PVC Flue Pipe

[dexmcdan]

I have been reading the forum for a while now and I am interested in putting the most efficient wood burning furnace I can find in my garage. Has anyone every seen a secondary heat exchanger on a wood burner like they have on a high efficiency gas furnace that condenses the water vapor out and the last bit of heat then is expels the cool burnt gas out a PVC pipe. they of coarse have a drain hose to ground or sewer drain. If you have an insulated secondary combustion chamber that is completely burning all the fuel what would be the harm?

 

[heaterman]

A condensing wood boiler or rather any condensation inside a wood boiler is something to be avoided at all costs. I don't know of a manufacturer that makes such a thing, and for good reason. There are way too many contaminants in solid fuel exhaust. Adding condensation to the mix is a recipe for disaster.

The best and highest tech condensing oil boilers require a special burner and maintenance that borders on being religious in nature. (Look up Buderus' condensing oil unit) Even the gas fired condensers need at least annual cleaning on the fire side of the HX and that's about as pure a fuel as you can get.

If someone tells you their boiler actually operates at anything over 85-86%, walk away. They are either lying, don't know what they are talking about or possibly may actually have a unit that will hit 87%+. You don't want to own it in any case. Wood fired boilers will begin to condense at flue temps around 280* and you will see that at the start of a burn, in which case the higher temps later on dry things out. That's an entirely different scenario than operating in that range through the whole load of wood. The lowest stack temp you want to average is probably 350* which will put you honestly between 78-84% efficiency.

 

[ewdudley]

If someone tells you their boiler actually operates at anything over 85-86%, walk away. They are either lying, don't know what they are talking about or possibly may actually have a unit that will hit 87%+. You don't want to own it in any case. Wood fired boilers will begin to condense at flue temps around 280* and you will see that at the start of a burn, in which case the higher temps later on dry things out. That's an entirely different scenario than operating in that range through the whole load of wood. The lowest stack temp you want to average is probably 350* which will put you honestly between 78-84% efficiency.

I've been reading up on stack temperature a little lately and have seen a couple places where they quote a 1% efficiency gain per 35 degF drop in stack temperature.

So +1 on what heaterman is saying. Don't flirt with condensing flue gasses down below 280 degF, it only costs about 2% efficiency to be safe and plenty hot at 350 degF.

 

[dexmcdan]

I can see the logic of that, burning at 1850 or so with 350 degress left over would be 80% of the heat captured for your house and with 20% up the chimney (350 degrees) that would be 2% for each degree you could capture from the exiting 350 degrees. I have owned a Vermont Castings Cat for years and it did great , have a bigger house now 3000 with 1500 enclosed dirt floor solarium and a 1000 upstairs loft to heat so I am thinging and reading, looking for something bigger to heat with. My logic went on a day dream reading about rocket mass heaters, which do exactly what you rightly point out is dangerous,

thanks for the input,