I’ve tried searching to see if anyone has done this, and although I found a couple old questions about it- nothing concrete.
So, my question is, has anyone successfully built a ducted RMH? I’m a heating professional (HVAC) and I’ve connected up lots of wood, coal and pellet furnaces to heat homes both alone and in conjunction with oil or gas burning rigs. I have an ultra high efficiency gas furnace and I want to build a large RMH with several round ducts embedded in it, which I’ll connect inline to the cold air return of the gas furnace. I’ll install a separate thermostat which will send a call to the cooling contacts on the furnace, turning on the blower at its lowest speed, drawing air through the RMH to distribute through the house ducting. If the mass gets to cool to warm the house the other thermostat will kick in and send a heating call (in stages) to the gas furnace; effectively making its first stage a second or backup stage.
Now, before I get preached at about radiant heat and the inefficiencies of heating and moving air- I don’t have the option. I have a mid 1800’s farmhouse, with many small rooms. None of the rooms have available space-even if the wife was willing to have a 55 gallon drum sticking out of a cob bench, which she is not.
So, anyone with experience in drawing out heat from the mass via ducting?
Hi Troy, Welcome to the forum! I personally have no experience with what your trying to do and the only example I can think of that may remotely resemble what your after is from Bigelow farms youtube channel. He was heating his greeenhouse with a RMH and had a metal shroud around the barrel to capture the heat then distributed it through ducting through his greenhouse.
Somewhere around 3 min. is where he describes this portion. RMH build
Wow I had the exact same thought yesterday! You voiced the question better than I could. I'm looking at "add on wood furnaces" to connect to my ducts, but would love to instead build a RMH in the basement with internal ducts to warm air that blown through and then distributed to the house like you described.
The heat exchange from the mass to the air you're blowing through it would be the key to efficiency. Seems to me a manifold of sorts with several small air tubes would work better than one large duct running through the mass (since air would blow right through a single 6 inch duct and not soak up much heat.) But I've never seen anything like that. I do wonder about coiling some flexible chimney liner around the steel drum of a RMH and blowing air through that and into your system, but still think embedding a heat exchanger into the mass would be better.
I lose all confidence when it comes to building my own RMH in the basement because there are so many factors to consider. The most tricky/dangerous thing is making sure your exhaust gas is still hot enough to create the draw up the chimney, but not so hot that you're losing out on heat capture. And rather than a RMH, I'm more interested in "batch box" design that can be loaded with more wood (tended less often) and even a masonry bell design. It might be easier to heat ducted air through a masonry bell design.
The wood-burning fireplace insert in my home has a plenum chamber above it, with twin ducts that run vertically, through the mass of the original fireplace, and tie into the hot air return for my HVAC system. The air intake for this plenum chamber is located in the brick facade, several inches above the doors of the insert. When I've had a fire going for a sufficient period of time, depending on how cold it is, the heat in the plenum chamber creates enough of a draw to slowly draw in air and "force" it through the balance of the duct work of the HVAC system. If I manually turn on the blower motor for the propane fueled heater, it takes about 8-10 minutes to normalize temperatures throughout the home. During those times that I keep a good fire going, I use absolutely no propane or electricity, while keeping our house much warmer than we normally would. It is nowhere near as clean or efficient as a rocket mass heater, but it feels great, just the same!
While that is not exactly what you are asking about, it's pretty close. I think the problem with pulling air through the mass of any RHM, and using it to supplement the heat from a conventional forced air system, is that the inherent flaws of a forced air system will quickly cool the mass of the RMH. I hope your experimentation proves me incorrect.
I have the same concern that you have voiced, which is why I’m reaching out before I build it. I have the exact same fireplace setup that you’ve described, and it, along with a type of room heater that I’ve seen in the past are the inspiration for this build. The heaters I’m talking about were popular in areas where folks got reduced electric rates during off peak hours. The heater used electric resistance style elements just lime an oven to heat up a huge mass of masonry blocks with gaps between them during off peak hours, and simply blew air over the mass through the gaps to heat the space during peak hours. They worked great and would hold heat even with air blowing through them for hours and hours.
The only way to gain confidence is to build one. I agree that the heat exchange efficiency will be key. I’m lucky in that I have a very well built chimney that pulls a draft even when it’s cold, so I plan to push the envelope as far as I can with my heat extraction. I’ll probably leave myself some wiggle room by building in a bypass so I can eliminate some flue footage if I have to. What you say about many small pipes is true. The larger the ducting is, the less it will warm the air. The trick is not causing to much drag in the airflow. I had planned to use 4” round and put in a sufficient number of them so I had about 1.5 times the cfm that’s being delivered through the warm air ducts.
However- after giving some thought to what Jason said above about simply using the furnace blower to move air around the house, I may very well just build a large mass and enclose it in well insulated walls with 2 or 3’ all the way around to create a “room”- Then I’ll pipe the house cold return air to that room and draw from it to the furnace plenum. So, all the air in the house will move slowly through that room. My aim is to create a very even steady temperature in the house with constant air movement, just like the new gasification wood furnaces do. They move WAAAY less air per hour but it’s a constant heating process rather than the on/off cycling that most warm air heating systems are infamous for.