Hello all, New to this forum. Wished I had found this several years ago when I was doing my first radiant floor systems. I've always done closed systems, but I have occasion to re-look at an open system that does both heat and DHW off a electric hot water heater. I know this topic causes lots of arguments, and I've read lots on both sides, but I'm putting this question out here because my specific situation warrants revisting this question. This new house HVAC is standard heat pump, but what with ducting in the ceiling and a tile floor in the slab on grade living room, we decided to put in radiant floor heat in the living room just to warm up the tile floor--not as a primary heat source. Radiant system thermostat is in the slab, not on the wall. Since the heat load for this small two-loop system is small and since the manifold is next to the hot water tank I'm thinking to join them together and save the cost/hassle of a dedicated small boiler or a heat exchanger and second pump. Using a check valve, the cold supply would be forced thru the floor loops which eliminates stagnant water. I know you've heard all this before, but I'm just not convinced that this would be wrong....but, of course I don't want Legionairre's disease for my penny pinching. My thoughts go like this: It's no different than running 300 ft of extra supply Pex to the house like a plumber would. Yes, this supply water sits heated up, not a cold 5' down in a buried ditch, but it sits heated up in the tank, too. And the water is constantly moved around and supply added and deducted...how is that bad iin constrast to water sitting for long periods of time in pipes off to the spare bathroom at the end of the house? I don't mean stir up trouble here. I guess I am asking if you all feel this issue has been put to rest or is it an on-going debate. thanks for your thoughts. ~martin

I doubt your loads are low enough to be covered by an electric water heater.

You will need to double the component cost to account for SS pumps etc.

After you service/replace the fouled components of your hydronic system you will not have saved anything.

Radiant "thermostats" placed in the slab, are almost never appropriate as the sole heating control and rarely as an adjunct.

It is the volume and perfect growing temperature of--typically non-chlorinated-- water that gets the Legionella crowd excited.

We commonly use storage water heaters hooked directly to fan coils but these are potable water certified, close-coupled and run at the top temperature of the water heater--typically 130°F and up.

In your case we would use a storage water heater with a SS plate heat exchanger between the expensive hydronic components and the potable water.
These systems will last 20 years with no risk of cross-contamination and all the benefits of combi space and hot water heating, including perfect heat transfer if you properly clean and treat the system with Fernox products or equivalent. Outdoor reset to keep the floors warm more of the time and burn less fuel. And finally, you will comply with all known building codes.

You will not find any arguments here, since the only people who promote the "open" hydronic systems are the people who sell them and I haven't seen them around for awhile.

Since he's claiming it's purpose is "...just to warm up the tile floor--not as a primary heat source." it doesn't matter what the total heat load is- that's what the heat pump is for.

But a low voltage mesh system would be about the same efficiency as a hydronic solution running off an electric hot water heater at a fraction of the upfront cost, and none of the contamination/code issues. (Not that I'm all that keen on resistance heating of any type.)

Agreed. But he might want to check the cost on the heating blanket...I mean low voltage before jumping in.

Hello, I've already placed PEX and poured the slab. I was going to go with a small electric boiler, but then got to thinking about doing the open system that I mentioned. The house will have conventional heat pump system as the primary HVAC, but since it's slab on grade and tile in a high ceiling great room, we want warm floors in the winter. Regarding the slab thermostat: I wouldn't do that for a regular radiant system, but since this is just for warm floor, I didn't want the regular thermostat/heat pump to be wrestling with the radiant floor and thought that having the actual slab temp being monitored would be the best way to go since the go is just warm floor. Perhaps with just warm floor in mind, going hydronic seems excessive, but I'm only say 600 bucks in and I've had resistance wire in a slab fail on me and that's a nightmare. I appreciate your help and I guess you convinced me that the open system is not a good way to go. Would you go small heat exchanger or dedicated small boiler such as a seisco or Hydro-shark?

We think, Thermolec or Electro and sorry I missed the floor warming. You can do that.

even for floor warming it's best if the thermostat has an air sensor. then it can automatically react to space heat needs to some degree at least.

If you already have a heat source I don't think electric mats save any money. usually per square foot they are significantly more expensive than hydronics. it's the heat source that usually makes the difference IIRC.

Thanks, Rob, How does it work if you have both an in-slab sensor and the trad wall unit? Do more complicated control units figure out proportional needs or anticipate? And how would all this jive with a trad wall thermostat for the heat pump? I was thinking to just let the heat pump do it's thing (and be the primary heat source) and let the floor slab thermostat moniter the slab temp. I know the air temp and the slab temp are interrelated, but I am trying to keep this simple. Thoughts? ~martin

it's a thermostat that can take a floor sensor. it will run on air temp as much as it can, and you set a minimum floor temp, and off you go.