Heat Pumps, Heat Strips and Saving Money
January 13th, 2018
Daniel Lofland, Energy Efficiency Solutions Specialist
A little energy discussion for the day. I have been running an experiment in my house. As some of you may know (if you follow my IG @linemandan85) I had some issues with my heat pump during the cold snap from December 22nd through January 4th. I had a leaky coil and our house ran off nothing but heat strips or resistance heating for that period of time. As an energy guy, this made me cringe. But I had heat, better than some so I shouldn't be greedy.
Nevertheless, I know heat pumps and how they work. I also know that they are far more efficient than heat strips. I think the coefficient of performance (COP) of my heat pump is between 2.5 and 3 (hard to get a clear cut conversion factor), and heat strips have a COP of 1. This means that a unit with a COP of 1 provides 1 unit of heat for each 1 unit of energy consumed (i.e. 1 kWh consumed would provide 1 kWh of output heat). My heat pump having a COP of 2.5 means for every 1 kwh consumed 2.5 kwh of heat are made. So when I found out that my heat pump settings when installed were to turn off at 35 degrees and not kick back on till 40 degrees made me question the installers reasoning.
Heat pumps pull heat out of the outside air and pump it in to your house in the winter, and the exact opposite in the summer. Well technically there is heat in the air down to -459 degrees (0 Kelvin). And if my heat pump pulls heat out of the air, why would I turn it off 494 degrees too early? Well there is a balance point of when, how hard the unit has to work to get that heat out isn't cost effective vs the heat strips COP of 1. Upon some researching that temp is roughly -4 degrees, obviously depending on the efficiency ratings the unit. So again I ask the question why would I turn my heat pump off 39 degrees before I need to? That being said, I asked the guys that came out to fix my coil leak to turn off the temperature settings that would cause my heat strips to kick on below a certain temperature. This way I could monitor my actual heat pump output temps at the registers, and not worry about auxiliary heating kicking on.
Moving on, one of the key reason installers do that is because people generally like to feel warm air coming through their vents in winter. A gas furnace can pump out temps of 135 degrees or higher at the register. Well according to some articles a heat pump pushes roughly 96 degree air at the register when the temp is 45 outside. I've been curious as to if this is accurate, and how that temp changes as the outside ambient air temp drops. So I've been keeping track at my house via a thermal imaging camera, and some basic weather data. The image below shows that even when the outside temp is 20 degrees my heat pump is giving me 87 degree air. I also did it at 42 degrees and got 96 degree air coming out, thus confirming the articles. I have also done this check when it was 8 degrees outside and I was getting 76 degree air at the furthest register from my air handler. 76 degrees! We all think of that number when it comes to maybe air conditioning in the summer time but not in the winter you may say. But if the thermostat is set to 68, is 76 not warmer? It will manage to keep the temp in the house, its just not the “warm” air we are used to feeling.
So let's discuss why people wouldn't want that 87 degree air vs heat strips, that can cost twice as much as the heat pump. The biggest reason is, people's body temps run roughly 98 degrees, therefore any temps below that feels cool. Make sense? I don't know about you but I will suffer the "cool" feeling to cut my heating bill in half. I need to note that due to this temperature (87 degree) only being roughly 15-20 degrees above your set temp, a heat pump will seem, and does run for longer periods of time. Yes, it takes more time for 85 degrees to warm a room than 135 degrees, obviously. Hehehe
So in short, if you have a heat pump, I would ask your HVAC installer what temperature setting they have the unit to kick off and turn on at. Now, personally I would wait until you need a service before I had them change the settings, so you're not paying for two trouble calls, but I would definitely think about doing it. I have some outdoor temp comparisons with my unit when we had a 25 degree average (Dec. 26th) a couple of weeks ago (with leaky coil, so all strip heating) vs yesterday (Jan. 12th) with very similar temps and just my heat pump running. I used around half of the kwh yesterday! Now I may not know much about you, but I bet if you could keep half your money in your pocket, you would. I also compared another 2 days January 1st and January 13th (also pictured below). Both days average temp was roughly 29 degrees. On Jan. 1st (all heat strips) I used roughly 175kwh of energy that day. On January 13th (all heat pump) I used roughly 60kwh. That’s almost 1/3 of the energy consumption! It was totally worth the 87-90+ degree air coming out of my vent!
Now I am not the end all of knowledge on the subject, and my experiments aren't perfect. But they are real world. Obviously everyone uses energy differently and with different systems, there will be different usage and price points. This is just a reference point.
For more information contact Daniel Lofland, Energy Efficiency Solutions Specialist at Daniel.Lofland@okcoop.org.