End of Season Report V3: HPWH, Zone Sync and More!

This is my 3rd season of monitoring/tuning/tweaking my 3-zone hydronic system (Part 1:

and Part 2:

The main changes this year were the swapping of a gas water heater adjacent to the boiler with a HPWH, and the addition of a DIY zone synchronizer and "De-Thunker":

The PLC performed zone synchronization, 'de-thunking' (where it inhibits the boiler/circulator before allowing the zone valves to shut), as well as a basic thermal purge (attempting to run the circulator after a call for heat until the water temp is just above the aquastat low-limit. The main questions I had going in were:

• Would the de-thunking work?
• Would the zone-synchronization help with the short-cycling? Would it make some floors too cold?
• Would the HPWH make the basement freezing and/or significantly increase the load on the basement zone?
• Would the thermal-purge feature do anything useful at all?

The bluetooth monitoring scheme was a little less reliable this year, so I stopped collecting data sometime in February, but I think I got decent answers to all of my questions.

• The de-thunking worked perfectly - we didn't hear a single water-hammer-type noise all season.
• The zone-synchronization did actually help some with short-cycling - you can see a small but clear shift towards longer run times, but the higher heat loss and less oversizing of the radiation on the 1st floor means that overall it's still dominated by ~90 second burns to maintain the temp while only a single zone is active. The zones normally operate with temperature setbacks at night and during the day, which wreaks havoc with the zone-synchronization algorithm. I wound up counter-productively reducing the use of setbacks to make things play nicely with the zone-synchronizer. The zone synchronization also makes it difficult to just turn up the thermostat temporarily, as the system must still wait for all of the other zones to call for heat, which was not popular with other members of the family. The 'de-thunking' mechanism also introduced its own short-cycling issues (which also shows up in the histogram) because I interrupted on-going burns in order to inhibit the circulator when a zone needed to shut off. All in all, it 'worked', but probably qualifies as too clever.
• Happily the HPWH was fine, and somewhat surprisingly, the overall heat loss for the house in terms of BTU/HDD-Hr seemed to stay pretty much unchanged (or maybe very slightly higher). Maybe that means I actually managed to increase the efficiency enough to 'pay' for the heat the HPWH was extracting from the space? Removing the gas water heater did mean the always-open flue got closed. Any discomfort in the basement I would attribute more to the zone synchronization than the HPWH itself.
• The thermal purge feature seemed to work okay. The aquastat high-limit was 160F, and would drop down to 140F before it fired again. I used a strap-on temp sensor on the supply pipe to guess when the boiler was sensing 141-142F, and after the last zone was done calling for heat, I would continue to run the circulator right until the aquastat was about to tell the boiler to fire again - usually an extra 5 minutes or so on average. I never noticed any sort of over-heating, although occasionally my temp sensing was a little off so it wouldn't shut off the boiler until it had already started firing again for a few seconds.

A histogram of burn times (as a fraction of total) for this season and last season shows the small but clear shift in burn times (as well as a much longer tail of 'long' burns).

And a scatterplot of this season vs last season shows the pretty much unchanged heat loss in terms of BTU/HDD-hr.

So mixed results overall. I haven't decided if I'll go back to a simpler setup or double down and try to improve things some more. I think my main 'wish list' improvements (that don't involve plumbing) would be:

• Make the thermal purge inhibit the burner but not the circulator, so that I can purge it all the way down to room temperature.
• Make the PLC aware of whether a burn is in progress so that it lets it complete.
• With the better thermal purge, maybe ditch the zone synchronization (or try to make it a little more clever).