheapo filter.

Can't suggest anything else without seeing it and testing.

Last night it took 2.5 hours running constantly to go from 74 to 73 degrees which just doesn't seem right.

This may very well be normal, if the system is sized properly.

 

[mepstein]

I don’t think dropping 1 degree over 2.5 hours is normal. I’m in SE Pennsylvania. It’s been 90+ during the day, 70 at night with afternoon thunderstorms keeping the humidity around 90-100%. Feels like a sauna outside. If I turn the temp down, I get cold air from the vent that I can feel anywhere in the room and the temperature probably drops about a degree every 15-20 minutes. Mine is just a normal 3 ton unit in a 2400sq/ft house. I’m in the woods so I don’t get the sun blasting on us all day but the trees hold in the humidity like a sauna. I do have a big dehumidifier helping out in the basement.
The OP knows something is wrong. Don’t give up.

 

[TheOtherGoose]

Thanks for your replies. I'm sorry for the blow-by-blow updates, but I think it's the only way I'll stay sane.

For the record, I tried the MERV 11-to-MERV 2 filter swap with no reduction in temperature drop time.

Re-reading all your helpful posts reminded me of what Aileron said. With this in mind I'll try to be patient and wait for a pro to review the situation sometime this week.

Until someone actaully puts a set of gauges on it, your beating your head against the wall. You can change a clean air filter all you want , if it has non condensables in it, low on ref, too much ref, resticted filter drier, hell a inefficent compressor itself,a plugged orfice, stuck txv , it isn't going to help. You will not know until a set of guages go on and most importantly and this is the hard one these days it seems, getting someone that knows how to use them and knows what the little numbers mean.

 

[BillK]

Still would be nice to know what the temperature of the air coming out of the vents is. A $10 digital thermometer would tell a lot

 

[TheOtherGoose]

I swapped out the thermostat and things may be working a bit better. I'll be very happy if it's just a bad thermostat.

For what it's worth, I put my basic digital room thermometer (probably not the best measurement device) on a register:

Output was 66.2 degrees @ 52% humidity at the end of a rather long run set to maintain 74 degrees. Outside temp is about 86 degrees. The A/C was off for five minutes before kicking on again.

Obviously, I have no widows or doors open in this heat - is the A/C kicking back on quickly saying my eighteen-year old house simply has crappy insulation?

 

[BillK]

I swapped out the thermostat and things may be working a bit better. I'll be very happy if it's just a bad thermostat.

For what it's worth, I put my basic digital room thermometer (probably not the best measurement device) on a register:

Output was 66.2 degrees @ 52% humidity at the end of a rather long run set to maintain 74 degrees. Outside temp is about 86 degrees. The A/C was off for five minutes before kicking on again.

Obviously, I have no widows or doors open in this heat - is the A/C kicking back on quickly saying my eighteen-year old house simply has crappy insulation?

I dont know the exact formula but I just stuck my thermometer in the vent in my upstairs bedroom and the air coming out is about 60 degrees. The air at the return is 75 and it is 93 outside right now. My house was built in 1978 so it is probably not as well sealed up as yours. My HVAC system is going on 25 years old. I might be wrong but I dont think your unit is cooling as good as it should be.