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Radiant mixing valve < .5 GPM?

I have 3 water temps (estimated @ 132, 91 and 74) and approximately 6 zones I'm looking to set up. The system is a basement in floor slab with about 7 zones of 1/2" pex anywhere from 250-300 per loop. A garage with a similar setup but 6 zones and a staple up (w/transfer plates - "Upstairs" on chart) covering the first floor with various floor types, tile, laminate and carpet (I know not ideal).

Currently I plan to have 3 temps. The temps are estimates I got from building my system using the Uponor software. The highest temp will be on its own branch manifold (BM) with no mixing valve runing off a mod con boiler with ODR. The 91 will be on its own BM with a t-stat mix valve and a circ on the mix side to pull through the valve. Same situation for the lowest temp. I'll be doing a heat exchangers for the garage for freeze protection but this isn't the reason for the post.

My concern as circled in red below is if only my T-stat 2 calls for heat on my 91 degree BM that my flow (.47 gmp) at design condition will be just below what t-stat mix valves seem to rated for. With ODR on the boiler reducing water temp as outdoor temp rises I assume my flow will stay somewhat consistent, but this isn't my main worry right now.

My larger question / concern is that most t-stat mix valves recommend a min flow of .5 GPM. Some like caleffi and Taco rate down to 1 gpm. Talking with some of the folks at their tech support it sounds like the solution is a motorized mix valve that doesn't care about min flow. Curious if this is the most cost effective solution? The rough mix valve pricing ~$115 ish give or take. Moving to motorized approached 3x and adding in ODR is 4x. Not to interesting in going this direction if I can avoid for two reasons, price and complexity. Everything breaks eventually and I'd much rather have a dumb valve with not electronics to fail if I can. Additionally the guys at Taco and Caleffi know there stuff and have been super helpful in their support or online presence to help DIY'ers. I'd prefer to stick with their brands if possible however in this case they don't seem to offer a "dumb" valve that gets down to .5 gpm like others.

Thanks for the input and perspective.


  • ZmanZman Member Posts: 6,002
    There is no reason to go that low on the flow rates. There is no downside to .5 GPM per loop.
    As for the mixing. why are you going with thermostatic valves? That will effectively disable ODR for those those zones. If you want to keep it super simple and low tech, use non-thermostatic valves and those zones will track the ODR of the boiler.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • schultzey11schultzey11 Member Posts: 2
    I clearly used the wrong terminology. Thanks for clarifying. I should have been saying just non-tstat valves. Got it.

    I'm not certain I follow on the flow rate though. Right now my .47 gpm is calculated. I will be using a Taco VT2218 circulator (for this circuit at least) likely set up for deta T. I'm specing my parts list of items to order so I don't have anything setup and working yet.

    My concern is if that one zone (.47 gpm) is calling the flow is low compared to the min flow rating on the non-tstat mix valves. Are you saying, who cares? In other words the valve might not perform in the "ideal" range but it will still give me a mix temperature that will heat the room? I may be at a set point above or below due to lack of the min flow rate but it will still work?
  • ZmanZman Member Posts: 6,002
    What I am saying is that the flow rate the program is calculating is based on a design delta T in the input side of the program. if you increase the flow, there is no negative effect and the mixing valve will be within it's design flow. Higher flow will mean a tighter delta t. This may mean that the design SWT will be a couple degrees high and can be reduced when balancing.

    Keep in mind that these programs make estimations based on a bunch of estimated assumptions, heat loss and ACH being the big ones.
    You are absolutely doing the right thing by designing with software and you will have a very comfortable system as a result.
    Don't expect the system to run at the exact numbers being spit out by the program. It NEVER happens.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
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