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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.

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Your goals are somewhat at odds. Even where radiant floor heating is used as the only source of heat, the floor typically does not get anywhere near "toasty." The floor temp is typically only 5 to 10 degrees above ambient (75 to 80 F), and the vast size of the radiator (i.e. the entire floor) provides enough energy to heat the entire space with such a small differential. So, the floor will feel better than an unheated floor, but since it is still colder than your body temp, it won't feel "toasty" by any means. So, if you piped enough energy into your floors to make them feel warm under foot, you'd likely overheat the house!

The only "toasty" floors I've experienced is a heated bathroom floor. In that case, there was electric coils under the tile (several hundred watts worth for a large bathroom), and indeed, the floor felt warm underfoot.

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  • A surface does not need to be above body temperature to feel warm. quora.com/… "Less flow than the normal low outwards flow feels warmer than normal, even if the object is colder than our body."
    – David Pfeffer
    Jan 7, 2015 at 20:00
  • I suspect, though I have no evidence at this point, that the BTU/hr to keep the floor at a temperature that a human would perceive as warm is still lower than the heat loss rate of the house. Thus, I'd just be lightening the load that the forced air heat would have to make up. However, I don't know how to calculate this to verify if I'm right or if you're right.
    – David Pfeffer
    Jan 7, 2015 at 20:03
  • Well, the article you linked to shows rather well, I think, that we are dealing with sensations that are subjective and relative. Thus, reasonable minds may differ on what constitutes "toasty." Even if you could calculate whether a human would perceive the floor as warm even though the floor was generating only a fraction of the heat loss, why bother? Research radiant floor heating. Most people who have such floors, and those who install it will tell you... the floors are not truly warm to the touch. Just not cold.
    – bobfandango
    Jan 7, 2015 at 20:12
  • Last thought.... I think you could accomplish what you want if you (a) don't run tubing under the entire floor as with a typical hydronic install, and (b) run higher temps so the floor is maybe 85 or 90. So, much like the bathroom install I mentioned, just certain areas would be toasty. e.g. the bathrooms, the kitchen areas perhaps... something like that. Since only a relatively small portion of the floor is radiating, albeit with a higher temp, you probably would not completely satisfy heat loss during winter, and the forced air furnace would make up the difference.
    – bobfandango
    Jan 7, 2015 at 20:14
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    I would take a look at this: bit.ly/142RW7u It shows you a design procedure for an entire floor. You are starting from a desired floor temp rather than a satisfying a worst case heat load, so it isn't completely on point. But, you could probably derive the equations you need to make an educated guess at what you'd need BTU wise. I will tell you now, however, it will probably be far less than even the smallest boiler will provide. You'll need a buffer tank and heat exchanger. Also bear in mind, wood is generally limited to 80 degrees, and no more than 90 for other floor materials.
    – bobfandango
    Jan 7, 2015 at 20:45

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