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Is there some kind of calculation I can follow to determine if the heating and cooling system in my home is rated (adequate) for the size of the house?

I assume there is some way to say something like “for X square feet per floor you need Y kind of system” for heating and cooling.

I recently bought a house (basement + 2 floors - ~1020 square feet per floor excluding garage) and I have the following system:

  • Rheem heat-pump (Model - RPLB-030JAZ) which is rated as 30,000 BTU/HR (8.79 kW)
  • Amana 15 kW Electric Furnace (Model – MBR1600AA-1AA)

So, trying to determine if this system is sufficient (or not)…

Thanks,

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    The calculation is complex, and includes the number of windows facing each direction, wall thickness, insulation, climate, etc. Square footage is only a single variable, in a much larger equation. You can probably find an online calculator, or maybe a smartphone app. – Tester101 Feb 10 '15 at 16:44
  • If it can keep the house at 70, when its -20 and 110. – Mazura Feb 10 '15 at 22:48
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There are a couple of ways.

  1. Are you comfortable when it's really hot or cold outside? If so, then it's sufficient.

  2. Do a Manual J heat load calculation. You can do a crude approximation of this yourself using http://www.loadcalc.net/

  3. Use "rules of thumb." These rules depend heavily on your area and become less accurate the better or worse your house's thermal envelope is.

You have a roughly 3,000 square foot house, assuming the basement isn't insulated/sealed off from the first floor. And you have a 30,000 BTU heat pump. That means the primary HVAC system can provide 10 BTUs per square foot. If you house is average to below-average construction in terms of insulation and air-tightness, this would most likely be insufficient in Minnesota, Maine, or Montana. But it would be probably be fine in California, Florida, or Georgia. In the middle third of the country, it might be okay, might not be. Might depend on how cold it got.

The fact that you have a (relatively) low BTU central ducted heat pump coupled with an electric furnace tells me that you live in a cold climate, and that the people who installed this system suspected that the heat pump would occasionally not keep up during cold snaps, and added the electric furnace for back-up. It woulda been a heck of a lot cheaper to simply calculate the load correctly and install a right-sized heat pump in the first place...

Another thing to consider is that a lot of older heat pumps don't work very well below certain temperatures. So if it gets to be -10 outside, the heat pump might be incapable of producing any heat. Newer better Japanese heat pumps can manage this without electric, gas, or wood backup heat, but most older American ones can't. So if it gets really cold, you might be forced to rely on the electric furnace, which will cost you big bucks to run.

I would highly suggest you improve your house's insulation and air sealing. After all of that, you might discover that your heat pump works just fine to heat and cool the house.

  • makes sense, but when we buy such a system, how do they determine what you need? I doubt the company will spend time and money on a barrage of tests - so how do they determine it? I had assumes it was simply based on the home square footage - at least for a good approximation. – Jonathan Feb 10 '15 at 17:18
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    They more or less guess. The only way to actually determine how big of a system is required is to do the Manual J calculation. It's even legally required in many places for new construction, but nearly all HVAC contractors routinely ignore this because it's more work than not doing it, and they're lazy. It's really not very hard anyway. Just visit the link I gave and you can do it yourself in about 20 minutes and have a reasonable approximation. – iLikeDirt Feb 10 '15 at 17:20
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    Typically speaking, although not always, the HVAC contractor is not the same "person" who does the engineering design of the system in a new building and so their "laziness" really doesn't have anything to do with it. Depends on where you're at but I doubt many HVAC companies have the licensing required to sign and approve plans. – ChiefTwoPencils Feb 10 '15 at 19:02
  • It ain't about the plans. It's about sizing the HVAC system to the house as designed or built. That's the residential HVAC contractor's job. – iLikeDirt Feb 10 '15 at 19:25
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    Anyway, to get back to the OP, he needs to do the calculation himself using the link I posted. And if he's in climate zone 5 or colder, he may find that his heat pump doesn't work so well in the dead of winter and the electric furnace is shockingly expensive to run. – iLikeDirt Feb 11 '15 at 2:50

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