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Old 12-16-2010, 08:05 PM   #1
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Heat pumps for dummies queation


Someone just told me it is a waste of electricity to run your heat pump if the temp is below 40 degrees. This is the 1st time in my (long) life I have ever heard this. Can someone please explain? I know nothing about electrical issues so please speak slowly and clearly...lol

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Old 12-16-2010, 08:18 PM   #2
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Heat pumps for dummies queation


They lose efficiency as the temp drops and there is a balance point depending on your local elec rates vs gas whether it is cheaper to use a different fuel. Google: air to air heat pump and read up on how they work.
http://oee.nrcan.gc.ca/publications/...sheatpumps.cfm
Read the section on efficiency and use your calculator to convert deg C to F.

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Old 12-16-2010, 08:35 PM   #3
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Heat pumps for dummies queation


when your system is cooling in the summer..the inside coil runs 40F on the very typical even commercial units up on roofs of malls.then in the winter the system switches the coils so the condenser is inside and that cooling coil is outside...so as the outside air drops near and below that 40F..the unit will start to lose its efficency.. down into the 32F freezing range ICE starts to form.mean while your heating the house with that condenser...but the ice builds and then the unit goes into DEFROST MODE switches the coils and melts the ice....during this deicing..your stat is still calling for heat...so they slide in a resistance heating section to temper that 40F now inside...like a toaster coil.if your electric is cheap then its OK for a HP...Florida and out west are design applications outside air above 40F unit does minimal defrosts cycles...
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Old 12-16-2010, 09:41 PM   #4
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Heat pumps for dummies queation


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Originally Posted by woodgirl View Post
Someone just told me it is a waste of electricity to run your heat pump if the temp is below 40 degrees. This is the 1st time in my (long) life I have ever heard this. Can someone please explain? I know nothing about electrical issues so please speak slowly and clearly...lol
Depends on what you have for back up heat.
Generally electric elements cost 3+ times as much to run at 40 degrees as a heat pump(COP of 3 or more). At 10 degrees it's twice as expensive to run the electric elements as the heat pump,COP falls to 2 because of lower BTU output. At below zero the cost is nearly equal but some still have a COP over 1 at -10F. A house with electric back up will always hit the thermal balance point before the economic balance point.

Gas back up the other hand can hit the economic balance point well before the thermal balance point. In areas with high electric cost and cheap gas the economic balance point can be 50 degrees. Cheap electricity and high gas priced areas will want to let the heat pump run untill it can't keep up the house temp, the thermal balance point.

My house for example has a 95% eff furnace with a heat pump. When gas was $1.07 a therm the heat pump was set up to run and run. At 9 degrees the house hit the thermal balance point and the gas back up had to come on to maintain the thermostat set point. Now gas is 49 cents a therm and the heat pump is set up to run only when the outside temp is above 40 degrees which is closer to the economic balance point.
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