Should boiler shuts off while circ pump is still on?

[mhmmofro]

I have a 4 zone hot water boiler. I reticently added the 4th zone. I have noticed that occasionally when the 4th zone is calling for heat the boiler will turn off while it is still calling for heat. When this happens the circ pump for that zone is still pumping and the boiler is up to temp accoring to the gauge on it.

Is this normal for the boiler to shut down like this? This zone is much smaller then the rest of the zones.

[electrichillbilly]

yep, boiler is reaching set temperature probably 180 degrees, then shuts down, circ pump will continue to circulate until room thermostat setpoint is reached, in the mean time when the boiler water temperature drops to the set temperature probably around 170 degrees, the boiler will fire up agian and run until the 1800 degree temperature is reached, hopefully your zone is not configured in such a short loop that the boiler will short cycle meaning go on go off in a short period of time, or your temperature set pointys are so close together that this could happen, sounds like your system is operating like it was designed to do, is it a wonderful thing

[Dana]

Even on small zones if you oversize the radiation you can limit the short-cycling, but simply bumping up the high-limit up to attain longer burns would also increase the standby losses related to the short cycle on that zone call. If this is a high-mass cast-iron type boiler running 4 zones, it's probably more than 3x oversized for even the largest zone, and most burns are shorter than 10 minutes, maybe even shorter than 5. It could easily be an order of magnitude oversized for the micro-zone. If this is the case the efficiency hit from the short-cycling on all zones is considerable, and installing a retrofit economizer control such as the Beckett Heat Manager or Intellicon 3250 HW+ can result in a double-digit reduction in fuel use.

These work by maximizing the temperature hysteresis of the boiler, and by heat purging the boiler at the start of a call for heat by running the circulator until the boiler's temp hit's the programmed low, and "learning" from recent burn history when to cut the burner prior to the thermostat being satisfied, for a partial heat purge at the end. The result is lower standby loss between burns (since the boiler is never parked at the higher temps), and fewer burns but of longer duration, but a lower overall duty-cycle on the burner. On a call from very small zone with a low heat if the boiler had been parked at say, 155F from a previoius burn, and it was programmed to not fire up the burner until it hit 130F (OK for most gas-fired boilers- 140F would be necessary for most oil burners), it's possible that there is sufficient residual heat in the boiler that it could skip the burn entirely. (It depends on the mass of the boiler, the heat load, and the hysteresis of the thermostat on that zone.) On larger zones where it COULD run continuously at 170-180F without tripping the high-limit it heat purges to 130F at the beginning of the call for heat, but also anticipates the end of the call for heat, and kills the burner ahead of time, pulling heat out of the boiler and into the zone for the finish. There will still be times when it short-cycles on the smaller zone, but the efficiency losses from those cycles are still reduced by the heat-purging.

Under smart-economizer control the effective high-limit will vary with the overall heat load of the house, and the low limit is whatever you set it to. The AVERAGE temperature of the boiler will be lower though, reducing distribution and standby losses. The strategy is distinct from "outdoor reset" control where the output temp is automatically adjusted relative to the outdoor temperature, which works very efficiently for single zoned systems with boilers that aren't radically oversized for the peak heat load, but ODR can make the burns per hour go way up on micro-zoned systems, making it less efficient. Exercising the thermal mass of the boiler to maximum effect is almost always as more more efficient than ODR in high-mass systems with more than 2 zones, and easier on the equipment (fewer ignition cycles, etc.) ODR is dramatically more efficient with low-mass condensing boilers (which is why modulating condensing boilers are almost always controlled that way), but even with those efficiency can be robbed severely by short-cycling on tiny zones.

The only reliable way to stop short cycling is to add thermal mass (usually water volume, but also high-mass radiators or concrete slab floors) to the system or zone in such a way that that all zone calls result in long burns. Ideally with a high-mass boiler you'd be at 5 or fewer burns per hour under any load conditions, with minimum burn times in excess of 5 minutes (10 minutes, even better!)

[Tom Sawyer]

Tis working perfectly, leave it alone

[mhmmofro]

Yea i figured that it was normal because the boiler had reached operating temp. I just had never had it happen with the other zones and i am far from a plumber so i was thinking i may of screwed something up. This zone is a real low volume baseboard loop so it makes sense.

thanks
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