Radiant 3-way Mixing Valve Return Inlet Temp?

CDecker

I am installing a couple of low-temp radiant zones, and was planning to use a 3-way mixing valve to temper down the supply to my desired temp. The valves I purchased are Taco 5000-HX's. They seem widely used for this purpose, and the manufacturers website even specifies "an ideal choice for a radiant heating system, whether in slab, over pours, or in joist bay heating installations."

Looking at the Instruction Sheet provided with the valves, I noticed they specify a "cold water inlet temperature between 39º-80ºF." Maybe I am mis-understanding, but if I have the valve outlet set at 120º for my radiant, with a 10-15º ∆T, won't the return water temp be 105º+ and way out of spec? I realize these valves are also used for tempering down domestic hot water, so maybe this specification doesn't apply to radiant? Will the valve still function as intended?

It also specifies a "Minimum Temperature Differential (between hot supply and outlet) of 10ºF." So should I assume that the valve will not function properly if set to 120º but my incoming water temp is below 130º?

Thanks in advance,
-Chris

ewang

A good question, that I was going to ask at some point.

Also, what happens with outdoor reset and even lower supply temps?

hot_rod

A bit confusing but you are correct in that the valve needs or wants about a 25-27° difference between the hot inlet and mixed out.
Now if the valve does not have or see that exact differential it does continue to flow, but the mixed outlet will vary some as the valve hunts around for it's required delta.
The valve will maintain accurately if the boiler or source on the hot is 25- 27° warmer than the 20F you are looking for. So if you mix down to 120, you want 145 SWT.

Also be aware of the valves Cv, the gpm it will flow at a 1 psi drop. As you flow above say 2.3 gpm pressure drop goes up.
5 gpm would be about 11 feet of head in a 2.3 Cv valve.

mattmia2

Taco also makes a motorized thermostatic mixing valve that is relatively inexpensive that solves most of these problems.

mattmia2

Look at this series. There are outdoor reset versions as well:
https://www.supplyhouse.com/Taco-I100T3S-1-1-3-Way-Setpoint-I-Series-Mixing-Valve-w-Sensor

ewang

@hot_rod
So if you have one system (radiators) designed to run at 140* and staple up radiant that runs at 120*, is there such a mixing device that can handle this narrow differential?

ewang

@mattmia2
This is a mixing valve that only has a 3*F differential requirement.
https://www.supplyhouse.com/Honeywell-Sparco-AM101-US-1LF-3-4-Sweat-Union-Mixing-Valve-LF?gclid=CjwKCAjwqML6BRAHEiwAdquMnYopkl-EdIdGBLvtc76Gwuo--AuTcYFDqca-f1EV1rwPQSmWz-fEhBoC3kYQAvD_BwE

CDecker

@hot_rod

hot_rod said:

A bit confusing but you are correct in that the valve needs or wants about a 25-27° difference between the hot inlet and mixed out.

I had no idea there was a minimum for this. The spec sheet of the 5000 Series valve I purchased shows 10ºF. It's a 5004-HX-C3. When I look up the same valve on Taco's website however, it shows 27º like your saying... Ugh...

hot_rod said:

Now if the valve does not have or see that exact differential it does continue to flow, but the mixed outlet will vary some as the valve hunts around for it's required delta.
The valve will maintain accurately if the boiler or source on the hot is 25- 27° warmer than the 20F you are looking for. So if you mix down to 120, you want 145 SWT.

Wow. That is much higher than I expected. I am using a Flat Plate Heat Exchanger as these zones will have glycol in them. I was planning on setting the boiler to 140º, and expecting less than that to make it to the zones after the loss through the Heat Exchanger. There is no way I'm going to have 27º (or perhaps even 10º) between my supply and desired temp. I was installing these just to give me the future option of adjustability, in case I wanted to dial things up or down a bit.

hot_rod said:

Also be aware of the valves Cv, the gpm it will flow at a 1 psi drop. As you flow above say 2.3 gpm pressure drop goes up.
5 gpm would be about 11 feet of head in a 2.3 Cv valve.

Another great point... Now you're really making me wonder if I should install these things at all! I was planning on using (and already purchaed) Viridian VT2218 ∆T pumps on the radiant zones with the mixing valves, but with the increased head as mentioned above, they may no longer be in spec.

Is there a better way to do this? How else do I control the temperature to my staple-up zones to protect my floors?

ewang

I wonder about using a non-thermostatic mixing valve? Basically a three-way valve that gets locked in position to always give you a differential flow. Obviously this would have to be tweaked if your boiler supply temp were to increase beyond your original design.

hot_rod

I don't know of any thermostatic mix valve that can be accurate with a small difference between H in and Mix out, 25- 27 is the common requirement.
The valve can and will maintain a tight delta, maybe 3° with no problem, as long as it has the correct delta to work with. That is a thermostatic mixing valves main purpose in life, maintain a tight fixed output temperature. they are built and listed for that purpose.

If you want accurate control with any hot supply temperature, above the requirement of course, a motorized mixer is best.

Yes a manual valve could work, but once set it is good (accurate) for that one specific condition. As the load increases or decreases the temperature will be too warm, or two cold, it's called a "dumb" mixing device for that reason, it has no brain, either a motor or thermostatic motor
A smart mixing device can automatically respond and adjust to a specific condition.

ewang

So, what will a mixing valve do if it's set at 120*, and sees 140 & 130? Will it error to the cold side or warms side? I guess I have no good understanding of how these things work.

hot_rod

ewang said:

So, what will a mixing valve do if it's set at 120*, and sees 140 & 130? Will it error to the cold side or warms side? I guess I have no good understanding of how these things work.

Are you saying 140 on the H and 130 on the C ports?
It needs a supply of cold water, lower than 120 of course, to mix down.
A 3 way thermostatic on a water heater for examples always has the incoming cold, say 55F to blend with the tank 140F to achieve 120F

On a hydronic application the only cold at the C port is the return water temperature.

If the valve is set at 120 and has that 25° differential between hot in and mixed out, plus a return colder that 120F it will maintain within plus or minus 2°

Here is a cutaway to show what happens inside a Caleffi 521, other brands are similar. This specific valve and many others cannot close the ports off 100%. Our 520 series will fail safe if it loses cold supply.

Notice the openings (light blue color) to the center on the H&C ports. One is up high, one down low. As one opens, it closes the other. The temperature sensed at the bottom Mix port is what opens and closes the windows. It cannot close one window less than it opens the other.

Some motorized valve has the ability to close ports off 100%, so that required differential is not as critical. If it is set at 120, and 120 is returning, then H port closes and it continues to circulate 120 until it drops in temperature to whatever differential is required and opens H port.

GroundUp

I do a whole lot of radiant fed by high temp boilers and have been around the mixing valve spectrum, and have run into the same issue with the 5000 series valves- will never buy another. The Webstone W series and Honeywell AMX/AM-1 series are the only ones I've found that will work with a tight delta. I've gotten both down to within 5-6 degrees on either side of setpoint before they start getting weird. The Webstone seems to have better control, but feel cheaper made and the gaskets are REALLY easy to tear and are completely unavailable through any supplier. The AM-1 are huge and every one I get has loose packing, plus they tend to wander a few degrees from setpoint, but they're robust and use nice paper gaskets easily sourced most anywhere. Both have their ups and downs.