That is a thorny problem and it's all about BTU. When I compared wood stoves, I found it hard to believe all the numbers. Some mfg's are pretty honest, but some quote max BTU that is unattainable unless you babysit the stove and keep it burning to it's max. potential. With masonry heaters, it's a similar story. The wood has a maximum BTU and you are supposed to just multiply by efficiency HOWEVER, it does work in practical terms, just like wood stoves. Are you building one fire per day? Two? Four? That's why BTU numbers are difficult to pinpoint. Add to that, some people are perfectly happy in a home that is 65 but just like you, I am not comfortable in winter until it's mid 70's.
Mere mortals can't afford to invest in a masonry heater and find out it doesn't generate enough heat. As you say, thermal mass means zip if there is not enough heat. I bought my Equinox and it is supposed to be too big for my needs but we found it to be just perfect, especially this winter. BTU heat loss calculations are based on norms and are useless when it's way colder than norm for 2 months in a row. When I spoke with one of the top masonry heater builders in North America, he said I should count on roughly 56,000 BTU. I knew that was insufficient and got a wood stove that is over 100K instead.
They are great ideas but the owner must do all the homework before investing and quite honestly, most people will need a heat loss specialist to do that for them. When I was working on the final designs for my house, the architect hired an engineer to do my numbers and he was dead wrong. So wrong, I refused to pay his bill. Sounds like I'm an arrogant SOB but after living in the house for 30 yrs, I can say in all honesty, the engineer was still dead wrong and I'm glad I didn't listen.
Moral is - masonry heaters are not something you can swap out if you are wrong. It seems most members here on the forum have owned at least 2 stoves to find what suits them the best.
