Furnace stops after about 10 seconds

Hi,
I'm trying to save some time sitting in front of my furnace. I have a five year old Dunkirk Mod 3E.75C furnace that I installed with a Beckett AFG50MB burner, a Honeywell R4184 D 1027 reset unit and a Honeywell Tripple aquastat relay type L8124AC. There are two zones with taco gold zone valves that are about two years old and two taco thermostats that are about thirty years old.
About once out of five cycles the furnace starts, runs for about 10 seconds and stops. It doesn't require a reset to start the next cycle which is usually a few minutes later. I've cleaned the furnace and photo cell and adjusted the spark gap. It sounds normal both on startup and shutdown. I do have sludge in the tank. If the fuel wasn't steady wouldn't the photo cell detect this and require a manual reset? Thanks for any advice.

 

The safety control will only lockout the burner if there is 45 seconds of burner run time with NO FLAME. If the unit starts and stops 50 times in 5 minutes, it won't lock out. Your problem could be the anticipator setting on one of the thermostats. It should be set at .9 amps and will cause short cycling if it is set lower than that.

Ken

 

I not sure I understand. It's not just the flame that stops. It's the burner motor. It doesn't constantly short cycle it is about once out of five times it runs. Many times a day I hear it start and then stop a short time later usually less than 30 seconds. I've checked the zone valves manually to see if they are actuated electrically when it stops and they are both closed so I think that might eliminate the thermostats. Is that right?

I'll try to check on the core temp when it fails to see if that will give us some clues.
Thanks again.
Frank

 

This is not uncommon with Taco zone valves. Like Ken said, make sure the heat anticipator on the thermostat is set to at least .9amps. Another thing that can cause this short cycling is if the zone valves are not wired correctly, the circulator & burner can come on before the zone valve opens. Since there is no place for the water to go, the boiler hits it's high limit & the burner shuts down.

 

I just noticed that the temp gauge on the front of the furnace is showing the furnace starting at 205 and going off at 225. I know these aren't the most reliable but I used a 200 degree melt stick on the discharge pipe feeding the heating system. It melted. The aquastat is set at 140 180 with a 15 degree differential. Other times I notice the temp drops to 175 before the furnace starts. Any thoughts. Thanks

 

I can only think of one thing that makes those aquastats that inaccurate. If they are installed in a very long well and the bulb is not pushed back into the end of the well. You have to pull more cap tube out of the back of the
control in some cases to reach the end of the well and if you don't, half of the bulb is not sensing boiler water temperature. Check out if that may be the case and it could account for some of the trouble.

Ken