Oil burning but furnace trips

erictomlin

Hi pros,

I have an oil fired furnace that was serviced toward the end of last season and worked fine for the remainder of the season.

Now upon startup of this new season I am experiencing an issue. When the temp drops and the thermostat calls for heat, the burner starts normally and there is fire in the furnace. However, after about a minute, the box on the furnace trips and the furnace shuts down. If I hit the reset button, the furnace fires up immediately and there is good fire in the fire box. At this point, sometimes the box trips again after 30 to 60 seconds and sometimes it doesn't. If it does trip, I hit the reset and it fires right back up again with good flame (so I'm not building up oil in the fire box).

The interesting part is that once the furnace warms up and the blower motor kicks on, I'm good to go and the box won't trip again and the furnace will warm the house to the desired temp.

The problem starts again the next time the COLD furnace needs to run. I tested a warm furnace by turning up the thermostat and calling for more heat while the furnace was still warm. The furnace operated as it should. This is consistent with the furnace being tripped the last three mornings when I wake up since the furnace doesn't run all night and has time to cool down.

Would love to hear your thoughts on this one. Why would the blower motor running or not be the deciding factor?

Thanks.

-Eric

EBEBRATT-Ed

Stop pushing the reset button. You need to get your technician back. Could be the primary control or the cad cell or..............any number of things.

Big Ed_4

Well depends on location of reset button ? If button on vent pipe (Stack Relay) Fire can be too small . If button is located on burner (cad cell relay) The cad cell is not seeing the fire enough on a cold start . Why then with blower running ? inspect fire before and after if there is a change there may be a hole in the heat exchanger or vent pipe if furnace is a low boy

Jellis

Put in a call to the service tech.
as mentioned there are a handful of things that could cause this issue, a competent tech should be able to test and properly identify the problem
without the knowledge and tools to test the equipment you would just be throwing parts at it which will cost you more in the long run likely.

EdTheHeaterMan

I like Big Ed_4's analysis. It was the second thing I thought of! The first thing was the flame sensor is not sensing the flame. On stack relays (the ones on the vent connector from the furnace to the chimney) the bi-metal might be clogged with soot. On the newer Cadmium Cell flame sensor The eye with the yellow wires connected to F F may be dirty.

Once I read further down to realize that your burner stays on after the fan operates, then I thought of the cracked heat exchanger scenario. When the fan operates, the heat exchanger is located in a pressure zone that is quite different than that of a fan off condition. This pressure change should have no effect on the pressure on the fire side of the heat exchanger... UNLESS IT HAS A HOLE IN IT... That could be a crack or a failed gasket or the fire inspection opening (if you have one) not being properly sealed. when the cleaning was completed last spring... were the heat exchanger clean-out openings properly sealed?

If you have a hole or crack. the change in air pressure on one side of the heat exchanger (Fan off then fan on) will cause a change in the air pressure in the fire box. this air pressure change will make the burner blower wheel deliver a different amount of combustion air to the fuel. This may make the flame change brightness or burn hotter. This change will effect the flame sensing and if you are near the limits of the control's sensing capability, that small change may keep the burner operating. This is a temporary condition. As the burner operates and more build up and accumulation of byproducts of combustion form... you will get to the point where reseting will no longer work.

If you don't have the proper instruments to check the flame signal, and combustion by products such as Smoke, Draft, CO2 or O2 and vent temperature, you need to call someone that does!

Good luck with your problem solving!

mattmia2

Could just be a semiconductor or solder joint that is intermittent and sensitive to heat...

STEVEusaPA

Could also be losing its prime, could be an unstable flame due to a venting and/or combustion air setting.
All can be checked by a competent tech with the proper tools/instruments.