Tankless Water Heater used as radiant heat

jp_2

i do not think you will get any replies, very few people are kean on 'open' systems, no 'regulars' here will agree with such a system.

go to a closed system, figure a heatloss or heatload for your project and you will get some help. be more specific, there are numerous ways to do underfloor heat, all have different needs.

Garritt

Tankless Water Heater used as radiant heat

I've got a unique situation where I'm trying to help out a friend of mine get some radiant heat (two 7' slant fin radiators and 120 sf of joist heat) as well get a new water heater. He is thinking about buying the model with about 4.5 gpm of constant hot water. I understand that he must order the model with the temperature controller goes above 120 for the radiant demand. My question is... should I be using a 3 way zone valve on my water heater hot outlet (for my radiant takeoff) and after my T install a 3 way mixing valve on my domestic side (to adjust the domestic water down to 120). Also, should I be putting in a spirovent and/or expansion tank since it's a open system? I am thinking of zoning with zone valves and having a circulator pulling through the 3 way zone valve. If I zone with zone valves do I need to install a pressure differential bypass? I know that I will have use components for an open system. On my radiant heat tap (on the return) should I install a check valve (swing or weight) before it goes into the Water heater cold tap? And how would I wire a controller, possibly a Taco SR to the tankless water heater. I would have to give the dhw priority, obviously.

Thanks.

Unknown

a system that small will short cycle any on-demand unit like crazy anyway, nevermind an open system being a bad idea in general.

Once you know what water temperature you need for that baseboard, options will be clearer.. if you can run it cool enough, you could do the heating with a cheapo tank water heater, and keep the on-demand for what it's good for, DHW production.