Heatmor outdoor boiler

[FBWNDR]

I'm trying to determine the pros and cons of having a wood boiler installed on my property. If you haven't heard of these, the boiler is built to circulate water around the firebox. The water gets pumped into a heat exchanger which is located in the main heating duct of the house. The existing indoor furnace pushes the air around the house.

Anyway, I'm looking at a Heatmor boiler, and I'm now looking for people who have experience with these. The website says that the boiler has a forced draft fan (75 cfm). It also says that the boiler can heat a 3000 square foot house. I know a little about combustion, and I calculated that with 75 cfm, you cannot burn more than 10 lbs of wood an hour. And if the wood is 45% moisture, then the available BTU is around 40,000 BTU. A 40,000 BTU furnace is very small for a 3000 square foot house.

So, has anybody dealt with these Heatmor furnces? Can you tell me anything about performance and maintenance?

thanks, FB

[dpb]

FBWNDR wrote: ...

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Not specifically, but...

Couple question re: your post, though.

Don't they have a Btu rating on the boiler? All units I've any experience w/ do.

What did you use for the pressure/density for the cfm calculation you did? Where did you get that performance figure and does it match theirs?

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[George]

Friend put a heatmor in last year. That one is probably the best of those style units available. Most of them are low quality very poorly designed units (no firebrick etc) which means you get lots of stinky smoke.

I did a heat loss calc for his house but as you stated they simply tout ridiculous meaningless specs such as "heats 3,000 square foot house". I don't remember which version he has but I can ask.

There is even better technology available which uses wood gasification for even higher efficiency and more complete combustion (lots less smoke too).

[dpb]

dpb wrote: ...

That is, specifically, if you used STP I'd expect actual to be higher by fairly significant factor depending on specifics of draft design...

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[FBWNDR]

I did use standard pressure and temperature for the CFM calculation. I would assume that the fan's rating is also given as standard pressure and temperature. And I also assumed perfect combustion, and so I determined how much wood (at 45% moisture) could be theoretically combusted per hour with an air input of 75 cfm. There may be a BTU rating on the heater, but I have not seen one. I am still trying to obtain a user's manual, which is not available on their website. At a very rough guess, I would assume a 3000 square foot house in the midwest would require at least 100,000 BTU/hr furnace, probably a little larger.

[ransley]

If boiler is outside full of water , you must put antifreeze in the water, antifreeze doesnt transfer heat as well as water, is that included in their btu output calculation. Do you have to feed it every hour that would become a hard full time job

[FBWNDR]

The anti-freeze heat transfer difference is pretty small, as least compared to some other assumptions, so I didn't include it. But what I have heard is that most people don't put anti-freeze into the tank. I don't understand this at all. The system holds 85 gallons, which would require 42.5 gallons of anti-freeze. This is not a small cost, but it is a very large risk to run the system without antifreeze. The furnace area is large enough that it can hold 12 hours worth of wood at a time.

[ransley]

If pipe is outside it will freeze fast if its cold enough out, it could be buried and insulated. 50% mix is -37f you dont need -37 unless it gets to -37. The reduction in efficency is actualy large , maybe 5% im guessing. Antifreeze conducts heat poorly compared to water, I just got a used car that some idiot had 100% antifreeze in it, I only checked its % because I got almost no heat inside, im down to 40% antifreeze now and air temp out of the cars vents is maybe 40 degrees hotter now, so antifreeze looses you efficency but I would think even 5-10% would help get you well below 32F, You cant expect it to be fired all the time , what if you are sick or want to take a vacation, you dont want to loose a expensive boiler from freezing, it has to be winterized to your local temp

[George]

Those boilers were pretty popular in my general area for a while. Unfortunately they got a bad reputation because folks were buying them based on price only. The "big box quality" versions have essentially a huge steel firebox with a coil wrapped around the outside and a small motorized damper. The idea being you would stuff it full of wood to last all day and when your stat called for heat the damper would open. Since the wood was in contact with the steel it gets cold. So the unit belches dense acrid smoke for a really long time until it heats up.

