Is my central ac wearing out?

[Weslsew]

Looking for some input or thoughts on my central ac. This is a house built in 2002 that I just recently bought. About 1700sqft downstairs and 2000 upstairs. It has two 2.5 ton 11 seer tempstar heat pumps, one upstairs and one down.

I am in Florida, and it's recently been getting in the 90s by noon and climbing until the sun goes down. With the thermostats on 80, the upstairs unit will kick on around 12-1 pm and run nonstop until late at night. The upstairs supply air is around 10 degrees cooler than return air, compared to the downstairs unit that is about 13-15 degrees cooler than its return.

The condenser coils are clean and look like new, the evaporator coils show some age with a little crust and corrosion but are relatively clean. I use the cheap filters and change them often.

From my little knowledge, I would think the unit either works if the compressor is running, or it doesn't. I don't know if air conditioners can slowly "wear out." So I'm not sure if I have a problem or if the upstairs unit is just undersized. The upstairs is fairly comfortable as there is cooler air blowing out, I'm just very concerned about electricity costs. Is there anything to be gained by having someone come out and check it, or will they just try to sell me a new system? Can I clean the evap coil myself, do the foam spray cleaners make any difference? Thanks for any help.

 

[yeldogt]

It could be down on freon -- only way to check is to put gauges on it. How cold is th return line at the compressor? do they both sound the same.

But -- if the unit is in an uninsulated attic ... It's basically running outside with R3 insulation around it. Placing AC units in uninsulated attracts has to be one of the dumbest building ideas .. ever. Only dropping 3 degrees .. sounds good.


AC units can live a long time -- I replaced a 4T York unit 2 years ago that was still running great from 1986. I insulated and sealed up the house -- even with a large addition 4T was too big.

 

[Weslsew]

It is located in an interior closet. I'll check the lines and see

 

[EOC_Jason]

Heat rises & cold air sinks, so your upstairs is going to be working a lot harder in the summer...

Your delta-T does sound low, but you can only do so much with visual inspections. I would make sure you don't have any air leaks around your coil or ductwork first. Usually if it's clogged or low on freon (depending on the level) it will actually get colder and ice over...

Being a heatpump the reversing valve could be leaking, or the valves could be out. A technician can check for those things.

 

[Falcon67]

What the techs do - from what I've seen since we just went through this - is to check the amp draw of the compressor. Ours was drawing well more than the spec as well as killing caps. So then the decision is to try and band aid an old system with new parts - if possible - or bite the bullet and move to a more modern maintainable system. Our old system did the same as yours - kicked on about 1 or 2 and ran 100% until around 11 PM. And could not hold the set temp. Ours in a single story house also.

 

[zmaxmotorsports]

When was the last time it was serviced?

 

[Milton Shaw]

For a quick check of the refigerant levels, check the suction line(the big one) at the outside unit.Is it sweating and cold. If it is sweating and cold then the charge is withing 15% or so of the correct charge. If the big line is hot and not sweating you have lost refigerant and need to get it serviced. A 2.5 ton unit is a little small for a second floor in deep south locations from what I have seen in other houses. If there is any openings (doors/stairs) then a lot of your cold is going downstairs and Hot air from downstairs is coming up. Close doors or use drapes or something to keep the air where it is supposed to be.

 

[bazar01]

It also depends on how the unit return and supply air are ducted.

Do you have individual return ducts on each room or just one big return grill in the upstairs hall way?

 

[Weslsew]

thanks for the replies. The suction line on the downstairs unit at the condenser is ice cold and sweaty. The suction line on the upstairs unit is warm. I guess I need to have someone come check it. Is it common for a leak to develop after 15 years?

Also, the house has an entrance where the first floor is open to the second floor above, so yes, a lot of the hot air probably goes straight upstairs. I suspected it was strange that both units were sized the same. There are no return ducts.

 

[EOC_Jason]

I think you are lucky that it lasted for 15 years in the first place.... My money is probably a small hole in the evap coil...

 

[6768rogues]

Our Florida house is 10 years old and the system is limping along on its deathbed. Everyone I talked to in FL including my brother who lives there full time has told me that 10 years is the expected life of an AC unit. Heck, in NY we have them for a generation but they only see about 90 days of use per year.

 

[LS6 Tommy]

X2 on the low charge on the second floor. You're really not doing yourself any favors with an 80* set point and a schedule that doesn't start running the machine until noon. You're better off starting earlier, setting it for a lower temp (maybe around 74* tops) and letting the systems get ahead of the load and then just maintain temperature as the outside air temp goes up. If it was my place, it would be set at around 74* 24/7...

