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I have two heat pumps, in an all electric house. Unfortunately they both seemed to have stopped working at nearly exactly the same time. I have called for qualified repairman, but they won't be out to my house for 3 days.

In the interim, the emergency heat mode works just fine. I have not however, been able to find the wattage of the heat coils in the air handlers for the emergency mode.

Is there a general answer as to whether or not the emergency heat or electric space heaters will be more efficient for short-term heating of my house while I'm waiting for the heat pumps to be fixed? If it makes any difference, I'm setting the temperature at the thermostat to 16 C.

Is there a place on the air handlers, where the wattage for the heat coils can normally be found?

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Q: "emergency heat or electric space heaters...more efficient for short-term heating?"

A: The efficiency is identical because they both supply heat from resistive electric heating elements. The operating cost of the emergency heat is likely to be more expensive because they are large heaters that heat the whole house, whereas the space heaters can be used to heat only the occupied rooms.

Q: "Is there a place on the air handlers, where the wattage for the heat coils can normally be found?"

A: The information is not always easy to find since most fan-coil units accept a wide range of heating elements. It is most likely on the electric heat coil unit itself, inside the fan-coil box, downstream of the refrigeration coils and the blower. Very roughly, the emergency heat is likely about 1/2 to 2/3 the rated capacity of the heat pump. To estimate this number in KW, divide the BTU/hr of the unit by 3412. For example, a 48000 BTU/hr heat pump would be 48000 / 3412 = 14 KW. An electric heater about 2/3 that size would be 10 KW.

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Emergency heat and electric space heaters are about the same efficiency: 100%. They don't have a smoke stack, there's nowhere else for the energy to go. But before you get all excited, a heat pump has an efficiency of 150-400%. So 100% is weak tea.

The space heaters have the advantage you can put heat where you want it, so you're not heating unused space like the forced-air system does. Their disadvantage is they cause annoying breaker trips when you try to use them on a circuit with any other large load (hair dryer, laser printer, gaming PC, almost any heat-based kitchen appliance) and have a risk of exposing any yet-undiscovered wiring defects, trouble from extension cords or power strips, things like that which sometimes burn houses down.

If you can't easily discern the wattage of your emergency heat, look at the breakers. Expect more than one. Generally, these large breakers come in fairly fine increments: 40, 50, 60, 70, 80. The breaker must be 125% of the actual load, so multiply by 0.8 (e.g. 32, 40, 48, 56, 64) and they generally size heaters to use almost all of it. (50A breaker = 40A derate ~= 39A heater). Multiply by modern US line voltage of 240V and that's watts. Repeat for each emergency-heat breaker.

E.G. for dual 70:

 70A x 0.8 derate = 56A max
 56A - 1 margin = 55A likely
 55A x 2 heaters = 110A
 110A x 240V = 26,400W  ... really. 
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