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Mixing Radiant Floor and Rads

Al_H Member Posts: 2
House is 2400 sq ft with fin style radiators built into the walls. 100000 BTU 84% eff gas boiler runs at 180 F, heating 2 zones - kitchen and the remainder of the house with zone controller.

I am adding a basement - excavating the crawl space and would like to put radiant heat in the floor, but know that 180F is too hot.

Options I have considered are to use a mixing valve, but there is no guarantee I could get the right temp of approx 120F needed for the floor heat. Similarly if only the basement is calling for heat (as the main floor has a wood stove) the mixing valve isn't much good.

2nd option is to run a circ loop off of the separate hot water heater. The water temp might still be somewhat high at 140F, but I would run the piping so that the hottest water would be on the perimeter of the room, and/or use a mixing valve as well. Not as efficient but since the area is only about 600 sq ft, I think I can live with the lower efficiency.

Would appreciate any help on this, or pointing to an existing thread.

I installed the boiler myself 12 yrs ago with help from a forum like this, and it has run pretty well ever since.


  • Gordy
    Gordy Member Posts: 9,546
    Some things to consider

    One is out door reset for the boiler. You probably don't need 180* water to the radiators everyday of the heating season. IF your rads are oversized you may not even need 180 on the coldest days.

    Your radiant will need a mixing valve it's done that way all the time. Taco I series comes to mind.

    It would not hurt to do a room by room heat loss calculation. See if your rads can match the calculated loads with lower water temps.
  • Ironman
    Ironman Member Posts: 7,354
    Radiant floor

    Is the floor gonna be a slab or wood framed? Either way, you need a second loop of the boiler with its own circ and temp control. Allowing the boiler return water temp to go below 130* can also result in flue gas condensation. Your control strategy must also address this.

    If the floor is a slab, then variable speed injection mixing is the proper way to control the slab temp while protecting the boiler. Look up Tekmar's essay on this from their site as well as info on their 356 control. It's something you're probably gonna need a real hydronics pro to do or guide you through.

    Don't even consider running the floor off your water heater! You'll create a Legionella breeding machine. There's a long thread on the "main wall" right now, as well as many others, dealing with this. It's titled "How Prevalent is This".
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Snowmelt
    Snowmelt Member Posts: 1,405
    Mixing valves

    Mixing valve will work in theory all the time, think about it. What I would do if I was you, after you run your plumbing but be careful you need a ball valve with a boiler drain to bleed the system. Also need a bypass valve so you don't let low temp. Water back in boiler creating flew gases to condensate.

    Mixing valve - when filling the zone up use below 120 degree water in other words have boiler not running, so now with the zone filled with water and all air is out of the pex tubing and you start the boiler the mixing valve will go up to 120 or what ever temp your running the water at. The return of the loop will go into the cold side of the mixing valve, that should be at least 10 degrees cooler then the water going into the mixing valve. So the loop water will never go above the 120 that it's set for.

    By-pass valve you will have to get a gate valve and do more research but in theory have to have supply water by-pass the zones and come back to the return with high temperature water so the return temp is high enough not to make the flue gases condensate.
  • Al_H
    Al_H Member Posts: 2

    Great info guys - thx much!!