Tommy

 

[DC73]

The upstairs supply air is around 10 degrees cooler than return air, compared to the downstairs unit that is about 13-15 degrees cooler than its return.

A properly charged system typically has a temperature differential of 17-20 degrees. I'd say both units could use additional refrigerant. But, dirty indoor and outdoor coils could be responsible for the numbers of the downstairs unit.

It's not a bad idea to clean both the indoor and outdoor coils any way. You can do this yourself if you're handy.

DC

 

[DC73]

You're really not doing yourself any favors with an 80* set point and a schedule that doesn't start running the machine until noon. You're better off starting earlier, setting it for a lower temp (maybe around 74* tops) and letting the systems get ahead of the load and then just maintain temperature as the outside air temp goes up. If it was my place, it would be set at around 74* 24/7...

Tommy

If he's trying to save money, he's better off leaving the system off until he needs it. The only drawback is waiting for the house to cool off once you let it get hot. If his main focus is comfort and cost is not an issue, then by all means set it for an ideal livable temperature 24/7. I gained considerable experience studying this a couple of decades back. We ended up with two new identical unoccupied houses we studied for a month. One had the thermostat set 24/7 at 76 degrees. The other used a programmable thermostat to simulate a family that was gone during the weekday. This one turned the unit off during the day and then back to 76 one hour before the first person was expected back home. The house with the programmable thermostat used considerably less energy over the course of the month. Forgot the particulars - had beers since then.

DC

 

[LS6 Tommy]

If he's trying to save money, he's better off leaving the system off until he needs it. The only drawback is waiting for the house to cool off once you let it get hot. If his main focus is comfort and cost is not an issue, then by all means set it for an ideal livable temperature 24/7. I gained considerable experience studying this a couple of decades back. We ended up with two new identical unoccupied houses we studied for a month. One had the thermostat set 24/7 at 76 degrees. The other used a programmable thermostat to simulate a family that was gone during the weekday. This one turned the unit off during the day and then back to 76 one hour before the first person was expected back home. The house with the programmable thermostat used considerably less energy over the course of the month. Forgot the particulars - had beers since then.

DC

Oh, I'm all for doing a setback. I agree it doesn't make sense to leave it at the chosen comfort set point 24/7, but turning the system off all day and then turning it on with a 85-90 box temp is just a waste of money. Even having too much of a set back delta quickly creates diminishing returns on savings. No disrespect, but unless the houses in your study weren't subject to any off cycle heat gain, it has been disproven by thousands of energy efficiency evaluations for decades. If just turning the system off when the space is unoccupied really worked, there would be no set back thermostats, just ones with a timed on/off sequence and only one set point.



Tommy

 

[acmikee]

since you probably don't have gauges-- just check the temp difference you want around 18-20deg split on supply and return. are you building up ice on the evap. if yes low on charge or air flow issue (dirty filter)

 

[Weslsew]

So a little more info... I took the evaporator coil cover off and the expansion valve and about the lower third of the evaporator is wet and cold, about 35 degrees. The upper portion of the evaporator coil however is warm, about 77 degrees. This is measured with return air temps of 81 and outside air temp of 76. Is this more evidence of low refrigerant? The coil overall looks good to me.

 

[DC73]

. . . but turning the system off all day and then turning it on with a 85-90 box temp is just a waste of money. Even having too much of a set back delta quickly creates diminishing returns on savings. No disrespect, but unless the houses in your study weren't subject to any off cycle heat gain, it has been disproven by thousands of energy efficiency evaluations for decades. If just turning the system off when the space is unoccupied really worked, there would be no set back thermostats, just ones with a timed on/off sequence and only one set point.
Tommy

I've never read one valid study that suggests you can save money by leaving your system running all day rather than turn it off when not needed. That's just an old wives tales concocted by people who don't want to be bothered with setbacks and such. Our study proved otherwise as well. However, you can find numerous articles written by people who don't know any better and just perpetuate the myths.

Look at this way. It takes the same amount of energy to lower the temperature in a house by one degree regardless of time of day or whether the temperature is being lowered from 76 to 75 or from 86 to 85. A typical AC system has a two degree run time - that is if you set the temp for 74, the system comes on when the temperature reaches 75 and runs until the house is cooled to 73. That happens multiple times over the course of a day. It's not unreasonable for it to happen 6 or more time per hour of operation.

