Advice on central air issues


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Old 05-20-13, 05:44 AM
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Advice on central air issues

I am looking for some general advice on the central air conditioning in my home. We recently moved into a new house, and the previous owner had a new furnace and central air installed just prior to this. The house is approximately 2200 sqft, and the units installed are a Goodman GMH950703BX and GSX130241 (I believe this is a 70,000 BTU furnace and 2-ton A/C, the latter of which at first glance seems to me to be a bit low for the size of the house).

The living room is fairly large (probably 25'x30' roughly with a cathedral ceiling that's around 15' at its highest point), and ductwork was not run to the room prior to the new system being installed. Previously it was heated with a gas fireplace in the colder months and cooled with two ceiling fans in the warmer months.

The contractor that installed the system ran three ducts to this room; he did not add a return. The house is older (1950s) with 2 additions made in the 1990s. A heat load calc. was not performed prior to the system being installed. The ductwork is flex duct with a single run off of the pre-existing metal ductwork, split into three runs under the living room. It's a long run, probably around 50' away from the furnace, all ducts on the outside walls. One gets moderate airflow, the other two essentially have no airflow at all - not enough to even stir the sheer curtain on the window above it.

I have been running the a/c the past few days and it has been struggling. The first day I set it to 72, and it couldn't get the temperature below 74 with an outside temperature of 75. The next day I closed the living room vents and covered them to minimize airflow, closed the living room doors and set the thermostat to 65. It was around 78-79F at the warmest part of the day; the a/c wasn't able to get the temperature below 72F at any time, and ran nonstop for the entire day. Airflow on all of the ducts in the main house is good.

I will freely admit that I am not particularly knowledgeable on HVAC sizing, installation and maintenance, but after closing off the living room and finding that the a/c still couldn't hit 70 degrees, I am having a difficult time believing that the unit installed was powerful enough, or that the ductwork is adequate. The contractor is going to split two of the three living room ducts off onto a separate run straight from the plenum, but if the a/c isn't cooling just the main house, I can't see how doing this will make much difference. The living room also has 4 skylights and large windows on 3 sides, and will absorb a lot of sunlight over the summer.

The contractor maintains that if I leave the LR door open with the ceiling fans on reverse, everything will resolve itself, but if the a/c is running nonstop all day (it ran 16 hours yesterday, never going below 72 and only shutting off when I shut it off), I suspect I will have both an astronomical electric bill and a warm house.

Basically I'm looking for input on the situation from those that are more knowledgeable on the subject than I am (which probably describes the bulk of the forum).
 

Last edited by sengar; 05-20-13 at 06:07 AM.
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Old 05-20-13, 06:17 AM
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What is the average delta-T ? (the difference between incoming register cold air temperature and your room temperature at that time, take 3 register's cold air temperature and get average)
 
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Old 05-20-13, 09:02 AM
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First, what state and large city do you live near?
There should have been a heat gain heat loss Calc before installing any air-conditioning equipment.

We don't know what level of efficiency your home has, however, I feel like you should have a ductless system to cool that large room, that appears to have a huge heat gain load.

It appears you may not have nearly enough airflow through the evaporator coil, also the supplier and return air duct system may not be properly sealed.

Buy a low-cost humidity gauge at a local hardware store, the percent of relative humidity is extremely important to any troubleshooting procedure.

At 85 Fahrenheit degrees outdoors 75 and 50% relative humidity indoors the heat rise off your 13 seer outdoor condenser should be around 18 above the outside air temperature.

I'm using 2.5 ton 13 seer Goodman expanded performance data, because I couldn't find the 2 ton hardcopy.

The indoor temperature drop between the supply air and the return air should be around 18.

I am surmising that the delivered performance to and from the rooms is not close to the achievable level of 90% of the units rated performance.

Every contractor should be testing the airflow across the evaporator coil and the airflow to each and every room. Then the entire air-conditioning system should be checked to see if it's getting at least 90% of its rated performance delivered to and from the rooms.

Here is a free whole house online load-calc you & your contractors can use; you can't save the results; print it when you get it right; use 0.4 ACH for infiltration rate; as you need to get it down to that level.
***This reply was typed with Dragon recognition software; I have severe carpal tunnel in both hands and can no longer type with my fingers without using the eraser end of pencils.
 
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Old 05-20-13, 01:23 PM
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Okay, I measured the temperatures at 3 registers, which averaged out to 62.6F with a room temperature of 78.4 per the thermostat (which is just outside the living room) and 74.0 according to the temperature display on my kitchen clock. The two ambient measurements were much closer together when I shut the living room and closed off the vents. Average was calculated with kitchen, downstairs bathroom, downstairs bedroom; as an afterthought I measured the output from the single living room register which has measurable output, which was several degrees warmer than the other three measured at around 66F.

I've also been taking data throughout the day when possible to compare interior/exterior temperature and humidity (thermostat has a humidity reading). These readings are taken with the living room open, ceiling fans in reverse drawing air in, a floor-standing Vornado fan blowing into the living room to try and increase air exchange from the main house, and the thermostat set to 65F.

8:07AM: Outside temperature 68F, interior temperature 72.1F, relative humidity 41%

9:00AM: Outside temperature 72F, interior temperature 72.2F, relative humidity 41%; outside humidity 69%

10:00AM: Outisde temperature 75F, interior temperature 73.5F, relative humidity 41%, outside humidity 66%

11:00AM: Outside temperature 78F, interior temperature 74.7F, relative humidity 41%, outside humidity 62%

1:20PM: Outside temperature 79F, interior temperature 76.0F, relative humidity 42%, outside humidity 61%

2:35PM: Outside temperature 82F, interior temperature 77.6F, relative humidity 42%, outside humidity 54%

3:53PM: Outside temperature 82F, interior temperature 78.4F, relative humidity 41%, outside humidity 57%

The A/C has run nonstop all day as the temperature has steadily increased, with the fan set to always on.

