AC Sizing versus finished square feet

[Erby]

Any suggestions on this comment:

I am concerned that the size of the heat pump may be inadequate for the size of the finished area. While I note that the only way to properly determine Heating and Air Conditioning size of unit to size of house is to do a load calculation (outside the scope of this inspection), I also note a "rule of thumb" (NOT A LOAD CALCULATION) generally indicates a range of 500-700 square foot per ton of cooling. The model numbers (Interior: WestingHouse B4VM-X60K-C & Exterior: Nordyne FT4BF-060K) indicates that this is a five ton unit. 5 x 700 = 3500 square feet. You told me the finished area was, first floor - 3122 sq.ft - basement - 2,000 sq.ft. Total finished square feet of 5,122.

I recommend that you consult with a heating and air conditioning contractor to ensure that this size and efficiency of unit can handle the 5,000 plus square feet of finished area.

[Jim Katen]

The heat pump is smaller than I'd expect to see for a building this size. It might not be large enough to adequately heat and cool the house. To find out for sure, hire an HVAC pro to perform a load calculation and determine whether or not the heat pump is adequately sized.

[Terence McCann]

How old is the home Erby? Electric furnace for backup? Finished basement?

[mgbinspect]

"Consult with a licensed HVAC contractor regarding the heating and cooling system, which might be undersized for the dwelling, based upon common standards." (I usually go with 600 - 800 SF / Ton, but have been cautioned by a couple of HVAC guys that those numbers aren't as applicable, in newer tighter homes, as they used to be.)

[Marc]

In this south Louisiana climate, using that rule of thumb will often put you far off course. Too many other, more prominent factors in the equation. I never use it. Instead, I make a judgement based on everything I've learned about the house. If I say anything at all about the adequacy of the HVAC system, it would be something along the lines of Jim K's comment.

Marc

[Scottpat]

Erby, it looks like they sized the HVAC unit for the main floor and did not take into consideration the basement or the basement was unfinished by the builder and the owner later finished it off. I have found this a few times when the owners finish the basement after they move into the home. I use the same SF numbers for the "rule of thumb" measurement.

[mgbinspect]

Erby, it looks like they sized the HVAC unit for the main floor and did not take into consideration the basement or the basement was unfinished by the builder and the owner later finished it off. I have found this a few times when the owners finish the basement after they move into the home. I use the same SF numbers for the "rule of thumb" measurement.

Hm. Sounds like I may need to back my numbers down from 600 - 800, which I got many moons ago from an HVAC contractor. Anyone else out there using something other than the two spreads discussed here?

[Tim Maxwell]

I had two houses like that in the past month. Both had finished the basement area tapping onto the existing HVAC equipment.

The other concern on both the ones I looked at was no thermostat for either basement area. Zoning the system with two thermostats and the automatic dampers may be the only way to make something like that work.

I also just left it to the HVAC technician to figure out...[:-magnify

[plummen]

Any suggestions on this comment:

I am concerned that the size of the heat pump may be inadequate for the size of the finished area. While I note that the only way to properly determine Heating and Air Conditioning size of unit to size of house is to do a load calculation (outside the scope of this inspection), I also note a "rule of thumb" (NOT A LOAD CALCULATION) generally indicates a range of 500-700 square foot per ton of cooling. The model numbers (Interior: WestingHouse B4VM-X60K-C & Exterior: Nordyne FT4BF-060K) indicates that this is a five ton unit. 5 x 700 = 3500 square feet. You told me the finished area was, first floor - 3122 sq.ft - basement - 2,000 sq.ft. Total finished square feet of 5,122.

I recommend that you consult with a heating and air conditioning contractor to ensure that this size and efficiency of unit can handle the 5,000 plus square feet of finished area.

most people i know dont figure basement when sizing equipt due to it being underground

[dad211]

load calculations are different for basements, due to the fact of most of the exterior walls being underground less windowsurface area and less doors. I was in the heating and air trades for 16yrs and have worked for many a contractors that use "rules of thumb" to me its a dangerous practice. most salesmen in the industry for residential units use 500sqft/ton to size for replacements instead of just taking a couple hundred bucks and a half hour to perform a true load calcs.

[Erby]

Thank y'all for your input. Yes, it appears that the original unit was sized for the first floor and then the basement was finished later.

When asked about it by the buyer, the seller said he'd had load calculations performed when the basement was finished. When the buyer asked for a copy of them, they were nowhere to be found and the seller couldn't remember who had performed the load calculations.

At last contact, the buyer was asking for load calcs to be performed and provided to the buyer.

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