I border on a less densely populated area and the nearest ones I have seen are a mile from here. They make such heavy smoke that on a calm morning it is like driving through fog. Most municipalities here have now banned them. The heatmor that was mentioned is probably the best of that type system. Their is a firebrick liner and it has an induced draft so there is less smoke. Unfortunately the "big box quality" units killed off all installs.

Installers typically haybale the install. They don't use water-water heat exchangers and just tie in directly and don't use antifreeze.

[bud--]

"A 3000 square foot house" has no meaning unless you know what the outside temperature is. It may be a 'true', but useless number.

Your calculation is a real interesting approach.

[Tony]

The antifreeze isn't loosing much total efficiency if any at all. If it doesn't transfer the heat it simply cycles around through the boiler and gets hotter again, hotter than it would get if cold antifreeze was circulating. It's not as though the heat that wasn't transfered goes off into space and is lost. The heat is still there being recirculated for the next time around.

[Bob F]

This whole class of heaters is illegal in Washington State, I assume due to the pollution.

[dpb]

What would you get if you consider combustion air at intake of (say) 20F or so? That density difference, while I didn't look it up, would be moderately sizable I'd think.

Not sure, seems like 45% might be a little pessimistic on moisture, too, maybe? I don't have data for seasoned firewood, just a thought. Being no trees around here, the fad is the corn-fed boilers around here. If one had the grain anyway w/ the automated feeders that seems to make some sense to me altho larger producers are going to the waste oil and/or the on-farm small (relatively) capacity biodiesel conversion if they also have soybeans as a normal rotation. I've heard of a couple that estimate they're replacing all their off-road diesel requirements from about 8-12% of their production acreage depending on the annual yield.

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[Ed Pawlowski]

Here in CT they are trying to outlaw them. Many com plaints about pollution, smells and smoke drifting over to the neighbors, etc. Much more than a regular woodstove. Check your local code for any restrictions before you buy.

[dpb]

dpb wrote: ...

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Actually, don't have to look anything up to get close--it's only about

10% change in absolute temperature so that's about all the density change is as well for ideal gas. So, not a big deal...

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[Stormin Mormon]

Considering that the OP wants to heat a 3,000 foot home with tongue and groove exterior, no insulation, and no tyvek, and no intererior drywall, in the northern reaches of Nebraska, I'd say more information needed.

[Bob F]

45% seems awfully high for moisture content. From Seasoned firewood contains less than 20 percent moisture and generally takes from six to eight months - and sometimes up to more than a year - to dry, or cure. Much depends on the weather, the type of wood you have and how you prepare and store it. Hardwood such as oak takes longer to dry than softwood like Douglas fir or pine.

[Andrew]

From

I do a lot of work on wood fired boilers - the large ones used in power plants. A 45% moisture content is not unusual. Kiln dried wood is only 10-20%. But in looking over my numbers I noticed that made a chemistry mistake, a big one, confusing moles of air for lbs of air. I have recalculated my numbers based on ambient air of 20 degrees (down from 40 degrees), and I used a series wood moisture contents. This is assuming perfect combustion, and all the air is used is combustion.

Fuel moisture content Wood fuel lbs/hr Available BTU/Hr in fuel

45% 71 246918 35% 60 260936 25% 52 268641 15% 46 275632

So, I guess that little fan does push enough air to burn enough wood. So now I have to look into the pollutant problem. According to this thread, there are a few states where these are being outlawed.

[George]

I am in PA. The state is not involved as far as I know but at least in my area most/all of the local governments have banned them. Unfortunately there are some really cheesy versions that are just a huge firebox made out of steel plate with a coil wrapped around it and a motorized damper. When the house isn't calling for heat the damper closes and since the wood is in direct contact with the steel it really cools off. When the house calls for heat the damper opens and it takes a really long time to get back up to temperature and the unit belches tons of acrid smoke. There are two of them about a mile from here and on a typical calm winter morning it is like driving through fog when you drive on that road.

[Bob F]

That chart says something significant for drying your wood well, expecially when you consider the cleaner burning of dry wood. Even if you calculate in that 15% wood is lighter than 45%, you still get way more heat out of less wood.