Mysterious Short Cycling--T-stats end call for heat after 5 minutes firing; then boiler starts again

D107

Needless to say very frustrated at this. Peerless MI-03 with Turbomax indirect. DHW is fine. DId a heat test all three zones ten days ago everything heated up fine. Now tonight two zones called for heat and in both cases t-stats shut down after five minutes. We never had any anticipator issues before--usually when room temp was about to drop just below setpoint, boiler would fire and stay on. I know this is a borderline season and temps on t-stats do reflect the target temp, but boiler is going on and off every five minutes. It's mostly one zone over and over again. Hydrostat does have the circulator hold off feature which seems to be working correctly. The T-stat in question is the White Rogers 1F78

Zman

Is the thermostat satisfied or is the boiler cycling on high temp?

D107

T stat satisfied room temp matches target. Cuts out when boiler hits about 132. Hi temp limit is 160

Zman

It looks like the anticipator is adjustable.
https://www.alpinehomeair.com/related/1F78 Digital Programmable Thermostat Spec Sheet.pdf

D107

Thanks; apparently when I opened it it's a IF90-51 probably 60 series. This is probably from the early 80s. has always worked great. I have the manual. I see that anticipator adjustments are possible by pressing set clock and set day at the same time. Perhaps that was inadvertently done and changed things. Otherwise I would think this thing is just getting old.

D107

I checked the anticipator setting and it was the factory default of 5, but they should be 15 minimum for hydronic systems. So somehow it slipped into default--not sure how. Now I will have to see if the other thermostat Honeywell MagicSTat/32 is doing the same thing or sometimes short cycles because it's right over the uninsulated boiler piping--intend to insulate soon or will throw some insulation up between the rafters. Thanks much for your response at this late hour. Tomorrow morning will tell the tale.

HVACNUT

Well we are on the shoulder months so some short cycling is to be expected but one zone turning off and on so soon does seem like a thermostat issue. Also with a cold start boiler, when one zone satisfies the boiler shuts down. 10 seconds later another zone calls and the boiler starts and cycles again. It might be time to upgrade your thermostats regardless.

D107

@HVACNUT thanks. It did seem to be the anticipator setting perhaps being accidentally set back to default setting of 5--which would be very frequent cycling. I tried 15 and now 20, which is much better.(40 is the highest setting.) I can tweak it as we go. As you know, too high and it will stay on longer but take longer between cycles. I really like this old White-Rogers unit--has a lot of anticipator fine tunings that the newer units don't have. FYI my system wiring is set up for two wires only. Also with the new correctly sized boiler, we removed any setback and keep it 65º 24/7, so we don't get that great morning boost from 63º that felt so comfy. This way our cast iron boiler won't have tons of very cold return water hitting it every morning startup.