View Full Version : Auxiliary Heat Comes On?

The Money Pit
02-06-2007, 09:57 PM
In my house my emergency heat comes on only when there's a problem with my system and I have to have someone come out to find and fix the problem. I had an issue like that fixed a few years ago and now the new heat pump in my garage is going into "auxiliary heat" mode.

What causes it to do that?

Is that normal?

Doesn't it cost a lot more to run the heat when it comes on?

We are having record low temps here in VA. If that helps diagnose the problem.

Thanks for any info.!

02-07-2007, 08:58 AM
The heat pump switches over when it cannot provide the heat that you require.

First up, the temperature of the outdoor air passing over the outdoor coil is reduced by about 10º F. This means that even if the outdoor temperature is above freezing (say, 35º to 40º F), the air closest to the outdoor coil will be reduced to below freezing. So if you are below those temperatures, it will switch over to emergency heat. No need to "fix" the problem - when it warms up it will go back to the heat pump. Second, "costing more" depends entirely on what your backup heat source is.

This is why the groundsource heat pumps are more advantageous in Northern areas - you have a stable temperature to deal with instead of the variables of air temperature.

So basically, what you are describing is completely normal and just an issue inherent with air source heat pumps.

02-07-2007, 09:06 AM
Sizing a heat pump is tricky. For best efficiency, you want to minimize the starts and stops. For best comfort in cooling mode, you want to avoid oversizing the heat pump. An air source heat pump loses capacity as the outside temp drops (just when you need the capacity the most). Smaller units cost less than larger units.

With these factors in mind, it is common to design a heat pump such that it cannot keep up with heating demand as the outside temp (and thus the heat loss) drops below a certain threshold. The "emergency" heat (in this case it is auxilliary heat) is set up to come on to help the heat pump keep up. The thermostat normally controls whether the aux heat kicks in or not, usually by the difference between the set point and the indoor temperature.

The heating coils use about 2 to 3 times as much electricity per BTU as the heat pump (considering the loss of performance at the low end of the operating temps). So it does cost a lot more. But if you look at historic weather averages, the temp doesn't get to that threshold all that often. A smaller heat pump performs more efficiently when the temp is above the threshold, making up for the losses (at cold temps) when the electric coils come on. You can game this out with historic weather data to see how a typical heating season plays out to see what the optimum size would be.

Your house may have a large heat pump to keep up with summer cooling demand, and it has enough capacity in this weather to avoid needing the aux heat.

02-07-2007, 10:51 AM
What they said with a few clarifications....

Aux heat kicks in for three reasons:
1. Heat pump is not working(system reverts to aux/emergency heat source).
2. Outside temperature is too low for heat pump to be efficient(this is a set point by the installer of the system. It is based on the unit efficiency, secondary(emergency) heat source, primary energy cost and secondary energy cost).
3. The difference between the actual temperature and the requested temperature is too great-- usually about 5 degrees. (You are requesting a significant temperature increase and it would take too long with just the het pump alone).

Since there is available heat to be pumped from the outdoors at temperatures down to almost zero degrees with the latest high efficiency units, you shouldn't need aux heat until then. Your installer should have done this calculation to determine the setpoint where the cost of aux heat is equal to the cost of heat pump heat.

02-07-2007, 11:24 AM
One more comment, just to amplify the previous point, -- heat pump systems do not benefit nearly as much from a thermostat setback strategy as other systems. If you set the thermostat really low when you are not using the garage, then come in and bump the thermostat 10 degrees as you start to work, you can almost guarantee the aux heat will come on to "help" fix the wide disparity. Does the setback "save energy"? You use less energy overnight due to the lower temp, but the recovery heat is less efficient than the steady-state heat. Whether the net is a win or loss depends on the timeframe involved. With gas or oil heat, setbacks for any length of time are a win, but with a heat pump it isn't as clear.

Contrast this with your house, where the nighttime setback may be much less, if any at all. The aux heat is then not needed because the system keeps up with the steady thermostat setpoint without the extra "help".

The Money Pit
02-07-2007, 06:19 PM
Thanks for the info guys. Now I'm beginning to wonder if my home heat pump is set inefficiently. My electric bill is through the roof. Given it's a 10 yr. old system and it's bottom of the line courtosey of my builder. So if I want it to be 55* in the garage, and it's set at 45* now, I should bump it up a few * at a time until it reaches the 55* mark so the aux heat doesn't come on?

02-17-2007, 06:17 PM
If I am not mistaken, should your unit call for heat require more than 2-4* the unit will automatically go to "EM" heat This should be listed in your owners manual.