Advice on a wood boiler system

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Thomas_sefton

New Member
Nov 27, 2019
3
43760
Hello ladies and gentlemen, new here. Seems like there are a lot of knowledgeable people here and I was hoping to get some opinions!

I have a newly built home, it's 1500 sq foot, 12' ceilings, built on a monolithic slab. When I did it I used polystyrene forms around the slab edges, and 4" of foam underneath. R19 air sealed walls, r60+ attic, triple pane windows all that fun stuff. Heat load calculation done myself shows around a 15k btu heat load.

For the hvac system I have a single point mini split 24k btu, but that's not the subject here. I put 1600 feet of 1/2 pex, 7 loops roughly 225 a loop. Single zone single 15-58 circulator flows about 2.8 gpm on low. I have it set up with a 38 gal electric water heater with 2 30 amp feeds running the elements. Bladder tank, 15 psi or so in the floor loop. It will meet the heat load when it's 0 outside with a single 4500w element but it's all it wants, so 15k btu design load is fairly accurate

I like the floor nice and warm, I don't like using 9kw to heat the floor! but I did it that way so I have a 38 gallon buffer tank, and backup heat for the mini split with the intent of installing a boiler later (now)

I have access to wood and want to do a wood boiler. I really dont want one inside a. Young son b. Liability c. Practicality, I don't have an equipment room, I have a 4 foot crawlspace under 250 sq foot of the house with regular 8 foot ceilings for my water heaters (regular hot water and radiant, I have them completely separated)

Any experience with something like an Eko 25 and placed it in an insulated out building? I am familiar with owbs but not super impressed by much short of a 12k gasification unit...parents have a "nature's comfort" smoke thrower but creosote and wood usage is through the roof.


Also any input in "thermal cycling" the slab? I know from kw/btu inputs it takes 400-450k btu to heat the floor from 70-84 or so... Would using the floor as a buffer and running a gasifier flat out then letting it die out and slowly drop be harmful to the floor? I could continue to use the mini split to make up the btu when the floor is cooler and then bank heat when I can tend a fire around the house.

Another consideration is I work 12 hour shifts hour drive both ways so 14-15 hours away from home 4 days a week is the norm. I figure anything I'm looking at will require starting a fire when I get home, going to be hard to get 15 hours out of a wood burner without a ridiculously large storage tank, or massive creosote problems.

Any other wood boilers that would suit my situation that I'm not aware of? Holes in my logic, suggestions, comments, concerns? Criticism welcome also!

Sorry it's long winded but I wanted to be clear with my situation and what my goals are! Thanks for any help and your time
 
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maple1

Minister of Fire
Sep 15, 2011
10,809
Nova Scotia
If you're set on a wood boiler, a 25 kw boiler with 500 gallons of storage should be a decent match. Should get you around 16 hours of coasting time on charged storage. Can always go bigger on the storage if you want. You might be able to use your slab for some flywheel effect, but I would try to run it at constant temp. For your heat loss, that should only be 4500w/hr.

You could put that all in an outbuilding. Make it big enough & your winters wood could go in there too, along with a work shop. Good underground piping ($) would be very important. Not sure the Eko situation these days, but a Varm 30 is just under $5k.

Air to water heat pumps are also kind of on the verge too.
 

peakbagger

Minister of Fire
Jul 11, 2008
6,597
Northern NH
This course is free and would be great way to get to speed


It sounds like you have a very energy efficient design. The same guy who teaches the course I linked to is major fan of air to water heat pumps. He has written multiple articles in the last year or two. Here is one https://www.pmengineer.com/articles/94022-john-siegenthaler-renewable-heating-design
The issue I and other folks on this board is that the available options for air to water heat pumps are not great in the US. The problem is they do not integrate well with conventional high temp radiators so the market isnt that big. I think this will change in the next year or two. If you have sun exposure and your state has net metering, I think a combination of solar panels and an air to water heat pump may be the best fit if you can wait a year.