Burner shuts off before high limit is reached

gitoby

I had a new PB WBV series oil boiler installed with a Riello burner, my system has 3 zones 2 for heat 1 for an indirect water heater. The burner has been cycling on and off several times when the water heater zone opens instead of just running until the zone is satisfied is this normal??

Steamhead

This boiler should have a HydroStat or AquaSmart control on it. Did they wire the indirect zone so it bypasses the "economizer" function?

pedmec

its not suppose to run till the tank is satisfied. its runs until the boiler water temperature satisfy's the aquastat setting unless the tank is satisfied first. it will cycle until the demand from the water heater tank aquatstat is satisfied.

EBEBRATT-Ed

Sounds like the boiler is making more heat that the indirect can absorb.

Are you calling the "high limit" the boiler control or the control on the indirect??

Alan (California Radiant) Forbes

The burner has been cycling on and off several times when the water heater zone opens instead of just running until the zone is satisfied is this normal??

Have you been in the house long enough to say that this didn't happen on a regular basis before? It could be that the heat exchanger inside the indirect has a mineral build-up, preventing proper heat transfer. What is the brand and model of the indirect? What is your water quality like?

EdTheHeaterMan

Depending on the control you have on the PB WBV, you have a high limit control that also acts as a low water cut off control. The Beckett 7600A or the AquaSmart or HydroStat will determine when it is safe for the burner to operate based on water level and water temperature in the boiler.

The sequence of operation of all these controls is about the same.

1. there is a call for heat from a thermostat, or the water heater temperature adjustment control (a liquid thermostat).
2. The zone valve for that zone (or zones) opens and the end switch closes a set of contacts once the valve is open
3. This end switch will activate the Boiler control to start the burner and operate the circulator.
4. As the boiler water heats up, the hot water will be pumped to the zone that is calling for heat (could be space heat radiator or could be the water tank)
5. The burner will continue to operate until the zone is satisfied or until the water in the boiler gets hot enough to open the high limit switch.
a. If the zone thermostat is satisfied the burner will stop and the circulator will stop and the zone valve will close.
b. If the thermostat is not satisfied the burner will stop however, the circulator will continue to pump heated boiler water to the zone (or water tank).
c. If the boiler water temperature drops about 10° and the zone thermostat is still not satisfied, the burner will restart and heat the water until the zone is satisfied or the high limit is reached again.
6. Once the thermostat is satisfied the zone valve closes, the burner stops and the circulator stops.
7. If another zone thermostat calls for heat during any of the above operations, the zone valve for that zone will open and the burner and circulator will operate as described above until all zone thermostats are satisfied.

Is it possible that item 5b and c is what is happening? If so then that is normal.

If you do not have zone valves however you do have separate circulators for each zone, the sequence of operation is about the same!

Is it possible that the boiler water zone piping for the indirect water heater is too small? I always used 1" or 1-1/4" piping to the indirect even if the tappings on the tank were smaller. You see you can get more boiler BTUs to the Tank heat exchanger in a bigger pipe. Rule of Thumb is that 3/4" will handle about 40,000 BTU of heat, while 1" Pipe will deliver 80,000 BTU. If your boiler capacity is 80,000 or 100,000 BTU and the only zone that is open is a 3/4" pipe, the boiler will not be able to move all the heat it can make, so you will have the equivalent of a BTU traffic jam. The water in the boiler gets to hot and the burner stops until that small 3/4" BTU highway can let the traffic out to start the burner again.