motorized mixing vs. manual valves

singh

Thinking about using Tekmar mixing valve with 441 Tn4. However,
boiler has ODR and was debating if a three way valve (not thermostatic) can be set to give a "proportional" mix temp in conjuction with the boiler temp off ODR.
Other than price, and simplicity any pros or cons to either approach.
RFH install .

TIA

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Brad White_194

Application

If the goal at your emitter is a fixed temperature or limited range (say a concrete mass, low-R value covering type radiant floor requiring very low water temperatures), a self-contained mixing valve is a decent way to get radiant control on a budget. Not the best way, but acceptable.

If on the other hand your RFH needs higher temperatures, has lower mass, higher R-value coverings, perhaps sometimes has solar gains, that the lower mass requires faster changes up or down and across a wider range, you should use a motorized valve and proper, responsive controls. That is where the savings and comfort lie.

The Steam Whisperer (Formerly Boilerpro)

I prefer the fixed valve setup....

With ODR and have a slew of them installed. One of the problems with a motorized valve is that if it suddenly calls for a large amount of heat (such as bumping up the thermostat), it can draw off every btu the boiler can put out and other higher temperature zones go cold. The fixed valve automatically limits the amount of btu's that a zone can draw, keeping system loads much more stable and the supply of heat to other zones stable. Figure alot more boiler if you plan on using motorized valves.

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scott markle_2

While I have no first hand experience with such an arrangement it seems to me that if the system you have in mind has a constant flow rate (single zone) a manual mix valve could be set up to follow your main supply temp fairly proportionately, if are running on basically the same curve just shifted down. I have watched motorized valves do all kids of gymnastics (hunting) to maintain a determined supply temp and ultimately settle into a fairly consistent position, perhaps this level of control is not necessary for high mass emitters, however it is nice to have a separate high limit for protection. as far a proportionality a good mod con can provide consistent supply temperatures but the return water you will be mixing may not be so consistent. In (a single zone)constant circulation system I have a feeling you might find that your mix temp follows your boiler with reasonable proportionality.

If it didn't work out you could always add an actuator and separate control later on.... explain that to your customer!

Unknown

You are describing a boiler that is undersized for a setback recovery situation.

the vast majority of the time that's a non-issue (I don't even know how many motorized systems I have out there that have never reported this kind of problem, and I am routinely accused of undersizing boilers). The times it is an issue it would pretty much be only gigantic setbacks or high-mass setbacks; neither of which are appropriate uses of setback. the solution is stopping the setback, not upsizing the boiler.

other than setback, talk to the user about proper system operation (find a temp and forget it) before you just upsize their boiler on an assumption.

-tiny boiler and motorized mixing fan.


to the OP, you can do proportional mixing. to do it right though you have to do it under "cruise conditions" or run the math and use a balancing valve if you're trying to set it up cold or under startup conditions. no need for 3-way valve at all in that case.