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Best way to create small differences in water temperature?

My radiant design calls for 115 degree water on the upper, very well insulated level, but 125-135 on the main living level, maybe less if we supplement with our mini-split cassettes on the coldest days. Additionally, most of the home will be heated from under the subfloor, but I may heat directly under the tile for the upstairs baths. I know that I can use a mixing valve to have both temperatures on the different levels, but is there a way to get two temps on the same zone? Specifically:

1. Can I use a mixing valve coming off one circuit of the same manifold? I've not seen a diagram where this is tried. The alternate solution is to pull down a little more ceiling and heat the bathrooms from under the subfloor too.
2. I intend to use an outdoor reset system with my Navien NCB-240, and already have that installed. From what I’ve read, I’ll need some sort of intelligent mixing valve (Taco iSeries?), but am not sure if there is a way to enjoy the savings of modulating the boiler temps, but still have the mixing valve adjust water in different zones? Is there a way to program a valve to get a temperature, say, 20 degrees cooler than the other zone?
3. So this seems like a hack idea, but… if two zones could be expected to mostly run at the same time (as they should with an outdoor reset setup, could the cooler zone be plumbed in series with a warmer zone? My system was designed with a 20 degree delta T, so if I ran a “cool” zone off the return water from a warmer zone, could this in theory work? Mostly musing here, since I plan to go with a proven solution for the final design.


  • pecmsg
    pecmsg Member Posts: 4,842
    W H O
    is saying 135°F supply for radiant?

    110°F max, 120°F for special applications!
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    Depends on the type of radiant @pecmsg.

    Try running it all at the same temperature. Or design the higher temp system for a lower temp. 
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • hullabaloo
    hullabaloo Member Posts: 9
    I’d hoped to run cooler temps. When I plugged everything into LoopCAD however, it said I’d need 135 to avoid supplemental heat on that level at my design temp of -6. But I have a gas fireplace (currently old and drafty) plus a couple mini-splits on that level (not great at -6F) so I don’t necessarily need to go that hot. The system is staple-up light gauge 4’ heat plates under the subfloor. I hadn’t heard of an absolute ceiling for temps and just planned on easing into it gradually, ideally staying at 120.
  • pecmsg
    pecmsg Member Posts: 4,842

    Depends on the type of radiant @pecmsg.

    Try running it all at the same temperature. Or design the higher temp system for a lower temp. 


    If floor thats too HOT
  • motoguy128
    motoguy128 Member Posts: 393
    Could run the secondary in series off the primary.  Hottest zone gets water first and the 135 setpoint.  The 125-120f then feeds second zone.  Water returns are 100-105.  
  • Zman
    Zman Member Posts: 7,569
    edited November 2020
    With a new construction, why not design it the run at the same temp? Tighten up the tubing in the high temp zone. Maybe a little less tubing focused on the windows and walking area on the low temp zone.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hullabaloo
    hullabaloo Member Posts: 9

    Not new construction, just a new heating system. The house is a '67, heated up to now with fin-tube baseboard. I've got two rows of 5" transfer plates in each 16" joist bay, can't really do a third. I will focus on energy loss on that level, and supplement heat if needed. We did a insulation refit on the upper level a couple summers ago were we replaced insulation and added rigid insulation board underneath new siding. The main level is brick however, so fewer options to improve insulation, though I still have some air sealing to do.
  • hot_rod
    hot_rod Member Posts: 22,118
    I would try running them at the same SWT.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • hullabaloo
    hullabaloo Member Posts: 9
    Thanks Guys. I decided to just run everything on the cooler water temperature and see how it goes. I bought a manifold with an extra zone that can be enlisted for a possible radiant wall on the cooler zone if needed. I also have a few insulation ideas and Will eventually pull out my gas fireplace logs and do a more efficient insert. I don’t think it will be a problem. I should know in a couple of weeks :-)