Replacing A/C - get a heatpump?

megawhizz · Feb 11, 2009

I have to replace an a/c unit for my house, for just the upstairs area (downstairs furnace/ac, upstairs furnace are fine). I've been mulling over the benefits of doing a heat pump vs. a/c, and this is what I've determined:

* Heatpump works till about 30 degrees. So will work in spring/fall in my area (Detroit, MI)
* Carrier/Bryant "HybridHeat" provide electric heating in addition to the standard heat pump functions - this will qualify me for a lower electric rate (non-interruptible) meter for the pump
* I can just replace the a/c with a heatpump and will not have to have major surgery done on the inside units, (except the indoor coil will be replaced for a/c or heatpump)
* Cost saving is likely to be not significant, though the carbon footprint will be lower with heatpump
* Heat pumps do not require increased maintenance compared to a/cs, and have similar lifespans.

Are my assumptions incorrect? Any sage words of advice?

motoguy128 · Feb 11, 2009

"Hybrid" heat is a gas or oil furance paired with a heat pump. There is no direct elctric bakc-up.

Heat pumps provide heat below 30F, but if sized for a area Detroit cooling load, this will be the thermal balance point. The point where the heat pump doesn't produce enough heat to maintain temp in the home. This could be closer to 35F in your eare IF...IF...did I say IF, the A/C is actually sized correctly. It can get humid up there (I grew up in Novi) and rarely does it get really hot, so you'll want the A/C sized correctly.

Carbon footprint is not much better with a heat pump. While overall you use less energy, natural gas actually has a smaller "footprint" than coal. Most energy in Michigan and the midwest grid is generated from coal. Personally... I still think global warming is a unproven theory only... and more political than real science...but to each his own. I fully support alternative energy because they make wind turbine blades down the street and Iowa is #2 in wind power overall.

In theory the lifespan of a heatpump is lower because it's run more often. But more units get replaces from obsolescence, misuse, or from leaks in coils ... than because the compressors wear out.

IF often gets above 30 or 35 in the afternoon even in Dec.-Feb. in Michigan. You'll still see some run time between noon-6PM. Actually... it's a little know fact that it's actually slightly colder in Iowa and Illinois than Michigan in Jan. because of fewer cloudy days. The clouds hold the heat in and reduce day/night temp swings.

It's best ot repalce the indoor unit coil and air handler along with the outdoor unit so they are a matched set. You will not get any rebates if you cannot prove the effeceincy of the unit. They only test matched components.

I don't know about the "lower rate" in MI for heat pumps or hybrid heat. Again, I'm guessing they are refering to haing a furnace so that on the colder days, you are not using electric resistance heat for the aux. Utlities like to give incentives to reduce peak demand. That's the whole reason for rebates.

badtlc · Feb 11, 2009

motoguy128 said:
I don't know about the "lower rate" in MI for heat pumps or hybrid heat. Again, I'm guessing they are refering to haing a furnace so that on the colder days, you are not using electric resistance heat for the aux. Utlities like to give incentives to reduce peak demand. That's the whole reason for rebates.

Not quite. Electric companies will provide lower rates if you go with a heat pump (or some place require electric strips) during the winter because they need to "increase" demand during the non-cooling seasons.

beenthere · Feb 11, 2009

Do a load calc.
And see what your thermal balance point is.
Might be 30 or less.
Could be 40.

motoguy128 · Feb 11, 2009

In theory, if a unit is sized correctly overall and sized for cooling load, the balance point can be estimated within a few degrees based on the region it is installed. The further north, the lower the cooling demand, so the heat pump is proportionally smaller to the heating load. In the south, the opposite is true.

IF the unit is a little oversized, it will be a little lower.

I'm not trying to say load calcs aren't improtant. Just pointing out that some things are proportional and you cna make some reasonable estimates. Besides the IAQ and can only set the balance points by 5 degree increments (which is a little annoying and pointlessly limiting IMO).

beenthere · Feb 11, 2009

Moto.

Its that kind of reasoning, that many companies ues to ballpark sizes of equipment.

No reason to be too lazy to do a load calc, and make up a balance point graph, and plot it(less then 5 minutes for the graph).
To find out if there will be a real payback. Instead of making a guess.

Replacing A/C - get a heatpump?

megawhizz

motoguy128

badtlc

beenthere

motoguy128

beenthere

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