Mixing Valve Question

REKBDR

I have a question about a Honeywell mixing valve that even Honeywell’s rep was unable to answer. Hopefully, someone can help out. The question boils down to what happens if and when the temperature at the cold inlet reaches or exceeds what is rated for that port?

I’m installing a single loop Warmboard floor in a small kitchen remodeling job. It taps into a conventional high temperature boiler loop, so I need a separate loop for the tubing, using a mixing valve to reduce the water temperature. Standard stuff.

Given the oak floor that will go on top (80 degrees max), Warmboard’s heat chart tells me that my average temperature in and out of the loop should be 92 degrees. I am shooting for a flow rate of .5-1.0 gpm, which corresponds to common practice and the range of the flow meters in most manifolds that are available. For the heat load that the Warmboard can handle, this computes to a Delta T of somewhere between 7 to 13 degrees.

For the mixing valve I need one rated for mixing at a low flow because my single loop dictates the total flow rate, and that is why I am looking at the Honeywell AM101R-US. It supposedly can handle flows as low as .5 gpm (compare Caleffi 521’s minimum of 1 gpm).

I noticed that Honeywell’s specs provide a temperature range of 120-212 degrees for the hot inlet (which is fine), and 70-180 degrees for the mixed outlet (also fine), but only 39-80 degrees for the cold inlet.

Given the constraints, I would be operating at best at the margin of the valve’s 80 degree limit for the cold inlet. My question is simply what happens if and when the return water goes above 80 degrees?

My fear is that if I set the mixed temperature at the top end of my Delta T (call it 100 degrees for simplicity) things will work fine only at start up. As the water temperature in my radiant loop heats up to the 80 degree cold inlet limit, will the valve shut down the hot inlet entirely, in which case my mixed water temperature will then be only that same 80 degrees—until the heat loss in the tubing lowers the cold inlet temperature to below 80 degrees, at which time the hot port will open to try to reach the 100 degree setting? Would the valve be caught in a vicious trap where it oscillates around the 80 degree level instead of putting out 100 degrees during sustained operation?

The rep I talked to seemed baffled and apparently didn’t know how the valve’s internals worked so as to explain what happens mechanically at the 80 degree limit point for the cold inlet.

Does anyone know, or can you recommend a different valve that would avoid these issues? Might I need to install some sort of “dummy second loop” in a two loop manifold so as to be able to use a mixing valve that can handle a higher “combined” flow rate?

Harvey Ramer

A standard mixing valve is not the proper equipment for a radiant floor. What you want to use is an electronic valve that has ODR capability. Taco I-series mixing valves would be great for your project.

REKBDR

Thanks, but that is pretty pricey.
What did people do before electronic valves came out?

Harvey Ramer

> @REKBDR said:
> Thanks, but that is pretty pricey.
> What did people do before electronic valves came out?

They didn't have Warmboard then either.

Many standard mixing valves don't completely close either port. With such low flow rates, and such a large temp difference between hot and mixed, and such a small temp difference between mixed and return, you may have to add 2 balancing valves too stabilize the mix temp. By the time you get it right, you paid for the i-series.

hot_rod

Thermostatic valves are perfectly fine for mixing radiant as long as you work within their specs.

Inside is a simple wax filled cartridge that expands and contracts with temperature change. As the temperature moves the spool inside one port opens as the opposite closes. As long as you have some temperature differential the valve will blend the two flows to the desired temperature within a few degrees, plenty accurate for hydronics or domestic application.

One mis application is trying to move high flow rates through the valve. Most typical thermostatic valves have a Cv around 3, easily accommodating 5 maybe 6 gpm.

At higher flows, say 10 gpm for example you have a 11 psi delta P
around 25' of head!

The Caleffi 5213 is a point of use valve .5 gpm, 2 Cv, but limited to 120F at the outlet. I have this valve on my own system.

Paul48

The spool moves, based on the "mix" temperature, not the "hot" or "cold". You're pumping out of the mix side, so as the temperature approaches your setpoint, the spool shifts to try and add cold, which it can't. It can only add your return temp from the emitter. Which it will continue circulating until it cools enough that more heat is needed, and the spool shifts to add hot. Make sure you install it per instructions.

Ironman

For a small loop with a wood floor, a thermostatic valve may suffice, but in general I would agree with Harvey about using ODR - especially if it was a slab.

However, the MAIN ISSUE that your missing: if you're tapping off of a high temp loop the floor will probably NOT perform as expected. If this is the sole heat source for the kitchen, it's not gonna give satisfactory performance.

Think about it: high temp emitters heat an area quicker than low temp. The floor is low temp and requires much longer run time than the high temp zone. But what you're proposing would make the low temp zone dependent upon the high temp zone's thermostat to supply heat for it.

REKBDR

Thanks, hot rod. The 120 degree limit would not be a problem. I checked out the Submittal Sheet for the Caleffi 5213 series, and the maximum inlet temperature is 85 degrees (a step in the right direction). However, the sheet notes that the minimum flow is 1 gpm (vs .5 stated elsewhere in the literature, which makes me nervous).

What about my idea of doing something to increase the flow through the mixing valve, especially since manifolds always come in two loop minimums, and I would have to cap off the second loop anyhow? I'm picturing a “mini-loop” with the flow adjusted to equal what I want running through my floor, giving me a total of 1-2 gpm through the mixing valve and circulator.

In any event, what happens in a mixing valve if the return is hotter than what the specs say for the cold inlet?

REKBDR

Ironman raises a good point, but it turns out that this radiant loop is essentially supplemental heating for the kitchen.

Decades ago a family room was added to the house, blowing out the entire back wall of the kitchen so that the previously square kitchen is now functionally just one half of larger rectangular room. The family room part (several steps lower) has fin and tube baseboard units fed by the high temp lines that I now want to tap into, and there are no emitters at all in the kitchen proper, nor any room for anything but a radiant floor. For the last dozen years we've made do—which has been fine except on very cold days when some toe warming would be much appreciated. I decided to use Warmboard because it is supposedly quicker in its response time

I'm taking advantage of a current “total kitchen gut” job to put in Warmboard S panels in place of the old sub flooring. The area of panels (not under the cabinets) is only 168 square feet. The fact that this such a small radiant heat system seems to make this a bit of a challenge to figure out appropriate piping and controls.

hot_rod

There are a number of ways to pipe a low temperature zone off a boiler running high temperature emitters. here is one example. It may take some repiping.

WB is a fairly fast heat emitter. All that aluminum with covering directly over it is considered a low mass, dry system.

Probably most days the kitchen will be running when other zones are also, so micro loading the boiler may not be a concern.