Using an Ecobee with a wood boiler (single thermostat!)

We live in the country, where natural gas is not available (half mile away, but may as well be 100) . We have a gas furnace and water heater that use propane, but it is crazy expensive. For the summer to heat water, it does fine, but we try to avoid it for heating the house. When we moved in we replaced the furnace and AC and opted for an air-to-air heat pump (an AC that can reverse itself to bring heat from outside in, rather than only inside to out) to help with heating in the spring and fall (and even mild winter days). The previous owners though, installed an outdoor wood burner which is awesome (because it works well, and we didn’t have to buy and instal it). If you’re not familiar, you build a fire in a small shed-looking building to heat water, which is pumped underground into the house where your furnace blows air over a coil filled with said water, heating your house. Ours also has a coil to heat water, and you can use them to heat your hot tub, pool, etc (I hope to use it for our pole barn some day).

The problem with wood burners, is you generally end up with one thermostat for the furnace (that you set for 68 or so) and one for the wood burner (that you set higher at 72). In the summer, the furnace thermostat runs the AC, and in shoulder seasons or whenever you don’t have the fire going, it runs the furnace. If the wood burner is operating though, you turn on the second thermostat and it takes over, turning on the furnace blower to send air over the hot water coil, heating your house. If the fire goes out unexpectedly, the house cools to the lower furnace thermostat temp and it will kick on. It works, but its funky and ugly having two thermostats.

This season, I got tired of the pair and decided to try using one. We have had an Ecobee for several years (got it for our last house and brought it with us) and figured there had to be a way. Ive been playing with it for 2 months and it seems to be working, so here’s what I did.

The Ecobee allows for a heat pump, aux stage 1 and aux stage 2 heat. If you don’t have a heat pump, just ignore the heat pump wiring (the brown O/B reversing valve wire). For stage 1, I connected a jumper wire to the blower wire. Stage 2 then goes to the furnace, and voila! OK, that seems really simple, but I dare you to find instructions for it anywhere else (I had a hell of a time finding anything for about 2 years). Here’s a photo of my wiring.

Thermostat wiring

Thermostat wires

In the settings, make sure you have all your wires showing in your equipment screen. I set my aux heat threshold to 10 degrees so stage 2 would come on as little as possible, but you can set a smaller threshold and it will kick on the furnace to help heat when bigger changes are needed. I also upped my “away” temp some to minimize the temp swing when I come home so it wouldn’t use the furnace to heat the house faster. No, its not energy efficient, but neither is burning several cord of wood.

Thermostat wiring screen

Thermostat wiring screen

Thermostat thresholds

Thermostat threshold settings

You can barely see it in the photos, but my old second thermostat is still installed below the Ecobee with one of the wires sticking out. I’ll probably wait out the full winter before removing that and patching the area.

This should work for any 2 stage thermostat (will have a W1 and W2 terminal). Our furnace is a 2 stage, but the furnace itself controls what stage (high or low) to run at based on runtime. If its been say 5 minutes and there is still a heat call, it switches up to high. In theory, the point of having 2 stages in the thermostat is to take this over and factor in the temp change needed as well. If its currently 65 because you were away and now you are home and turn up the heat to 70, the thermostat can make the decision to go straight to high due to the large swing needed. Our furnace has a jumper to let me tell it if it should handle 2 stage automatically (it decides) or manually (the thermostat will tell it), I assume this is standard among 2 stage furnaces.

All this because, in reality, furnaces are very dumb. They send out power on the red wire and wait to see if it returns and on what wire, which will indicate if you want heat (white wire), cooling (yellow) or just the fan (green). If I were using the thermostat for 2 stage, the power would return to the W2 (high heat) or Y2 (high cool) terminals. The furnace has no actual knowledge of the set or current temperature, only if it should be on or not (which is why cranking it to 80 because you want it to heat faster is stupid).

So what I’m doing here is letting the thermostat decide Stage 1 (wood boiler) or Stage 2 (propane furnace), but telling it to use Stage 2 very infrequently. Meanwhile, the furnace has no knowledge of this. If the propane heat call is ever made, it will still make its own calls on stage 1 or 2; starting in stage 1 for a set time then turning it up (which incidentally is only a blower speed change, not actually hotter).

So, there you go. Like I said, I’ve been playing with it for a while, mostly balancing the temp settings for Away vs Home vs Sleep and how to read the remote sensors. If this can help someone else though, awesome.

Finally, even if you don’t have a heat pump or wood burner, an Ecobee is awesome and will pay for itself, I cant recommend it highly enough. I haven’t used a Nest, but it should be similar.