Best Of

This project was great fun and put all my 40 years of hydronics expertise to the test. This client has a small Koi breeding operation. The Koi in these two ponds are worth close to 100K. They are his babies. He wants to see them all year round. The piping pictures to follow. Mad Dog

This is a 28 year learning and experimentation lab for me. As I learned better hydronic techniques and strategies, I would repipe accordingly. The Home has staple up Radiant, Mud Job Ceramic tile Radiant, Buderus Panel Radiators, Modine Heater for Classic Car 🚗 area. Mad Dog

Now let's move on to efficiency. To address the efficiency problems associated with low duty cycle, we had to consider what the actual causes were:

1. The boiler was relatively separate from the main conditioned space, and it was surrounded by bare concrete block walls. Some BTUs 'trapped' in the boiler would make there way to conditioned space, but not nearly as effectively as if the boiler were in the center of my living room.
2. The oversized nature of the radiation meant that calls for heat were satisfied relatively quickly, so frequently the boiler would heat up from room temperature to 180F in a few minutes, satisfy the call for heat after delivering a few thousand BTUs, then cool back down significantly before the next call.
3. The cast iron boiler, although not 'high mass' by historical hydronic standards, still contained quite a bit of thermal mass compared to the baseboard loops on each floor. The boiler had a few gallons of water and a few hundred pounds of cast iron in it, vs a total of 150' or so of baseboard in the rest of the house. The boiler by itself, with ~180F water, stored 8-10K BTUs when a call for heat was done, vs maybe a few hundred stored in the water and baseboards for a given floor.

The larger aquastat differential sort of helps with #2, by slowing down the delivery of heat so that more time is spent radiating heat via baseboards instead of just the boiler itself (this is tempered in the other direction by the boiler staying 'hot' for longer, but even the reduced output still satisfies the calls for heat pretty quickly). After a lot of experimenting and researching, I settled on adding an 'all-zone' thermal purge at the end of a heating cycle. What's the quickest way to transfer the BTUs trapped in the boiler into the conditioned space? When the last zone is done calling for heat, open all three zone valves and run the circulator until the water drops back to room temperature. Any zones that had been inactive will likely be sitting on a loop of pretty chilly water that can rapidly mix with the water from the active zone and the boiler itself, and the large differential means that frequently the water is closer to 120-130F when the call for heat ends, rather than 170-180F.

I detailed my experiments in a previous thread here. The effect on measured efficiency (my monitoring setup tracks the average temperature of each zone once a second and estimates delivered BTUs) was pretty dramatic, although I don't have a good way of estimating how many BTUs were finding there way into conditioned space just via convection from the boiler.

The best graph came from a series of isolated experiments where I just tracked delivered BTUs vs the temperature when I stopped purging:

BTUs escape much easier from radiators than the boiler itself, so it's extremely effective to just keep the circulator running. Looking at the daily results from when I started tracking all of this vs now shows that there is almost no dependency on estimated efficiency vs runtime:

What about overheating? The rate that heat is delivered depends on the temperature difference between the baseboards and the air temperature. Every zone is constantly losing heat, even if that zone's thermostat isn't calling for heat (ignoring things like solar heating for now) yet, so preemptively delivering heat at 1k-2k BTU/hr during a purge has very little chance to overheat things. I've never noticed any overheating in practice - I think the primary effect is to slightly delay when those zones will next call for heat. This scheme also has a very nice benefit in terms of freeze protection (particularly when using deep setbacks on some zones). When using an overnight setback of 6F on the main floor of my house, the purge cycles from the other two zones mean that water is never sitting still for very long.

To address the short-cycling, I effectively wound up modifying the aquastat (using an additional PLC controller with a strap-on temperature sensor and an extra relay to inhibit the burner) to use a 60F differential, rather than the default 20F. This lowers the average water temperature in the baseboards, but they could actually meet the heat loss at the design temperature with 120F SWT. With my PLC, the aquastat high limit remains at 180F, but now waits until it falls to 120F before firing again. This obviously triples the run time for 'short' burns, but it also lowers the average heat output during a call for heat and allows for more overlap between the different zones calling for heat. The cycling is drastically reduced.

120F is nominally in the 'condensing' range for natural gas, but due to low gas pressure my boiler is slightly underfired, and the higher excess air puts the condensing temperature down in the 116F range when I had it combustion tested this fall. I have a 'cold-start' cast iron boiler, so it's frequently operating in the condensing range anyway (and I have zero evidence of any problematic condensation in the boiler's 10 year lifetime so far), but this was as low as I felt comfortable setting it.

This idea wound up being unnecessary, but I had initially considered doing an 'indoor reset' scheme, where I modulated the aquastat low-limit (rather than just leaving it fixed at 120F) so that I could increase the average SWT (and thus heat output) as necessary, based on how long the thermostats were calling for heat. I think this would be a very effective way to modulate the heat output of a CI boiler with zones if it's merely 'pretty oversized' rather than 'wildly oversized.'

The mixing station and Flat Plate HX and pond filtration

The Williamson on the left has been the dedicated pool heater/boiler for the Gunite pool for the last 25 years. I repiped for boiler protection (in a very tight corner and around his tool shelves) and to provide pond warming, so this boiler now gets year round use. WIRING BY OTHERS. Next time I'm there I'll bring a bag of zip ties. Mad Dog