New build heating system recommendations – GreenBuildingAdvisor

First, good for you for going with a modern level of insulation. A house built to that standard, especially if you can achieve the air sealing, will be comfortable, draft free, quiet, less dusty. Just all-around more pleasant.

To my comment about responsiveness, I’d expect a house in built to 21-century standards to have a design heating load around 15 BTU/sf. The heat output of a heated floor is entirely determined by the temperature difference between the floor surface and the room in the air, to meet the design load you’d need the surface temperature to be 7.5F above room temperature. Four inches of concrete weighs about 50 pounds per square foot. It has a specific heat of about 0.5, so it takes about 25 BTU per square foot to move the temperature by one degree F.

When the heat is completely off, the surface will be at room temperature. So to go from full off to full on that slab has to be warmed by 7.5F, which will take 187.5 BTU per square foot. But your design load is 15 BTU/sf — and presumably you sized your supply piping for design load — so it will take 12.5 hours of the system on full capacity to warm the slab. In the meantime, the house cools.

Something similar happens when you want to turn the heat off. The only way to keep the slab from releasing heat into the house is to allow it to cool off. There’s only one place for the heat that’s in it to go, that’s into the house. You’re going to see a similar delay when you want to turn the heat off. What happens then is the house warms up.

How realistic is it to need to go from full on to off in a short period? Here’s I post from a couple years ago: https://www.greenbuildingadvisor.com/question/hows-this-for-solar-gain . The point of the post was that on a day when the outside temperature was at design temperature, on the shortest day of the year, I was able to meet my entire heating load and then some from solar gain.

But on a day like that, when the sun goes down, all of a sudden you go from a heating load of zero to a heating load of full design load. And it happens pretty quickly, what I typically see is the air handlers turning on around 5:30 on a sunny winter day.