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Weil McLain Aqua Plus differential control - tepid to scalding

ddunn Member Posts: 12
Hi everyone,

I have a fairly new 80gal Aqua Plus attached to an Evergreen 105 boiler. A recirculation loop is also installed.

https://www.weil-mclain.com/sites/default/files/field-file/WM Aqua Plus Series 2 Water Temperature Controller Instructions 082718.pdf

The differential on this seems to be about 15F. When I set it to 120, I've gotten nearest sink readings from 108 to 120. Raised setpoint to 130 and have seen 112 to 125. Raised again to 135 and see 117 to 132. These are all taken when the tank is not calling.

The instructions don't say that you need to always install a tempering valve. But with this big of a differential, I don't really see how our family can safely use it without one.

80 gallons of water at 108 is not what I'm looking for. Calling for heat when sink drops to 117 seems more appropriate. However, the 132 seems risky to me. We only have adults and teens in our family, but I worry about elderly visitors.

Is this normal for these tanks? Or do people usually see better differential control?




  • Jamie Hall
    Jamie Hall Member Posts: 23,275
    The differential is fairly normal. Perhaps a little wide, but not out of line -- and it may be adjustable.

    That said, however, there are some nasties which grow in hot water, particularly Legionella, which you truly don't want. They can't survive water at 140, though, so our recommendation is to set the tank to reach 140. Then, of course, use a mixing valve with thermostatic control to bring the water delivered to the users down to something safe -- 110, perhaps.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Robert O'Brien
    Robert O'Brien Member Posts: 3,541
    You need a mixer, it's code in every AHJ I know of. The control logic attempts to minimize stacking effect.
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • ddunn
    ddunn Member Posts: 12
    Ok. Thanks. I'll ask them to install appropriate mixing valve.
  • kcopp
    kcopp Member Posts: 4,431
    That tank is particularly susceptible to poor temperature control.
    A mix valve is good insurance. The cash acme ones are simple to install.
  • Le John
    Le John Member Posts: 226
    I like the Caleffi 3 way mixing value with built in temperature gauge that gives the temperature of the mixed water.