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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?