Let's just assume it happens only once per hour. Over the course of a 9 hour day when no one is home, the unit would use energy to cool a total of 18 degrees. Well insulated homes would never get 18 degrees above normal set point if the unit was off all day. In our study, both homes were well insulated and the temperature at the end of the setback (off) period would only be 8 degrees or so above normal set point. That's 10 degrees less cooling needed than the approach of leaving the unit on all day. That's being very conservative as to expect the unit to only cycle once per hour is unrealistic on typical summer days.

Programmable thermostats don't have a programmable "Off" period so the only course of action is to put the setback temperature high enough the unit will not run.

I know I won't convince you. Old habits and old wives tales die hard. But, I've never had anyone not save money when switching from your way to my way.

DC

 

[brewchief]

So a little more info... I took the evaporator coil cover off and the expansion valve and about the lower third of the evaporator is wet and cold, about 35 degrees. The upper portion of the evaporator coil however is warm, about 77 degrees. This is measured with return air temps of 81 and outside air temp of 76. Is this more evidence of low refrigerant? The coil overall looks good to me.




Does the joint where they 3 lines meet the larger line feel oily? If so that's most likely where it's leaking.

Yes it's low on refrigerant.

 

[Weslsew]

Does the joint where they 3 lines meet the larger line feel oily? If so that's most likely where it's leaking.

Yes it's low on refrigerant.

Yup, oily and sticky there

 

[OccupantRJ]

It could be down on freon -- only way to check is to put gauges on it. How cold is th return line at the compressor? do they both sound the same.

But -- if the unit is in an uninsulated attic ... It's basically running outside with R3 insulation around it. Placing AC units in uninsulated attracts has to be one of the dumbest building ideas .. ever. Only dropping 3 degrees .. sounds good.


AC units can live a long time -- I replaced a 4T York unit 2 years ago that was still running great from 1986. I insulated and sealed up the house -- even with a large addition 4T was too big.

8 Carrier Rooftop 9 ton units on my workplace each lasted a minumum of 47 years. When they started failing, they all did so inside a 4 year period. I guess we got our money's worth out of them......

 

[acmikee]

Yup, oily and sticky there

it should bubble if you squirt some soap on it

 

[EOC_Jason]

Yup, oily and sticky there

A competent tech can check with a leak detector (or soapy water), if it's there he should be able to patch it.

 

[LS6 Tommy]

I've never read one valid study that suggests you can save money by leaving your system running all day rather than turn it off when not needed. That's just an old wives tales concocted by people who don't want to be bothered with setbacks and such. Our study proved otherwise as well. However, you can find numerous articles written by people who don't know any better and just perpetuate the myths.

Look at this way. It takes the same amount of energy to lower the temperature in a house by one degree regardless of time of day or whether the temperature is being lowered from 76 to 75 or from 86 to 85. A typical AC system has a two degree run time - that is if you set the temp for 74, the system comes on when the temperature reaches 75 and runs until the house is cooled to 73. That happens multiple times over the course of a day. It's not unreasonable for it to happen 6 or more time per hour of operation.

Let's just assume it happens only once per hour. Over the course of a 9 hour day when no one is home, the unit would use energy to cool a total of 18 degrees. Well insulated homes would never get 18 degrees above normal set point if the unit was off all day. In our study, both homes were well insulated and the temperature at the end of the setback (off) period would only be 8 degrees or so above normal set point. That's 10 degrees less cooling needed than the approach of leaving the unit on all day. That's being very conservative as to expect the unit to only cycle once per hour is unrealistic on typical summer days.

Programmable thermostats don't have a programmable "Off" period so the only course of action is to put the setback temperature high enough the unit will not run.

I know I won't convince you. Old habits and old wives tales die hard. But, I've never had anyone not save money when switching from your way to my way.

DC

I think what you're considering to be "my way" is not what I was driving at. I agreed there was no savings in leaving it running all day. I said using a setback thermostat is widely proven and that just turning it on when you get home is not better.

It absolutely does NOT take the same amount of energy to reduce the temperature by 1* regardless of time of day or return air temperature. I agree the same amount of heat energy is transferred for a reduction in temperature of 1*, regardless of starting temp, but the same amount of energy is not used. I think you're confusing energy usage and heat energy transfer... You have to move 1 Btu of heat energy for each pound of air you cool by 1* F. That's a constant for SENSIBLE heat. You still have to consider LATENT heat. That's where humidity comes into play. You need to add in the load of the latent heat of vaporization, which is dehumidifcation.

