Boiler gas valve on/off

Glenn_5

I've noticed that my hot water boiler (natural gas) will sometimes try to fire but then immediately shutoff. This usually happens after the boiler has been running for an extended period. I'm assuming the water temperature is too high and keeping the boiler from firing. I have 4 zone valves (Honeywell) and not sure if it's isolated to one zone or not. Any thoughts on how to correct/troubleshoot this? Thanks!

Docfletcher

Tell the guys what brand boiler. Post some photos. How is it vented? Could be a number of causes. But I doubt water temp is at fault.

DZoro

A little more info is needed,
What size and type of boiler?
How large are the zones?
What water temp are we dealing with?

HVACNUT

With a cold start boiler and four zones, what's likely happening is before a heat zone shuts off, the limit differential will fire the boiler to raise the temp back to High limit. At about that same instant the thermostat will satisfy and shut off the burner again. And when other zones come into play, it's a vicious cycle.
It's fairly common when micro zoning without a buffer.

JUGHNE

Does this boiler have a flame sensor rod and if so, has it been cleaned lately. Dirty flame sensor can give you the short firing issue.

Glenn_5

It's a carrier 140000 btu boiler.

Glenn_5

High temp shutoff is 170 degrees. Zone 1 is a bedroom, Zone 2 is a bedroom, Zone 3 is living room & kitchen areas, and Zone 4 is basement area.

DZoro

Pretty large boiler, especially with small bedroom zones. So it's short cycling really hard. What is the sqft of your home? How large are the bedrooms? The boiler piping looks to be only 1" and 3/4" If that is the case it cannot even handle the boilers capacity
D

Glenn_5

Sqft is about 2700. Bedrooms are 11 x 15. Boiler is about 15 years old

DZoro

Don't know where you live, but if the 2700sqft includes both levels, a rough estimate is that the boiler at least 50,000 too many btu's to run the entire home, let alone a small bed room. So yes there is definitely short cycling going on. Probably not much you could do without a major re pipe/new boiler/buffer tank.
D

Glenn_5

DZoro.....What's the downside to the short cycling?

HVACNUT

That's over 41 BTU's per sq. ft. Way oversized. How long have you been in the home? Have you been there for years and this is a new issue? No changes or zones added, repiped, etc?
Any interruption in the burner circuit will shut off the gas valve. At that point, the vent damper opens contact on the end switch, and the damper must make a full revolution in order to complete the circuit again. Keep an eye out.
I would also be interested to see this experiment. Turn off the thermostats in the bedrooms and basement. Manually lock those zone valves in the open position. Operate the heat with the Kit/Liv rm. thermostat only. Less short cycling. Especially in the shoulder months.

Glenn_5

Been in home for 20+ years. Started noticing the issue last winter. I'll try the experiment with the zone valves too. What is there to watch with the damper; CO, odors, etc? Disappointed to hear the boiler is oversized....

HVACNUT

The purpose of the flue damper is for efficiency. Damper closed on the down cycle. In order for the burners to fire, the damper (among other controls) must open and "prove" through and end switch. This is normal operation any time there's a demand for an on cycle. Just something to watch to see if its acting funky.

DZoro

Down side to excessive cycling/oversized equipment:
High gas bills
Discomfort in home, usually heating over run
Hard on equipment, on/off cycling
D