When it comes to modulating electric micro boilers like Seisco for radiant floor heat in a cold climate (zone 6a) with a well insulated garage and workshop space (R22 sub-slab, R46 above grade walls, R66 roof), is it better to choose a unit that just meets estimated Btu needs or go with a unit that will provide more Btu’s than estimated? (The units I’m considering the Seisco 09 or 11, both cost the same, so there isn’t a direct financial drawback to going with a larger unit outside of a larger gauge wire and larger amp circuit breaker.) The units self modulate so most of the time the kW use should be much lower than maximum rating.

Is there a drawback to going with the larger unit?

I’m also trying to see what folks think about thermostats – air vs. sensor (in slab). My initial thought is to go with the sensor (in-slab type) so that every time the garage door opens and cold air blows in, it doesn’t make the micro boiler cycle (as I assume it would with an air based thermostat). I’m assuming once the radiant slab is up to temp, it will buffer those instances where the garage door is open for a bit.