Lochinvar Knight and DHW cycle hysteresis

tropostudio

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?

JohnNY

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.

SuperJ

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_rod

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

tropostudio

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.

tropostudio

Thanks again for your suggestions - after trying them, my friend's boiler still modulated rapidly up and down near the end of the DHW cycle. I dug into the service manual. Her system is set up primary/secondary with a 1-1/4" hydraulic separator, and DHW piped across the primary loop. 3 each 3-speed pumps. A couple of techs had been over adjusting it to no avail. It had been set up with DHW as Zone.

I programmed it to set DWH as Normal, with priority, and turn off the boiler and system circ pumps during the DHW call. Supply dropped to 170 at DHW, and it still recovers plenty quick. Could play with that some more, along with adjusting the outdoor reset curve, and see how it holds out during cold weather. All works fine now!

Is there a reason this system could not have been direct piped, with one circ pump for CH and another for DHW? That set-up is shown in the install manual as an option for this boiler. All the CH is on one 'zone' with one tstat, and low pressure drop (cast radiators and large gravity system piping. Gotta think it would have worked fine with less hardware...

clammy

Usually when i install a mod con or any boiler w a indirect ,from the first time it heats the tank I write down the tanks temperature ,what’s it set for and how long to be satisfied .it cuts down on the I think your tank is scaled up big time and helps in the future when your starting to have issues . Easiest way is to be sure if throw some temp gauges on your tanks supply and return see what your temps differences are from 1 st calling to when your short cycling starts ,from what I ve dealt w it usually lime and calcium build up on the coils ,your boiler temps rise quickly your temp difference gets narrower instead of 15 to 20 your down to 10 td and then it’s just hitting the limit and riding it on and off .i ve noticed on most systems w a per purge pump set up that after hit the set point while the pumps is running and boiler off even w a mod con you will see a increase in tank temp,usually when its dirty there is less of a rise in temp if any at all and it will be much slower . Timing your indirect is the easiest way to track where or not it time to de scale or replace .hope this helps peace and good luck clammy

tropostudio

Clammy -
Makes total sense. Thanks!