Thermostat short cycling dropouts

Mike Kusiak_2

Has anyone noticed similar short cycling with Honeywell programmable thermostats and steam systems? I noticed that the burner drops out for about one to two minutes after either 8 or 20 minutes of firing. The periods seems rather consistent although it appears to occur at random times. It is not pressure cycling and definitely caused by the thermostat.



Is this something inherent in the control algorithm, or a defect. If it is there by design, what possible purpose would it serve?

nicholas bonham-carter

honeywell visionpro peculiarities

i've noticed that the boiler will fire, and if i blow down the lwco, with its resultant cutting off of the burner, the thermostat will not re-fire the burner for a longer time than i would have thought. also noticed the short-cycling from the visionpro of which you spoke. i wonder if the occasional static shock from my finger may have confused it!--nbc

Mike Kusiak_2

Peculiarities

I don't think its by static or external causes, as the boiler was completely unattended when this data was taken. The off periods seem quite constant in duration at about 1.5 min, but my sampling interval is 30 sec, I can't resolve it any more accurately than that.



I probably would never have noticed the cycling if I had not been logging the data, but on one occasion actually observed a dropout when standing near the boiler and wondered what caused it.



I found a similar thread from a while back, where someone else noticed the same behavior with the Honeywell thermostat:



http://www.heatinghelp.com/forum-thread/134814/Thermostat-vs-Boiler-Short-Cycling



Here is an expanded trace of the cycling occurring at about 8 min intervals, with 1.5 min off times.

jpf321

are you sure ...

that you don't have one of those fancy Cyclegard/Hydrogard/Watchdog Probe LWCO thingy's that shuts down for a "foam test" every so often? 

Mike Kusiak_2

LWCO

No fancy electronics. Only a M&M 47  float type combination automatic feed valve/ LWCO. At first I thought it was shutting down momentarily on low water, but the valve seems to first add water and then only shut down if it cannot maintain level. I tried draining water while firing, but makeup water was added faster than I could drain it and it never shut off the burner.



The only possible reason I can think of for purposely dropping out the burner periodically relates to the use of TRV's. If the boiler steams for long periods without cycling, air will never reenter the radiators, and TRVs controlling venting would be ineffective. Perhaps Honeywell added the dropouts to the steam setting control algorithm for this purpose?

Rod

Thermostat Dropouts.

You might try Honeywell Tech support 1-800-468-1502 and see if they can give you an answer. The TRV idea is interesting. I've often wondered about this problem with TRVs myself when using  a typical "normal" thermostat. If Honeywell actually thought that far ahead I'm impressed.

- Rod