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Converting baseboard to underfloor... Mixing valve location?

Is ok to have a mixing valve located downstream of circulator pump?

(I just found this site, my first post... I'm about to install under floor radiant, converting from baseboard heat. I think I have basic design/layout figured out, but I'm sure I'll have some questions.) I'll post some pics and drawings too.



  • BultarBultar Member Posts: 8

    Here's a shot of the existing setup... Quietside QVM9 boiler (installed about 2 yrs ago, to replace an old oil boiler that gave up, tied into old baseboard radiators), plan was always to switch to underfloor radiant, but the boiler died unexpectedly (of course), so went ahead and had this installed for me, with intent that I would later do the radiant install.
  • BultarBultar Member Posts: 8
    rough plan

    quick drawing of planned layout, and source of my question on mixing valve placement...

    it would be easier for me to do it this way, instead of moving the pump so the mixing valve comes first in the system...
  • GordanGordan Member Posts: 891

    Not to be snippy, but it's not particularly respectful to the professionals at this site to use them as your primary means of answering questions that a little research would have answered. Doesn't the Quietside manual have any diagrams in it for a mixing valve circuit?

    In short, no, it is by no means OK to pipe a three-way mixing valve that way; your arrows show how you want water to move but you don't provide any means to get it to move that way.
  • BultarBultar Member Posts: 8
    sorry to bother you...

    Well Gordan, I thought I was being really nice, and only posted that question after reading all the manuals for that boiler and other equipment I already have installed, and spending days researching online, and watching dozens of videos... and I've seen plenty of diagrams of mixing valve circuits...

    Maybe my sketch didn't make sense, but it seems like it would work, depending on the inner-workings of the mixing valve, and it would be easier given the setup I have to start with.

    I certainly wasn't trying to be disrespectful, but this isn't what I expected from a "help" site. I guess I'll delete my posts and take my discussions somewhere else, good day.
  • SteveSteve Member Posts: 486

    If a thermostatic mixing valve was used why would it not work?
  • GordanGordan Member Posts: 891
    edited December 2010
    It's not that the mixing valve wouldn't work.

    It's that there's nothing there to make water move in the mixed circuit. You always need a circulator on the load side of a mixing valve, as every circuit diagram with a mixing valve (or other means of injection) shows.

    The fault probably rests with me, and I shouldn't speak for others here - for that I apologize. It's not that this question is a wrong question to ask, but to me it just seems like a question that someone who's actually ready to make changes in their heating system (as opposed to merely learn/inform themselves from a layperson's perspective) should not have to ask. I'm a do-it-yourselfer myself and I would certainly not begrudge someone asking questions where the issues are muddy, but this is pretty basic and it would make me, personally, reconsider tinkering. It is possible that I'm being unduly stern here, in which case, again, apologies.
  • SteveSteve Member Posts: 486
    will the

    circulator not move the mixed water when the mixing valve allows.
  • GordanGordan Member Posts: 891
    What mixed water?

    In order to get mixed water, you need some way of forcing water to go from either/both of the inputs to the output of the valve (mixed circuit.) There is nothing in that piping scheme to induce a pressure differential between the "cooler" input and the output. The little arrow on the "bypass" that's pointing back at the mixing valve? That's wishful thinking.

    If you pipe that mixing valve as it is in the diagram, the only time it will allow any flow at all is when the supply water temperature is cooler than the setting on the mixing valve. In other words, when you don't need a mixing valve.

    Long story short: circulator goes on the output of the mixing valve. Just as every piping diagram out there shows it.
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