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Wood boiler

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Wood boiler

Postby Alfred » Fri Feb 04, 2011 9:26 am

I have decided to put a wood boiler in the basement. Here's the deal: We have propane hot water baseboard right now. The last 3 fuel deliveries were about 22 days apart each to the tune of $600. Ouch. A cord of wood, cut, split, and delivered is about $150 and should last about the same time, so the boiler (total installed cost at $4500) should pay for itself in about 3 seasons.

Here's my understanding of how the heating system works: The thermostat triggers the water pumps when air temp drops below, say, 70* F. Once the water starts to circulate the aquastat sense the water is below temp and starts the boiler firing.

Here's what I want to do: Plumb a wood boiler in the return line to the propane boiler. That'll preheat the water entering the propane boiler, so it will circulate but not fire. This way I can heat with wood during winter and use propane spring and fall when you only need heat at night. Then I don't have to try to maintain a low fire or start a new one every evening. Water lines will be valved to bypass the wood boiler spring and fall so the propane is not trying to heat that unit, too.

Here's the ?: Can I get a wood boiler without the therm and aquastats controls for less $, and is that desirable? Anything else that's being overlooked? Opinions, please.

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Re: Wood boiler

Postby minime » Fri Feb 04, 2011 10:42 am

If that's a TRUE cord 4X4X8ft you're likely right and that's a decent deal. If it's a face cord you're likely gonna burn it up in about 10, 12 days even if the weather is mild. Wood burning is generally not a very good deal unless you're getting it almost free. Like if you own the woodlot or if you get to clean up behind an operation that isn't chipping for a flakeboard plant or papermill.
I owned my own woodlot for years. I sold firewood and used the cash to buy oil. Safe, reliable, and handfree.








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Re: Wood boiler

Postby vb » Fri Feb 04, 2011 11:19 am

using two different heat sources is a good idea, but does have difficulty.

I typed up a long response on how I'd design the system, but decided I'd just draw it up quick.
I'll post it up here as soon as its done.
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Re: Wood boiler

Postby vb » Fri Feb 04, 2011 11:47 am

you got the right idea, you just need to add a couple check valves and a low volume circulation pump to the mix.
you should get the majority of heat from wood as long as you keep the fire stoked, how much will depend on the size of the two boilers.
you can use the "dumb" wood boiler, as the gas boiler will regulate any heat that is lacking from the wood boiler.
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Re: Wood boiler

Postby myles » Fri Feb 04, 2011 12:57 pm

I have a multi source system in use in the house.

Oil fired boiler, oil fired range stove(with boiler) and wood/coal fire with back boiler. Mine is a mixed system with 2 domestic hot water tanks.

One hot water tank has double coils for the boiler and range cooker. The other hot tank has double coils for the boiler and back boiler.

The radiator system is fed off the oil boiler exclusively (in theory). With the boiler turned off, but with the central heating pump running, the radiators do run on the back boiler in the fire. It seems to use the domestic hot water tank as a heat exchanger.

I would recommend double coils or solenoid valves, as opposed to having the two heat sources in series.
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Re: Wood boiler

Postby manu de hanoi » Fri Feb 04, 2011 2:26 pm

try coal, it's dirt cheap
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Re: Wood boiler

Postby big worm » Fri Feb 04, 2011 5:08 pm

a pellet stove is very efficent.
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Re: Wood boiler

Postby airhill » Fri Feb 04, 2011 7:24 pm

Only problem I can see is that wood is an uncontrolled heat source whereas propane is not, I would be very careful about building in safeguards against overpressure if you are modifying a current system :)
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Re: Wood boiler

Postby Alfred » Fri Feb 04, 2011 9:33 pm

Minime, that's a full cord, not a face cord. Could get it for $135 elsewhere but the guy who quoted $150 customs saws cabinet grade lumber for me. He's poor and we have a good working relationship. I don't mind.

VB, why the pump and check valve in the bypass? about the dumb boiler...It'll be slightly oversized for the house in case of a future addition.

Myles, the reason for having them in series is that the main circulation pump is part of the propane boiler. It has a single pump and two zone valves. Doubling up on the rest of the hardware will be a much bigger job, but if there's a compelling reason I'd like to hear it. The old oil boiler had 2 separate pumps. They're in the upstairs of the garage.

BW and Manu, just checked the prices and wood pellets are awfully tempting. Slightly more $ but much less hassle with an auto-feeder. Same with coal. Will look into those options. Thanks men.

Airhill, that's a valid concern but both boilers have/ will have pressure relief valves from the factory. If something goes wrong they will dump the water on the floor. It's covered.

Local place has the Harman SF-260. for around $3600. Might do that for wood or coal.
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Re: Wood boiler

Postby vb » Fri Feb 04, 2011 11:23 pm

checkvalves are there to keep fluid flowing the right direction, when you wnt it to be flowing, and keeping fluid from bypassing the boilers.
the circulation pump is there to keep the two boilers acting as one, rather than the boilers working against each other.
you can do it without the pump, but you loose some efficiency.
you could also add a heat exchanger in the mix to preheat your hot water before it enters the hot water tank. the circulation pump loop word be a good place to do that.
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Re: Wood boiler

Postby vb » Fri Feb 04, 2011 11:32 pm

I was actually putting a little more though into it, the setup I posted is ideal for a large wood boiler and small gas unit.
your current gas unit is capable of heating your entire house, I'm assuming the wood boiler would be as well, so the recirculation pump isn't needed at all really.

I'm too used to working with engineers, if its not 2 levels more complex than needed then I didn't do my job.
but the hot water preheater is a really good idea if I do say so myself.
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Re: Wood boiler

Postby Alfred » Sat Feb 05, 2011 7:21 am

yeah, each boiler will be sufficient. Mostly I just want to be able to switch back and forth, using wood (or whatever) for high demand and propane for low demand. I'm not following you on the preheater. How would that work?
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Re: Wood boiler

Postby ArcticShine » Sat Feb 05, 2011 7:25 am

have you checked something like this out?
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Re: Wood boiler

Postby Alfred » Sat Feb 05, 2011 7:42 am

An outdoor boiler like that costs around $12-13K installed. :(
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Re: Wood boiler

Postby ArcticShine » Sat Feb 05, 2011 8:09 am

holy smokes!

pun intended
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