As a scientist-turned-efficiency nut, I have railed against people who believe the conventional wisdom when it comes to environmental conservation, often ignoring the data. I’ve talked at length about corn-to-ethanol, cash-for-clunkers, and other programs that are supposed to save the earth, but do the opposite when you really take a look under the hood.
Thus, it is with great shame that I admit I fell for that old trick in my own home.
When I bought a house last year, I immediately installed a new programmable thermostat to replace the mercury-filled wall-wart that came with the house. I even set it up to cool down during the night and times when we were away, thinking that the automation would save me money and pay for the upgrade. Everyone knows setbacks can save you energy, right?
Well, what everyone knows requires some qualification. After a year of relatively low electricity bills in the summer, and very HIGH bills in the winter, I installed a TED 5000 energy monitor. That’s when I realized why there was such a seasonal discrepancy: my heat pump’s auxiliary heat was turning on!
A heat pump is a very efficient machine for heating and cooling, because it uses a compressor and refrigerant line to shuttle heat between the outside air and your house. It’s not burning anything, just moving what is already there, so it can actually move 3-5 units of energy for every one unit of electricity it consumes. That’s called the Coefficient of Performance, or COP, and higher is better.
When it’s too cold outside and there just aren’t enough BTUs to pick up, the heat pump uses electric resistance heaters like the coils in a toaster oven, to create the heat. This has a COP of less than 1, because each unit of electricity runs through a resistor and is converted to one unit of heat, with some efficiency losses. Because these auxiliary strip heaters have such a low COP, it’s generally better to let the heat pump run, even for a long time, than to switch over to the faster-but-less-efficient strips.
By watching the display on my TED 5000, I realized that my programmable thermostat was programmed to turn on the inefficient strip heaters any time the room temperature was more than 2 degrees below the setting. Think about that for a minute: by letting my house cool down during the day to “save energy,” I was actually forcing the HVAC to switch to an inefficient form of heating! I would’ve likely saved more money by keeping the old non-programmable stat and keeping the house parked at 72 degrees!
I searched high and low for a way to solve the problem, but found little on the internet. What I needed was a thermostat that would let me control the logic behind the auxiliary heat, rather than just letting it take over when there was a 2 degree difference. Most of the commercial offerings and major brands you’d find at the hardware store are just not designed with heat-pump efficiency in mind.
My initial impressions:
The thermostat allows you to control two important setting: the temperature difference that triggers auxiliary heat (called the “deadband” by my electrician friend) and the amount of time the heat pump should work before the auxiliary heat kicks on to save the day (we’ll call it “lag”). A traditional deadband, as I mentioned, is 2 degrees, but I’ve set mine to 7 to allow for a 5 degree setback during the day. I’ve also set the lag to 59 minutes so my heat pump has a chance to do its job before the energy hungry auxiliary heat kicks in.
We don’t have a humidifier/dehumidifier, but with data, more is better. I did check the humidity this weekend before I baked some bread!
7 Day Programming
My wife works part time, so with seven days of individual programs, I can automate the setbacks at night and on days we’re not home. Many thermostats only allow you to program a “weekday” setting, not individual days. The “Copy” feature made setting up the schedule pretty painless.
The color of the screen changes based on what’s happening: red for heating, blue for cooling, etc. There are 64 colors to choose from, and it’s giving me a quick visual sense for what my system is up to.
TED 5000 comes with a load profiler that is supposed track the energy consumption of individual appliances, but it’s had trouble with the heat pump. With the Energy Watch feature of my new thermostat, I just input the power consumption of the heat pump, strip heaters, and fan into the Enersaver, and it will keep track of run-time, and therefore energy consumption, for me.
The Not as Good
While I like having control over the deadband and lag on the auxiliary heat, their logic is disconnected. In other words, with my setting, the auxiliary heat will come on if the temperature is 7 degrees below setpoint OR the heatpump has run for 60 minutes. I would prefer to make this “AND.” Let the heatpump run as long as it likes, as long as it doesn’t drop 7 degrees below setpoint. I may end up disabling the auxiliary heat entirely if this turns out to be a problem.
The touchscreen is okay, but there were moments where it seemed unresponsive. (Do I have cold fingers?) Also, setting some of the values is a pain because you touch the screen for each increment, and there’s no reverse button. When I overshot one of the time values, I had to go all the way through 59 to get back to zero and start over, no small feat, especially when the screen wasn’t cooperating. The spin and click version of the thermostat probably works better, but it didn’t have all the features I wanted.
The wiring diagrams seem pretty complete in that they show many systems and setups, but they are not infallible. If I had simply followed the wiring diagram, I would’ve had some incorrect connections, as my original thermostat called the white wire X2, the programmable stat called it Aux with a jumper connection to E, and on the Enersaver I have it wired to W1! Before you install this stat, I’d recommend understanding what the individual wires DO, not just their color codes.
In my next post, I’ll tell you about the battle I fought with the heat pump, and show the data that determines the victor.