How to Diagnose Garage Heater?

[Shiny]

Dayton 3E226C natural gas heater. Thermostat clicks when turned up. After a delay the blower comes on, but gas does not get to burners. The pilot is lit and stays lit. The blower never turns off until unplugged. How do I tell if this a problem with timer, limit switches, or gas valve?

Thanks

[HayZee518]

Having the schematic is helpful. One side of the control circuit will be neutral and will feed all devices that open or close something else. The hot will feed a switch of some type [ limit switch, thermostat, permissives etc] Start with something you know, i.e. the output of a transformer. check it with a meter. should read control volts. Then go to the first device, say a fuse, read one side of the fuse then the other side. If fuse is good, voltage will be the same on both sides. Next go to a switch inline with a device. Thermostat is just a switch for the hot. It is never used in the neutral line. check one side of the "stat" operate the "stat" and measure the output from this device. If it is good, the input voltage will equal the output voltage. Next check any limit switch inline with the gas valve [ the something that operates something else] If it is intentionally closed it will say NORMALLY CLOSED on the diagram. Input to this will equal output from the device. If it is NORMALLY OPEN it must be manually closed to check its output voltage. This is basically the steps in troubleshooting any circuit. Relays complicate the procedure in directing the voltage to different places at the same time in the circuit. Just handle one at a time.

[Shiny]

Thank you HayZee. I will do as you suggest. I just got off phone with tech support at Grainger, who sold the unit. Do you know where I can get a schematic? Apparently this unit model has been discontinued and they were not able to give me a phone number for the manufacturer. The tech had access to a diagram that I do not, and was able to make the following suggestions:

1. with thermostat calling for heat, check voltage between terminals TH and TR on the control valve. If activated (24V) and no gas, then the valve is bad

2. if no voltage to the valve, then check the blocked vent switch (spillover valve). He said it will trip if the vent is blocked by sensing an overtemp condition in the flue or at the heat exchanger exit. He said it has a "reset" button, so there is a chance this can be corrected as simply as resetting this switch. If it trips again, then there is an airflow problem in the vent that will need to be corrected or the switch itself is bad.

3. if the blocked vent switch has not tripped, then check the hi limit switch. I will use your suggestion to chase the 24V with my meter if I need to test this switch.

Wish me luck.

[Shiny]

I was fortunate ! The blocked vent switch was tripped. I reset the button and my furnace is working again. If it trips again, I probably will have to clean a blocked flue... maybe a bird nest?

Thanks again for your help.

By the way, I found out the Dayton brand is actually made by Sterling. I called Sterling 413-564-5540 and they were very helpful, giving me the equivalent Sterling model number and providing both an online installation and owner's manual in addition to e-mailing me a wiring diagram.

All good now !!