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Question DetailsAsked on 1/28/2017

Is 16seer recommended for roof gaspack in AZ condo. Inside temp was 68 outside temp 32 with no furnace.

Replacing gaspack for 1st floor unit in 2 story bldg. Must be on roof. Has 2.5 ton for 948 SF. Is 2 ton OK?

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1 Answer


Minimum SEER rating for your area is 14 by law (and 11 EER for heat pumps), so 16 SEER would be about a 12% reduction in energy cost on the cooling side. Would likely pay off in a hot desert area - whether it would pay off in your specific location depends of course on the local climate and shading and total annual usage, but since replacing one already thereyou might have an idea (or can get one from local power company or state energy or environment depart website) of the power cost your air conditioner runs per year. Figure 12% savings on that for maybe 10-12 years and compare that number to the difference in cost between a 14 and a 16 SEER installed unit to see if it makes sense or not - or for a better answer run an amortization calculator on the capital cost over 10-12 years at your borrowing cost or assuming time value of money against the 12% annualized energy savings number to see which comes out best.

Whether a 2 ton unit would work would require the contractor to run through the ACCA Manual S (for Sizing of the unit) - which depends on local climate, desired indoor temp, how well the house is insulated, its size, etc. Off the cuff, around 400SF/ton is a broad rule of thumb for much of the US - so by that, 2.5 ton would be likely around the right size. If in a hot part of the state, certainly more likely that 2.5 ton or even larger (think Tucson) would be the answer, while up in the mountains in a shady area a smaller unit might be adequate. Type of construction makes a tremendous difference too - a thick-walled adobe house needs a lot less air conditioning in your area (hot days, cool nights) than a conventional 2x4 studwall house, for instnce - but running the calculation would tell.

You said 68 inside with 32 outside - which is actually likely below the operating temp of your heat pump so I presume it was using the supplemental gas or electric heater element to maintain that temp - which depending on how cold it can get in your area, might say that auxiliary heat capacity was just adequate (if that is as cold as you can expect it to get for any extended period of time), or might say it is undersized if you want the indoor temp higher or if it gets a lot colder at times.

As far as A/C goes, one would have to look at the existing A/C capacity rating and whether it maintained desired indoor summer temp without excessive runtime (generally about 1/3 max for extended periods, 1/2-2/3 for a few hours a day on the hottest days is the recommended max portion of time actually running). Again, Manual S calculation would show what size unit is needed for your case. Certainly, the ground floor unit will get heat from the upstairs unit in the winter and have a warmer surface at the flooring so it hasless outdoor surface area than the top floor units (and might get lateral heat from adjacent units on same floor too), and likely does not see as high a solar heating load for the same reason, so looking at the capacity of the other upstairs condo units might be instructive - I would expect that your tonnage rating would not be any higher than their unless you have unusual construction. If this building has a lot of units and yours is an interior unit - only one wall of outside exposure, it is not uncommon for them to have as little as 10-40% of the heating/cooling demand of corner top floor units which are exposed on two sides and the roof.

Answered 1 year ago by LCD

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