RMH that heats the floor and water

Hello.

First. I made this post about a year ago to Redit and someone (Pollyannapermaculture) proposed to post ot here. However I never got around it till now... Purpose of this post is to offer some user experience and possibly tips for similar builds... Cheers!

As I am writing this, my 2018 RMH is heating the room, roaring steadily in the middle of the room an keeping my floor and home nice and warm. I wanted to share some pictures of the project and my notes on the build.

The guidelines for building the system were mostly taken from Ianto Evans and Leslie Jackson book Rocket Mass Heaters III.

Luckily for me, I started the build when my COB/Cordwood/Straw wall house was unfinished so I had liberty of playing around with it. I decided that RMH should be able to heat up the floor beneath the house as for mainly straw house, there is not much heat capacity in the walls. The floor is my main heat capacitor. The basic idea is hypocaust floor. All the hot gasses from RMH are pushed through series of pipes buried under the floor and slowly but steadily heat up the whole mass. When we had outside temperature of -25 degrees celsius, we often heated up the room up to 25 degrees. Floor was so nice and warm that our cat decided to move there instead of the sofa. The heat capacity of the floor is so good that even if the RMH is not used for several days, the floor will still remain comfortably warm.

1) My system has "J" shape core. Inner cross section volume of \~314 cm2. Heat riser, vertical tunnel and fuel box are 15.4x20.4 cm in size and the output exhaust consists of series of tubes with cross section of roughly 160cm placed 2 side by side ("double barrel shotgun layout") to give appropriate cross section to the system (I did not have large enough tube to burry at the time). I used cast iron Soviet age metal pipes meant for wastewater in the ground. Outside the house I have 1 x 200mm cross section flue pipe - chimney- that rises to 3m. And it has induced draft - I use chimney ventilator on top of the external part to ensure flawless, easy and foolproof ignition. I also have tube of cold air coming into fire box from my hallway, (cold airflow is induced with 120mm, 20W computer fan)

2) All in all there is roughly 10 meters of pipes inside the ground, under the floor, \~30cm below the surface. The floor is uninsulated but there is a \~500mm deep trench around the inner perimeter of floor, filled with insulating granules. Under the pipes I used a padding made of lecca, on top of the pipes the clay rich earth is tamped and some big stones are added to improve the heat transfer and increase heat storing capacity. Floor is made of COB, closed with linseed oil and it gets nice and warm. There are in total 4 access points made into the floor to allow cleaning of the pipes and also a cleaning access point for the manifold. The reasoning behind uninsulated floor is to keep the heat mass at maximum + it was too much of a hustle to dig out \~30 cubic meters of soil just to put it back in later. Decisions were made and I am happy with the result.

3) I am using dried wood (mainly alder, aspen and elm) from around my house (i live in the forest) gathered at winter and dried over the summer. I use 50 cm long firewood which I chop into quite thin pieces but if I am lazy, it will eat up quite large diameter wood easily.

4) I have added a hot water system to the build. It consists of DIY heat exchanger made of big soup pot (\~40 liters) with copper pipe coiled inside. Water inside the pot gets hot - sometimes close to boiling - and the water inside coils is pushed around by circulation pump, feeding a modified boiler (in current setting it is just an insulated tank for hot water with inlet and outlet). There is a second coil of copper pipe in it as well that heats up the floor of my bathroom (used cars coolant fluid for the medium in this system, pushed around by another circulation pump). Edit: now i also hooked the circulation pumps to mains through a sonoff switch which measures temperature of the water. If water reaches 55 degrees (celsius) it switches on automatically and when it goes below 33, it switches circulation off.. lazyness is motivator.

Why draft inducer, you may ask. Well, I have 11 yo son who is sometimes responsible to light up, keep an eye on the fire and add firewood. Draft inducing vents are voice controlled (Sonoff switches and Google nest). This also helps my wife to start the fire. There is a world of difference in getting the fire going with or without the draft inducer especially in such system where hot gasses must find a way through 10m of cold/warmish pipes in the ground. However, most of the time I keep it on all the time because it just keeps the flow through the system at steady pace and heats up the barrel much quicker. The system does work without the vents but it is unreliable and can smoke especially when we have wind or wrong atmospheric pressure on the streets. The total power of the motors that keep the flow running is \~50-60W which would be deducted from total efficiency of the system but I think it is too small impact to fuzz about. The draft inducing system is 230V but it is backed up with UPS with around 1400 Ah of batterie capacity in case we have power outage. Even if the lights are out around the village, I still have heat and comfort in my home. I think the added benefit of comfort and peace of mind when my kid is home alone is well worth the somewhat lower efficiency.

I want to take time to thank Ianto Evans and Leslie Jackson for their wonderful book that helped me to get the system built. RMH is one of the greatest DIY builds for a home - a really fit heart for the sustainable house.

NB! I do have some pictures of building process which I will try to upload soon.

Hi Andres, welcome to Permies!

What a creative and effective RMH build!
Congratulations with having your home cozy and warm even at -25C outside temperatures, it's a super idea to use the floor mass to heat your home.
I'm looking forward to the additional photos of the build.
Thank you so much for sharing, this is very inspiring - I've yet to build another RMH into the other little house (they're connected with a corridor) and my imagination is now totally fired up to make something similar to yours.

The first build is here. I'm going to make some modifications to make it more efficient, and will add the edits to the original thread in the future, next summer/fall.

That riser is calling out to be further described! Looks like a very tight gap between it and the barrel, did you calculate that out precisely?

I look forward to details about the floor construction. I'm set up to attempt something similar once weather conditions are more favourable.

Andres, I'm really inspired by your build for my next project.
Does your rmh exhaust tubes end vertically in the chimney or horizontally, outside?
And for getting the system going, you need the draft inducer also if the system is warm? Would a candle or a torch warm the system up enough not to have to bother with a draft inducer? Probably not, since you're using it, but I'd like to read about it in your words

Coydon Wallham wrote:That riser is calling out to be further described! Looks like a very tight gap between it and the barrel, did you calculate that out precisely?

I look forward to details about the floor construction. I'm set up to attempt something similar once weather conditions are more favourable.

Riser and gap between riser and manifold were calculated precisely. It was a bit of math and tinkling to get the cross section area right but it is decent. It only looks thin as it is dark and sooty. Works well..

Floor construction pics are coming but I don't have acces to  my laptop now.

Nina Surya wrote:Andres, I'm really inspired by your build for my next project.
Does your rmh exhaust tubes end vertically in the chimney or horizontally, outside?
And for getting the system going, you need the draft inducer also if the system is warm? Would a candle or a torch warm the system up enough not to have to bother with a draft inducer? Probably not, since you're using it, but I'd like to read about it in your words

I have vertical chimney outside, one large tube (zinked ventilation duct as the temp is not very high). I'll make some more pics of it + the draft inducer.

About draft. When I am using nice dry wood, nicely chopped up, when the system gets hot, draft inducer is not vital. It will run without it. But it is not roaring very rockety... Draft inducer just makes it work foolproof throughout the cycle. I have venturi vent draft inducer so it does not block the chimney when not working.