I am a firm believer in monitoring internal exhaust gas temp rather than external surface temps.
A readily available candy thermometer inserted in the exhaust stack a few feet above the mass is all it takes.
To my knowledge this information is not in the builders guide, but rather was innovated by curiosity (as all good ideas are).
Took some readings this morning at 10 am. RMH in the greenhouse / studio has not been fed since 8 pm last night.
Room temp is a balmy 50 F (non insulated building) external temp is 34 F
Thanks to my Tom & Gerry Dragon breath monitor (also known as a candy thermometer) We can see that after over 13 hrs with no fire, my internal gas temp is still over 110 F
While only the startup kindling is roaring (apx 2 minutes in,gota love a warm mass) the gas temp is already past 150 F external pipe temp is 68 F...
15 minutes into startup, gas temp is over 210 F external pipe temp is 76 F.
First full load of wood is in.
In comparison, the brick bell RMH in my shop (also uninsulated) plummets in gas temp over night, to brick levels.
Brick temps ave 40 F in the AM and external pipe temps match.
The brick bell however takes time store its heat. I expect 20 minutes plus before internal gas temp rises over 130 F
This can take even longer if there has been no fire for several days.
If you have the room, a solid mass hold's your heat longer, but a beautiful brick bell can be constructed to fit the area you have.
Hi Thomas, Ever since I learned about the boundary layer as described by Peter here: Boundary layer and by Satamax here: Boundary layer2, it has helped me to visualize whats going on inside the exhaust chimney and why reading the surface is an inaccurate reading of exhaust temps. The T&G thermometer should be a part of every RMH setup as it is cheap, easy to install and is fairly accurate. I use to use a digital battery operated one but it became a drag having to turn it on and off all the time and having to replace a battery every so often. The analog ones are much simpler, just as responsive and always ready to go. Makes a great Christmas stocking stuffer for that hard to buy for person too!