Unsure of how a Wood-Fired Boiler Operates

There are various ways to hook up a boiler and integrate with an existing system, but I'll explain mine. My house is a typical 2500 sq. ft. or so two story, which was equipped with an oil-fired boiler and baseboard heat. Last year I installed a Tarm Solo 30 and 670 gallons of storage, which is simply a large rubber lined box which holds water. Inside the storage tank are two heat exchangers. One of them transfers heat into the tank by heating the tank water when either my wood furnace or oil furnace is running, and it draws heat from the tank to heat the house when the house is calling for heat and the wood furnace is not running, and the temperature of the water in the tank is above the "setpoint" at which my oil boiler would kick in to heat it. My understanding of the thermodynamics of the system is that when the house is calling for heat (at least one zone open), if a furnace is running most of the heat being produced will go to the house, but some will transfer to the tank.

The other heat exchanger in the tank supplies my domestic hot water. Water comes in from my well via copper pipe which runs to the bottom of the tank and comes back to the top via a 180' coil which allows the water to heat in the pipe to heat up along the way. There is a mixing valve as it exits the tank to bring it back down to 120 should it exit higher than that.

Theoretically, I don't need to tend my wood furnace, as the oil burner will kick in as necessary. As a practical matter, about 1 fire a week supplies my wife and I with all our domestic hot water, around the first of October I start using about a fire a day to heat the house, and by December I have a fire in the morning and another at night. The furnace does not usually need to run steadily, as an initial filling followed by a topping up will usually last the 8 or 10 hours I need until the next burn.

amkazen said:

Ok, so my thoughts were close to being correct.

You do need to ensure boiler is stocked with wood at least once per day of you want to use the boiler to help heat water. And, when it gets really cold it could be that the boiler would need to be stocked twice a day.

If you did not want to use the boiler to heat the house or the water, like in the summer months, or when you go on vacation, you just do not stock it. In that case, if the house calls for hot water or heat, the propane boiler (in my case) kicks in.

And, the storage people talk about is hot water storage.

And, boiler & furnace set points determine which boiler or furnace kicks on to supply the hot water or heat.

And, this compares to running 2 - 3 wood stoves in the house to heat the house. And, tending to one fire in a wood-fired boiler 1 - 2 times per day beats tending to 2 - 3 wood stove fires, especially in that a boiler will burn for 8 - 10 hours without being watched while a wood stove needs to be looked after a lot more frequently than just every 8 - 10 hours.

Am I close?

Click to expand...

You are close, but I don't think the typical fire lasts 8 - 10 hours. I will burn for 3-5 hours, and the heat from the tank will be sufficient for another 8 - 20, depending on how cold it is outside.

And the gassification boilers burn very cleanly with very little smoke (except for 6+ minutes when lighting a new fire). Many people (like me) put their boilers in an building outside and pump the hot water through underground pipes to the house. Others put the boiler in the basement. I prefer the outside building for keeping the inevitable smoke (when opening the doors) and mess (wood, dirt, ash) out of the house. Others like not having to put on a coat and boots to fill their boiler. Hot water storage allows the boiler to run at 100% producing very little smoke. Without storage, the boiler will cycle between heating and idle. Each time it goes in to heating from idle some smoke will be produced.

If you run the boiler without storage it will have an idle stage (depending on your loads). Most propane boilers today have electronic ignition (no pilot light). Take some time and look at the manufacturer manual for a few models. My manual is here http://www.ahona.com/Orlan_Paxo/Manual Paxo web.pdf