# Woodstove and thermal mass questions



## happycat47111 (Nov 23, 2013)

Hi, everyone. We just moved into a log cabin that has two hearths for wood stoves. We had one cleaned and are wanting to install a stove on it. Thing is, after looking and looking at things, I'm in love of the idea of a rocket mass heater. I don't feel confident in our know-how just yet as far as building one from scratch, so here's what I was wondering. Can we get a regular wood stove (am eyeballing the Pleasant Hearth 1,200 sq ft model from Lowe's) and then build a masonry box of some sort around it and the inside portion of the flue to make a hybrid? I wouldn't interfere with the design of the stove insofar as it pertains to drawing and all that sort of thing, but just insulate it so we have a thermal battery to store heat and slowly release it through the night. We work at home, so we'd be here during the day when the fires would be going. 

As far as the materials go, I'm thinking concrete blocks -->sand?-->stove. Put some pretty tiles on the blocks. Same thing for the flue. 

Thoughts? Am I dreaming/crazy/genius? All of the above?

Thanks!


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## Solar Geek (Mar 14, 2014)

HI Happy cat! Well, what you call a "rocket mass heater" is referred to in WI at least as a masonry heater. Very common but very costly. Here is a link to an incredibly talented group who build them and homes. http://gimmeshelteronline.com/heaters-and-bake-ovens/masonry-heater-portfolio/

That being said, we had a log home (2400 sq ft main and loft upstairs with office, 2 bedrooms and bath and 450 sq ft down) and used only the LOPI Liberty Woodstove freestanding to heat all winter. Also cooked a lot on it. 

Temps of -30F and high winds were no problem. We had built an aggressively passive solar home and combined with closing all shades at night, being fully open during sun, having tile floor with 4" concrete underneath as a heat sink, and using the wood stove, we were toasty!(But since the downstairs was a walk-out level with high exterior exposure, we did use the smaller LOPI ANswer wood stove there as needed. Upper level could not warm lower level).

The reason we decided NOT to do the masonry/rocket heater was it just got too hot for too long. Building a small fire is a no - no as you don't want creosote buildup and need the really hot fire to warm all the stone/mass.
Problem is, log homes if well built are really temperate - cool in spring/ summer, warm in winter/fall.

We now live in an ICF (insulated concrete form) home and are again using the Liberty. Because this house is one big thermal mass and we have 1/2 slab with 6" concrete and 1/2 full poured basement, we only need a fire if there is no sun or the winds are super high (our windows here are IMMENSE as we don't care if we have to burn wood).

Any great freestanding wood stove would work. We chose the Liberty as it is and was the most highly EPA rated wood stove for low particulate matter. 

They make excellent woodstove inserts for fireplaces.


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## Nimrod (Jun 8, 2010)

A neighbor put a barrel stove in the basement and built a bunch of bricks all the way around it. It keeps his house toasty even when the outside temp is -40. Just build a fast hot fire in the morning and again before bed.

There is a huge difference between a rocket heater and a Russian/Finnish stove. Google both.

Time for the news.


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## happycat47111 (Nov 23, 2013)

Solar Geek - oooh, ICF. That was our dream for years. Never came to fruition, but kudos to you!

This house has been sorely neglected for about twenty years. It's not in bad shape, but there's a lot of maintenance that wasn't done to it. So we're playing catchup there - sealing everything that isn't sealed, etc. We have a heat pump now, and it does pretty well. Amazingly well. But it won't do us one bit of good if the power goes off, hence the desire for the woodstove/rocket mass heater. Plus, I'd prefer to not have to pay $$$ to stay warm this winter. 

What we're thinking would be more of a hybrid, a distant-cousin kind of hybrid, of a masonry stove. Like Nimrod says, there's a huge difference between the RMH and the masonry stove. We want the battery but aren't confident enough to do the full-size RMH. 

I think we're going to experiment around with a couple of things we found on YouTube last night. Build it outside, see how well it runs, see if it's feasible, and if so, move it inside, seal it up, and go from there. What we found was well put together, well thought out, and not as um&#8230; questionable as some of the RMH designs. So fingers crossed. 

I did more research on the wood stove we were looking at - the construction is just so pitiful on it, I can't see getting it. Even if we rehab it and fix some of the issues the stove has (according to reviews) I still wouldn't feel safe with it in the house, and that's before we added the battery. 

Thanks, y'all! More opinions and thoughts welcome.


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