Slimline RMH

Hi all,

I’m Jon from Manchester, England 🏴󠁧󠁢󠁥󠁮󠁧󠁿
My heating bills are now £250+ a month (approx $313) so I’m trying to research alternatives to gas central heating.

RMH seem like a viable option, but I have a few questions:

1. I have a space on an internal wall 260cm long, that could be 40cm deep and 100cm high ish. (Higher at the sides if needed, up to the ceiling). Would that be enough space?

2. Can the mass be tall and flat rather than wide and flat (as if the bench was folded up against the wall)

3. Can the exhaust be lower than the RMH and exit the mass at the far end downward under my subfloor then travel a few meters horizontally under the floor before exiting out the side of my house and then go up an external flu past the roofline?

4. I’m thinking of trying to incorporate the mass into a media wall, is this a daft idea 👀😅 obviously insulating electrical parts.

Please see pics attached. Thanks

40 cm (16") deep is really tight. What is the material of the wall behind? If it is masonry, your dimensions might work. If wood framing, you need at least about 10 cm clear for air circulation to cool the wall. For heat circulation inside the mass cavity to work well, you need around 20 cm or more clear. Brick walls need another 10 cm thickness in front, more for large flat spans.

Your idea of a mass that is higher at the sides than at the center can work fine, using a "bell" or stratification chamber, which is essentially a hollow box for hot gases to fill, give up their heat to the container, and fall as they cool to exit near the bottom.

Exiting below floor level, traveling several meters, and rising on the outside wall is problematic. It might work if conditions are right, but may well not work and want to backdraft smoke into the room. Rising to near the ceiling and exiting on the outside wall has a much better chance of working. What is the structure like? How many floors is it and where are you located in that? What is the prevailing wind in relation to your space?

Glenn Herbert wrote:40 cm (16") deep is really tight. What is the material of the wall behind? If it is masonry, your dimensions might work. If wood framing, you need at least about 10 cm clear for air circulation to cool the wall. For heat circulation inside the mass cavity to work well, you need around 20 cm or more clear. Brick walls need another 10 cm thickness in front, more for large flat spans.

Your idea of a mass that is higher at the sides than at the center can work fine, using a "bell" or stratification chamber, which is essentially a hollow box for hot gases to fill, give up their heat to the container, and fall as they cool to exit near the bottom.

Exiting below floor level, traveling several meters, and rising on the outside wall is problematic. It might work if conditions are right, but may well not work and want to backdraft smoke into the room. Rising to near the ceiling and exiting on the outside wall has a much better chance of working. What is the structure like? How many floors is it and where are you located in that? What is the prevailing wind in relation to your space?

Thanks Glenn.

The wall behind is brick 🧱 and plasterboard.
So 40cm minus 10cm brick could leave a 30cm wide internal stratification chamber gap.

What sort of WxDxH external dimensions would I be looking at for the right hand side corner for the burn chamber and riser?

The house structure is all brick, it’s a 4 bedroom semi-detached house. 2 floors but I’m only concerned about heating the ground floor as it’s open plan.
Upstairs are the bedrooms and a bathroom.
The gas central heating gets it to about 20 degrees, I have the heating going off by 7pm (it’s 18.5 degrees now at 9:30pm) and it drops to about 13 degrees over night. It’s 1 deg outside now, it will be zero tonight.

Wind wise I don’t know, but I’m in a pretty inclosed area surrounded by other houses and over head height fences so I’d guess it’s fairly well covered wind wise.

I’ve just measured the distance the exhaust would have to go horizontal under the subfloor and it is just over 2m. The height on the right hand side of that wall I’d have to play with is about 2.2m x 56cm.

I don't have time for a full reply right now, but the structure sounds good. I would want to put a layer of fireproof material spaced a bit off the existing wall to make sure the other side of it doesn't get too hot... something like cement board in moderate heat areas, sheet metal in high heat areas behind the J-tube and riser and up. Plasterboard with its paper surfaces would need to be stripped off the brick, which would add a tiny bit to the available working depth. What size system are you thinking of? 150 or 200 mm? The mass space you have may not be enough to extract and store the heat output of a 200 mm system. I can make a sketch of possibilities and post it, but that will take a few days until I have the spare time and tools accessible. The column of space on the right should be usable.

Jon,

If you built 150 mm batch box, with the riser on the side (side winder) and assuming firebricks to be 230x114x64 laid as shiner and skin bricks to be 100 mm wide and 10 mm gap between the firebox and the skin:

Dimensions of the firebox and single skin
depth 720 mm
width 780 mm

The height would depend on the bench dimensions, but could be around 1.7 m tall to get the recommended 5.3 m2 ISA (area of the heat absorbing skin walls).

Good approach would be to remove plasterboard to bare bricks and plaster the wall with clay based plaster

Glenn Herbert wrote:I don't have time for a full reply right now, but the structure sounds good. I would want to put a layer of fireproof material spaced a bit off the existing wall to make sure the other side of it doesn't get too hot... something like cement board in moderate heat areas, sheet metal in high heat areas behind the J-tube and riser and up. Plasterboard with its paper surfaces would need to be stripped off the brick, which would add a tiny bit to the available working depth. What size system are you thinking of? 150 or 200 mm? The mass space you have may not be enough to extract and store the heat output of a 200 mm system. I can make a sketch of possibilities and post it, but that will take a few days until I have the spare time and tools accessible. The column of space on the right should be usable.

Hi Glenn, no problem time wise and thank you again I really appreciate any suggestions at all. Yes I was thinking 150mm and agree that the plaster board would come off and go back to brick. There’s the column of space on the right and also the same space on the left that could be used if more mass is needed.