undersized a/c unit

darryllee9February 2, 2011

I bought a foreclosure last year 3800 sq feet. (Charlotte NC) a/c unit is 3.5 tons.(only one unit). Have a dual zone system (two thermostats one upstairs one downstairs). The system heats fine but cooling during 90+ days is not adequate. Will cool entire home by about 10 degrees on hot days so we only use one zone when it is hot out. It will cool one zone down about 20+ degrees making the other zone basically uninhabitable. My furnace is a Bryant 310aav048090 (the system is still under warranty). The hvac company that installed the system states that the 3.5 ton a/c system is the right size for my home but i am nearly positive that this is not so. Based on what i read it should be around a 5-6 ton system. What are my options? Would the HVAC company be responsible in anyway for such a undersized system?

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You may be undesized, but you can't be sure without a Manual J load calculation. You can hire someone to do this for you, or you can use programs like HVAC Calc and calculate it yourself. The HVAC company should have done this as part of their installation. I suspect they did not. I would not trust them to do it properly for you.

Here is a link for the software:


What happens when you shut down the downstairs zone? Are you able to adequately cool the upper floor? Do you get good air floor to the upstairs registers? Is the system located in the basement or the first floor?

You may have an inadequate duct system. You should ask the HVAC company to do an evaluation. If it turns out you are undersized, then may need to improve the duct work in order to handle a bigger system.

    Bookmark   February 3, 2011 at 9:28AM
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you may be undersized but be aware that your furnace only has a 4 ton rated blower so even if evap coil and outside condenser were replaced, you could only go up to a 4 ton condenser unless furnace was also replaced.

yes a load calc is in order especially on the cooling side. But also your zoning controls should be checked out and make certain they are operating correctly in cooling mode. The zoning controls need to be eliminated as the source of your problem.

you have adequate supply and return ductwork that is sized correctly for home? where is ductwork and registers for upstairs?

lots of variables that need to be checked. of course AC charge should be checked out in the spring when temps moderate and reach 70 degrees.

how old is system?


    Bookmark   February 3, 2011 at 9:57AM
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You might try reducing heat loads as a cheaper way of dealing with being undersized (if indeed you are). A tint layer on west windows; air seal the attic (remove fiberglass and spray some foam); get leakage test and fix leaks; radiant barrier.

I bet you could spend less than $5k and make it quite nice.

I think it would be hard to get an HVAC company to replace a system after the fact but maybe if you have a guarantee.

    Bookmark   February 3, 2011 at 7:47PM
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This news really isn't horrible if it seems to be undersized in comparison to oversized. I would run the manual J or have it done. I would then have an energy audit done with blower door and thermal imaging.

I would around and really look into how many air leaks you have like how many recessed lights there are and that unless they are sealed units tend to leak quite a bit of air (could be 4-7cfm each) which can be pretty well sealed with new sealing type trim rings.

If you have an unfinished basement you can look at all the penetrations where wires,pipes or lines go through and the size of the gaps for air leakage. Even small gaps add up but the main thing to keep in mind is the same gaps are in the attic which causes huge losses. A bunch of caulk and cans of foam is in order to seal it up.

The ductwork.... The connections need to be sealed with brush on mastic or mastic tape to stand the test of time. Regular foil tape will not cut it.

I know these things are or can be extremely time consuming but the material cost is pretty reasonable if you are handy and have some DIY ability. The time is what will eat you alive on a bid for these items but it can almost be like finding a couple windows that had been left open and shutting them.

With the manual J and audit with thermal imaging scans you will be able to figure out where your biggest leaks are and start there.

Your utility might have some programs for the audit that will help with cost and if so take advantage of them if you get to see the images or have copies.

You will want to consult with an HVAC tech for his input on the whole picture and while getting the system checked for charge and overall health and do it over 70 degrees so the 'freon' charge level is accurate. They should also look over the ductwork situation also.

Energy Auditor..This is where pictures do not lie and it will be good to see. These people usually do or have others available that will do the insulating and sealing that is needed for you. I would still get at least one other bid before giving the shake for them to do the work.

This is still much better in my opinion of a over sized system as there is no where to go other than add an addition or get a smaller system. The air leaks would still need to be addressed anyway in all honesty. In this instance you could seal and insulate your home so the system works.

    Bookmark   February 3, 2011 at 11:12PM
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Agree with countryboy. I have a custom built house from less than 2 years ago. Built to energy star standards. The leaks they consider acceptable into the attic are shocking. But after that, in the summer, solar gain is a big issue - shade trees can help and solar screens. I'd find out if you have low e windows - if you don't, then you should have no problem improving the situation to an acceptable point.

    Bookmark   February 4, 2011 at 6:21AM
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I wanted to go back over some issues I found with my energy audit and on my own with my 'old' builders grade system on my 06 house.

The system was a 3.5ton ICP/Tempstar 13 seer 8hspf on 1750 sqft with basement that in the summer could not maintain temp and any sunny day that was over 75 the system would run seemingly forever and struggled to maintain the inside temp in the heat of summer.

A digital CFM hood calculated return air/supply air at every register and the auditor checked the recessed lights individually with this also which I will get into later.

