I have a forced air heating system that works well, and I'm not looking to replace it. However, I'd like to get the "warm floors" effect of radiant floor heating. I've heard that with a lot less BTU/hr, you can still do "floor warming" by using a lower hydronic temperature. Since any energy put into the floor to warm it needs to go somewhere, I would expect that this will supplement my forced air heating.
Given that the desire is just to warm the floor, rather than heat the house, I don't think that figuring out how many BTU/hr I would need to supply to keep the floor toasty involves any sort of heat loss calculation for the house. Any heat lost is already replaced by the forced air system. As a result, how do I determine how much I need to just keep the floor warm?
Note: I am aware of the labor involved in installing hydronic radiant floor pipes, but I've just gutted my basement for a remodel and the first floor subfloor is accessible for a time. If I'm ever going to want to do floor warming or potentially future radiant heating, the time to install the pipes would be now. Eventually when we replace our second floor's finished floor, I'll probably rip up the subfloor and put in hydronic structural panels at that time too. For the purposes of this question, assume I already have unused radiant floor heating pipes under all of the floors.