Best Of

Sometimes it is easier to see how the water flow will work when you add some temperature numbers to the water and some pressure numbers to the water. Here are some Temperature numbers in the attached diagrams. There is a problem with @Mad Dog_2 diagram, (not just upside down). When you get to the inlet of the cold water to the mixing valve... the water temperature may be as high as 160° if the baseboard is ramped all the way up to the maximum temperature during a design temperature day. The reset curve may yield 180° supply to the baseboard with a 20°ish ∆T at the return. Since the mix cold inlet is attached to the baseboard return, the lowest temperature the mix valve could provide has to be no lower than the 160° inlet to the C on the valve.


I have modified @Mad Dog_2 's diagram in the second file. This will take the C on the mixing valve from the return of the low temp loops. That will let the Mix on the mixing valve have temperatures closer to the desired temperature because the C inlet will be drawing from the coldest water in the entire system.


In the second diagram, the low temp pump can run with the internal boiler pump without using the system (Baseboard) pump. Also the system pump can run without the low temperature pump if only a baseboard zone is calling. I believe this will eliminate the concern that @hot_rod mentioned "pumps in series".


I was trying to make the least amount of change to the existing design in order to make the contractor more likely to agree to the repiping. There is no way around placing a circulator within the MIX to radiant tubing to C on the Mixing valve and boiler return. There will be no pressure difference to force the water thru the radiant loops after you repipe the C on the mixing valve to the proper location.


Your plumber should get familiar with this Plumbing and Mechanical article https://www.pmmag.com/articles/88035-the-do-s-and-don-ts-of-three-way-thermostatic-valves-br-john-siegenthaler-pe/. This is the trade journal for his trade!


See how the pump location is between the Mix port of the mixing valve and the tubing manifold. This is how the professional should do the job.

In the next paragraph (diagram 2) it specifically states that the piping design your plumber used is NOT the "Best Practice" for radiant floor piping design

By adding a high temperature system to the mix, the cold inlet will not have access to any lower temperature water in order to "Mix Down" the floor temperature.

Yes..as HR says, you want the Rafiant zone Circulator pulling away (pumping away!) From the Mixed port of the 3 Way Valve.  The Taco Circs I use have Flow Checks in them. IFC (integral Flow Check), so You don't need a Swing check if that's what you mean..Its maybe 4-6 hours work to fix it.  If the guy insists on his Cockamamie piping, move on.  Mad Dog 🐕 

I did testify as an expert witness in 2 court cases in my career. I believe that Cockamamie is a technical term @Mad Dog_2. This can become overwhelming for some persons like Judges and Lawyers that are not in the trades. You may want to use more generic terms for those that may not be well versed on the trade slang. I might suggest "Bone Head" or "Idiot"