Thanks for taking a look. Please see the PDF drawings, and tell me if I have overlooked something important.
The existing design has two heating zones with excessively long 600' loops of pex. Not much heat is being transferred from the pex to the floor because it is hung between basement joists with those plastic Wirsbo pex-clips. The tubing sits in a 3" airspace covered with only R13 batting. I believe this style of radiant floor was meant as a warmer, and not intended to provide heating. The major flaw I see is the continuous loops which should be no more than 300' long for 1/2" pex. It is either exceeding the minimum return temp for the boiler, or else not putting enough heat into the zones.
My proposed changes would replace those plastic clips with aluminum Joist-Trak, and split the long loops into three 200' loops on manifolds. The tubing would be covered with R19 instead, filling the joist cavities. To save cost, I'd like to use the Mr. Pex manifolds with flowmeters and dials for balancing the flow by hand. After running the heatloss calculations with Uponor's ADS software, I've determined that the loops will call for a max of 97 degrees. So, my plan is to use one of the existing circulators as a primary pump, and feed two secondaries from it, all with 1" copper. The temperatue of each secondary loop would be controlled by a hand-set thermostatic mixing valve.
So, naturally, each thermostat will control its respective secondary loop circulator. But either one will also trigger the primary loop circulator and fire the boiler.
Existing Design (PDF)
Proposed Changes (PDF)
Apart from any mistakes or oversights I may have made, my questions are:
1) Is the location of primary loop circulator acceptable downstream from the secondary branches as shown?
2) Could this type of setup lead to short cycling the boiler?
3) To control the secondary loops as described, can I use a spare two-zone Taco control box? It would recieve the signals from the two thermostats, and switch on the secondary loop circulators. The relay output from that 2nd box (which is normally used to fire a boiler) would instead provide signals to the existing Taco box, controlling the primary pump and boiler. Seems like it would work, but am I missing something?
I understand that without all the fancy electronic zone valves and motorized mixing valves that I might lose some efficency, especially from the lack of automatic seasonal reset.