Heat Pumps and Setback Thermostats

by Gary Foreman


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Dear Dollar Stretcher:
I have two home heating questions. First having to do with using pre-set thermostat offsets and when a heat-pump becomes ineffective due to outside temperature. Off-set opinions range from don't use them (heating the house back up wastes whatever you saved), 5 degrees lower during the night-time hours, don't use the off-set unless you can set it for at least 8-10 hours, use 10 degrees off-set or not at all. I guess then the "controversy" stems from 3 different aspects:

  • whether or not to use an offset period;
  • how much temperature difference actually makes a difference;
  • how long an offset period is cost effective.

The other side of my question involves heat pump cost effectiveness at various temperatures. Here in Maryland the winter temps fluctuate widely; for example, the forecast for today calls for a 58 degree high. Next week it could be in the teens. My question is: at what temperature does a heat pump become cost ineffective? Should I rely on the furnace assist or just set it for furnace only below that temperature?

My new-fangled thermostat obviously gives me too many future shock choices. Don't you agree?
Bonnie S.

Bonnie asks a couple of good questions. Let's look at the heat pump question first since it's a bit simpler. A heat pump is a device that can take heat from outside air and then transfer that warm air inside your home. Believe it or not, there's even some heat in 20 and 30 degree outside air.

But, just as Bonnie suspected, a heat pump becomes less effective as the outside temperature drops. According to the U.S. Department of Energy, a heat pump can be twice as efficient as conventional heating at 50 degrees F. But it rapidly loses it's efficiency and effectiveness when you get into the 30 degree range. In fact, most pumps will automatically engage your regular heater when it gets to 35 degrees outside and many will shut off when you get to 15 degrees.

Now, lets look at offset thermostats. These are thermostats that are programmed to change from one temperature setting to another for a preset period. For instance, you could have your heater turned off from 8 am to 5pm during working days. Or you could have it only heat the house to say 60 degrees instead of your normal setting of 68 degrees.

I went out looking for an answer to Bonnie's question about the appropriate use of setback thermometers and found a surprising lack of data. When you think about it though, it's really not that surprising. We're asking for an answer to a question with quite a few variables. What type of heater do you have? How cold is it? How much difference is there between the regular and offset temperatures? How long do you use an offset?

There is a way to test your own furnace so that you can answer the questions that Bonnie asks. The only 'equipment' that you'll need is a watch, a thermometer, a note pad and the ability to do a little math.

Here's what we're going to do. First, we're going to figure out what percentage of time that the furnace runs to keep the room at an even temperature for the higher setting. To do this we'll make sure that the room is already at the desired temperature. Then we're going to listen to our furnace to see how much time it's actually burning fuel.

Remember that most heaters actually start burning fuel for a few seconds before the fan starts to move warm air through your vents. With a gas or oil furnace you can listen for the 'whoosh' that the burner makes when it lights. If you have an electric system you'll have to refer to your owner's manual to find out what the delay time is (probably about 10 seconds).

You'll be making notes saying when the furnace turns on and off. Do this for about 20 to 30 minutes. Enough time to get a representative sample. Then total the time the heater was actually consuming fuel and calculate the percentage of time ('on' time divided by 'off' time).

Next you need to check the amount of time that the furnace is running at the lower 'setback' temperature. Drop your thermostat to the lower setting. Wait long enough for the room temperature to reach the new level. After it gets to that temperature check the amount of time the furnace runs just like you did before. Calculate the percentage just like before. Naturally, if you've turned it off the percentage 'on' is zero.

Now compare the two percentages. Subtract the 'setback' percent from the higher percent. How much difference is there? If the setback percent is 40 and the regular is 60% then you would save 1/3 of your heating costs for the time you use the setback temperature.

Clearly if you save that much it doesn't take many hours to save some real money. But if the difference is only a few percent then you'll need to use the setback for more hours to achieve real savings.

Now how about the question of what it takes to heat the house back up after the setback is over. Do you lose all your savings? Let's start by going back to our original test. During normal operation the furnace was on 60% of the time or 36 minutes each hour (60 minutes X .60). During setback operation it's on 24 minutes each hour. That means we save 12 minutes for each hour of setback use.

Let's see what it takes to heat back up to the normal temperature. Once your house has stabilized at the setback temperature manually reset the thermostat for the normal temperature. You're going to monitor the furnace just like you did before. Note how long the furnace runs before it reaches equilibrium at the normal range.

Suppose it ran for 35 minutes before it stopped at the normal operating range. You really need to subtract the amount of time that the furnace would have run anyway during normal operation. In this case it would have run about 20 out of the 35 minutes. So you actually ran it about 15 minutes more than normal (35 minus 20). Remember you saved 12 minutes per hour using the setback. That means you'd need to use the setback for a little more than one hour before you actually saved any money.

Now there are some variables that we haven't taken into account. You'll get somewhat different results on a 50 degree day than a 15 degree day. If you have people opening outside doors that, too, will make a difference! But you'll quickly get an idea of whether it makes sense in your home to change the setting for only a few hours.

You can translate the info we collected into actual dollars saved if you know your cost of energy (gas, oil, electricity) and the rate that your furnace consumes fuel.

Generally speaking, you can save money by keeping your thermostat as low as possible. Even if you'll be raising the temperature later, you'll save money by turning it down for whatever times you can. Thanks again to Bonnie for asking a thought provoking question.


Gary Foreman

Gary Foreman is a former financial planner and purchasing manager who founded The Dollar Stretcher.com website and newsletters in 1996. He's been featured in MSN Money, Yahoo Finance, Fox Business, The Nightly Business Report, US News Money and he's a regular contributor to CreditCards.com. You can follow Gary on Twitter or visit Gary Foreman on Google+. Gary is also available for audio, video or print interviews. For more info see his media page.

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