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I have a hydronic heating system in which water heated by a boiler is pumped around the perimeter of the house using circulator pumps. Currently the flow rate is too low, causing the boiler to repeatedly cycle on and off. Unfortunately, increasing the head/flow rate of the pumps causes cavitation.

What are my options?

Pumps: https://www.xylem.com/siteassets/brand/bell-amp-gossett/resources/manual/671075208_ecocirc20_18.pdf

Boiler: https://s3-us-west-2.amazonaws.com/catsy.782/X-2+IO+Manual.pdf

Additional info from comments:

The expansion tank is on boiler output before the piping splits to each zone. The pumps, however, are on the individual zone returns.

The pressure gauge on the outlet of boiler reads 17 PSI.

Regarding cycling, the boiler is cycling not because the thermostat turns off the pumps, but because it very quickly heats the water to 180 deg faster than the pumps can move in colder water from the return. To heat one zone 2 degrees, I'd say it cycles about 10 times (2 minutes on, 4 minutes off).

Also, only the first baseboard radiator in each zone gives off significant heat.

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  • The expansion tank is on boiler output before the piping splits to each zone. The pumps, however, on on the individual zone returns. The pressure gauge on the outlet of boiler reads 17 PSI. Regarding cycling, the boiler is cycling not because the thermostat turns off the pumps, but because it very quickly heats the water to 180 deg faster than the pumps can move in colder water from the return. To heat one zone 2 degrees, I'd say it cycles about 10 times (2 minutes on, 4 minutes off). Also, only the first baseboard radiator in each zone gives off significant heat.
    – user169686
    Sep 8, 2023 at 15:42
  • Page 28. "When Priority Time parameter is set to “on” and Domestic Hot Water (DHW) call for heat is “on”, the DHW demand will take “Priority” over home heating demand. During Priority Time, system circulator will be forced “off”." - "Circulator Pre-purge Time (PP_) See Table 11- 5. When boiler is warm (boiler water temperature higher than 140°F (adjustable using start temperature parameter) and there is a thermostat call for heat, system circulator is started and boiler firing is delayed [by the] Circulator Pre-purge minutes."
    – Mazura
    Sep 9, 2023 at 22:10
  • What's the Circulator Pre-purge Time set to? Don't let it come on until it's long since been taking 80 degree water and mixing it with +140. And don't let it go into priority mode because that stops the circ pump. - If it can't keep up, then it has to at least keep moving.
    – Mazura
    Sep 9, 2023 at 22:11
  • @mazura the pre-purge is set to the default. Do you recommend lowering it so that the boiler fires on sooner? The DHW isn't the source of pump shut-off. In fact, my pumps don't shut off, only the boiler.
    – user169686
    Sep 10, 2023 at 13:19
  • The opposite. It only cavitates if you mess with the pressure. The problem is it short cycling. Means its burner is too big. So set the temp as low as possible, delayed for as long as possible. Then one or two of the short cycles gets caught in the delay. HL = 140. dF = 20~30 instead of 15. Or=10. PP=10m instead of 2m? Set the differential on your thermostat to 2 instead of 1. - Note, (with deff that last one) we're starting to trade your comfort so as to be nice to the boiler.... Also not my specially; grain of salt here. dF could be key, that's tank temp differential. Try just that with 30.
    – Mazura
    Sep 10, 2023 at 18:56

2 Answers 2

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This seems to have been not well set up by whoever designed it. Stock advice on the relationship of pumps and expansion (aka buffer) tanks is (as @Solar Mike already said) that the expansion tank should be on/near the intake of the pump, precisely to prevent cavitation. With the pumps on the returns and the tank on the boiler outlet, they are sucking through a very long straw. They are much better behaved if blowing into a very long straw.

If your current pressure is 17 PSI and your pressure relief is a typical 30 PSI setting, you may get some improvement by raising the system pressure to something like 25-27 PSI - of course if it gets too close to the relief setting it may go over under some conditions and release a bit of water. If your valve is in good condition that's not a big deal, if it's old and corroded it may fail to re-seal after operating. So you might want to approach the limit slowly rather than jumping very close to it if not sure of your valve resealing itself. Or you may want to proactively replace it if it seems dubious. If replacing it, use one of the same rating to maintain safety, or consult your boiler manufacturer on a safe replacement specification.

That's the easy one to try, since it doesn't involve much (or any, if the valve is good) plumbing; by raising the static pressure, the point at which cavitation (due to negative pressure) begins is moved by the same amount.

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  • Thanks! This work was actually just recently done, so I may be able to get the pumps moved from the return to the supply at no charge if I can make the proper case to my plumber. Just curious: does the current setup have any advantages? (pumps on return, expansion on supply)? For instance, wouldn't moving the pumps to supply also raise the temperature of the pumped water and hence encourage cavitation? Seems non-obvious to me which is better.
    – user169686
    Sep 8, 2023 at 21:42
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    Pumps on supply side get hotter and age faster, but have less cavitation problems. The best solution is to have pipes of the proper diameter according to desired flow rate. Unfortunately that's a lot of work to change... If the pipes are old, perhaps they're a bit clogged, or your radiators valves may be stuck, or there may be other flow obstructions.
    – bobflux
    Sep 8, 2023 at 21:55
  • Having the pumps on the hot end does increase the odds of cavitation due to higher temperature. In your case they'd also be lowered to to the proximity of the tank, so that would be a trade-off. With zone valves a single pump on the return can be served by a single expansion tank before it - with individual pumps, you need more expansion tanks to have one per pump. In any case, the person or company who designed this should make it work correctly, and it pretty obviously does not, from your detailed description of symptoms.
    – Ecnerwal
    Sep 8, 2023 at 21:56
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So, often the pumps suffer from cavitation due to poor inlet conditions.

Improving the inlet supply will sort this, one method is to have a buffer tank to give adequate supply.

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  • I have a buffer tank. However it is on the supply side of the boiler while my pumps are on the return.
    – user169686
    Sep 8, 2023 at 15:57

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