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In another thread, I posted

Jean-David Beyer
Jean-David Beyer Member Posts: 2,666
"I designed a possible installation and asked their Install Manager what

he thought of it. I did not insist they do it my way. He said that they

would not have done it that way, but that my way should work, and would

probably be a little more efficient than his way."

I do not think this is a question. But I want to expose my thinking about this point.

The W-M Ultra 3 boiler has a very interesting controller, that they call U-control. It accepts three thermostat inputs. The thermostats have three priorities. Associated with each priority are a bunch of user-settable  parameters that I assume most users do not even look at. Desired water temperature is one of them, and the reset curve is another. There are lots more. There are also outputs, including the running of up to 3 circulators. The circulator #1 is for DHW, #2 is the boiler circulator (P/S piping) is #2,  and the radiant heat downstairs is #3. I wish they had 4 outputs, but a standard Honeywell relay box handles that one.

So I hooked up  the indirect water heater to priority 1 and set the boiler to run at 180F.

I hooked the downstairs up as priority 2 and set the boiler to run at 120F (on the coldest day). Downstairs is slab on grade radiant heat.

I hooked upstairs as priority 3 and set the boiler to run at 140F (on the coldest day).

Upstairs is Slant/Fin that is much larger than you might use at 180F, but I wanted to run it as low as I could, so the Slant/Fin goes the entire width of the two rooms.

The contractor had not thought of doing it that way (the U-control is rather new). His view was to run the whole thing at between 160F and 180F, not replace the undersized heat emitters upstairs (saving money in the short run), and use a mixing valve, such as a Taco 5000 Series mixing valve, to reduce the temperature of the water to that required by the radiant heating zone.

My thought was that in avoiding the mixing valve and using the U-control to make the temperature what I actually wanted in the floor, the boiler water temperature would be less, and because of that, more heat from the fire in the heat exchanger would go into the water because of the increased temperature difference across the heat exchanger.
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