Oversizing the HVAC in a residence pros and cons?

[rgramjet]

HVAC is starting. Basement and first floor are 1800 sf each, first floor is 2,200sf. 2 x 6 exterior walls, R-19 wall insulation, R-30+ in attic, 9' ceilings throughout. Also has a 2 story open foyer.

My HVAC guy is telling me I need (2) 4 ton units to condition this house! Seems like a lot to me and other people that Ive talked to.

Ive been told that the units will short cycle. Are there any other advantages or disadvantages to think about?

Thanks again,
Roger

[HDNord]

Is your first floor 1800 or 2200 sq.ft.?

Not sure what the climate is like in your area but assuming one unit is for the first floor and the other is for the basement is sounds suspicious to have the same size units on both floors. The cooling requirements for the basement should be considerably less. I'm not in the A/C business but this forum has a few posters that are so it will be interesting to see their comments.



We are having a similar home built, main floor about 2200 sq.ft. and daylight basement below about the same. The A/C's being installed will be a 3 ton for the main floor and only 2 ton for the daylight basement. In our area I doubt that the A/C unit in the daylight basement will have to run much.

What were the bids for the two units they proposed? Ours will cost about $13,000, 93% efficient gas furnace's and 12 SEER A/C's.

[roger g]

Where are you? South Texas or northern Canada? Do you have huge south facing windows that you cannot cover? Is the basement deep in the gound or half out? Is the basement wall insulated?
The questions just keep coming. I suggest getting several quotes. Basements are notorious for being cooler with a/c. On some houses we put in a standard a/c unit and where there were huge heat gains ( large windows etc) we started putting in small ductless heat pumps in the locations where the a/c needed extra umph. It's hard to size a complete house with only one unit when you have sized it for one huge heat gain area. You could get into zone controls but it's a lot of money and complicated. Of course not complicated for the guy who is trying to sell it to you.

roger

[rabadger]

There is no way of knowing without seeing the constractors room by room load calculations. Every home is different. Heating and cooling is not sized by square footage. To design a system with the right size ducts a
room-by-room manual J load calculation must be done. This calc takes in considerations of room volume, type of windows, doors, insulation, wall construction materials, roofing materials, and location of home along with wall and window exterior, shading and eave overhange. It is the only way you can determine the size and quantity of supplies and returns needed for each room.

I have 3001 square feet of heated and cooled area in my home. The load calculations were 34 pages long. In my 27 years of the HVAC industry running load calculations and designing systems for contractors I do not remember having two systems coming out to be the same size on a residential job. One is amost always larger than the other, (I am not saying it doesn't happen). If you put a tremendous amount of money in it he may have matched the rooms for load, in wich case more duct work would probably be needed and the contractor was not the low bidder. He would have sold you the system by explaning explaning it, showing you the duct layout on paper and custom designing it.

Ask him to show you the room by romm load calculations and explain them to you. You should have had HVAC drawings and specifications at the time of bidding so all the contractors would bid apples to apples.

[rabadger]

On the over sizing, it will hurt not help. The unit has to run long enough to remove humidity, and over sizing causes shorter equipment life.

[rgramjet]

Duh....the first floor is 2,200....sorry about that...I live just outside of Wash DC.

[rabadger]

2200 sq st ist floor, 1800 sq ft basement, and 1800 sq ft 2nd floor. Is this correct? If so and he is putting one unit in for the 1st floor and basement,
the second unit for the 2nd floor. (this is the normally accepted way) I think you could be in trouble my friend. I would have to see his calculations and duct layout to know for sure.

If you read my post on my 3001 sq ft home it is about 12 years old. I am in northwest Indiana about 40 minutes outside Chicago. I am using one
85,000 output 80% furnace, one 3 ton A/C, 3 thermostats and 24 volt zone dampers. No problem.

[rgramjet]

Just spoke with HVAC guy again reiterating the concerns. His main point is, going with an oversized unit allows one to slow down the fan speed and the system doesnt have to work as hard. Also he said the units would most likely not be the same size...

The only reason why I trust this guy is because he has done several systems for me in the past and theyve turned out magnificently. Most of his projects are in the 3-20 million dollar house range and he uses no flex which I like.

[rabadger]

Are you saying his is going to over size the furnaces so he can run them on low speed and deliver the required air volume through the A coil according to the recommended CFM per ton that the manufacturer's engineers published? If so what will happen when you get to such temperature rise and the gas valves start cycling off and on because the limit switches open and close.

OR

The BTU output requirements are going to be correct but he has ordered the units with larger blowers? If so will the temperature rise through the furnaces reach min. requirements.

I agree with one thing, no flex.

What happened to the two 4 ton units? Is he or is he not going to show you the room by room load calculations and explane them?

Matched systems are matched systems. That is the way thy air designed.
there are furnaces that have larger blowers for applications when the cooling load is higher than normal. Like say south of the Ohio river. But you are in just outside Washington DC?

[Sparks]

I'll second everything rabadger said. This sounds suspicious to me. Why would you purposely re-engineer the unit that degreed engineers have spent endless hours designing. Oversizing equipment and then compensating by lowering blower speed???? I think that equipment should be operated as it was designed to be operated, in most all cases, especially a new install. When you say his previous install worked beautifully, are you considering everything, like utility bills, over the long run? I would never oversize, it creates problems. Tell him you want a room by room heat gain/loss calculation performed taking everything into consideration, windows, insulation, everything. That is the only way to go, rules of thumb no longer apply to hvac.

[rabadger]

I must admit I have recommended changing out good equipment to the larger CFM and same BTU equipment. But I did it because the contractor cheated on the duct work and it was cheaper for him to eat a furnace for the more expensive model than pay for the drywall to be taken down and the homeowner's hotel bill for 6 weeks.

[Sparks]

Oh I know sometimes it happens for various reasons but I guess we would need more specifics. Apparently, the contractor has done it before with success. So who are we to argue with success? Hopefully, there won't be any coil freeze-up problems and hi-temp cutout issues with slowing down the blower speed. I guess it all depends exactly how he matches these sub-systems together.