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Old 10-20-2009, 08:54 PM   #1
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Gas Furnace with HP or plain A/C

This is a new install. I am looking to put in a 95% efficient Gas Furnace for heat and new A/C system using same ducts for both. I am not sure whether a HeatPump or simple 2 Stage Compressor is the most efficient system. My installer commented, because Electricity is 18-20cents a kilowatt on Cape Cod, that having a Heat pump means on days when temps are above 35 that I'll use lots of Electricity, and that burning a little gas in winter(to maintain 55degrees when not there) is cheaper due to the high efficiency of a good furnace. Is he right. I enjoy learning about these things, like most DIYers.


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Old 10-20-2009, 09:51 PM   #2
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Your terms are a little loose here. An air source heat pump uses the compression, pumping and evaporation of a refrigerant gas to generate heat. When the system is ran in the reverse, it cools an area. It can easily be thought of as a reversible AC, because an AC uses the exact same process.

In the heat mode, a heat pump is removing the latent heat from the outside air, and transferring to the inside of a dwelling. When the outside air temp is below a certain point, additional heat is needed. This point is (was) 52*, but has varied depending on the heat content of the air, and the characteristics of the refrigerant. The main factor determining airs heat content is humidity and it temperature. The additional heat source can be electric coils, or a gas fired heat exchanger.

So you see, you generally never have a gas furnace mated to a HP. You either have a HP, or a furnace/AC combo.

HP's are common in commercial work, because there is such a need for cooling, because of the greater heat generation by machines, lights, technology, and a denser occupant load. In residential applications, at least in Washington State, heat pumps are common only in areas which receive very very cheap electricity from the Columbia River hydroelectric systems.


Last edited by Anti-wingnut; 10-20-2009 at 09:54 PM.
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Old 10-21-2009, 12:30 AM   #3
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It's not unusual to have a dual fuel setup (HP + gas furnace), though it might be in Massachusetts. I don't know about that. You have very high electricity rates, but it's the relative cost of competing fuels (like gas) that matters, not the cost of electricity alone.

#1: Heat pumps are the most efficient form of heating equipment available. They can be 300-400% efficient instead of just 80% or 95% like furnaces. In other words, they use less energy to deliver equivalent amounts of heat.
#2: That efficiency may help to overcome the initial price disadvantage of electricity vs. gas or oil. Then again it may not.
#3 That efficiency (and the HP's heat output) will decrease incrementally as the outside air temperature goes down.
#4: Depending on your home's heat load and the ratings of the unit you buy, there will come a point (usually somewhere between 25 and 40 degrees of outdoor temperature -- certainly not 52) when the HP can no longer supply 100% of your home's heating needs. That is called the balance point. The HP may still be able to produce cheaper heat below that temp, just not enough to maintain 100% of your thermostat set point***. If you have a multi-stage T-stat and a backup furnace, the t-stat can stage the HP and furnace so that you use the most economical heat possible.
#5: The point at which the HP is no longer economically cheaper to run is variously called the cutoff, the break-even point, or the economic balance point. With such expensive electric rates as yours, your cutoff point may actually be at a higher temp than your balance point. Nevertheless, if you get a large enough percentage of heating days above the cutoff, the HP can save you money because its heat will be cheaper in that temp range.
#6: The additional cost of a HP is not that great if you are already going to have a split A/C system anyway. In essence, the HP is just an A/C with a reversing valve. Be aware that you may need larger ducts when going with a HP. Have your contractor evaluate that.
#7: Some utilities offer a more generous KWH baseline amount for folks who rely primarily on electricity for heat. In essence, that would provide more juice at the lowest rate. Without that benefit, increased KWH usage may cause your rate to spike rather substantially, negating any benefit you might get from the HP.
#8: At the moment, gas is pretty cheap nationwide, but that can change. Having a dual fuel system offers you the chance to hedge your bets and go with the cheapest heat depending on outside temps and the market price of competing fuels. That's why I own one.

***If you only want to maintain 55 degrees in the winter months, the balance point for a HP will be much lower than it would be for folks who want to maintain 68 or 70 degrees.

Last edited by HPGui; 10-21-2009 at 12:39 AM.
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Old 10-21-2009, 11:08 AM   #4
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My bad on two points

1) I thought I wrote 42*, not 52*. "Around 40* is the temperature I learned years ago, and it certainly appears that modern HP's can wring heat out of the air more efficiently than 20 years ago. I missed this mistake on my part in more than one editing

2) My experience with HP's is as a commercial superintendent, as well as working for a large mechanical contractor (large as in we didn't do HP's, we built power plants). My information is dated, and specific to a segment of the trade. I have not seen an air-source HP without built-in auxiliary heat. Apparently they are common in residential applications such as yours.

You will have to run the numbers to decide what is the most economical way to condition your house: HP, Furn + HP, Furn + AC
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