Using Mixing Valves for Radiant Heating

HeatingHelp

In this excerpt from his Classic Hydronics seminar, Dan Holohan shares his knowledge about mixing low-temperature radiant heating with a higher-temperature hot water heating system. He talks about how mixing valves work, using diverter valves, and more.

Watch here

Rocky_3

See, even after 30 years, you still can learn something new every day. Never even considered that aspect of a 3-way isolating the pump away from the expansion tank. Thanks, Dan!

mowgs

Hey, I've never used a diverter valve on radiant like this so, I'm kind of confused as to how it keeps the temp at design temp if its mixing hot boiler water downstream of the diverter. My brain makes me think its going to be hotter than intended. Also, where can I find these valves? I might have to mention it to my boss because all I ever put in were thermostatic mixing valves. Thanks!

taubbri

Guess I'm a little late to this party. Just watched the video. I can't see the mixing valve being a problem in the way described. The only time I could see this problem arising is if for some reason the mixing valve drastically overshoots the target temp. In normal operation, I would assume that by the time the water cooled to the point of potentially causing cavitation that there would be a demand for more hot water to be mixed in, thus reconnecting the path to the expansion tank. Would love to know why this thinking is wrong

hot_rod

Mixing/ diverting are pretty generic terms. One good way to determine the flow path is with a cutaway as shown.

In the case of a 3 way thermostatic valve, as one port closes, the opposite opens the same amount, so there is always flow out the mix port, from A B or a combination. It has to operate like this as the sensor inside needs to always have flow moving around it so it can can "see" and regulate. It is sometimes called a smart mixing device.

Manual mix valves are know as dumb valves as they cannot sense and adjust on their own.

Often with a manual mix valve used for hydronics it has an actuator added to give it a brain, make it smart. That actuator could be capable of preventing a deadhead situation as Dan described. By limiting the stroke so a port is never completely closed, for example.

Tekmar was alway adamant about using actuators on their 3 &4 port mix valves so they can work, control accurate mix temperature over a range of conditions based on sensor input. Without an actuator is is only accurate at one flow and temperature condition.

So you need to look at the design of the specific valve you intend to use to assure it will do what you are expecting.

The placement of the expansion tank connection is important also, so all circuits alway have a means to always reference that PONPC and expansion device. That gets more complicated on primary and tertiary piping arrangements. This piping shows the tank connected in different points in the piping.

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RascalOrnery

So does a thermostatic mixing valve (that is manually set, like for domestic hot water) really apply here? Wouldn't the only time the circulator would have cavitation problems be if the HOT side closed? Wouldn't this only happen if the loop wasn't giving up any heat (nearly impossible). I don't understand when the HOT is going to ever be closed? Thanks in advance

hot_rod

A couple thoughts. Generally speaking in this pic from Dans presentation, there is not usually a need for that check. The concept of properly applied (close) tees is there is no un-wanted flow into the secondaries. So IF there was not a check in the crossover, then the tank is never isolated from the secondary circ.

Looking back through many Idronics, I haven't see that we ever show a check in that line? An iso/ purge valve, yes, but no check.

As for the 3 way thermostatic, if the H was closed all the way C is open all the way, so how would the circ starve?

And yes the H would only be "near" 100% close with it seeing return near the valve setpoint, plus the differential built into the valve, 7- 20° or more. Manual 3 way valves can seal a H or C port off, as seen in these pics.

Also it is really tough to cavitate a small wet rotor circ in a pressurized loop. I've seen them run with plugged strainers, one iso valve closed, and still not cavitate. I've built a number of clear tube demos in the hopes of showing cavitation in a small circ and have never been successful. Even pulling a negative on the suction side gauge is tough to do without really chocking flow. Especially a circ after a mixing device set to low temperature. A good discussion about cavitation in Idronics 16

One place to see cavitation is a high head circ on the back of an open system OWF. High temperature, some times boiling, and only a psi of suction pressure.

So it comes back to the designer or installer knowing they flow path through all the components under all conditions, I suppose. So certainly the design of the valves, location of checks, etc needs to be evaluated.

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