I am just outside of Buffalo, NY. The living room itself is well insulated, but is very bright (4 skylights, bay window, glass doors in back, two 6' x 2' windows on the side).
 
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Old 05-20-13, 03:25 PM
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Your delta-T is average at 15.8 (78.4-62.6), this is not great, but it is acceptable. So your AC system is working fine. the problem is the insulation (both the AC duct insulation and house insulation, also large windows, West face windows, skylights, etc.) unless you can put in a larger AC unit, the only other thing you can do is to add more insulations, window shades, and redo all your duct work, The theory is simple, 'if you can not produce more cold air, then try to save the ones you already have'. In fact 2 ton is the right size for a regular 2200 sqft house in upstate NY, my sister lives in Rochester, they have a 1.5 ton unit, works fine for their 2100 sqft house (the difference is their house is only 3 years old and is well insulated). You should call those pros back and ask them to do a load calculation for your house.
 
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Old 05-20-13, 03:40 PM
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Just as an immediate remedy. Have you replaced the air filter recently ?
Airflow is very important and the filter can reduce it quickly.

Can we also assume that this was originally a heating system that the AC was added to later ?
That would mean all registers are low in the rooms.
 
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Old 05-20-13, 06:39 PM
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Well, I just lost a nearly completed post!

I have the data on your condenser GSX130241D model; at INDOOR 80F, 800-CFM, 50% RH; & 85F outdoors the indoor temp-drop shows 20F.

I needed the condenser temp rise above outdoor temp, for diagnosis reasons...
Now that data will need collecting again during the same time frame, or forget the diagnosis...
 
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Old 05-20-13, 07:08 PM
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I did replace the filter the other day. The old one was clean, but was something the previous owner had put in (washable) and was a fairly dense, thick foam. Right now I have it running with a throwaway Home Depot filter to maximize airflow. PJMax is correct in that prior to the new install, the home had heat only. All of the registers are either baseboard or floor, including the newer ones in the living room which are all on the floor.

Apologies for not including condensor temperature readings, I will gather samples of that tomorrow and update the thread. Also, I would like to thank everyone for their advice, it is very much appreciated.
 
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Old 05-21-13, 02:08 PM
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I haven't been home enough today to take as many readings as yesterday, but here is a measure of the interior/exterior temp and humidity, along with heat rise from the condensor.

5:00PM: Outside temperature 81F, interior temperature 79.2F, heat rise 98.6-98.8 (thermometer fluctuated), relative humidity 41%, outside humidity 60%

System set to 65F, fan always-on. The thermostat data says the a/c ran 21.75 hours yesterday.
 
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Old 05-21-13, 05:05 PM
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What was the temperature drop indoors?

The 17.6 Fahrenheit outdoor condenser the rise sounds reasonable, however, the return air system could be drawing unconditioned air from the attic or garage or some other area; or you could have supply air duct leakage.

The system should not be running that much, it should bring the temperature down on the thermostat inside all of within a reasonable short time.

You need a complete checkup of your home's energy efficiency, the duct system, and delivered BTUH to and from the rooms.
 
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Old 05-22-13, 07:12 AM
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The temperature drop between the ambient indoor temperature and the register output was about the same as my measurements on the previous day, averaging out to 15.5-15.7 degrees depending on which registers I measured from, although again omitting the living room registers, 2/3 of which have no measurable output and the last which is around 4 degrees above those in other rooms.

I checked the stats for yesterday and the a/c ran 23 hours (not looking forward to the electric bill). Temperature in the living room is above the thermostat's reading (not surprising given the anemic airflow in that room), and was reading higher than the outside temperature with low humidity. There are a few spots in the basement ductwork where it feels as though air is leaking through.

I am going to schedule a home energy audit through a different company, and then try and get whatever problems are a result of the new install dealt with by the installer.

How much should this sort of system be running per day under relatively optimal conditions, in order to maintain a temperature of 72-74F?

Edit: I spoke to the contractor. He is going to come out on Friday and split 2 of the living room ducts off and run a new line to improve airflow, and go over the exposed ductwork and try to seal off any leaks. He claimed that the a/c running all day nonstop was normal for hot days (84F may be a bit warm for this time in May but it's hardly out of the ordinary for Buffalo summers), and that the best I could expect from central air is a 12 degree drop from the exterior temperature. In my experience with central air conditioning in the past, this isn't particularly true.
 

Last edited by sengar; 05-22-13 at 07:41 AM.
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Old 05-22-13, 12:15 PM
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In Buffalo NY, 2.5% summer design is 85F dry bulb, 70F wet bulb or around 48% RH.

Well, 90F - 12F is 78F room temp, hopefully at 50% or less RH. You probably get quite a few +90F days...Use a floor fan & dress cool on those extra hot days.

At 85 Fahrenheit degrees outdoors; 75 and 50% relative humidity indoors the heat rise off your 13 seer outdoor condenser should be around 16.5F above the outside air temperature. I posted a wrong temp-rise on that, last time...Indoor temp-split should be around 20F.

At 85 Fahrenheit degrees outdoors & 80 and 50% relative indoors the heat rise off your 13 seer outdoor condenser should be around 20.6 above the outside air temperature. Indoor temp-split should also be around 20.

This is the GSX130241D Condenser's Expanded Cooling Data chart


Hopefully they will make enough improvements in the system & your home so you will be comfortable enough with the 2-ton system.
 
 

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