[Stoves] CARITAS stove model TJ4B

[Crispin Pemberton-Pigott]

>From Frank:

 

“It always seems scary to me to have a small coal burning stove inside a home due to possible CO poisoning. Seems all one needs is a cold stack that reduces air flow, wet coal or the top covers are not closed tightly - and problems. So is there a lot of deaths during the night with people burning coal?”

 

>From Nari

 

“In India during winter months there were quite a number of such deaths. This was mostly true in 1950s and 60s when coal stoves were common.”

 

Of course all chimney stoves have dangers. Cecil Cook was almost killed by a wood stove in Ohio in the 70’s. I have had a poorly installed chimney on the windward side of my office in Swaziland reverse flow in the night (when I wasn’t there) running the CO up to >500ppm in the room (I had a CO logger running at the time to see what happened at night).

 

Chimney reversals are the greatest danger. This has been known for centuries which is where there are regulations about the chimney height relative to the roof (etc).  If a stove has a chimney and it is made from brick, it is far less likely to reverse as the brickwork retains heat and keeps the draft pressure negative.

 

It is not just coal fires that make CO of course. There is no difference between a dying charcoal fire and a dying coke fire, other than it is more likely a coal stove is hot with retained heat than a wood stove. The CO/CO2 ratio, as I said, hovers between 12-14% in such cases.

 

The best performance I have seen with respect to this is the TLUD stoves sponsored in Ulaanbaatar which have ceramic liners and in some cases, cast iron shells. They tend to be able to control the excess air very well and at the required 90% mass burn point, have far lower CO than the baseline or light body stoves. I could post examples of the CO levels being below 2% (safe level, usually) far past a 90% burnout. We had stoves that reached 97% of fuel mass burned while still being below 2% CO/CO2. 

 

Thus one cannot generalise about fuels or stoves. If a stove is used unattended in a room with sleeping people, the chimney has to be correctly installed. There are ways to prevent very late fire accidents with CO. One is to make sure all air entering the secondary section runs through a vertical conduit that is heated by the remaining fire. For the TJ3B this would involved making a shell around the stove body at that point (the height of the air holes) and running a supply tube downwards, perhaps formed out of a U-shaped metal strip. This causes the air to enter the chamber under positive pressure. If there is any heat in the chamber, it warms the air and ‘does the best it can’ to push air into the chamber instead of letting it leak out.

 

In Mongolia we tried to imagine various forms of failure and leakage of CO into the room. The traditional stoves are quite leaky and that means at every joint. What we found when measuring draft and efficiency is that all of them even in the poorest looking designs, drafted the warm air out of the room rather than drafting CO into the room. The efficiency dropped to as low as -250%. We heard tales of people being poisoned by traditional stoves under certain circumstances – the public is very aware of the issue. But we didn’t observe anything lose to an incident if the chimney top was above the peak and there was heat in the chamber.

 

In South Africa the number of people killed by CO from unvented coke stoves – a burning bucket of coked coal – burning indoors is high. I believe Prof Lloyd in the 1990’s measured indoor CO concentrations with people living in 1600 ppm CO which was hitherto considered impossible.

 

Cautiously yours,

Crispin

 

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