Wood Burning Fire Box Plumbed Into Oil Boiler and Baseboard System

CoolTommeh

Hi All, looking for professional opinion on the operation on my houses baseboard system. House was built in the 50s, but the boiler system was added in the 90s by the looks of things. 3 zones all work fine and heat the house well. We purchased the house October of 23 and after some renovation moved in in February of 24. So I missed the bulk of chilly season for running my boiler and heating the house last year. This year though, we're looking to max it's potential.

My boiler system has an unusual feature (at least that's what the chimney sweep, boiler service tech and boiler plumber I had in tell me). There is a wood burning stove that has heating coils plumbed directly to the boiler. The wood burner is completely isolatable (valve at either end of the loop), an independent pump (plumbed in brand new this past September and operated via a switch on the wall) and the heat coils run inside the stove, not outside all along the walls of the box. There is also a pressure relief valve and a thermostat plumbed into the loop so we know what the water temperature is. Additionally, my house has 3 flues, one for the fireplace in the living room, one for the boiler and one for this fire box. Again cleaned and fire rated pipe installed from the box to the flue.

It's getting colder now, and the boiler has been running more consistently to keep the house warm and I'm looking to help my oil costs by lighting this stove. Talking with my boiler service tech (a friend of my father-in-law) he says that the dampers (one at the entrance and one at exit of the box) can be used such that the fire inside never burns too hot and the water will stay a relatively regular temperature. Meaning, if my boiler regularly heats the water to 190°F I can use the dampers, with a little experimentation, to get the water to consistently be just below that, maybe 170-180, so the boiler doesn't need to heat the water all that much.

My concerns are: 1. Is that accurate? Can I really consistently set the water temp with a fire as long as I keep feeding it? 2. This fire box loop is not large. The box sits right next to the boiler and if the zones are not calling for hot water, the water appears to just circle right around back to the box for heating again, thanks to the pump constantly creating flow. Would this not eventually result in boiling of the water? Basically, there is no dump zone for the hot water, am I at risk of this thing boiling the water and the pressure relief valve shooting steam everywhere.

When we renovated we took out a ton of material inside and tree all around the property. I have wood to burn! Would be amazing if I could burn it and heat the house, but only if it can be done safely. Really appreciate your thoughts and help!

Jamie Hall

I don't really like it, but I dare say that if you are careful and tend the fire at all times when it's going you can get some useful heat out of it. How much? I doubt very much that you will get enough to make much of a dent on the overall oil costs…

Now one critical question: is the pressure relief valve for that arrangement on the firebox side of the isolation valves? Please tell me it is — because you need one there. In fact, it should be a pressure and temperature relief valve, not just a pressure relief valve. Why? Because on a cold night with a fire in there, if the power goes out you will no longer have circulation through the box — and the water in there will get very hot if you are not there to close the dampers. Same thing if somehow the isolation valves get closed, only more so.

Please be careful.

EdTheHeaterMan

I have actually done this for 2 customers. One had an outdoor wood stove boiler, the other had something similar to your situation. Th goal is to get used to your wood stove's heating habits. By adjusting the inlet damper (called the draft damper on old coal boilers and furnaces) and the outlet damper (which may be the Check damper or the Shut off damper depending on its location) you should be able to get that stove to maintain 190° to 160° water temperature in the boiler for several hours. The game plan is to Let the house get too hot when using the hand fired boiler to heat the building. That is because you don't want the circulator pump to stop heating the house if there is still fuel to be used in the wood stove.

That is the way you find out how much wood to fire each time you stoke the fire box. If you get the house to 78° and then it drops to 60° hours later, to let you know that you need more fuel in the wood stove you are getting it right. That's how out grandparents lived when steam and hot water heat was coal fired.

The option of automatically operating the Oil Burner to heat the home when the temperature drops below 60° to keep the home from getting too cold, is the trick of thermostats, aquastats and relays. Your idea of reducing the fuel oil usage by heating the oil boiler with some wood, is a little backwards. You use the wood as the primary and only use the wood as a backup. This of course assumes that the wood is free or at least at a lower cost. Also the wood stove will make the room that the stove is located in warmer also, so that room does not need any radiators.

If this logic of operation is what you are looking for, then I can make that wiring diagram after you show me the piping connections (a drawing or a photo(s) of the pipes) so I can get the controls set up for you. There will be a learning curve to get the wood stove to give you sustained 190° to 160° water for several hours and get used to the higher indoor temperatures. My customer with the internal wood stove like yours got all his wood for free since he lived next to a sawmill and there were lots of scraps for the taking.

This illustration shows the three different dampers you may have on your wood stove.

Of course this is a illustration of a hand fired coal boiler from the 19th century, but the concept of making it burn for extended time periods is exactly the same. Keep the air over the flame to a minimum and let it just simmer for hours like a slow cooker or crock pot. Use hard woods that burn longer. And pile up as much fuel as you can fit in the firebox. (click on the photo to zoom in)

CoolTommeh

Thanks for the feedback! Appreciate the notes. See some attached pictures below. Thermo Control is still a functioning business and I'm hoping to hear back from them on how to work this as well. Please let me know if the pictures shed some more light on the best way to work this system.