Need advice sizing a hydronic radiant floor on-demand heater

wjg108

We have a 720 sf house that was built with a radiant floor system on slab on grade. We decide when we installed it to go with a hybrid electric tank water heater. I would say this was a failed experiment. Although the water heater did ok heating the floor, at the end-of-the-day I would have to say it is not the right heat source for this application.

So, we would like to switch to a propane on-demand water heater. I cannot afford to spend a lot on this right now, but I will buy whatever is necessary. Given the size floor (720sf), and a climate where it rarely goes below 30F, and the average low is around 35F, and we have a mini-split for backup heat on the rare occasions it would get down to 20 or so.

What is the minimum size on-demand propane water heater we could use?

hot_rod

well a tankless is not the right appliance for the job, regardless of the fuel source. Size-wise, even the smallest tankless is 8 times the btu you probably need. I’d guess you load on the coldest day is about 10,000 btu/ hr?

Do you have LP in the home? If not the cost of adding a tank, piping, venting etc?. Is it worth it for such a tiny load.

A small electric boiler is an option.
A small inexpensive tank type water heater, electric or lp is another option.

Hot_water_fan

Just use the minisplit. Don’t throw good money after bad!

EdTheHeaterMan

Tankless water heaters are designed to make potable water hot enough to use for a shower. That would be making 50°F water into 120°F water. That is a temperature difference (∆T) of about 70°F rise. A space heating boiler is used for making water about 10°F to 30°F ∆T. and most radiant heat systems are closer to the 10° ∆T. That may not be a large enough difference to make the water heater gas valve to do the job you need it to do.

On a small job like yours, I would recommend that you use a tank type propane water heater. you can get then very inexpensive that vent thru a roof with B Vent. You can also get side wall vent ones for a little more $$$ that need to be plugged into the wall. They can operate at 120° on a thermostat. All depends on how much you want to spend.

The correct heater for the job would be to use a boiler that operates on propane or electric that is designed for space heating, but they are more expensive than a tank type water heater. Since your project is only 750 SqFt, I believe you can get away with a Tank style in this case. If the project was larger and required a 50,000 BTUh boiler or larger, I would not use a water heater at all.

Ironman

An instantaneous (tankless) water heater is definitely not the right tool for the job. They’re not designed, controlled, or approved for space heating. And as Bob said, the smallest on would be 8 times too large.

In this scenario, a gas tank water heater would be the cheapest and most reasonable choice. But caution: only connect the radiant floor to it! Do NOT try and make it dual purpose by connecting your domestic to it. You’d be creating a Legionella breeding machine.