I need to select a 1" thermostatic mixing valve to reduce my 120F supply to 100F for the slab portion. Reading the specifications of the major brands I find some problems selecting an appropriate unit. The two common incompatibilities are: minimum delta T between supply and mix temperatures (typically 25 or 30 degrees), and minimum operating pressure (typically 30psi, whereas my system is 15psi). Can someone recommend units that are known to work well in the radiant heating application with delta T of only 20F and pressures of only 15psi? Thank you in advance for your experience in this. John

My plumber installed Watts 1170C-M2. I can say that the pressure is not an issue. My system run at 12-15 PSI and the valve work fine. For the dT mine is 160-80 °F for a mixed temp of 90°C So i cannot say if this will be an issue.

your slab is running at 90F? If I set mine to 90F then my delta T is 30F which might satisfy the thermostatic mixing valve. I have the same valve as you, in one inch size. I pulled the check valves out as they reduced the orifice to about 3/8" !

Why a mixing valve?

Yes, why indeed?

I have different emitters for some zones. The less efficient under floor plates require 112F according to the calculations. The efficient slab only 90F. Therefore the electric boiler is set to 112F. I need a thermostatic mixing valve to reduce the boiler supply temperature to 90F for the slab.

Why did you use the less efficient floor plates in lieu of efficient thin-slab? You can typically use an average supply temp for emitters that have supply temp requirements within 5-10F of the average supply temp. So you might be okay with using a supply temp of 100F and adjusting the balance valves to provide the required flow rate to produce the required heat gains. You would of course need to design for this and select appropriate pump(s).

Efficiency is a relative term.

Mixing valves are something you use when you have to, not an inevitable component of every hydronic heating system.

The thermostat is the fine tune on any zone. Unless the lower temperature zone is regularly overshooting the thermostat setting, especially in low-load areas such as basements, there is little benefit to mixing down to design.

This is especially true of radiant floor systems driven by electric boilers, like my own, where the only benefit of a lower SWT would be the slight, imperceivable and unmeasurable, lower of the heat load itself.

When you run less than 140°F SWT you are in the sweet spot of hydronic heating and all else is just bragging about minutiae.

We focus on comfort with practical heating systems for real people.

Once your hydronic radiant floor heating system is installed and operating, it can be used to access the performance of both your building design and your hydronic radiant emitter design.

With regard to building design performance and with an indoor temperature of 70 degrees F and outdoor temperature of 20F, we consider a hydronic radiant floor surface temperature of 70-75 degree F as excellent performance (i.e., a low-load, energy efficient building), a floor surface temperature of 75-80 degree F as average performance, and a floor surface temperature greater than 80 degree F as poor performance (i.e., a high-load, energy inefficient building).

With regard to hydronic radiant emitter design performance, we consider a 10 degree F or less temperature difference between the hydronic radiant floor heat source supply temperature and the hydronic radiant floor surface temperature as excellent performance, a 10-25 degree F temperature difference as average performance, and greater than a 25 degree F temperature difference as poor performance.

One should always keep in mind that you have to live with the performance of both your building design and your hydronic radiant emitter design forever.

The is utter and complete nonsense.

Performance is not a technical term and has no meaning, even if arbitrary numbers are offered.

The floor surface is a function of the heat load, the tighter the house, the lower the load. Higher delta T often increases overall system efficiency.

Engineering out of a book, indeed.

Nothing lasts for ever, thank the good Lord for that as I have had enough.

Posted By sailawayrb on 07 Feb 2015 08:27 AM
Why did you use the less efficient floor plates in lieu of efficient thin-slab? You can typically use an average supply temp for emitters that have supply temp requirements within 5-10F of the average supply temp. So you might be okay with using a supply temp of 100F and adjusting the balance valves to provide the required flow rate to produce the required heat gains. You would of course need to design for this and select appropriate pump(s).

I already have knee problems, slab everywhere? No thanks.

At 112F I am definitely in the sweet spot. Curious how you handle mixed systems, like some baseboard radiators, staple up, and slabs? TMV seems the least complicated solution.

What is the problem you are trying to solve with the TMV? Ie, what do you think will happen if the water is moderately higher temp than required?