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Gas furnace w/ ? heat pump


Jerry Lozier
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Home with (possible heat pump) and mid efficency Rheem gas furnace. Could not find/access data plate on outside unit to verify if it is a heat pump. 3Thermostats (big house) all were heat pump type, were set to heat and the little red light glowing indicating aux heat?

about 30 degrees outside today

Question: I've never seen both heat pumps and gas furnaces together on same system.... always one or the other.... so does gas furnace come on when temp gets low to indicate need for aux heat (rather than go to HP aux heat) or is this an a/c unit with heat pump thermostats or??

inquiring minds need to know??

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Bill

I set system to off on all thermostats.... waited a while until nothing running then set to emergency heat and gas furnace started.... shut down again, waited and set to 'heat' a couple degrees over room and still no heat pump as gas furnace fired again, ran until thermostat was satisfied. Breaker was on, and fuse block in outside sub was installed...

Guess you answered question, as have not seen a HP go to a gas furnace for emegency heat before, although certainly makes sense and probably better.

and you are right usually I see HP working into 20's or below before emerg heat starts, nothing out of outside unit today

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Home with (possible heat pump) and mid efficency Rheem gas furnace. Could not find/access data plate on outside unit to verify if it is a heat pump. 3Thermostats (big house) all were heat pump type, were set to heat and the little red light glowing indicating aux heat?

about 30 degrees outside today

Question: I've never seen both heat pumps and gas furnaces together on same system.... always one or the other.... so does gas furnace come on when temp gets low to indicate need for aux heat (rather than go to HP aux heat) or is this an a/c unit with heat pump thermostats or??

inquiring minds need to know??

I have a Ruud heat pump (same as Rheem) with a Ruud Criterion II gas furnace for back up. Mine, and every other system like mine that I've seen, is set to lock out the heat pump when it's about 38 degrees outside. Unlike a heat pump with an electric furance for back up, these heat pumps can not run concurrently with the gas furnace. It's one or the other, so at the outdoor tipping point, the heat pump shuts down.

In addition to the reversing valve that Bill mentioned, you can spot a heat pump several ways. The heat pump's outdoor unit will require five control wires, while most air conditioners only need two. So look for a larger low-voltage control cable. Also, the Rheem & Ruud units always say "heat pump" on the brand label as well as on the data plate. If there's no marking on the outside, you can remove 4 screws and get access to the wiring compartment where it's labeled again. Also, if there are heat pump thermostats in the house, those are almost certainly heat pumps. The heat pump stats are more expensive than plain ac stats, so it's unlikely that someone installed them by mistake.

- Jim Katen, Oregon

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I have a Goodman--easy, Terry--heat pump with gas auxiliary, and the manual override gizmo was an add-on. Just as a postscript, it saves money because when it's too cold for the heat pump to effectively condition my house, the condenser doesn't run for half an hour before it times out and senses a need for auxiliary heat.

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In 10 years of inspecting I've seen maybe 4 heat pumps. I've always assumed that it's because we have relatively cheap gas here. We certainly have periods of moderate colder weather where a HP could be effective. The cheap gas thing could change, of course.

I'll be needing to replace my own A/C this spring, and I've considered a HP. What brands would you (Terry, or anyone else) suggest? More important, what questions should I be asking the installer to know that they're really familiar with how they work in my climate?

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[brIMine, and every other system like mine that I've seen, is set to lock out the heat pump when it's about 38 degrees outside.

Why 38°? Is it to avoid including a defrost cycle?

No, it's not related to the defrost cycle. There's a point at which the heat pump, working alone, can't keep up with the heating requirements of the house. That point is well below 38 degrees, but the installers around here add a fudge factor. I could probably mess with the outdoor thermostat on my system and find out exactly how low I'd have to go before the system ran all day just to keep up with the indoor set temperature, but I kind of like the extra warm boost that we get when it switches to gas.

You could design a heat pump that would adequately heat the house at far, far lower temperatures, but then that same system would be vastly oversized for cooling in the same climate. Since cooling is more critical than heating (you can add backup for heating), they size the system for cooling. That means that it comes up short during the heating season.

- Jim Katen, Oregon

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Heat pumps often have a coefficient of performance (COP) of approximately 3, which means that it produces 3 units of heat for each unit of electricity invested in it. I have a hard time imagining electricity from any utility in the USA costing 3 times as much as gas, when compared on an energy content basis.

Marc

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You could design a heat pump that would adequately heat the house at far, far lower temperatures, but then that same system would be vastly oversized for cooling in the same climate. Since cooling is more critical than heating (you can add backup for heating), they size the system for cooling. That means that it comes up short during the heating season.

- Jim Katen, Oregon

Such a system is already here. Click here.

ONE TEAM - ONE FIGHT!!!

