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Camelot One
Premium,MVM
join:2001-11-21
Greenwood, IN
kudos:2

HVAC sizing and SEER

Looking for some direction on a new HVAC system, or more particularly, who to call to get better information.

Cape Cod style house built in 1985, 2400 square feet in the 2 stories above grade, plus 500 square feet of finished/ducted basement, located in central Indiana. It still has the original 90k btu gas furnace and 3 ton AC, both of which are still working somehow. But the size seems wrong to me. (we haven't moved in yet, so I have no idea how well the current system heats or cools)

I would rather not rely on the guys trying to sell me a system to tell me if I need to buy bigger units, but who else can I call that will look at the house and tell me what the ideal system would be? And can I assume going to a bigger unit is going to mean duct work for the extra air flow?



Msradell
P.E.
Premium
join:2008-12-25
Louisville, KY

The only way to know for sure if those units are correct size or what size you will need is to have an HVAC professional do the calculations for you. They will take into account installation, number and type of Windows, doors, etc. Just a gut feeling tells me that your sizing is probably pretty close for that area, but that is only a guess not knowing much about your home. Without a lot more information nobody here is going to be will help you much.


averagedude

join:2002-01-30
San Diego, CA
Reviews:
·Cox HSI

1 recommendation

reply to Camelot One

Please stay away from any contractor that "only" basis his heat/cooling load on the square footage of the residence.

You can get a very rough starting point using square footage, but it is only a rough starting point.

That being said and given no other information...3 tons X 400cfm/ton = 1200 cfm.
That is .5 cfm per sq foot...that is low average for residential homes. It will cool, but you might see long run times, and very little "pull down" capacity.
IF you are going to run an AC that is undersized, then I would not use the set back function more that 10 degrees - remember it MAY not have the capacity to "pull down" the high temp fast enough for your "comfort level" - it will still work but may take a long time to cool down.

With a 90,00 input btu furnace at apx 1200 cfm (based on ac size) will give you a 55 degree temp rise. Please note that just because it makes hot air does not mean it will heat your house on a very cold day. My gut says that the heater is close to being ok, but I have almost nothing to base that on.

The wild card is how much the basement will affect the AC/heat. (painting with a large bush so be warned) Most new homes that don't have a builder finished basement have AC/heat based only on the finished part of the home when it was built. That may explain why the AC/Heat seems a little low. Take the basement out of the equation and the sizing based on limited information seems Ok.