[mr.magoo]

If you can jump start a gas furnace at the control board ... does that mean the control board is good or can it still be defective? Thx.

 

[TarheelTerp]

If you can jump start a gas furnace at the control board ...

Crossing the red & white to bypass the thermostat and those wires?

...does that mean the control board is good or...?

If the furnace is running...
then everything inside the cabinet is likely fine.

 

[surferdude2]

Sounds good to me. In reality, the external controls, such as thermostats and timers, are merely environmentally biased or time based jumpers themselves, so simulating them can be done quite easily, in the majority of cases.

 

[mr.magoo]

Appreciate the reply.


The situation is the thermostat won't operate the furnace. I can start the furnace by jumping the R and W wires on the control board.


I have replaced the thermostat and have even run new wires from the thermostat to the control board. No luck.


When testing the wires on the thermostat with a multi meter I still get zero volts.

 

[surferdude2]

Must be wired wrong. Jumper the R terminal to the W terminal at the thermostat and see it starts the heat mode. Perhaps you connected the control leg to the wrong R terminal and actually need to install the required jumper between the Rc and the Rh as the old thermostat did. Check the old one to see.

 

[TarheelTerp]

I can start the furnace by jumping the R and W wires on the control board.

With that complaint I'd suggest you ohm the t-stat wires for continuity (even a brand new cable)...

Next up is the t-stat itself.
Be sure you have one that suits the sort of furnace you have and the correct number of conductors.

From there it's about instruction reading & workmanship.

 

[mr.magoo]

appreciate the reply. Nothing happens when trying to jump the system at the thermostat.


When the furnace initially stopped working I checked for power at the thermostat and had zero volts. Then I checked the control board which seemed to be working fine but the lights showed no call for heat. The control board status lights showed normal. I then tried to jump the system at the control board and it started.


I thought the thermostat might be bad so I bought a new thermostat (the old one had a jumper between R/Rc. The new thermostat says it does not need a jumper). Installed the new thermostat and still no voltage and no heat. Then I thought a wire from the thermo to the control board might have gone bad so I ran new wire from the new thermo to the control board. Still no voltage at the new thermostat and no call for heat, but I can still jump the system at the control board.


It's driving me nuts.

 

[surferdude2]

Read carefully what the new stat requires. You will likely see that you now need the C terminal from the furnace board to connect to the stat. If not, it will require batteries... are they installed?

Even so, you still should be able to connect a jumper from the W to the R at the stat and bring up the heat mode... something fishy with the wiring... either broken wire or wrong identity.

 

[mr.magoo]

The old stat had 5 wires connected R,W1,Y,G,B. The new stat has a different connection layout (a couple of added connections like U, A and K).



The new stat doesn't have a B but has C and I put the blue wire (which I assume is the common wire) in the C connection on the new stat. The wires at the control board I swapped color for color.


I tried the new wiring with the old stat first and when it didn't work that's when I tried the new stat (and it does have new batteries installed).

 

[surferdude2]

The voltage between the C and the R wires at the stat should be 24 vac. If you don't have that, recheck it at the furnace. It can't be disappearing without a trace. Voltage drop shouldn't be a problem so that would be clutching at a straw. An intermittent connection at the control board could be possible. Use your voltmeter and chase it down.

Verify that the line voltage is 120 vac and the wall switch is not failing intermittently. Likewise check the panel circuit breaker. Also the safety interlock switch at the furnace access cover.

 

[mr.magoo]

I'll have to crawl under the house and try again later. I appreciate the replies. The control board is only about a month old. It's a White Rodgers 50v54-820 for a Trane furnace. I replaced it myself and it's been running fine.


I was working under the house running a wire for a new doorbell ... at the same time I was removing some old wiring from an old security system. I was mindful not to cut anything coming from or going to the furnace. As a joke I told my wife I'd better check the stat to make sure I hadn't cut anything important .... and wouldn't you know the furnace wouldn't come on.


I crawled back under to verify I hadn't touched the wires from the stat. Which I hadn't.



I checked the breaker, I shut the furnace off for 30 seconds in case I needed to restart the control board. I've run brand new wiring and tried a new stat.


I coincidence between working under the house and having the furnace stop working is almost too much. But I can't, for the life of me, see anything I might have done.

 

[mr.magoo]

Thanks to everyone who tried to help. After laying in the crawlspace and looking at the wiring until I was glassy eyed ... I saw a likely wire going around to the back of the furnace.


After low crawling 40 meters around all the obstacles to get there it turned out the 'float switch' had been jostled and tripped the furnace off.


Heat is back on ... I guess at least I have new wiring and a new thermostat.

 

[surferdude2]

Those Little Giant float operated pumps have made for several easy money service calls for the lucky HVAC techs for years. I'm ashamed that I forgot to mention that as a possibility. It usually doesn't happen in the heating season so it never came to mind.

Glad you got it fixed! Good job! Thanks for reporting back!

Best regards, SD2