Well first off, I've got to nail you on your terminology (it's for your own good).
The terms "furnace" & "blowing off" are incompatible ... so I'm going to assume that you have a BOILER, and not a furnace (I was guilty of this too my whole life - it's a MA thing I guess
). A furnace blows hot air ... no safety devices on the air. A boiler heats water - hot water can "blow off" - plenty of safety devices for the water.
I've got a Harman Mark III with 2 stainless coils I bought from a ripoff artist in NY. I don't recommend buying anything from those crooks in Schenectady, NY.
Had to get that in - I never forget!
The coils are tied into my oil boiler, so once the stove is running full-tilt, it will heat the boiler water enough that I can leave one zone's T-stat on 70° 24/7, and heat hot water on top of it. When it's 50° out, even my drafty dump of a home will get unbearably hot.
Stoves just burn basically the same amount of coal (and as such, give off the same amount of heat) whether it's -40° out ... or 65° out. Stokers are better able to gear way down in output in warmer temps by altering settings. Hand fires basically, with a very narrow range of adjustment, give off a bunch of heat regardless of outside temp.- for that reason, I wait until nighttime temps are in the 30's, & daytime temps are in the 50's before lighting up. IF I had anywhere near normal insulation in this place, I'd roast out of the house lighting up even then. It all depends on your house, and your stove's characteristics. If you have a hand fired boiler, you'll have to constantly monitor your water temps until you get an idea of what the unit wants for a given load, and outside temperature. It's an art. With practice, it'll be as second nature as riding a bike to you ... but remember, that won't be the case for the "officials".