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GDO Member Posts: 1
I'm Installing a Weil Mclain Ultra 105 which includes an Ultra Plus, 4 fin tube zones, & 2 radiant under slab zones. There are 3 priority zones, 1 for DWH, & 2 for other space heating applications. If I would use 1 of the priority zones to control my under slab zone, wouldn't that interfere with my fin tube zones if they were calling for heat. Could I use a manual 3 way mixing valve to control the temperature under my concrete slab? If so, what would be the best way of piping this in to my secondary/ primary loop?


  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I have a system like that, but it is only 2 heating zones.

    I am a homeowner, not a heating contractor. I have a W-M Ultra 3 80k BTU/hr model. It runs a 40 gallon indirect fired hot water heater also W-M.

    I use all three thermostat inputs, and it is zoned with circulators. It is piped primary-secondary just like in the I&M manual, so you can see I am a relay contact short. I do not use any mixing valves. On design day, I run 112F water to the slab and 130F water to the baseboard. I deliberately have more baseboard than usual so I can use lower supply temperatures and get lower return temperatures and therefore more condensing.

    The domestic indirect is piped across the boiler's supply and return pipes. Its thermostat goes to the Priority 1 thermostat, so generally, if the indirect is calling for heat, it gets the heat that priority is set for. This times out at 30 minutes, so if the indirect is hogging heat, the rest of the house is not cut off, it just has to wait. I have never seen the indirect run over 10 minutes, so this is not an issue. When running at priority 1, the circulators to the two heating zones are cut off, so all heat can go to the indirect.

    The radiant zone and the baseboard zones are zoned with circulators and are across the secondary circuit. The radiant zone thermostat is connected to priority 2 and the baseboard zone thermostat is connected to a Honeywell relay whose main contacts run the baseboard circulator. The other contact on that relay goes to the priority 3 input of the boiler.

    So, ignoring the indirect, if the radiant zone asks for heat, it gets it at the temperature it wants. If the baseboard wants heat at the same time, it gets some heat, but only at the (lower) radiant zone  temperature. If both zones call for heat, the higher priority zone gets it for 30 minutes, then the baseboard gets its heat for 20 minutes, rinse and repeat.

    If the baseboard zone calls for heat and nothing else is calling for heat, it gets hot water at the temperature it wants.

    (I am leaving out some minor details so this does not get too long.)

    If you think about it, this may not work well in all situations. If the baseboard zone requires more heat than the radiant zone, the radiant zone would get more heat and thus the baseboard zone might get too cold. In my house, the radiant zone wants 24,000 BTU/hour when it is 0F outside, and the baseboard zone wants only about 6,500 BTU/hour, so this works just fine. But if the situation were reversed, you would probably have to run things at the baseboard temperature and mix it down for the radiant zone. I think this would be less efficient.
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