Calling all Woodstove Mathematicians

I think what you are looking for is a wood fired boiler, not stove. Stoves should not be retrofitted for heating water. I assume you knew this since you posted in the boiler room......

You're highest efficiency would likely come from a gassifying boiler with thermal storage. There are several brands here that you will hear mentioned all the time. Those include EKO, Tarm, Garn and a few others. I think you'll find excellent reviews on most. It all comes down to how much room you have, how much time you want to spend processing wood and how much money you want to spend on the front-end.

Good choice with in-floor heat, by the way. That's a great start....

For that kind of money I think that you will be happier / better served with something like an EKO , Tarm, etc... gassification boiler with storage. Up front cost will be a little more but efficiency and convenience will be greater. Do a little more searching in "The Boiler Room" and ask questions this site has a wealth of information from knowledgeable and experienced folks.

Many of the boilers BTU/Hr ratings are stated with ideal conditions and almost none will get to the maximum output for a sustained duration. I am sure that others will be commenting soon.
As far as additional pieces parts go.... for a gassifier(and most all other boilers) you will need Class A chimney pipe, water to air heat exchangers (if forced air), manifolds/pumps/reflective insulation in joist bays/oxygen barrier PEX(for in floor radiant), insulated oxygen barrier PEX(if boiler is outside), copper or black pipe or PEX for indoor install, side arm heat exchanger if you want to heat your Domestic Hot Water (DHW). Thermal Storage(water tank) is also what helps gasifiers get maximum efficiency as well as convenience.
Since you are interested in going Radiant from the get go, the PEX options will be the most likely to apply.

It gets worse.....

You're looking at 37k btu/hr at PEAK demand. Your average will be about half of that. Here's how I would approach it:

1) Think about how often it's convenient for you to build a fire. Once per day might be reasonable, but with your heat load once every two days would not be out of the question. We'll assume 24 hours for this example.

2) Calculate your average heat load for that period. For instance, if you want to go 24 hours at an average heat load of 20k btu/hr, that's 480,000 BTU.

3) Calculate how many hours your boiler will need to burn at average output to provide that many BTUs. Average output is usually about 75% of rated output. If you had a 120,000 BTU/hr boiler, ass an average output of 90,000 BTU. At that rate, it would take about 5.3 hours of burn time.

4) Subtract this amount of time from the period we chose above. That gives us the number of hours in the day that we'll need to heat from storage. In this example, it's 24 hours - 5 hours = 19 hours.

5) multiply the average hourly heat loss by the storage hours - 20,000 X 19 = 380,000 in this case. This is the heat storage BTU capacity that you'll need to carry you to the next day.

6) Determine your minimum 'useful' storage temp - the minimum temperature that will give you 20,000 BTU/hr heat transfer into your living quarters. For radiant floors, that could be 100 degrees or less.

7) Determine the maximum average storage temperature that you can easily attain. For pressurized storage, 170 degrees is reasonable.

8) Calculate storage: storage BTU capacity needed / 8.3 / (max storage - min storage) : 380,000 / 8.3 / 70 = 654 gallons

Simple, really ;-)

Keep in mind that these calculations are the minimum to cover 24 hours on an average day. You'd likely want a bit more so that you'd have some flexibility about actual start times. Additional capacity would also allow you to skip days more often. With a good house, any sunny day will be minimal load and you'll be able to skip days. The worst case assumes a bitterly cold and sunless day. You need to plan enough output and heat exchanger to cover it, but average days are a lot more common.

Bottom line - 130 gallons is not enough to be useful.

try www.nofossil.org or go to post #9 and click on the link in his signature ;-P

At the risk of exposing a wealth of ignorance, I'd bet that you couldn't go wrong with the smallest unit by Tarm, Econoburn, etc. and a typical 670 home built tank of unpressurized storage (4x4x8). If it'll heat a 2500 sq foot home with older insulation standards, it'll only keep yours warm that much longer before the next fire.

steelejones said:

Hi Rory, thanks for replying.

Where can I find info about home built unpressurized storage tanks.

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Maybe hearth.com ;-)

Do a search there are a lot of threads with photos and construction details. I chronicled mine on my site, but it's not entirely home-made.

If there's any way to do it, I'd suggest pressurized instead, using used (or even new) LP tanks.

