Boiler mixing valve adjustment - advice needed

Ronanmd1

Hey guys - First off - Hello there from frozen Calgary! Hoping you can help me understand how to adjust the mixing valve for my boiler. Despite searching the forums (and web), I have a few questions about my system.

I have a Weil-McLean cast iron boiler supplying the in-floor heat to my basement slab, and garage, and in-floor heat to several bathrooms in my home. The house also has a natural gas furnace and a separate hot water tank.

I just replaced the 4-way manual mixing valve for the boiler (the old one had a leak). I am slowly figuring out how this system works but I could use some advice on adjusting the valve to optimize supply/return temps. My boiler kicks on when its internal temp reaches 165F and shuts off at 185F. Ambient outdoor temperatures here in Calgary are -5F to -10F.

At my current valve setting, when all zones in my system are at their target temp, the supply temperatures are approx 170F and the return temperatures stay just above 150F. At this return temperature, the boiler seems to cycle pretty frequently. Based on this information, I was planning to open the mixing valve a bit to allow for warmer supply temperatures (175-180F). Presumably, this would lower the return temps closer to 140F, and may allow for longer boiler cycle times. However, when one of my zones calls for heat in this cold weather (especially the large garage concrete slab), the return temperatures briefly drop down as low as 125-130F. They climb back up above 130F in 5 min or so as the boiler kicks on.

So....should I open the valve to allow higher supply temps, even if it means that there will be short-duration, return temps lower than 130F in this cold weather. Or should I keep the valve where it is, and have supply temps that are closer to 170F and return temps that are closer to 150F most of the time, knowing that the return temps will always be above 130F (but with shorter, more frequent cycle times)...

Thanks for your advice!

hot_rod

The 4 way mixers do two things for a system. They adjust the temperature to the distribution, but they also provide return temperature for the boiler.

It really should have an electric actuator on it, with a sensor on the boiler return pipe to assure the boiler runs warm enough.

The time to keep an eye on it is when a cold slug comes back from the garage slab firing on

Concrete slabs rarely need more than 100- 115f supply? Especially if a furnace is carrying some of the heat load? Try closing it down some, which also helps return. 82F is about as warm as you want the floor surface temperatures

You might be able to run the boiler at lower temperature also if it just supplies in a slab radiant, maybe 145- 165. Lowering the boiler operating temperature will save some fuel consumption. As long as the return stays at or above 130f

The return may drop below that with the garage kicking on, but within 10 minutes, warm the return. Looks like it is operating that way now. Probably the boiler is oversized and catches that cold load quickly

If you did have a motor operator, the system could run on outdoor reset control, better comfort and usually less fuel use, as the temperature modulates based on outdoor conditions.

Long run cycles with low supply temperature is best for slab radiant. Hitting it with excessive temperature can result n the flywheel effect, over heating the rooms as weather warms and loads decrease.

One step at a time, seems like it is dialed in manually fairly close, but maybe excessive floor temperature with that high supply temperature?

Ronanmd1

Thanks Bob. That makes total sense. We have wild fluctuations in temps up here with our winter Chinook winds. Yesterday it was -10F. Today it is +34F. An external sensor and motor actuator for the mixing valve would likely make things run more smoothly. For now, I will follow your advice and gradually close up the valve until I see the supply temps closer to 110-120F, provided the return lines stay where I need them. Hopefully, this will also lengthen the run cycles a bit.

Cheers

Jason

Dave Carpentier

If you have wood or laminate floors, you dont want in-slab temps too high.
I send mine down into the slab at about 115f. I have laminate in 3 bedrooms, and try to hit a target of 80f on the slab below them. I've run it a little hot in the past, and the (23 yr old) laminate (tongue and groove w/glue type) is now starting to separate. Its near end of life anyhow, so Im happy enough with the approach.
The rest of my floor is ceramic, love the feel of the warmth on that.

HomerJSmith

In you situation all cast iron boilers will condense on start up. Depending upon the water capacity of the boiler five min may not be that bad, however the shorter the heat up the better. It is the prolonged operation under a return temp of 130 deg that is really damaging.

You should really have a sensor controlled motorized 4 way mixing valve. You need two sensors, one on the sys out flow to the load and one for the regulation of the return water to the boiler.

I have added a Taco I-series setpoint mixing valve with sensor as a boiler bypass in this kind of situation set at 135 deg.