Best backup heat for massy solar house?

I’m about to start building a rammed-earth solar-heated house. This climate has cold winters, most January nights at -20C (-4F) or lower. It’s perfect for passive solar heat, being high, dry and sunny, and only 34N. I’ve lived in solar heated houses at our school nearby for about 20 years without backup heat, but now that I’m designing a personal space, I want to eliminate those January nights in rooms below 60F / 15C.

The house will stay warm enough almost all days, but might want a few extra degrees, perhaps just in the living room, for some nights of January. The house will be made of thermal mass heated by an attached greenhouse. I’ve heard that perhaps a mass heater is not ideal for a situation where you want extra heat for only a few hours at a time.

Three possible sources of backup energy:

1) Wood is common fuel in the area. I won’t have my own trees for several years, but I could buy firewood.

2) Electricity is unreliable and weak in winter. It’s from Ladakh’s first largish dam, which didn’t submerge any habitats so it’s fairly benign, but the Indus is so low in winter that the supply is limited. I’m thinking of putting electric heating tape in the floors because it’s so cheap to install, and if the power situation improves someday I can use it.

3) Bottled LPG gas, which I’ll buy for cooking anyway, and heaters are cheap. Minimal investment cost and instant flexible heat, but high running cost and it’s fossil fuel ....

So I’ve been pondering three wood-heat options. The room I’m thinking of heating is the ground floor living room, and any vertical chimney will have to go through the bedroom above, and then out the flat roof. The subfloor is dry packed earth up to plinth level, and it’s easy to put a foundation in if adding something heavy.

1) A woodstove. People here use lightweight wood stoves and just set them up from October to April and then store them outside for the summer. I would only need it for January. It requires very little permanent installation except a hole through the inter-floor and roof.

2) A Rumsford fireplace. Nobody here uses open fireplaces with masonry chimneys, so it would be exotic and, I think, charming. I find Erica Wisner’s discussions of them very compelling. Would this be a good choice for quick heat when needed, when the rest of the house has lots of mass that is semi-warm? But I’d have to add a two-story masonry chimney into the building plan. I have no idea about the availability of chimney tiles -- the local materials are earth, rough stone masonry, and various metal materials. Also, there are no chimney sweeps here: people normally dismantle their lightweight modular chimney pipes to clean them. So I’d have to clean the 2-storey chimney myself: is that easy for a klutz like me?

3) A rocket mass heater. Well, of course that’s appealing because I’ve been hanging out here at Permies. Is it possible to install one in a ground floor room and then run a chimney pipe up two storeys to the roof? The rocket would be against an internal wall, the bench would run 12 feet along the south-facing wall that is shared with the greenhouse, and then it could go out the west wall at knee level or have a pipe going up two stories to the roof. I could have the vertical pipe either built into the wall, or just run a bare pipe up the corner of the rooms, with a hole in the interfloor and roof. If the pipe doesn’t have to be built into the wall, I can add it later and not worry about designing it into the building, except for keeping a knee-high hole in the west wall, which would be easy to do.

There is a chance of such good solar gain that I never want backup heat at all, but somehow I doubt I’ll be so lucky. But this chance means that maybe I shouldn’t go for a major investment but just tough it out the first winter and see how much backup heat is actually needed before choosing a system.

So the basic question I’m asking you is, for a house with plenty of thermal mass that will be just a little chillier than wanted for some evenings, would you recommend a woodstove, a fireplace, or a rocket mass heater?

Thanks for your comments and advice!

If electricity was reliable, a little heat tape and electric blankets or mattress pad do wonders. Kind of an extension of Paul's heat experiments.

A light stove is nice if you are in front of it. Only the radiant heat works when the mass around you is cold, but eventually it will warm the mass. Your chimney going up through the bedroom will add heat there. But two stories of thin pipe is a recipe for disaster, it gets too cold and creosote builds up fast.

My dream home would have two rockets. A bigger one built into the mass, with a tight damper(s) so I could selectively cool the mass when it gets too warm in the summer. And a small one in the bedroom with the bed platform as the bench and the barrel heating the adjoining study/reading area.

Since you don't need lots of heat and only over a short period, what would you think of a small J-tube rocket or batch box inside a barrel? Has been done at the Innovators Gathering and it works. Small footprint, no mass to speak of, you can use handles on the sides to relocate it for summer. Same idea but even simpler: what Erica christened the Nymph, a small pocket rocket(ish) thingy which doesn't eat itself over time with something like a batch box port in there.
Inevitably, you'd need refractory materials to build a rocket.
In short: a low mass, moveable and apparently very clean burning rocket device?

Of course I am partial to a RMH, but for your situation any of the choices would probably work fine. Even the greater efficiency of a rocket would be minimally relevant in something seldom used. I think it would come down to making an educational demonstration of something you could prove worked excellently for your neighbors who use stoves more. The Nymph or something similar, if it proves easy to build and move, would have the advantage of essentially eliminating stovepipe cleaning. I don't know if that would impress people there, or if chimney fires ever happen. I'm sure more efficient use of wood would be a serious benefit in your region, as well as reduced smoke pollution in populated areas.

Sounds to me like all your options could work. And now that you've asked, you have MORE options.

With any of these, I would go ahead and do the 2-story internal chimney, if you can get the metal stovepipe for it. You can vent a Rumford to a properly-installed metal chimney (you'll want an insulated collar to go through any tight-clearance spots like floor and roof).
Doing an out-the-wall-and-up chimney is harder to keep warm, and harder to support structurally. No guarantees that anything short of roof height would draft properly for you, so 2 stories outside is a lot colder and prone to chimney stalls from damp condensing inside the pipes.
A 2-story building typically has a lot of draft (negative pressure at the bottom, positive pressure at the top) so a rocket side-exhaust doesn't work well in that situation. Unless you get really lucky with the lee-side of the prevailing wind, and can shield it from any eddies or gusts, and even then it's likely to be a fitful performer. Not worth messing with, in my opinion.

Buckley Rumfords (www.rumford.com) has the original Rumford essay on how to build these. And they can be built with adobe. I think that's a good bet for year 1, if you want a quickie: adobe Rumford, careful attention to the throat detailing so it burns clean, and install your pipe. (The opening should be about 10x the pipe CSA I believe, but check with Buckley's site to be sure. You could also check with him about the likelihood of creosote, and how to minimize that factor. They usually burn very clean for a fireplace, but of course, that's not necessarily clean by Peter's Test-o standards.)

With all occasional-use stoves, paying close attention to good, clean priming fuels (dry newspaper or well-dried hardwood slivers, not pitchy "fat-wood), laying the fire so it burns candle-wise and not smoky, and getting your chimney warm before loading your main fuels will help a lot.

-Erica

The Nymph is pretty reasonable to move, and as clean or cleaner than the Rumford at a guess. Our version weighed about 110 lbs / 50 kilos.
If you don't have fire brick on hand, you might be able to do a cob version of the liner, if you're very good with cob. You can inspect periodically when you take the ashes out.

However, inspecting and cleaning may not be as easy as for other stoves; it has no lower openings since it's designed to be submersible in water up to its chin.
Might have to be un-hooked from its chimney a little more often than is really convenient, or invent your own cleaning port that doesn't compromise airtight function, using local materials.

-Erica