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Radiant Floor Heating System Help for Discouraged Individual

Hey all. Bit nervous to post because I installed a heating system without fully understanding everything. From reading posts here, there are some very smart people who know this stuff inside out and always do their homework. The more I learn the more I realize how much I don't know. I installed the system myself from scratch last year and had a very successful winter last year. Was feeling really good about it. Now I turned it on for this year and running into some issues.

The general diagram of my system can be found here: https://docs.google.com/drawings/d/1tvHL9YLxkWCqzTuODKeEZHZN1EGRBnqPAoYRpeenfZE/edit?usp=sharing

8 port manifold is fed by 3/4 pex to a tankless water heater. There are 4 circulator pumps, each one running 1 zone which is made up of 2 ports on the manifold. For example, kitchen and dining room are 1 zone, so have one pump, but each port is a 250 ft run of 1/2 pex. The pumps are UPS15-58FC, 3-Speed Circulator Pump, 1/25 HP, 115 volt

Last winter it worked pretty well, averaged about 1.2 GPS and kept the house pretty warm. Right now, it averages .5 GPS and so the heater shuts off. If I bleed the system, it gets back up to 1.2 GPS, but after an hour or two, is back to .5 GPS.

The GPS was never great and now it is too low to keep the tankess water heater on... so not sure how I can improve that.

2nd issue, not related but need to keep in mind. I have two more zones to add to finish the whole house. So I was going to run 3/4 pex to a new manifold which will have two 1/2 pex runs per zone.

I am wondering if I should replace the smaller circulators with 1 big circulator? or can you have multiple circulators on a system (like a pump on the supply side and a pump on the return side).


  • Zman
    Zman Member Posts: 7,466
    Do your zones have check valves? Probably built into the 15-58?
    I am thinking that repiping it primary/secondary would be the best fix.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • icy78
    icy78 Member Posts: 404
    Buffer tank or hydraulic separator or like @Zman said.
    Then you always get your required boiler flow.
    Use constant pressure pump on heating loop side.
  • neilc
    neilc Member Posts: 2,221
    edited November 2020
    mraupie17 said:

    , , , a tankless water heater , , ,

    what changed from last year to this one ?
    did HWH foul up and it needs to be flushed ?
    known to beat dead horses
    rick in Alaska
  • Ironman
    Ironman Member Posts: 7,169
    The tankless water heater is your biggest problem. It's not designed, controlled or approved for space heater. It's heat exchanger has way too much resistance to flow for hydronic circulators to overcome. That's why your flow rates are so low.

    As mentioned, piping it p/s with a large circulator on the heater side may give reasonable performance, but no guarantee there.

    It will also have a very shortened life expectancy because it's designed to have 50* inlet water, not 100*+.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • EzzyT
    EzzyT Member Posts: 1,269
    Exactly what @Ironman has said.