AC Runs and Runs and Runs...Help!


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Old 05-28-12, 08:58 AM
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AC Runs and Runs and Runs...Help!

So my wife and I just moved from Ohio to Atlanta, GA and purchased a 15 year old home. The house is about 3000 sq. ft. and has two 2.5 ton AC units. 1 2.5 ton unit cools the main floor and basement and seems to be just fine. 1 2.5 unit is for the upstairs only and seems to just run and run and run when the thermostat is set to 74. (85+ outside). Here's what I know:

Intake air is about 73 degrees, output air at top of handler is about 51, so a split of about 23 degrees. Air measured at the closet vent is 54 and air measured at the furthest vent is about 60. The outside condenser and the inside coil were both replaced about a year ago and both look to be clean, although I can't really see the coil too well.

I had an HVAC guy come out and take a look and he's saying the unit is undersized. Is it possible that it's been undersized for the last 15 years and the previous owner lived with it?
 
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Old 05-28-12, 12:57 PM
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You stated that the indoor and outdoor units were replaced a year ago... so, it may not have always been a 2.5 ton.
 
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Old 05-28-12, 01:12 PM
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Split numbers looks OK, so A/C unit may be fine. make sure you have strong air blow from the registers, you should feel it from 8 feet away. the furthest vent's temp should not drop that much, check to see if that duct is all sealed and no leak. Where is the air handler ? Do you have good attic insulation ? or big window face west, etc.. all these things counts.
 
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Old 05-29-12, 01:00 PM
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Looks like the vents all point toward the walls, so I definitely cannot feel airflow if I stand 8 feet away. If I lay on the floor under the vent, which is about 12 feet, I do feel cool air. I believe the air handler is in the attic with the farthest vent about 30-35 feet away. I can't really feel any leaks but didn't inspect the entire vent.

The room with the thermostat is on the west side, but there are no big windows, and those there are have blinds and curtains, which I've closed.
 
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Old 05-29-12, 01:17 PM
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unit is probably sized correctly. couple things to think about. your hot air will rise causing your upstairs unit to run much longer. and depending on what design conditions were used could play a big factor. in jersey we use 95 outside, 75 inside and 50% RH. if its 100 degrees outside people can't expect it to be 70 inside. what is the temp outside when your unit is constantly running?? just expect your unit to run all the time at the design conditions if sized correctly.
 
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Old 05-29-12, 01:54 PM
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Not sure of the RH, but the temps have been in the 80s, but I know that the summer will be hotter. Only asking for it to be 74.

Will a larger blower help? Can I even retrofit one or does the entire furnace unit need to be replaced?
 
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Old 05-29-12, 08:02 PM
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Something is not right. with 19 degrees split, it should cool the room down from 80F to 74F within one hour, 2 the most, unless the room/house is not insulated. You can not see inside of the wall, but is the attic well insulated ? how about windows, leaking ? If they are all good, and you have strong(??) cold air blows into the room, the temperature has to drop, simple physics. there must be a heat source somewhere, upstair is always hot of course, but in most cases, 2.5 ton should be enough. (you should not change the blower size, there is a standard air flow CF/minute, I forgot what the number is, but you need the right tool to measure it, by the way, IS it just as strong as the downstair AC's air flow ?)
 
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Old 05-30-12, 04:46 AM
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That was my thought too. Room and attic are well insulated. No window leaks. I wouldn't necessarily call the cold air flow strong, but that's why I was asking about the blower. Not sure if the flow is the same as downstairs.
 
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Old 05-30-12, 06:46 AM
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home is 15, so the insulation should be pretty good. you should have just about the same airflow as downstairs. if you can snap a few picks of the attic ductwork we may be able to help a bit more. check for kinks in your flex, long winding runs, insulation falling off your ducts, obvious things that are seen with your eyes that don't need test instruments or technicians to tell you.
 
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Old 05-30-12, 08:40 AM
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Just one more question. do you have an un-air-ed room (or may be a garage) under. One of my room is pretty hard to cool because that room is above the garage, and there is no insulation in between.
 
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Old 05-31-12, 04:30 AM
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Will try to take pics in the next couple days. The Master (where the thermostat is) is above the kitchen, but the Master Bath and Closet are above the garage, which is also on the west side, and yes it get's pretty hot in there. This definitely could be why that room is so tough to cool.
 
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Old 06-02-12, 02:21 PM
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400 cfm/ton of cooling and the tool is an anemometer. try saying that 3 times fast
 
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Old 06-02-12, 02:26 PM
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forgot you mentioned the the unit was changed last year. how was your cooling last year after replacement? or were you told of the replacement?? i guess you can be oversized but 2.5 ton is alot for an upstairs unit. i'm assuming the area thats hard to cool is getting the measured 60 degree air out of the registers?? how many, what type and where are your returns located? give us all the duct info you can and pics if possible, we'll figure this out.
 
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Old 06-03-12, 02:07 PM
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You have enough capacity - 5 tons is plenty.

Air measured at the closet vent is 54 and air measured at the furthest vent is about 60.

Read more: http://www.doityourself.com/forum/ne...#ixzz1wlaHyWNb
Ductwork may not be well insulated. (possibly leaking too)


A 23F split is high, so you're probably a little low on airflow - check air filter and coil. (possibility - undersized ducts)

It's also possible that your attic isn't properly insulated - you should have at least R30. (R50 is preferable)
 
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Old 06-04-12, 04:34 AM
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Thank for all the replies. I'm away on business but when I get back I'll post some pics.

