Best add on wood boiler?

First find out what your actual heating load is for the structure. Current boiler size can mean next to nothing in a lot of cases.
150,000 sounds high for a 3,000 sq ft house. Could be it was sized that way to provide domestic hot water and heating at the same time but there are better ways to do that.

You will need to incorporate some sort of thermal storage tank into the system so plan for that also.
Is there space for a tank around 300 gallons minimum in your basement?

Heaterman, your questioning of 150k btu being high raises a question for me which probably has been discussed somewhere. For a NG, propane or oil hot water boiler, isn't the theory/practice to "over-size" in the sense of having sufficient capacity to very quickly bring the water up to design temperature (160-180F), cycle the hot water through the baseboards until the thermostat temperature is reached (70F), and then shut down? Then repeat each time there is a call for heat? And to get this quick response, isn't a fast, high output burner needed?

With a gasification wood boiler, isn't the theory/practice to flow hot water through the baseboard, or other emitters, more or less continuously, or at least for fairly long periods of time, and to do this with somewhat lower temperature water, such that btu output of the gasification boiler can be less, and sometimes a lot less, that the existing fossil fuel boiler, and the gasification boiler with storage can have long burn cycles to most efficiently combust the wood fuel?

Therefore, a wood gasification boiler often and probably usually can be sized considerably smaller in btu output when it is replacing a fossil fuel boiler. At the same time, as you mention, determining actual heating load remains very important. But, if the gasification boiler and storage is to supplement rather than replace a fossil fuel boiler, might it be less important to determine actual heating load and optimally size the gasification boiler based more on 1) achieving minimally an efficient operating scheme and then 2) maximizing based on the homeowner's budget, installation space available, amount of storage capacity to be provided, and the frequency of loading to result in a high convenience factor in use of the boiler? The fossil fuel boiler always remains available to "supplement" the wood boiler as needed when storage temp is low and/or the wood boiler burns down.

A dramatic example of this was when Deep Portage switched from propane boilers with 2 million btuh rating to two wood boilers, each having sustained measured output of 500,000 btuh, and the goal was to eliminate a need to use the propane boilers, although they remained in place as backup. The heating contractor was so much opposed to this, believing that insufficient heat would be provided, that DP had to sign a waiver before the contractor would proceed. DP also installed 4000 gal external storage primarily to serve a Wood Gun E500 plus the 3200 gallon storage integral with a Garn WHS3200. The resulting outcome is that during the great bulk of the heating season, only one boiler needs to be operating, and only during the fairly brief really cold periods (-10 to -40F) do both boilers need to be operating (1 million btuh total output), more or less continuously, and DP never has experienced insufficient heat for its facility.

The short answer is its quite impractical to run an add on wood boiler without storage. Effectively you are installing a outdoor wood boiler indoors with the associated issues with high wood use and significant creosote and smoke. Alternatively you can do what I did for several years which is run a wood stove and only use the boiler in cold weather or when I had adequate demand to run it full out. Either stick with a wood stove or spend the money for storage and wood boiler.

We're thinking differently. A wood gasification boiler with storage is radically different from an outdoor wood boiler: much less wood usage, much less to none for smoke and creosote.

I think everyone here is saying the same thing - some re-reading might be in order.

As I read things, heaterman was implying that a wood boiler on its own shouldn't be the same output as a fossil unit (which was all the OP was seeming to go on - aside from sq.ft.-age, which isn't really quantitative given all the variables there), and should be less - and storage would be needed if it was sized the same. Which is also what I read jebatty saying - we all know what an oversized wood boiler leads to. Which leads to what peakpagger is saying about the issues in having a big OWB, and having an oversized IWB would lead to many of the same issues. Which then gets me back to jebatty's post that storage changes everything and boiler sizing to the oversize side isn't so bad if the storage is big enough to hold the load.

Which I would agree with all of.....

Good thoughts by all. *~< <---- me with my night cap on posting on Hearth at 2:30 AM.......

Sizing factors I consider:

1. I'll still go with the heat loss because that's the baseline for all other calculations regarding boiler size, pipe and tube size, pumps, additional emitters etc etc.
Without it you're just guessing. Maybe with experience and good back ground behind it. But still guessing.
2. Many times I''ll measure up the emitters in the case of rads or baseboard to see what is actually installed. Cant tell you how many times I've seen 60,000 btu worth of baseboard hooked up to a 150K or larger boiler. If you know what the system can transfer you can peg the boiler size pretty close just with that based on whether the occupant says "it heats great" or, "It don't heat".
3. A good question to have answered is if there are any plans for expansion of the structure or things like pools and hot tubs also.
4. Throw in a factor for DHW and you can get pretty close with just # 2&3.

