how the heating element works in heatpump system

langle1033July 20, 2009

My old heat pump does not have the heat strip, only the fan and the evaporator inside the air exchange unit. The new unit includes a 15KW heat strip element.

I read some where that during winter, when the outside temp below 25 degree F we should use the heating element instead of the heat pump in order to get the warm air output plus saving more money in electric. Is this statement correct?

The thermostat has 4 setting: Cool, Off , Heat, Emer. Heat.

If that is the case, then what I should do with the setting

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black_adder

Just set it to emerg. heat if you just want the strip heat to come on,

    Bookmark   July 20, 2009 at 1:21AM
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dadoes

Under normal conditions, leave it set on Heat. The strip element will cycle on/off as needed (the compressor and element run at the same time), when needed. In this case, it's known as auxiliary heat ... the element comes on automatically if the heat pump (compressor) alone can't maintain the indoor temperature ... typically when the indoor thermostat senses that the temperature is not reaching the setpoint.

Emergency Heat is intended for use if the compressor fails completely and there's no source of heat except the strip element. Emer Heat shuts the compressor off and ONLY the element runs ... cycling on/off as usual in response to the thermostat's control (assuming the element is of sufficient capacity to handle the heating requirement solo).

You can use the Emer Heat setting to purposely prevent the compressor from running in cases of icy/snowy weather, if you perhaps notice the heat pump is very frequently going into defrost. It's largely a personal preference, and depends on the local weather conditions that may occur. Personally, I've never done that, but I'm on the Texas mid-coastal area which doesn't get that kind of weather very often. In the few instances it has happened, I let the system continue to run in normal mode, works perfectly fine for me.

    Bookmark   July 20, 2009 at 3:11AM
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srercrcr

"we should use the heating element instead of the heat pump in order to get the warm air output plus saving more money in electric"

You'll use the heat strip to get warm, but it uses more electricity. The HP does a good job heating in weather down to about 40 degrees. Below that it can't keep up. It's more useful in southern climes where the occasional cold front blows through.

    Bookmark   July 20, 2009 at 5:48AM
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srercrcr

They need to change the name from HEAT pump to WARM pump.

    Bookmark   July 20, 2009 at 3:55PM
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dadoes

srercrcr, my HP does work quite well at temps below 40°F. I said in the post above that such weather doesn't happen often, meaning that we don't get continuous weeks or months of it. There are typically several incidents of freezing weather each season, lasting a couple days at a time. 2004 we had a couple inches of snow on Christmas eve. Lowest I recall in recent years is ~26°F, I had no trouble keeping the house 70°F, *without* engaging the auxiliary.

    Bookmark   July 20, 2009 at 4:44PM
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garyg

"The HP does a good job heating in weather down to about 40 degrees. Below that it can't keep up. It's more useful in southern climes where the occasional cold front blows through."

- Wow. Someone doesn't have a clue about heat pumps.

    Bookmark   July 20, 2009 at 9:46PM
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creek_side

I read some where that during winter, when the outside temp below 25 degree F we should use the heating element instead of the heat pump in order to get the warm air output plus saving more money in electric. Is this statement correct?

Sort of.

Heat pumps have a rating called Coefficient Of Performance, abbreviated COP. This number essentially is the amount of heat produced for the amount of energy used to produce it. Thus a HP COP of 3.0 means the unit will produce three times more heat than would be provided if the energy used to produce it was converted directly to heat.

Heat strips, the resistance heating elements that produce the HP's auxiliary or emergency heat have a fixed COP of 1.0

The HP's COP will vary by temperature. The lower the temperature, the lower the COP. At some point, the COP will be down at around 1.0 and there is no benefit to operating the heat pump over the heat strips, since the heat pump will still have to go into defrost mode occasionally, sending a shot of freezing cold air into the house.

For this reason, some HVAC pros install a lock out thermostat in the outdoor unit designed to prevent it from operating below a certain temperature. 25 degrees is often mentioned.

However, a lot of newer heat pumps still have usable output at that temperature and should be turned off at a lower temperature. The heat pump manual should have included a chart that graphs the COP versus outside temperature. Take a look at the chart to see what kind of temperature it takes to drop your unit's COP to 1.0. Consider switching the heat pump to emergency mode when the temperature drops the COP to 1.1 to 1.2.

But I wouldn't worry about it if the temperature isn't going to go down below that temperature and stay there for awhile.

    Bookmark   July 21, 2009 at 10:38AM
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creek_side

srercrcr, modern heat pumps produce hot air, not warm air, although it does drop some in temperature as the COP gets way down there. That said, my heat pump, which is not a high end model, produces usable heat down to 15 F. Below that, I use the strip heater pretty much exclusively.

Fortunately, we see those kind of temperatures very rarely.

    Bookmark   July 21, 2009 at 10:44AM
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kframe19

" The HP does a good job heating in weather down to about 40 degrees. Below that it can't keep up. It's more useful in southern climes where the occasional cold front blows through."

New heat pump systems operate well at temperatures far below that.

My Trane XL 12 won't start kicking into aux. heat until the temperature reaches about 25 to 26 degrees F. And it will go just a little lower than that on a bright, sunny day when the sun is shining on the heat pump case. Seems the solar gain gives it just a little bit of a boost.

    Bookmark   July 21, 2009 at 2:04PM
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langle1033

Based on Creek side's argument, can I make a statement like this:

Since heat pump effects only at the temp 20 F or above, and since the cost of electricity to run the heat strip is too high, we should has a heat pump system using gas/or oil as an auxiliary heat source instead of heat strip as it is now.

    Bookmark   July 21, 2009 at 11:50PM
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weedmeister

Depends on the cost of gas or oil. And whether you already have a gas or oil system already in place. You might find that the cost of installing a new 'dual fuel' system is high and would take several years to pay off with the 'savings' of gas/oil over electricity.

    Bookmark   July 22, 2009 at 11:56AM
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creek_side

Besides weedmeister's point, it isn't that simple. Since the COP of a heat pump varies with temperature, its effective cost of operation varies too.

With a gas furnace, heat strips, or oil furnace, the cost for a unit of heat stays the same no matter what the outside temperature is. You just use more units as the temperature drops. But with a heat pump, the cost of the unit of heat increases as the temperature drops, so you have a double whammy. It costs more to heat because it is colder out and it also costs more to heat because the heat pump's efficiency drops due to that cold.

You have to take all that into account when you are comparing prices between the heat pump and other heat sources. A rough way to do it is to assume some average cold winter day's median temperature and get the COP number for it. Then compare that to the other heat sources, figuring some days you will do better and some days you will do worse.

What you can't do is simply use the published (maximum) COP for the unit to compare heat sources. You will come up with wildly optimistic numbers for the heat pump compared to the others.

    Bookmark   July 22, 2009 at 2:08PM
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