Can i use a water heater as my source for the in floor Pex tubing system?

Question

I installed Pex tubing in concrete in 2 areas, both garage floors. I have been told 2 things.... one is that it will require a boiler, two, that a hot water tank will do the job. Cost is a huge factor here when compared. I am willing to try the hot water tank as it is a fraction of the cost and can use it on another property if not successful. Any thoughts on this would be appreciated.

Answer 1

It's all about heat load.

A very well insulated building may have a heat load so low that most boilers would be operating very inefficently to meet the load - sitting idle most of the time, even on "design days" when the heat load is at the greatest due to the outside temperature being the lowest temperature designed for (thus, "design day.")

As a practical example, I have a radiant floor system installed in a building with a design-day load of 30-36,000 BTU/hr. A typical "small" boiler is 100,000 BTU/hr, so it would be in standby 2/3 of the time even on that coldest day (which is typically for no more than a week, and not every year.) A water heater can be more closely matched to the load, simply because they are available in smaller sizes.

As for tank .vs. no tank, it's largely irrelevant - there are inefficient and efficient versions of both (and much of the hype for tankless heaters being "more efficient" due to no standby losses falls down in houses where hot water is actually used, at modern insulation levels which lower standby losses; especially considering greater complexity and maintenance expense that is often associated with tankless.) A common example (and commonly used for radiant heat) of an efficient tank type is the polaris, at something like 95% efficient and a condensing design.

If you have a larger heat load, an efficient condensing boiler might be a better choice - that has to to with the whole building construction, not the heat delivery method.

Answer 2

You could use either an on-demand water heater or a standard one if you want to keep 20-40 gallons of water hot on hand. The big thing is they will not run as hot as a boiler. However, running pex tubing in concrete doesn't require the water to be as hot as perimeter or radiator hot water heat as long as you leave it on long enough to heat the concrete up to operating temperature which will take longer.

You will also need a circulating pump, air separator, and an expansion tank. There are several companies that make a quick install board with all of these items, guages and valving included but they are not cheap.

Did you insulate under the garage floor? If not you will be losing a lot of heat into the ground for quite a while until you warm the soil up and it will be more inefficient in the long run.

If you want to DIY this project get yourself a book called "Modern Hydronic Heating". It is the textbook to have on this subject.

Good luck with your project!