Enthalpy, (the total heat content of the air including relative humidity and temperature) indoor and outdoor air temps all have a huge effect on energy usage. I'm sure you've used a psychrometric chart. It plots the dry bulb, wet bulb, RH, dew point and grains of moisture content of air. That's all involved in comfort cooling, it's not just the reduction in sensible temp.

Put an ammeter on a compressor with an outdoor air temperature of 90*, an outdoor RH of 90%, an indoor RH of 70%, an 86* return temp and then take another current reading when the return temp is 74* and the RH is 60%. According to you there should be no difference in current draw. That couldn't be farther from the truth. Do the same test again with a 55* outdoor air temperature, 60% outdoor RH, 60% indoor RH, 74* return air temperature and you'll find that across the board the current draw is much less that the first test.

Programmable thermostats don't have a programmable "Off" period because set back operation is more efficient.


Tommy

 

[Weslsew]

I've always assumed that it was much more efficient to have the ac run in the evening time to cool down the house than to have it run in the afternoon at the hottest time of day with the sun beating down on the condenser.

In my case, for my upstairs which is the problem, we don't go upstairs until 9 pm or so to go to bed, so i don't think it makes sense to let the ac run at any point during the day when it can just kick on at 7pm or so to start cooling the bedrooms off. I haven't done any testing (I don't have an energy monitor... Yet) but I'm happy the way it works.

 

[zmaxmotorsports]

I'd much rather have an air conditioner cyle on and off a few times an hr during the day than have it come on and try to cool the house from 80 plus down to 70 while trying to remove a truck load of humidity out of the house.

 

[DC73]

You still have to consider LATENT heat.

You are correct. For simplicity and to keep other folks eyes from rolling back in their heads at the complexity of the discussion, I ignored latent heat but that doesn't swing the energy savings back the other way.

My bottom line is if you want to save energy, turn the unit off the entire time no one is home. But, that results in coming home to a hot house. If you want maximum comfort and don't want to be bothered with setbacks and such, leave it set at your normal level all the time. The best compromise is to use a programmable thermostat to set the temperature higher when no one is home but have it reset at the appropriate time so the home is cool when the first person arrives at the end of day.

That said, if I lived in a very humid area, I would likely just swallow the energy penalty and leave my unit on all the time.

Good discussion. Apologies to the OP for getting off track.

DC

 

[DC73]

In my case, for my upstairs which is the problem, we don't go upstairs until 9 pm or so to go to bed, so i don't think it makes sense to let the ac run at any point during the day when it can just kick on at 7pm or so to start cooling the bedrooms off. I haven't done any testing (I don't have an energy monitor... Yet) but I'm happy the way it works.

You got it figured out. This is a good way to do it.

DC

 

[EOC_Jason]

That said, if I lived in a very humid area, I would likely just swallow the energy penalty and leave my unit on all the time.

It's funny, in Houston you know when winter has finally hit (we don't get "fall") because you go outside and the distinct hum of everyone's A/C constantly running has finally stopped...

 

[LS6 Tommy]

You are correct. For simplicity and to keep other folks eyes from rolling back in their heads at the complexity of the discussion, I ignored latent heat but that doesn't swing the energy savings back the other way.

My bottom line is if you want to save energy, turn the unit off the entire time no one is home. But, that results in coming home to a hot house. If you want maximum comfort and don't want to be bothered with setbacks and such, leave it set at your normal level all the time. The best compromise is to use a programmable thermostat to set the temperature higher when no one is home but have it reset at the appropriate time so the home is cool when the first person arrives at the end of day.

That said, if I lived in a very humid area, I would likely just swallow the energy penalty and leave my unit on all the time.

Good discussion. Apologies to the OP for getting off track.

DC

Don't forget, latent heat is the largest part of the load. almost 1.5 times the sensible heat load.

My apologies for the hijack, too...

Very good discussion DC!

Tommy

 

[DC73]

Don't forget, latent heat is the largest part of the load. almost 1.5 times the sensible heat load.

Around here, peak humidity is in the early morning and drops off during the day to the point where it's not that big of a deal by the time we get off work. So, our biggest latent heat load comes during the time my AC is programmed to not run (My setback temp is 90 so AC is effectively off - if a house around here gets indoor temps above 90 with the AC off during the day, something is very wrong with the construction).

Last week, one day around 5:00 PM, I noticed my outdoor humidity was 1%. For the most part, our homes maintain indoor humidity on the low side of the normal range without much effort during the summer but a lot of folks run humidifiers in the winter.

DC