How cold can you realistically run a HP

[pgb104]

I was wondering how cold an outside temp can a heat pump realistically be used. I live in State College and I thinking about my ceiling cable elecrical disconnected ( so the ducting can be installed) and putting in a heat pump. Will I save money? will I still save money after heat pump turns itself off and I am heating from the heating coil?

[dash]

The heat pump is more economical to run than the strip heat down to beloww zero.It should be setup for the heat pump to always run,if set piont hasn't been reached,with the strip heat cycling to maintain the indoor temperature.

So you will save money.Now I have seen homes with ciling heat,where each room has a stat,and they don't heat all rooms.In that case a heat pump can cost more ,because now you are heating the entire home.

[docholiday]

You can run a heap pump well below 0 but there becomes a capacity issue hence the need for aux heat. You currently get 100% efficiency out of your heat. A heat pump will operate anywhere from 250-350% efficient. When the aux heat is required to suppliment, it will operate at 100% efficiency but it only runs to make up the difference between your heating requirement and the lowered capacity of the heat pump.

[amickracing]

For what it's worth, I talked to an American Standard guy about this a few weeks ago. He told me he never saw any reason to shut the heat pump off ever. It's always making some heat (sometimes not enough, thus the heat strips), but it doesn't hurt it to keep running in the cold.

I don't recall what they tested thier HP units down to, something like -50, maybe more. He did have a good point though, the starting and stopping of the HP usually adds more wear to it than letting it run all the time, even if it's cold.

[BaldLoonie]

I hate to see time-temp defrost units run below 10 or so. If you are in a cold climate, you'll usually have to run defrosts every 50-60 minutes max or you'll have an icecube much of the winter. But in the bitter cold weather, there's little frost but that stupid board doesn't know that and for up to 10 minutes an hour, you sit in cooling mode pouring cold air in the house with your strips trying to overcome it. My pet peeve, those boards should be banned in favor of all demand boards.

[berad]

The realistic effective outdoor temp you can run your heat pump without aux heat is going to be different system to system. It comes down to two words BALANCE POINT! The balance point will be different system to system based on the sizing, efficiency, and some other factors of the design from the manufacterer.

[docholiday]

Yes, but, below the balance point you only have to suppliment the difference in load and capacity. Many people, including technicians dont grasp this concept. The heat pump will still be running and producing heat. (or moving it rather).

For instance say the heating requirements are 38,000 btuhs at 15 degrees outside to maintain 70 in the house. The heat pump (depending on size, etc.)Lets call it a 3 ton, will produce say a reduced 17,000 btuhs. This means the heat pump will continue to run and the suplimental heat will only need to make up the 21,000 differnce. Not the entire 38,000 required.

Many places get cold, no doubt about it, some just plain miserable, but short of the extreme north in the US, very few places stay at or below the balance point for long. I submit that you would have to be below 20 continuosly and have no spring or fall heating requirement to make a heat pump cost more than gas.

[berad]

Originally posted by docholiday
Yes, but, below the balance point you only have to suppliment the difference in load and capacity. Many people, including technicians dont grasp this concept. The heat pump will still be running and producing heat. (or moving it rather).

For instance say the heating requirements are 38,000 btuhs at 15 degrees outside to maintain 70 in the house. The heat pump (depending on size, etc.)Lets call it a 3 ton, will produce say a reduced 17,000 btuhs. This means the heat pump will continue to run and the suplimental heat will only need to make up the 21,000 differnce. Not the entire 38,000 required.

Many places get cold, no doubt about it, some just plain miserable, but short of the extreme north in the US, very few places stay at or below the balance point for long. I submit that you would have to be below 20 continuosly and have no spring or fall heating requirement to make a heat pump cost more than gas.

I agree, which is why I stated "without aux heat". I realy like to use outdoor stats to stage the strips in the airhandler to stage with the o.d. temps, makes for maximum efficiency.

[beenthere]

In other words,

As Dash said, if your heating your whole house, or most of it with your radiant ceiling heat, the heat pump will save you money.

Your winter design temp is 7°, leave the heat pump run!!!

[SBuckJ]

Hey pgb104. If you're in the State College PA area, give me a call at 342-6682 or e-mail at SBuckJ@hotmail.com. I'm only 26 miles away.
Spencer
SRJ Heating & Cooling

[mlbussert]

Originally posted by docholiday
A heat pump will operate anywhere from 250-350% efficient. When the aux heat is required to suppliment, it will operate at 100% efficiency but it only runs to make up the difference between your heating requirement and the lowered capacity of the heat pump.

I have a question: is this a typo (250% - 350%)? I am just curious, because I seem to remember from high school that it's impossible to do that - to be more efficient than 100% (one of the law of thermodynamics, I think the first). Maybe this is something else and I am confused. I am just curious.

[beenthere]

Doc is refering to the c.o.p. of a heat pump.

coefficent of performance.

its the ratio of watts consumed to btu's delivered.

A heat pump may provide 3.5 watts of heat, while only consuming 1 watt of electric at 47° outdoor temp.
And 1.1 watts of heat, and consume 1 watt at -17° outdoor temp.

The heat pump isn't making heat, its just moving it, from one place to another.