In-floor mixing valve, keep rebuilding or just replace entire unit

Dave Carpentier

So, I have a 1" Honeywell/Sparco mixing valve on my radiant system, which is 20 years old. Maybe 10 years ago I replaced the internals. It appeared to still be working when I shut the boiler down this spring (but I didnt check it with my thermal gun). I took the mixer apart and it looks clean enough.

We're taking the system down to swap out the old boiler, but I still need a mixer.
Would you..
Reuse it. Worry about it only when it give some trouble.
Rebuild it. It looks like the rebuild kit is still available and would be less than half the cost of a new mixer.
Buy new. Then you get a new "bore" and a shiny part.

mattmia2

Are you just hooking up what you have to the new boiler or are you replacing a lot of the near boiler piping and controls. There probably are better ways to do the mixing than what you did 20 years ago. If you aren't redoing all of it with more modern controls then I don't see a lot of value in replacing that part now over waiting until it fails again.

Is this a CI or other high temp boiler or is it a mod con?

Dave Carpentier

Its a modcon (rather, will be when I buy it). Ive got quite an assortment of loads including some that are below the boiler minimum, so it's going thru a buffer tank (2-pipe) and also the buffer tank is shared with an air handler that will trigger the boiler to higher temps. So the presented temp will vary some , hence the mixer to cap the in-floor temp maximums.

mattmia2

Others will know a lot better than I but ideally you would set up the mod con to produce the lower temp when only those zones are calling to get more condensing out of it. The thermostatic valve is going to want to see a significant differential, it might make more sense to use something like a variable speed pump or 3 way motorized valve to make the mix with a mod con. If you are using the boiler to heat the buffer tank up to the high temp then you lose the efficiency benefits of the mod con.

I think you have another post about the buffer tank and maybe you went over this in there.

Dave Carpentier

Yes, hotrod has been helping a lot with the boiler-buffer side of things.
Thanks for the tip about mixers and low temps. I was already very marginal on the flow thru the mixer (0.5-ish) for my longest loop.
So, I'll hold off on buying a rebuild kit for the mixer, and add it to the list of things when I talk to the supply house.

Zman

I would replace it with a non-thermostatic model. Your radiant would then track the ODR curve of the air handler loop.

hot_rod

First decide if you want to maintain the buffer at a high temperature say 140 or so. The higher the temperature the longer the drawdown before firing again. You store the most energy with a hot stratified tank. But you give up some boiler efficiency when return temperature increases.

Running on ODR will maximize boiler condensing and efficiency, but shorten the amount of energy you store.

So is the buffering short cycles a priority, or most efficient boiler efficiencies, lowest operating temperature more important?

If you run high temperature tank, you need a smart mix valve for the radiant, thermostatic or motorized.

With an ODR controlled tank, a manual mixer works well with a single zone, single temperature requirement only.

With a manual valve, an actuator could be added later.

You should have enough data now to crunch the numbers both ways, hot tank, or ORD regulated tank.

Wide deltas are your friend with buffer tanks.