Perhaps the ducts need cleaning because the filter is brand new (one of the first things I changed) and while I can't see the coil it should be clean as well since it's new. The Master bedroom is one of the closest rooms to the unit and that's where the thermostat is.

Oddly, when I was home last week during some 90 degree days, the temperature in the room actually went up...from 76 to 78, all while the AC was running.
 
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Old 06-04-12, 02:18 PM
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Duct cleaning doesn't improve airflow.

It's normal for the indoor temp to go up a little if the system can't keep up with the heat load.

If your attic isn't well insulated, bringing it up to R40 or 50 is a worth while investment.

Try keeping the main floor a little cooler to reduce the load on the second floor's system, since warm air tends to rise.
 
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Old 06-06-12, 05:31 PM
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78 on a 90 degree day isn't too good. you should easily be able to get into the low 70's. we will wait on pics. when you get a chance, take temp readings in all upstairs rooms. definately sounds like a duct issue. leaking return, leaing supplies, insulaton falling off, ducts not sealed. pics will tell us more
 
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Old 06-07-12, 04:11 PM
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While I was out I contacted the company that did the warranty work and they told me that they only replaced the outside condenser with an R410a(?) unit and therefore needed to change the coil on the inside. Nothing else was changed.

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Old 06-07-12, 05:08 PM
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Forget about that warranty company, they don't even know what kind of problem you have, they just want more money. With temp split at around 19-23degrees (a little too high as mentioned, that points to weak air flow), and your attic insulation looks OK, the only problem is still the air flow. you never asnwered if you have a strong air flow or not ???, what type of filters you use? take it out and see if that help.
 
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Old 06-07-12, 07:04 PM
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Air flow would improve by replacing that awful flex with hard pipe
 
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Old 06-08-12, 04:57 AM
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I have no real way to measure air flow but I would say it's not great. Fan speed does seem to be on high, and the filter was just replaced so I know it's not due to being dirty. I'll remove it and see what happens.
 
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Old 06-08-12, 10:52 PM
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Good: Supply runs have been strapped up.

Negative: Plenum not insulated, one of the return ducts is badly kinked. Furnace exhaust pipe appears to be single wall (is that legal?) and in contact with roof sheathing. (possible fire hazard) "Spider" type flex duct system with no trunk lines. Equipment in attic.

While I was out I contacted the company that did the warranty work and they told me that they only replaced the outside condenser with an R410a(?) unit and therefore needed to change the coil on the inside. Nothing else was changed.
The newer 13 SEER r410a units do require larger coils (different orifice too); reusing a coil from a 10 seer unit is a terrible practice.

Do not use the warranty company for future hvac repairs - cash out if you can.

-----------------------------------------

What kind of filter are you using? What size?

What's the rated hp of the furnace blower motor? (it should be at least 1/3hp)

With respect to airflow, only a tech who knows how to measure static pressure can be of any help.

Insufficient airflow can mask other problems such as incorrect charge/coil mismatching.
 
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Old 06-10-12, 07:49 AM
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You must have a matching R-410A coil installed.

The kinked return is a problem.
The duct systems needs a manual D performed on it using the existing air handler.
It appears there is a major low airflow problem. Did you say what SEER the condenser is?

When we know the SEER number of the condenser & take the humidity level of the indoor air, plus its temperature we can tell if it's absorbing enough indoor heat by the outdoor temp-split between the condenser discharge air & the outdoor air temp.

Everyone needs to have at least one humidity gauge which you can get at an Ace Hardware store for around $10. Humidity levels affect both those indoor & outdoor temp-split numbers.

It is a good practice to have a Home energy Efficiency audit performed so you know what needs to be performed to reduce the heat-gain & heat-loss of your home.

Immediately Replace that old evaporator coil with a new matching coil with a thermostatic Expansion Valve (TXV) metering device!

Address the duct problems & get the required 400-cfm per ton of cooling through the coil & to & from the rooms.

Test that outdoor condenser temp-split ASAP & post it with the SEER number.
 
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Old 06-11-12, 04:31 AM
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The condenser and coil were replaced with R-410a parts. The seer is 13.

I'm not sure what the humidity is, but we live in GA so it might be a bit higher than other parts of the country.

As for kinked return - I think it might just be the picture, but I don't see any kinks anywhere. As for the rest of the ducts, I agree the problem lie there somewhere. I haven't heard of a manual D procedure, but I'll have someone take a look.
 
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Old 06-11-12, 06:54 AM
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Well, a lot of HVAC guys don't know anything about a Manual D or how to perform one; therefore, they just need to have the instruments to check duct system static pressures & airflow CFM coming from the registers.

If the outdoor humidity is fairly high then the indoor humidity/latent load will probably be rather high much of the time.

The 2.5% summer design for Atlanta is 92F dry bulb, 74F wet bulb for around 43% relative humidity; for Savannah GA it's 93F DB 77FWB for 48% RH; for Valdosta-Moody AFB it's 94F DB 77F WB for around 46% RH. That is a heavy humidity zone when you factor in the high temps compared to an indoor temp of say 75F.

It needs a lot of long runtimes to get the humidity down; however, there may be too much air infiltration & other factors that call for a Home Energy Efficiency Audit.

Get a humidity gauge for indoors so you know what level you're dealing with; as that greatly affects the indoor & outdoor temp-splits of the coils.

If there is high indoor humidity then the temp spread indoors should be much less than 23F, because high humidity reduces the indoor split & increases the outdoor coil-temp split.
 
 

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