Troy Hatton

@ Matt Todd

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.  

@Jason Broom

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.

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?