Heating efficiently with wood is really a simple concept once you understand it. Unlike gas or oil fired appliances which can be turned on/off to provide instant heat when needed, wood as a fuel functions rather poorly when used in that manner. Yes, to be fair, a person can buy a small wood boiler and with careful tending of the fire, attempt to match the boilers output to the load of the house. Is it the best choice? I have to say no.

So what's the key to happy wood burning and decent real world efficiency? Simply put, the answer is btu storage. Some will argue that the btu's are best left stored in the wood but this is true only if that wood can be consumed at a rate directly proportional to the load. As you know from living in Maine that load can vary widely. So that leaves us with burning the wood in the most efficient manner and storing the btu's in some other medium usually known as water.

That leaves us all with one option that is best and that is incorporating enough btu storage to hold the heat generated by one or more load(s) of wood without cycling the fire. This can be done by adding a separate water tank to a boiler such as an EKO, Tarm or Econoburn or by purchasing a unit with integral storage like a Garn. The thing to remember is that the heat needed for the structure comes from the storage medium not the fire. You can dump heat into the storage at 200,000+ btu's per hour and let your heating system use it at whatever rate is needed to provide heat for your structure.

Rory said:

Why? The unpressurized seems awfully simple, and the internal heat exchangers should have an extremely long life.

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Simple. With pressurized, you don't need a heat exchanger at all. That means you can get the storage to a higher temperature more easily when charging and can get more heat out of cooler stored water when discharging. Also typically one less circulator running, so less power consumption over the life of the system.

But the heat exchanger for your DHW usually ends up being something more elaborate than simply coiling your DHW supply line through the tank, eh?

Rory said:

But the heat exchanger for your DHW usually ends up being something more elaborate than simply coiling your DHW supply line through the tank, eh?

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Yes. If you use pressurized storage, and indirect DHW tank set up as a zone would work, or a small flat plate HX with a circ would also work. I personally would like to set up a sidearm so that it would heat the DHW by thermosiphoning from the pressurized storage. That's a bit more demanding to configure, though....

Those thermocontrol units you pointed to on E-bay earlier are not gasifiers, but instead are conventional boilers. They aren't horrible, but they are nowhere near the efficiency of the gassifiers... I went up to the Woodsman show in Maine this spring, and looked at the several boilers that were on display, many of them outdoors with fires going in them. Thermocontrol was one of them, and the ONLY booth in the show that was putting out visible(and smelly) smoke...

Otherwise, I would say you are on the right track and have been getting a lot of good advice...

Mine would be to go with the radiant tubing, and put it in as part of the home build process. When designing the tube layout, go for one that will let you run as low a temperature water as you can, ideally in the 100-120*F range. Bear in mind that it really doesn't matter what the heat source is for the in-floor part of the design - the floor doesn't care where the hot water came from, so you can get that part in before you have necessarily decided on the setup for the boiler side of things.

A good heating company should have no trouble helping you with the design and install on the house side of things, though not all are as familiar with the wood boiler issues.

You also should plan on having some sort of "conventional" gas or oil heating backup system in addition to the wood - this is helpful if you want to go away on vacation, or can't feed the boiler for some other reason, and may also help with financing, insurance, code inspections and all that kind of thing. What many folks do is have a system where the wood boiler and storage system runs in parallel with the dino-boiler, and have some sort of automatic or manual switchover to pick which one to heat with.

If you go through the threads here, you will find LOTS of different plumbing diagrams, most of them with commentary about what is right and wrong with them.

I would also reccomend the "Idronics magazines" on the Caleffi website, and googling for John Siegenthaler, who has written many excellent articles on the howto's of hydronic design, mostly the house side. This site is probably your best source for the boiler side...

Gooserider

steelejones said:

Thanks for all of the replies...

I had a chance to check out the Garn WHS, that thing looks like a work of art! Its pricey however I called a guy downstate thats a dealer and he wants $12,400 for the smaller unit. I see they have horizontal and verticle chimney options. Will this not hook up to a standard chimney ?

Can anyone here comment if they have and love the garn? Its very pricey so im hedging.

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If you drop a stainless liner in the chimney I don't see why not. A Garn has pretty low flue temps so condensation would be an issue if the chimney was unlined. ......Actually, it would be an issue even if were lined but in a different way. Some of that water vapor is going to condense even in a SS liner and will run back down. Garn recommends Class A stainless chimney but I don't know what the dif would be as far as using it inside goes.