A simple, effective one pipe cycle timer

chuckNJ

19 years since I joined this site, 18 years since my last post (although I'm on the mailing list and do check in and lurk a bit periodically). I was very lucky to be introduced to the wisdom of Holohan by our then brand-new next-door neighbor almost immediately after we bought the house. After the first year of heavy lifting to straighten out the one-pipe system (with key help from a well qualified pro), starting with a new boiler, lots of work on pipe pitch, valves. vents, etc., culminating in the application of a vaporstat and a commitment to good maintenance, we've been happy enough with it that it that only minor tweaks to try to improve comfort across various conditions have been made periodically. Downsized some of the excess radiation (thus oversizing the boiler a bit), added some TRVs (causing some further variable oversizing), replaced the trusty Honeywell VisionPro with an Ecobee. Not perfect, but acceptable. However, I have been bothered for some years now by the short cycling, temperature overshoot and limited performance/function of the TRVs. Sooo…

I recently happened to see and was inspired by @PMJ's posts in this thread regarding his PLC based efforts at cycle management. Having a skill set that includes a considerable amount of electronics and systems experience, I set out to see if I could achieve at least some of his gains with a simple, inexpensive, robust and fail safe solution. I won't be able to fully judge how successful I've been until next winter since the deep cold has already passed this season, however I already judge it a significant success so I thought I would share.

Here are 2 pictures of my initial prototype, which is also the essential core of my current implementation - which now additionally wraps some status monitoring around it:

The module is a 24vac "multifunction programmable timing relay", the sensor is a N.O. bimetallic thermal switch. Including some wire, this represents a total investment of less than $100. The N.C. relay contacts of the module are in series immediately following the thermostat and ahead of the boiler safeties and control wiring. The power for the module is also taken from this line so the device is powered by the boiler throughout a heat call. The N.C. contacts pass the heat call until the relay actively interrupts it after its programmed delay is triggered and has completed. The dry contact "trigger" inputs are connected to the thermal switch which is mounted to the top of the return drop at the end of one steam main. With a cut-in at 194 F and cut-out at 158 F, this has worked well as a "steam up" sensor. This arrangement isolates the highly variable "time before steam is in the rads" from the time steam is producing heat in the rads. The relay is programmed to delay its activation by a fixed interval after "steam up" is achieved, this is the time interval steam gets to heat the radiator iron. I determined this time following @PMJ's suggested methodology, which took some study of system behavior over multiple days and varying weather conditions. Once the relay kicks in, the boiler is held off until the thermal switch drops away. While the relay module also has the capability to add a separate fixed delay to the "steam off" (or rest) time, I have not found that to be needed - the hysteresis of the thermal switch seems to establish an appropriate rest period. The rads are still putting out significant heat when the switch opens, the relay releases and the boiler starts up again (as long as the heat call is still on from the tstat). On my system this is running roughly a 40% steam time to 60% rest time ratio. This achieves about 2 cph AND the TRVs now can and do work much better. Temperature overshoots are a thing of the past, the vaporstat never operates because there is never any pressure build and comfort is noticeably improved. I did find that it was also beneficial to tweak the venting at some rads to accomplish optimum amounts of heating for each rad during the " steam on" time, this further improved TRV performance and evenness of heating and significantly reduced the issues associated with original but now oversized radiators. It remains to be seen next winter how much adaption occurs to the rest time in really cold (high load) conditions, and how well the comfort performance holds up.

If you have enough electrical knowledge to be comfortable with the basics of boiler wiring, this was simple, inexpensive, pretty easy to implement - and produced very perceptible comfort performance enhancement.

chuckNJ

To answer the question not yet asked: the relay I used is a Macromatic TD-88168. There are are certainly other options available capable of the same functionality, some with fancier user interfaces than DIP switch programming - for a higher price.

The thermal switch is a pretty generic part - I found these on Amazon where I could get a small quantity quickly and inexpensively (2 for <$10.) I did try one with a higher pair of cut-in / cut-out temp ratings but found it would not trigger on steam in the pipe reliably in my configuration.

EBEBRATT-Ed

Nice.