Frank B. wrote: any problems burning coal with a clay/terracotta flue?
I can answer that, No trouble at all. Anthracite coal chimney stays just as clean as if you burning gas or oil. There's 0 build up on the chimney walls except for fine layer of fly ash that doesn't
accumulate. Check it once a year for obstructions as you should any chimney, besides that the flue is pretty much maintenance free. This info is for anthracite only, soft coal may be different, I wouldn't know.
The only difference is that you will get a fly-ash build up in the bottom of the chimney and the exhaust pipe coming off the furnace/stove. There should be a clean out area below where you insert the exhaust pipe into the chimney to accommodate for this. Clean it out occasionally, same with the exhaust pipes. We do it once a year but have a 20 inch exhaust pipe and flue entrance. If need be I could allow it to go for 2 or 3 years, but would never consider that.
How much you need to clean this out depends. If you new to burning coal check it every few months until you know how often you need to do this. It is very important that you do this. Nearly every accidental death due to carbon monoxide poisoning is the end result of poor or non-existent maintenance.
Properly maintained, a coal heated unit is just as safe as anything else, I would even argue safer in some cases such as compared to gas or wood. You don't have to worry about gas leaks as far as gas is concerned or chimney fires for wood.
In the end proper maintenance and care should be taken no matter what your using to heat your house.