The Truth About Heat Pumps

November 29, 2012 Written by  Brad Eisenberg Comments Print
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Even though only 30% of homes in the US are heated by electricity, the majority of these all-electric homes use heat pumps. Not sure if your home has a heat pump? Well, if you don’t get a gas or oil bill, and you have a central system that you use for both heating in the winter and cooling in the summer, you’ve got a heat pump.

There are many misconceptions about heat pumps and they can be an efficient alternative for both heating and cooling as long as they are used properly. According to the Department of Energy, replacing an old inefficient heat pump with a new Energy Star model can cut the amount of electricity you use for heating or cooling by as much as 30% to 40%.

So How Do They Work?

Heat pumps work not by producing heat, but by moving it. Utilizing a heating cycle (think the refrigeration cycle of your refrigerator or air conditioner run in reverse), a heat pump is able to extract heat from outdoor air, concentrate it, and then deliver it inside of your home. In this way, a heat pump is able to deliver three or four times the amount of energy to your home than it consumes in electricity. Compare that to furnaces which only deliver 70-90% of the energy they consume in gas or oil.

So if this sounds so great, why is it that homeowners with heat pumps can still have such high heating bills? Well its because the efficient heating cycle is not the only source of heating that a heat pump utilizes. All heat pumps have a secondary backup heat source. You may know this as the system’s auxiliary or emergency heat source. And that heat source is 100% electric resistance heat, which is the most expensive kind of heating. The backup heat source is important for a heat pump to have, because if the heating cycle alone can’t deliver enough heat into your home, it relies upon the auxiliary heat to make up the difference.

A common misconception is that the auxiliary heat turns on when the temperature outside drops below a certain point, let’s say around 30-35 degrees Fahrenheit. Although that can be the case, it doesn’t tell the whole story. Heat pumps don’t measure outdoor temperature; there’s no sensor outside that triggers the auxiliary heat when the temperature drops. However, the heat pump does have a temperature sensor that triggers the backup heat, and that sensor is called your thermostat. Heat pump thermostats measure your indoor temperature and are designed to turn on the auxiliary heat if the indoor temperature drops 2-3 degrees below the setpoint temperature of the thermostat. There’s several cases in which this happens. It could be that it’s simply too cold outside for the heat pump to be able to extract enough heat (this is normal, it’s why the heat pump was designed with an auxiliary heat source in the first place). It could also be that the heating cycle of the heat pump is simply broken so the system has to rely upon the backup source (if the fan in your outside compressor isn’t running when the heat is on, you might want to call an HVAC company). But when customer’s call us due to high heating bills, those are not typically the reason. Most often, we find that there are two main causes for high electric heating bills - the home’s building shell or the behavior of the home’s occupants.

Think about the first case. Even if it’s not extremely cold outside (and especially if it is), if your home is very leaky and under-insulated, all of the heat that your system generates is just going to escape back outside. If more heat is escaping from the house than the heating cycle can deliver, your heat pump won’t be able to keep up and it has to rely upon the backup heat source. This is by far the most typical case for our customers. Our recommendation: get an energy audit to see what areas of your home’s building shell can be improved.

Most people don’t realize that how they use their thermostats can have a huge impact on the operation of the system. Did you know that you can unintentionally cause the emergency heat to run just by adjusting your thermostat’s set point? Remember, regardless of outdoor temperature, the auxiliary heat turns on when the indoor temperature drops 2-3 degrees below the thermostat’s setpoint temperature. So what do you think would happen if your thermostat’s setpoint and your home’s temperature is at 70 degrees and then you set your thermostat to 74? The house is still at 70 degrees, and now the setpoint is at 74 - a four degree difference - so the emergency heat turns on, even though it isn’t necessary. For this reason, we always recommend that people with heat pumps simply leave their thermostat settings alone in winter. Don’t manually change the setpoint and don’t use a programmed schedule if you have a programmable thermostat unless you ensure that the thermostat never jumps more than 2-3 degrees at a time. Otherwise, you’ll be paying for electric resistance heat when it may not be necessary to do so. Almost all thermostats will have either a light or will say on the display when the emergency heat is on.

So the lessons from all of this? An all-electric home is not such a bad thing. Heat pumps can be very efficient if utilized properly. But make sure to minimize the use of electric resistance heating in your home when it’s not necessary, and also make sure that your home is well sealed and insulated.Check out our infographic below with more information on heat pumps. Click the link to view last week’s blog Optimize Your Home Energy Use with Efficient Washing or check back next week for more energy saving tips.