Mike

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Heat pumps often have a coefficient of performance (COP) of approximately 3, which means that it produces 3 units of heat for each unit of electricity invested in it. Marc

Anything to back that up Marc? I'd be interested in reading the white paper.

EDIT: I found it while surfing around. From Wiki:

"The COP of heat pumps (300%-350% efficient) make them much more efficient than high-efficiency gas-burning furnaces (90-99% efficient), and electric heating (100%). However, this does not always mean they are less expensive to operate. The 2009 US average price per therm (100,000 BTU) of electricity was $3.38 while the average price per therm of natural gas was $1.16.[1] Using these prices, a heat pump with a COP of 3.5 would cost $0.97[2] to provide one therm of heat, while a high efficiency gas furnace with 95% efficiency would cost $1.22[3] to provide one therm of heat. With these average prices, the heat pump costs 20% less[4] to provide the same amount of heat. The savings (if any) will depend on the actual cost of electricity and natural gas, which can both vary widely".

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Is electric cheaper than gas there Jim?

It's always been my assumption that if you have to heat electrically then a heat pump will save money however if gas is available then it's better to just have a conventional HVAC system for cheaper operation (at least in this neck of the woods).

In my case, I have no access to natural gas. My system is LP. So the heat pump is cheaper to run.

In general, though, it depends on the cost of electricity and the cost of gas. The nearby town of McMinnville has really cheap electricity - about 4 cents per kwh. That town is full to the brim with heat pumps & electric backup coils. Super high efficiency gas furnaces can compete, but barely.

By the way, the folks in McMinnville were outraged a few years back when their electric rates jumped from 3 cents to 4 cents. They characterized it as a 30% jump in cost. (In my town, we pay about 8 cents.)

- Jim Katen, Oregon

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During our first energy crunch heat pumps were in fashion due to the fact that you couldn't get natural gas (among a lot of other things). It made sense, up here, to run a heat pump due to that fact. Once natural gas was free flowing again heat pumps fell out of favor. In warmer climates heat pumps make sense, in cold climates, not so much.

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No, it's not related to the defrost cycle. There's a point at which the heat pump, working alone, can't keep up with the heating requirements of the house. That point is well below 38 degrees, but the installers around here add a fudge factor. I could probably mess with the outdoor thermostat on my system and find out exactly how low I'd have to go before the system ran all day just to keep up with the indoor set temperature, but I kind of like the extra warm boost that we get when it switches to gas.

You could design a heat pump that would adequately heat the house at far, far lower temperatures, but then that same system would be vastly oversized for cooling in the same climate. Since cooling is more critical than heating (you can add backup for heating), they size the system for cooling. That means that it comes up short during the heating season.

- Jim Katen, Oregon

I'm familiar with sizing heat pumps and balance points. I've lived with them for over 20 years and installed them in 2 of my homes. I'm just questioning the 38° cut-off. There's a whole lot of dual-fuel systems here with the balance point in the mid 20°s.

Mine is sized perfectly for cooling and the QuadraFire stove on winter nights keeps the heat pump running well down below 20°. The staged electric back-up rarely kicks in. I would never consider LP back-up here.

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. . . Mine is sized perfectly for cooling and the QuadraFire stove on winter nights keeps the heat pump running well down below 20°. The staged electric back-up rarely kicks in. I would never consider LP back-up here.

I don't doubt it. But your climate is different. I suspect that your cooling loads are greater than ours, if only because of your high humidity levels in summer. Our air conditioners don't need to be as large as yours. Conversely, our heat pumps need a higher set point to go to backup.

- Jim Katen, Oregon

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At one time, the gas furnace and heat pump were billed as "dual fuel" so that you could select the most economical fuel based on the price of the week.

Of course every heat pump here needs a back-up since the COP factor drops off rapidly at temperatures below 30°F. The real issue though is not the COP but the actual reduction in the amount of heat available at lower temperatures. Right when you need the most heat, a heat pump has the least to give.

We don't get near as many cold nights as folks to the north but temperatures in the low twenties are common several times per year and the occasional dip to the teens or even single digits is normal. A properly sized HP for our 105°F summers just won't cut it on the heating side without a backup source. Maybe the two stage units would, but not worth the extra bucks for the 5% of the winter that the HP won't handle the load.

Most times the dual fuel systems I come across have cobbled together controls using the emergency heat switch to manually switch over to the gas furnace but there are very few systems left that use both fuels and even fewer new systems installed; likely due to the difficulty in marketing the concept; only geeks really get it.

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I don't doubt it. But your climate is different. I suspect that your cooling loads are greater than ours, if only because of your high humidity levels in summer. Our air conditioners don't need to be as large as yours. Conversely, our heat pumps need a higher set point to go to backup.

Oh, now I get it. Thanks for the explanation.
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