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Lochinvar Knight and DHW cycle hysteresis

A friend of mine has had a Lochinvar Knight WHN-085 mod con paired to an Amtrol CH41-ZWD indirect tank in her home for 4 years. It is a retrofit to an old cast iron radiator gravity system. Even though the radiators are all on one zone, the system is piped primary/secondary, with two 3-speed pumps. There is an additional 3-speed DHW circ pump. The system uses outdoor reset.

In the last year, she has noticed that the boiler modulates rapidly for 5-6 cycles near the end of a priority DHW cycle, going from 100% to as low as 25% burner modulation and fan speed before the cycle ends. I’ve observed this behavior myself. For the first 3 years, she says it ran full out and then downfired slowly, 1x, during a DHW cycle. She is a very observant and exacting machinist/welder, and I doubt she’s just now noticing something that’s been happening since the system was commissioned. The system doesn’t exhibit this behavior during space heating.

With the Amtrol Tstat set at 136F, and the boiler DHW setpoint at 180F, I observed the 5-6 modulations from 100% down to 50% and as low as 25% and back up to 100%. The cyples were fairly quick, with jumps on order of 20% – I assume partly because the boiler only has 5-step modulation. They started as the tank temperature was around 129F (5-6F away from tank setpoint, which is NOT monitored at the boiler). It didn’t make much difference to the length or extent of the fluctuations if the DHW or boiler primary circ pump speed was changed. In all cases, the boiler output temp varied rapidly from 182-187F, while the input varied similarly as the control maintained a delta T of 20-23F. I did not disable the boiler pump, but it was always confirmed on via the control panel display.

Does this much hysteresis in burner and fan modulation seem normal? My Westinghouse fire tube boiler fires up and down with no oscillations during a DHW call, with or without central heat running. Although her cycle ends with the DHW tank at temp, it is an extremely underdamped system. Is that the nature of this beast due to control algorithms, or is something else at play?


  • JohnNYJohnNY Posts: 2,324Member
    I've had this happen and solved it for better for worse by limiting my fan speed by about ⅓ during a call for DHW.
    For installations, troubleshooting, and private consulting services, find John "JohnNY" Cataneo here at :
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    John is a professional Master Plumber, licensed by The Department of Buildings of The City of New York, and works extensively in NYC while consulting for clients in and out of state.
  • SuperJSuperJ Posts: 501Member
    Sounds like it's bouncing off a high limit. Can you reduce the DHW setpoint to 175? Maybe there is some scale buildup on the DHW side, limiting heat transfer after a couple years?
  • hot_rodhot_rod Posts: 11,773Member
    lowering the boiler SWT during indirect call will increase efficiency
    Lower it as low as you can and still get adequate recovery time

    Get it into condensing mode for as much of the cycle as possible
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • tropostudiotropostudio Posts: 9Member
    Thanks for the suggestions! JohnNY - I saw the option to limit fan speed during DHW cycle in the service manual. I assume that is lowering your max firing rate instead of lowering boiler SWT.

    Seems like this behavior might be the nature of the controller logic? I'm not a boiler installer or plumber, but I have a bit of experience setting up PID controllers for equipment in my shop.
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