Copper coil around the chimney connecting to the radiators

Hello! First time posting here. Im trying to find a cheap way to heat my 20x40 garage. I bought a wood furnace for my house and was maybe thinking about using a copper coil around the chimney and a rad in the garage to heat it up. I’m not sure if that would work so that’s why im asking you guys. The idea is when im heating the house with the furnace i turn the pump on so water/glycol flow around the chimney then goes to the rad and heat up the garage. The garage is about 65' from the house. I would bury the pex piping in the ground. Would i be able to get the heat up in the garage or that’s too far away? I made a beautiful sketch😂 Any idea or tips?

Hi Guillaume, and welcome. If you're trying to extract heat from the chimney, you need to bear in mind that the stack temperature needs to be above a certain point for the system to draft properly. This is usually 200 C for wood stoves and similar equipment. If you pull too much heat from there, you could have problems like water vapour condensing and stalling the movement of gases to the outdoors.

If your furnace has a way to put in a water heating coil in the firebox (commonly called a wetback in this part of the world), that would be the way to go. If your heat source is at a lower elevation than the demand (your garage) then you might even be able to run a loop as a thermosiphon, but pay attention to venting and safety any time you're heating water to the point where it may turn into steam.

Hi guillaume and welcome to Permies!
In theory your plan works and I’m sure you could figure it out somehow, depending on your plumbing abilities. But water heaters involve a bunch of technology and I’m not sure if it’s an easy or cheap way to heat your garage. Or necessarily the safest.
Have you considered building a rocket mass heater in your garage?
Why do you want to heat a garage in the first place?
Are your measurements feet?

Phil Stevens wrote:If you're trying to extract heat from the chimney, you need to bear in mind that the stack temperature needs to be above a certain point for the system to draft properly. This is usually 200 C for wood stoves and similar equipment. If you pull too much heat from there, you could have problems like water vapour condensing and stalling the movement of gases to the outdoors.

I agree that this is something to watch out for. You can probably mitigate this risk by adding an adjustable valve to your pipe - partially closing the valve will restrict the flow and keep hot water in the system for longer, drawing less heat from the stove (the greater the difference in temperature, known as the "temperature gradient", the faster energy will be transferred).

Phil Stevens wrote:If your furnace has a way to put in a water heating coil in the firebox (commonly called a wetback in this part of the world), that would be the way to go.

This is called a "back boiler" in the UK. Lots of commercial stoves include them, particular older stoves before gas or oil-fired boilers became available. They seem to be more efficient than copper coils, although I have heard of copper wrapping the flue pipe quite often.

As Phil mentions, it's important to consider pressure in the system as, if it becomes too hot and the water boils and turns to steam, you may end up with pipes exploding or becoming scolding hot. You can get special valves that will release at a certain pressure. If you connect one of these to a pipe leading outside of your property, any steam or excess hot water will drain harmlessly away. It would also be a good idea to have a pressure gauge and a manual outflow pipe that you can use if things get too hot - just make sure you have a pair of good gloves stored nearby.

It's common practice to place the water tank higher than the heat source as this will drive the thermosiphon, taking hot water away from the heat source automatically. This is a good precaution regardless of whether you include a powered pump in the system too - if the power goes out, you'll want to keep the water flowing. It also means you have gravity on your side when you want to make use of the hot water.

Benjamin Dinkel wrote:Hi guillaume and welcome to Permies!
In theory your plan works and I’m sure you could figure it out somehow, depending on your plumbing abilities. But water heaters involve a bunch of technology and I’m not sure if it’s an easy or cheap way to heat your garage. Or necessarily the safest.
Have you considered building a rocket mass heater in your garage?
Why do you want to heat a garage in the first place?
Are your measurements feet?

Alright thank you everyone! I guess i’ll find an other way to heat my garage. I want to heat it so during winter i can work on my cars and i don’t want to have to go to the garage and start a fire every time i want to go work in the garage.. i want it to always be heated.

What about infra red electric wall heaters. They heat you and not the car!