Several days ago, I found myself without heat and hot water at 6 p.m., just as I was washing dinner dishes. My propane-fired modulating combi boiler, which does double duty, heating the house and my hot water, decided to stop working. It’s still chilly in central New Hampshire, where I live. The fan that sucks in combustion air and blows out exhaust gases stopped spinning. The sensors and internal computer lock out the boiler burner when this happens.