As to sizing smaller with wood....... As you have seen a Deep Portage Jim, design heat loss constitutes around less than 5% of the heating season. Often times it is far less expensive to go with a wood boiler that will do the job 80-90% of the time and use a couple Eden Pure type electric heaters or equivalent in cold spots for peak heating load.
This is especially true when no storage is used or anticipated.
The thing with gas or oil fired boilers is the ability to go instantly ON and instantly OFF.
With a wood boiler you have no such luxury.
If you want a good clean burn you are going have to add a zone or tank which can absorb extra output during times when the full capacity is not needed. People are learning this simple truth, usually by having to figure it out themselves but sometimes through their own fact finding mission. Using a dump zone to keep output in line with actual load can lead to much wasted wood and work.
Was yakking with Mark Odell from Econoburn a while back and he told me when they started shipping virtually none of the boilers went out with a storage or buffer tank in the design. Now nearly 80% of them do if I recall that conversation correctly.......... That should be a pretty good indication of how thermal storage is begniing to seep into the collective conscience of the wood burning community and that is a good thing the way I see it.

Doesn't change anything except improve accuracy. The DP propane boilers replaced were 2.4 million btu, not 2 million.

I believe Tarm will not warranty any boilers without storage and I think several other firms are the same.

I am not sure what BB is but if you go with unpressurized storage, you can hang two coils in one tank one for heating and one for hot water but the trade off is that you lose some flexibility. Ideally with a new house you go radiant heat or at least radiant baseboards so you can still heat with fairly low water temperatures. If you go with conventional baseboard, once you drop below 140 degree F, you get very little heat out of conventional baseboards. With radiant you can go down to 100 degrees (I have no radiant experience so someone else may want to chip in on how low you can go). If you have a hot water coil in the storage tank and heating with radiant you could be in a situation where your hot water is not hot enough.

The tough part of this is I dont think you have any hope of getting an installed system for less than 10K. 15K might be possible with a lot of labor on your part and getting a deal on a pressurized storage tank. If you pay a contractor, its going to be in the 15 to 20k range.

One of the reasons gas and oil fired boilers are so popular with new home construction is they are cheap, have a small footprint and go in fast. Wood or pellet options are the opposite. The trade off, once you make the investment for wood the long term operating cost is lower so in the long run the extra cost up front is paid for in fuel savings.

Start with a heat loss calculation. The builditsolar website has an estimator.
The Uponor Complete Design Assistance Manual and the Zurn Radiant Design Guide walk you through simple heat loss calculations that you can apply to your house.

peakbagger said:

... Ideally with a new house you go radiant heat or at least radiant baseboards so you can still heat with fairly low water temperatures. If you go with conventional baseboard, once you drop below 140 degree F, you get very little heat out of conventional baseboards. With radiant you can go down to 100 degrees (I have no radiant experience so someone else may want to chip in on how low you can go....

Click to expand...

Retrofit underfloor with Thermofin and outdoor reset. 30 degrees outside and the house needs about 90 degree water to keep it comfortable.

As most know on this site, I'm not a heating guy, just have a nice gasser and like this site.

But do as heaterman suggests and measure your total baseboard length. Thats important figure to have. Look at the height of the BB. Is it high out put(tall) or regular sze. someone can explain better than me. Also, I think heatermans correct(quite frankly, when isn't he?) you're boiler now might be oversized. a quick and easy thing to do is oversize the oil boiler, then customers are warm....no call backs. Plus it's a lazy/quick and easy way to design a system.

i got very lucky 20 yrs ago. when i had my house built, i gave no input design of heating. Just throw in a oil boiler and keep me warm. the heating guy had been around for a long time, old school, but he did know how to size a system. My house heated very well. 1800sq/ft, 2 story, moderate insulation, on top of a hill in northern Maine. He installed a 80k btu weil mclain oil fired boiler. Didn't short cycle in the jheating season, but it did run mostly full out in the deep cold weeks. You know that -20 to -40 january's that we used to have? Plus made our DHW for 3 kids. There might have been a time or two that the boiler mate didn't make everyone's shower scalding hot in the deep freezes, but that was a minor problem.

Also wood. It will need to be cut/split/stacked for at least one year before burning. Two yr seasoned wood is the catz azz.

Retrofit underfloor with Thermofin and outdoor reset. 30 degrees outside and the house needs about 90 degree water to keep it comfortable.


Or cut in some cast iron rads to the existing baseboard.

I would love to do that to at least the ends of our 4 zones - which would put one in all 3 of our bathrooms, and a first floor hallway. Just haven't been able to convince my other half about that...

The second floor has Buderus panel rads, sized to provide the required heat with the same temperature water.