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Old 08-12-2010, 11:58 PM   #1
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How are heat pumps more efficient then electric heater?


I saw a thread and did not want to hijack it, this is more a general question.

Maybe I'm mistaken, but a heat pump is basically a reverse air conditioner right? So the compressor runs "backwards" so the condenser gets cold (outside) and evaporator gets hot (inside).

Now using newton's laws, to generate said heat, don't you need the same amount of energy that would required to generate that same heat using a heating element?

Or does it have to do with the fact that even though it may be cold outside, there is still some heat, and it gets absorbed into the coils and "sucked" into the house? So this is energy that is already existent, it's just being moved and you are only using the electricity to move the energy, and not create it?

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Old 08-13-2010, 12:58 AM   #2
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How are heat pumps more efficient then electric heater?


basically: yes

first, an electric resistance heater is the most efficient heater you can have. It has nearly 100% efficiency. What you are concerned with is cost effectiveness.

and yes, a heat pump takes heat from outside (air or ground; depending on what type of heat pump it is) and transfers is into the house. You are not creating heat but transferring existing heat. That is also why a heat pump is not effective below a certain temp as there is less heat available to transfer. This is why many heat pumps have resistive heat elements built into the system.

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Old 08-13-2010, 03:11 AM   #3
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How are heat pumps more efficient then electric heater?


A heat pump is always more efficient than electric resistive heat. If you get a heat pump with resistive backup heat you wind up not saving a lot of money. I think a heat pump with natural gas backup is the best way to save money year round.


Heat pumps are between 3 to 5 times more efficient than electric heat over their useful range (65 to 30 degrees outside). Newton's Laws are not violated in a refrigeration cycle. In this cycle heat is not generated, it's moved. Heat generated with electric heating elements are close to 100% efficient, but when compared to a heat pump you get 3 to 5 times less heating capacity for the amount of energy you put in with resistive element heating when compared to a heat pump. Or another way, where a resistive element is 100% efficient, a heatpump is 300% to 500% as efficient trying to do the same function. You don't get to create heat energy with a heat pump, you do "work" with that energy to move the energy inside/outside. And it does this by making the outdoor coils even colder then it is outside in heating mode.

1. Take the datasheet of a heatpump:
http://www.goodmanmfg.com/Portals/0/pdf/SS/SS-SSZ14.pdf
2. Look at page 18.
3. See the COP (coefficient of power)
4. Notice how it is from 3.01 to 4.44 between 30 to 65 degrees outside.
5. That means that for the power coming in, you move the input power times that number worth of power.


If you want to study it more, here are some tips to interpert the datasheet.

1. They show the outdoor temperature in this chart on the first row. This is assuming the indoor temperature is 70 degrees.
2. The MBh is Thousands of BTU per hour. To convert to equiv KW of electric heat, Divide this number by 3.41
3. Delta T is how much cooler the outdoor coil is then the ambient outdoor temperature. This is so the heat will transfer, If you have a cool day and an even cooler coil, then heat will transfer from the cool air into the coil and thus go into the house.
4. kW this is how much power you're putting into the heatpump compressor and Fan.
5. Amps is how much current your outdoor unit and fan is drawing with a 220v input, other loads in the system include the indoor fan and the outdoor coil de-froster
5. COP: output heating ability divided by input heating power.
6. EER: multiply the COP by 3.41
7. Then just various pressure measurements in the system..


See my thread here:

Heatpump vs Natural Gas - Energy Cost Spreadsheet

Last edited by zootjeff; 08-13-2010 at 03:24 AM.
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Old 08-13-2010, 08:55 PM   #4
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How are heat pumps more efficient then electric heater?


Ahh so guess heat pumps are more for the south then. Here 30F is considered a hot day outside. The average around here is more like 0F and even lower. -50F is not too rare. So I guess at those temps it would not really run very well.
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Old 08-13-2010, 09:03 PM   #5
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How are heat pumps more efficient then electric heater?


Red. Check out the hallowell heat pump. Works at temps of -30F. No other air to air heat pump can match it at this time.
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Old 08-13-2010, 10:31 PM   #6
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How are heat pumps more efficient then electric heater?


Dang red you're in Canada for crying out loud. Most everybody in north america is " south" compared to you.
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Old 08-14-2010, 05:04 AM   #7
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How are heat pumps more efficient then electric heater?


Quote:
Originally Posted by Marty S. View Post
Dang red you're in Canada for crying out loud. Most everybody in north america is " south" compared to you.
ROFL
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Old 08-14-2010, 06:20 AM   #8
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How are heat pumps more efficient then electric heater?


So true
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Old 08-14-2010, 06:21 AM   #9
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How are heat pumps more efficient then electric heater?


My heroes. Bob and Doug McKenzie
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