I had SO much leakage on a system that should have been pulling @1300cfm total on the return grills and had less than 200cfm measured at the grills TOTAL! I had another stupid low number on the supply vents. It was fairly noticeable without poking around that the connections were leaky but after getting out and looking I found 1" gaps on the trunk connections and other assorted leakage areas.

The low return side still did not make sense to me or the auditor until I on my own pulled the grills and on some took downward pictures with my camera phone. The pieces they used to block from pulling air from the attic had huge gaps and worse yet the opening in the floor cavity was only 4-6" long when it should have been opened at least close to the full stud area approx 14" wide. After looking at the air leakage areas from the basement every hole for a wire or pipe was huge and the plumbing areas for the tubs and showers were cut completely open with gaping accesses into the shower pan and tub pans. The unit was pulling air from the attic and basement more than the house itself. The return air temp going into the coil for the unit to cool and send into the house was over 5 degrees warmer than the inside house temp which should be within a degree of house temp if possible.

I rebuilt all of the returns and opened up the accesses so the air could flow freely down towards the basement and sealed everything up and brushed everything down with mastic and this made a huge difference in noise and comfort when your not trying to cool and dehumidify all the air except the air in the house and the system was maintaining.


Here is a biggie to keep in mind through your adventure. My recessed lights were leaking 4-7cfm which who cares big deal right.... I thought... and was corrected by the auditor.

Time for Math Scare 101

I have 14 recessed lights and say each one only leaks on average 4cfm.

56 cubic feet per minute

3360cf in an hour

80,640 cubic feet per DAY.

I still attempted to shrug this huge number saying that with different wind conditions could affect the flow and he made a good point. Even if the average is half that is huge when you can cut it almost to zero yourself with little effort.

I still have the attic sealing to do but have been putting it off because I have R19 rolled out on top of cellulose and its going to be a nasty job even if the payback will be huge I havent jumped up there yet.

I have since upgraded the old system to a 15 seer 10.5hspf Ruud matched system with demand defrost,txv and staged heat strips but DROPPED the tonnage to 3tons and the strips from 20kw to 15kw and have not sealed and caulked the attic penetrations and it still works great in comparison.

It is easily possible on many homes to caulk foam and mastic your way to a smaller system.

I started my journey after a horrible bill and thinking I had an undersized system and caulked and sealed things down the point I was able to downsize even more.

    Bookmark   February 5, 2011 at 3:00AM
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Infiltration is a far larger problem than even low r-value insulation.

If you seal things better a smaller unit may be adequate.

    Bookmark   February 5, 2011 at 9:13AM
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wow wasn't expecting such a large response Thank You!
the answers to the questions asked.....
What happens when you shut down the downstairs zone? It cools fine upstairs and visa versa (very forceful air flow in that one zone. when both zones are going at the same time a few of the more distant vents basically have no air flow.
Are you able to adequately cool the upper floor? yes if the downstairs zone is off
Do you get good air flow to the upstairs registers? if the downstairs zone is off
Is the system located in the basement or the first floor? 2nd floor in the wash room
How old is the system? ...about 3 years the home is 3 years old.
where is ductwork and registers for upstairs? ductwork is in the attic, registers 1-3 per room, return there is only one located on the 2nd floor
no basement.
So my question is in the being that we have very hot summers, would a 3.5 ton system ever be adequate for 3800 sq home with a 4 ton condenser? if not and assuming that my duct work is adequate. what would be my options for adding another unit to the system say a 2-3 ton system, what would my ballpark cost be? Would I have to have a 2nd furnace even though the one in there now heats the home adequately. In the utility room were my furnace is located it appears that they were initially going to install a second unit as duct work the same size as what is currently connected to my system drops from the ceiling but is connected to nothing in the attic and nothing in the utility room.
Thanks Again

    Bookmark   February 5, 2011 at 11:01PM
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A 3.5 ton system could be adequate for 3800 sq ft home. There was someone building a 3000 sq ft home with 2 tons - although in a cooler climate. It seems like what is often the case is that airflow becomes an issue on bigger houses.

Is the house actually in Charlotte? I'm wondering if the permitting office keeps a record of HVAC permit and there might be some helpful info there. Have you been able to talk to the original installers? What you would like to see is a Man J. I live in NC and our jurisdiction requires a Manual J and Man D. I wouldn't be surprised if there is a duct diagram at the city where I live.

It sounds like you should focus on airflow at first. Poorly installed ductwork seems like your most obvious problem. You should always have airflow at all vents. One return in a house that large is absurd. But maybe you were saying there is only one on the 2nd floor?

A picture of your attic might be helpful for seeing the ducts.

I do think if you improve the airflow and work on infiltration, you can probably make this livable. And working on infiltration will lower your utility bills. It might be helpful to hear what those are - I know your rates are roughly the same as ours. It gives some idea of what your insulation and infiltration is like. Insulation is a easy inspection and likely to be fine - the infiltration is not tested and I believe has no standards.

I highly doubt that your ductwork would be adequate for another system. But if it was, you might be at $5k for another system. If you don't have a return downstairs, that would need to be added. I do think that money might be better spent on infiltration and duct work. Are you a DIY kind of person?

    Bookmark   February 6, 2011 at 4:48AM
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one return?
I agree with countryboymo also.
you can hire a blower door and duct leakage test and diy
a lot of it yourself.
we can walk you through most of it.

best of luck.

    Bookmark   February 6, 2011 at 11:25AM
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