Adding hot air plenum to the bench to blow warmed air through current forced air duct work?

A typical house has isolated rooms, a forced air gas furnace and lots of ductwork. My furnace has died and since it is March I need to replace it yesterday. But it makes me think I could build an RMH with a large enough bench, not for the purpose of warming butts, but burying a plenum to provide hot air to the current ductwork. Bench size is an issue because ductwork is about moving volumes of air, so a plenum is a substantially large item inside the thermal mass besides the exhaust manifold. The benefit-- you can heat multiple stories and closed off rooms quickly. Enough of the plenum would have to be exposed to provide space for attaching duct runs. It seems straightforward. It's all about having enough thermal cob and stone. Has anyone gone ahead down this path? Thank you, Tom.

Another route might be to heat a vat of mineral oil, circulate that hot oil through a radiator in your duct work, then back into the vat so it can reheat. That should provide heat to the house when air moves across the radiator.
The oil can be heated much hotter than water without excessive pressure, so the radiator can be hotter for better home heating.
Not sure why this isn't already being done at the residential level. It's fairly common at the industrial level.

Bill, oil sounds like a great idea but I think it has the same fundamental problem as heating water with a rocket mass heater. Hotter temps and longer burn times are required to heat volumes of water. The fire in an RMH is only on for a short amount of time. Whereas air flow through the bench benefits from the lower, longer lasting temps.

Thomas, i don't know if you're aware of the "half barrel system" but along thoses lines, i've been thinking lately about something similar. Everybody is on about monolithic mass. But Matthewalker developed the half barrel system, where you cut a barrel in half lenghtwise, and put the two halves in a bench, making a bell. I took on his idea, and thought, if using fist sized rocks above the bell, and having an air inlet and outlet , with forced air on one side, to exctract the heat out of the rocks. Ok, fist sized rocks, or a smidge bigger to allow convection of air in between them to charge them, and allow for forced air movement. Smaller, and you get traped air which insulates. Or you could make a criss cross of bricks too. Use a flat bell in that case, like an old home heating oil tank, (well we sometimes have square ones over here, i bet you do too.) Just an idea.

Hi Satamax, I am aware of the half barrel system. It appears to be a good workable variation on the theme. Your plan harkens back to a similar design in Mother Earth News in the 70's-- a cistern filled with heated rocks (from wood fire)-- air blowing through them is warmed and distributed through the house. I can see if duct work is the main distribution system and necessary for distribution to isolated rooms it would necessitate insulating the thermal mass to preserve heat for that purpose. Thank you for contributing to my missing links! New design is taking shape!

Thomas, yopu made me think.

If you insulate the mass, you could also use the horizontal batch feed and longer burn to heat the main room your stove would be in. And you can always regulate the primary heat by sizing your barrel. A little barrel releasing less heat and leaving more for the mass, big barrel doing the contrary. You can also use pozzolan elements, or fire bricks or else to make a bell, which has slight mass, instead of a berrel. Then you slow down the release of the heat in your main room, thus enabling you to have longer burns, which in turn charge more the following (second) mass.

Wow Satamax, you're a creative junkie after my own heart! I was just thrilled with the idea I could bring a plenum and a bench together in my head. A slow air moving fan taking warmth to the far corners of my house through the current ducting is gonna be my limited goal at this point. For all you're suggesting I'd need to soak up much more youtube and pictures-- always worth a thousand words. Thank you! Tom

Thomas, using loose rocks is my idea, but for the rest, i've learnt it all from other people. Like the idea of using a fan, with the loose rocks, that comes from having an electric storage heater. Which i think works not bad.

Hi Tom,
What type of furnace do you have that died? I had a working oil burning furnace that i hadn't used for a couple years since I've been heating primarily with wood. I decided to clear out all of my ducting and the furnace to make space in my ancient basement for other projects like a rmh. After removing the ducting, cold air returns and the furnace shell i was left with a very substantial steel heat exchanger. Removal of the injector revealed an intact refractory core! Seeing that, i just had to convert it to a wood fired heat exchanger!

I used fire brick to build a J tube feed and burn chamber feeding directly into the refractory core and fired it up. It works great! The design isn't perfect but the surface area is truly substantial and because it is attached to the chimney, there is always draw through the system. I extended the exhaust down to the floor and buried it in sand (temporarily) to steal as much heat as I could from the flue gases.

As far as the heat exchanger design goes, it consists of what is essentially a barrel with a vertical 3" wide opening to box surround. It is designed for air flow and creates good convective current on it's own. Built into a shell with a proper air gap it has massive surface area and could again be used as a heat exchanger for forced air. Since it is still in the middle of my space, I intend to move it to a new location. Next, I will build a masonry chamber around it essentially mimicking the old steel shell's inner dimensions, but with masonry, and finally using a cob mass bench to extract whatever heat is left in the gases.

I plan to document the build when i relocate it, but I have a feeling there are many of these old furnaces going to the scrap yard.

Hi Tom,
What type of furnace do you have that died? I had a working oil burning furnace that i hadn't used for a couple years since I've been heating primarily with wood. I decided to clear out all of my ducting and the furnace to make space in my ancient basement for other projects like a rmh. After removing the ducting, cold air returns and the furnace shell i was left with a very substantial steel heat exchanger. Removal of the injector revealed an intact refractory core! Seeing that, i just had to convert it to a wood fired heat exchanger!

I used fire brick to build a J tube feed and burn chamber feeding directly into the refractory core and fired it up. It works great! The design isn't perfect but the surface area is truly substantial and because it is attached to the chimney, there is always draw through the system. I extended the exhaust down to the floor and buried it in sand (temporarily) to steal as much heat as I could from the flue gases.

As far as the heat exchanger design goes, it consists of what is essentially a barrel with a vertical 3" wide opening to a "box-in-box" surround. It is designed for air flow and creates good convective current on it's own. Built into a shell with a proper air gap it has massive surface area and could again be used as a heat exchanger for forced air. Since it is still in the middle of my space, I intend to move it to a new location. Next, I will build a masonry chamber around it essentially mimicking the old steel shell's inner dimensions, but with masonry, and finally using a cob mass bench to extract whatever heat is left in the gases.

I plan to document the build when i relocate it, but I have a feeling there are many of these old furnaces going to the scrap yard.

Hey Colin,
I'm sorry I haven't gotten back to you. I must have overlooked the reply notice in email. Your stripped down furnace sounds interesting. Wonder how it's worked out for you-- steel holding up to the intense heat? Would love to see some pics. I've been spending alot of time in the basement to finally accept that building an RMH anywhere near my current plenum would be like building one in a submarine, and looking up three stories, everything is in the way- major renovations, just too cramped. The back of the house is always cold, too far from the furnace even for insulated flex duct. I'd have to beef the floor like Ernie and Erica did on their 8" RMH DVD to put one there. I've also read Erica's caveats placing it in the basement. I can't believe no one's tried it yet. I really wanted to be in the innovator's club on this one, Damn!!