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Is there a formula for calculating the mass needed in a RMH?  RSS feed

 
Patrick Freeburger
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Summary:
I want to heat a 700 sq ft house for 14 hours from ~32F outside to 68+F inside (moderately insulated) using a portable (wood framed) RMH. Is there a formula I can follow? Will a 2'x2'x6' bench work?

Details:
My family comes home at ~6pm to our small house in moderately cool Northern California. I want to put in a quick fire to warm up the house for the evening and keep it warm throughout the night. Since no one is home from 8am to 6pm, building a thermal mass to retain 3 or 4 days of heat does not seem efficient (correct me if I'm wrong) and we don't have the space for much more than a small bench heater. Also, Is there a curve where the RHM comes up to temperature and then slowly goes down - i.e. no one likes to get out of a warm bed to a cold house so I don't want to be too far down that curve. Alternatively, we could put the propane heater on from 6-6:30 pm in the evening and 6-7:00 in the morning to give the RHM a boost and I will still have reduced my propane (space heating at least) bill by 90+% to our current situation.

PS - I enjoyed the DVDs and I look forward to hearing more about the shippable core. Keep holding classes I still hope to make one.

Regards,
Patrick


 
Erica Wisner
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Location: Okanogan Highlands, Washington
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Patrick Freeburger wrote:Summary:
I want to heat a 700 sq ft house for 14 hours from ~32F outside to 68+F inside (moderately insulated) using a portable (wood framed) RMH. Is there a formula I can follow? Will a 2'x2'x6' bench work?

Details:
My family comes home at ~6pm to our small house in moderately cool Northern California. I want to put in a quick fire to warm up the house for the evening and keep it warm throughout the night. Since no one is home from 8am to 6pm, building a thermal mass to retain 3 or 4 days of heat does not seem efficient (correct me if I'm wrong) and we don't have the space for much more than a small bench heater. Also, Is there a curve where the RHM comes up to temperature and then slowly goes down - i.e. no one likes to get out of a warm bed to a cold house so I don't want to be too far down that curve. Alternatively, we could put the propane heater on from 6-6:30 pm in the evening and 6-7:00 in the morning to give the RHM a boost and I will still have reduced my propane (space heating at least) bill by 90+% to our current situation.

PS - I enjoyed the DVDs and I look forward to hearing more about the shippable core. Keep holding classes I still hope to make one.

Regards,
Patrick



Well, you had me all excited about the specifics of the calculation... until you brought in the portable part. We only have 4 data points for portables' heat storage, and they all perform quite differently from each other (different types of gravel / rock fill). We also have a few heaters using just loose or tamped mineral soil, and these perform a lot more like the cob heaters overall.

It sounds like someone is home at least 2/3 of the hours of the day, and since the biggest heat storage would be that period right after the fire (when you want that overnight heat) I think it's still worth doing.
For our 800 SF cabin (good insulation) up near the canadian border, we have an 8" system with 3 passes of 8" duct, inside about 2 cubic yards of solid (earthen mortar / fieldstone) masonry. we burn about 30-40 lbs of wood per day, and it holds the house within about 10 degrees F per day (drops from 75 to 65 by the following afternoon, then to 55 the next day, etc).

Your 2' x 2' x 6' bench might hold about half that amount of pea gravel, but unfortunately the heat transmission from the pipes to the pea gravel is not as reliable as through solid masonry / cob.

But given how much milder your climate is, you might come out OK, especially if you are willing to build up a bit more solid masonry (e.g. loose brick with a mortar core around the pipes) to transmit heat better.

You could also consider boosting the mass by building in some water jars or stainless water pots as thermal mass near the cool end of the system. For example, you could can some jars of water in a boiling-water bath, then remove the rings before installing them in the bench. They'd stay sealed while things were going well, but the lids should pop for safety if you did accidentally make them hot enough to boil.

Here are some useful calculations for home heat loss / load in general:
- compare your climate with mine (we have 6500-7000 HDD to make up on average. Annual HDD is a straight multiplier on your home's heat loss (insulation, exposed wall surface area, etc)) so it's a useful way to estimate the total amount of heat needed per month or per year. You can divide it down into days too - it's made from the difference between your average daily temperature outdoors and the one you want indoors, e.g. (68F-32F) x 24 hours http://www.huduser.org/portal/resources/UtilityModel/hdd.html

- figure up your heat loss using the building's insulation values, window areas, and so on. Hyperphysics home energy calcs or Engineering Toolbox heat loss calcs for buildings.
Now this won't be super-accurate (it's hard to know exactly how much you are losing to infiltration and so on without specialized equipment, and of course in an unusual cold snap you may need more heat than the monthly average) but it gets you in the ballpark pretty close.

- figure up how much wood you'd need to burn to compensate for that heat loss
ChimneySweepOnline.com wood BTU calculations
(note that your rocket mass heater may have an exhaust temp somewhere between the woodstove norm (350 F) and the rocket stove norm (100 F), depending on how effective your portable mass turns out to be.)

- use the heat capacity and conductivity of your thermal mass to estimate how much heat it can store at what temperature (target is between 80 F and 200 F, you will probably not get it much above 85).
Or compare your home's heat loss with known heaters, and choose a heater design that is likely to handle the anticipated load based on proven performance.

For ballpark:
- in 900 SF, poorly insulated in maritime climate (Portland OR) with single-pane windows all facing the shade, we burned less than half a cord per year. Ernie says 1/8 cord. I think it might have been as much as 1/3 cord, because we used shop scrap as well as the stacked wood.
In any case, "less than half a cord" has been consistently reported from small buildings in southern Oregon and up and down the west coast.
In Portland we ran our 6" heater maybe 2 to 4 hours per evening, burning up to 20 lbs of wood per day, and sometime skipping a day if we felt like it. In a real cold snap (teens) we burned for 6 hours for a couple of days in a row, then went back to 4 hours per day. That heater had a similar thermal mass, but stretched out flat (5700 lbs / 2.2 cubic yards).

- in the Okanogan Highlands with our higher heat loss due to colder climate, we burn about 1 cord per year for our 800 SF cabin (R-36 or better insulation, about 72 SF of double-paned windows).
We burn more like 4 to 6 hours per day during the coldest months, 30 to 40 lbs of wood, but still skip a day or two in spring and fall.

So there you have it! Lots and lots of tools for guessing with numbers.

Yours,
Erica W
 
Patrick Freeburger
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Erica, Thanks for the detailed reply. It is not so much that it has to be movable, it is aesthetics. My wife has agreed to it if looks like a normal couch and a put a wood box around the oil drum to hide it. If you (or anyone) has a picture of the most 'couch-like' looking RMH. I would love to see it.
 
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