How Does an Outdoor Wood-Burning Furnace Heat Our House?

The firebox

The outdoor wood-burning furnace has a huge firebox, You can fit large round logs all the way in there. It goes about four feet back. The way this works is that in the top half, there is a huge water tank that holds 400 gallons of water. 

We use this transparent tube to test the water level.

You heat the water up with the fire. Then the water gets pumped through two PEX tubes that go right underneath us and over to the house.

The pumps 

Here we have two pumps. These each pull about 100 watts.

The water gets pumped through this pump, through insulated PEX tubing.  

There’s one point of entry over there and the other one is way in the back and goes to our other furnace. We have two families here and there are two furnaces.

Once we turn the outdoor wood burner on these pumps stay on the entire burning season, constantly pushing the water through there. This makes sure the pipes don’t freeze.

The controller

I am going to light this fire up tomorrow. It will burn for six months straight. I don’t need to relight it over and over again. We will need to stoke it and add wood, but even though there won’t always be flames (sometimes there will be hot coals instead) it will continually reignite throughout the day.

How?

Right here we have a little controller. Right now it reads 51 degrees Fahrenheit, which is the current water temperature. We don’t have heat going to it now. We have been using propane to hold us over and keep it from freezing before we light the fire.

Say we want to go on vacation for a week. We leave the pump on so the water is moving and it’s less likely to freeze. But, we can also turn on the propane furnace as a backup. Water will go through the furnace and the hot air from the propane-heated fire will prevent freezing.

The damper

We leave our water temperature at 170 F. What that means is that this system will regulate the temperature of the water through the damper on the door here.

The damper opens and closes throughout the day based on the temp up here. If the temperature goes down to 160 this door opens back up and lets air back in to heat the fire up. 

The damper door is open right now, so you can see daylight through it. Once it hits 170 degrees that door will shut and won’t allow any more air in. The fire will last longer that way.

The thermostatic valve

Now I will take you into the house and show you how the outdoor wood-burning furnace heats our home and water. 

This is what it looks like where the water comes into the furnace. 

You may be wondering what this big black thing is here. It’s a thermostatic valve. It’s kind of cool. It checks the water coming from the furnace. If the water is less than 170 degrees, it reroutes back to the outdoor wood-burning furnace to reheat it again.

After the valve, the pipe continues up the wall, across the ceiling, and to the radiator.

The radiator

This is basically a radiator. The hot water flows through this little tube and heats the air pipe. You simply have to turn on the fan and the heated air from the outdoor wood-burning furnace blows throughout the house. 

We got a little temp gauge on here. 

When we are not using the outdoor wood burner to heat the house, in this little window there are burners that are propone-fired. 

The water heater

Another tube goes from the thermostatic valve to our water heater. When we aren’t burning wood, it uses electricity and can be expensive.

So, it’s basically a big tube over a little tube. The waters never mix with each other. They are just touching each other. When the outdoor wood-burning furnace is on, the large tube fills up with water and gets pushed from the bottom up to the top, constantly circulating hot water and heating the cold water in the little pipe. There is no pump pushing the water up because heat rises.

The next question is, why do we have two water heaters? It’s a safety feature. The reason for the overflow tank is that this water could get so hot it would exceed safety standards.

On an electric water heater, you can set the temperature, so it doesn’t get too hot to scald you, but on the other tank, it could get hotter and hotter. So, the water goes out of the wood burner tank, through the mixing valve, where cold water mixes with it, and into the electric, temperature-controlled tank.

When we aren’t burning wood, we are using electricity, and we are paying a lot of money. But when the wood-burning season comes, there's a circuit breaker I use to turn it off. All season we have no electricity coming to this water heater, and we are relying entirely on burning wood.

The thermostat

Right here is the thermostat setup. We have two thermostats. 

The one we use when we aren’t burning wood has A/C and propane. When we are burning wood, we use the other thermostat, with only heat. If we set this thermostat to 80 degrees, the fan will turn on until the room gets to 80 degrees, and then it turns off.

Wood is more efficient than propane. We don’t use a significant amount more wood to keep the house warmer. So, we keep the house nice and toasty when we are using our outdoor wood-burning furnace.

How an outdoor wood-burning furnace works

That's exactly how our outdoor wood-burning furnace works. How do you save money when heating your home? Let us know in the comments. 

For more on this topic, see our previous post on Is Buying an Outdoor Wood Burner for a House Worth It?