Heater Running But Thermostat Went Blank -- Help Please!

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e2139adfa

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My honeywell wifi thermostat went blank while my heater was running yesterday and now I can't shut off the heater unless I unplug the York furnace. Here's what I've checked so far but still can't figure out what's wrong and the cause of my blank thermostat. Would appreciate if anyone can give their expertise and suggestions, thank you in advance.

1. Changed battery on thermostat (it's powered by c-wire but changed battery just in case).
2. Used multimeter to get a low voltage VAC reading from the wall where my thermostat is installed - no voltage detected.
3. Checked circuit breakers. Not tripped. Checked voltage at breakers, confirmed breakers are working fine.
4. Checked fuse inside furnace - fuse is fine, not blown.
5. Checked furnace transformer. 120v coming in fine and there are two low voltage leads going out. One had 24v reading (#1 in picture) and the other had 0v reading (#2 in picture). Did not do a resistance test.
6. York furnace light blinking green at intervals of four.
7. Checked low voltage wiring leads on furnace board (wires that go to the thermostat, outlined in red in picture) -- no voltage on any of the leads. (heater was turned on and running at this time)

Tempted to just replace the transformer but concerned it might go out again if something is wrong with the system. Also tempted to just put in a new fuse (even though it looks fine) or resort to replacing the thermostat but it's only 4 years old and if I'm not getting power at the thermostat wall, highly doubt the thermostat is the issue.

Appreciate any/all help, thank you so much!


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Fitter30

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Terminal strip that you box if it has marking R, C,G, W, Y
Meter
R-C should have 24vac
R-W 0 vac thermostat calling for heat
R-W 24 vac thermostat not calling for heat
Disconnect W at terminal strip with power off, turn furnace back on heat should not run
If that works reconnect W
Thermostat
R-C 24vac stat has failed
Disconnect W at thermostat heat should shut off
 
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Reach4

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5. Checked furnace transformer. 120v coming in fine and there are two low voltage leads going out. One had 24v reading (#1 in picture) and the other had 0v reading (#2 in picture). Did not do a resistance test.
There is one transformer in that picture, and the 1 and 2 labels label output terminals.

To measure the transformer output, you want to measure between 1 and 2, which should be the same as measuring R-C as fitter30 said.
 

e2139adfa

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There is one transformer in that picture, and the 1 and 2 labels label output terminals.

To measure the transformer output, you want to measure between 1 and 2, which should be the same as measuring R-C as fitter30 said.

Thank you. I measured the 1 and 2 out from the transformer and got a 25 vac reading. However tried measuring R-C at the terminal strip like @fitter30 suggested and got only a 1.2 vac, likewise for the reading of R-W.
 

Reach4

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I think transformer #1 would make it to R, except for something blocking the path.
 

e2139adfa

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Where is 1&2 landed? Take a pic of the wiring diagram and the model number.

Hope these pictures help. 1 and 2 are the gray wires along the bottom of the second picture that land on the board.
 

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Fitter30

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9 pin plug on board unplug power off . With wire side facing you check continuity 3-8 top left to middle right both red wires
On the top of diagram there are 4 safety switches a limit ,2 roll out and rds (don't know what it is). If theres a open follow the red wires check each safety.
On diagram four flashes limit open. Doesn't make sense that the heat runs with limit open. If that is true pull board has to have burn marks on the back side.
 
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jadnashua

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The thermostat acts like a switch. WHen it is calling for heat, it applies the transformer output (R) to the furnace via the W wire. That 24vac goes through (usually) a relay where the other side is connected to C, or the other side of the transformer to create a closed circuit for the voltage/current to operate the relay. The switch in the thermostat may not be perfect, so when you measure some voltage across it, you're measuring what amounts to the resistance of the switch relative to the coil in the relay. If the switch was 'perfect', the voltage drop across it would be nearly zero. That 'switch' may be a triac, and that will have some voltage drop across it. That's essentially an electronic, solid-state switch.

As said, if you remove the W wire from either the furnace or at the thermostat, if the furnace doesn't turn off, the furnace is at fault. If the furnace does turn off when you remove the wire, the thermostat is bad.

Using test tools can mislead you to where the problem is without a bit more understanding of how to use it, and what it means. If you do have a triac versus a switch in the thermostat, you couldn't get a reasonable resistance measurement across the two lead, and, you can't measure resistance with the power on.
 

DavidDeBord

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My honeywell wifi thermostat went blank while my heater was running yesterday and now I can't shut off the heater unless I unplug the York furnace. Here's what I've checked so far but still can't figure out what's wrong and the cause of my blank thermostat. Would appreciate if anyone can give their expertise and suggestions, thank you in advance.

1. Changed battery on thermostat (it's powered by c-wire but changed battery just in case).
2. Used multimeter to get a low voltage VAC reading from the wall where my thermostat is installed - no voltage detected.
3. Checked circuit breakers. Not tripped. Checked voltage at breakers, confirmed breakers are working fine.
4. Checked fuse inside furnace - fuse is fine, not blown.
5. Checked furnace transformer. 120v coming in fine and there are two low voltage leads going out. One had 24v reading (#1 in picture) and the other had 0v reading (#2 in picture). Did not do a resistance test.
6. York furnace light blinking green at intervals of four.
7. Checked low voltage wiring leads on furnace board (wires that go to the thermostat, outlined in red in picture) -- no voltage on any of the leads. (heater was turned on and running at this time)

Tempted to just replace the transformer but concerned it might go out again if something is wrong with the system. Also tempted to just put in a new fuse (even though it looks fine) or resort to replacing the thermostat but it's only 4 years old and if I'm not getting power at the thermostat wall, highly doubt the thermostat is the issue.

Appreciate any/all help, thank you so much!

That, or a light should be continuously on, not blinking...Is there a Fault code sticker on the left/right wall showing what 4 blinks indicate? Something in the low voltage control circuit,is causing the furnace to run continuously, & quite possibly it is the board.


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e2139adfa

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9 pin plug on board unplug power off . With wire side facing you check continuity 3-8 top left to middle right both red wires
On the top of diagram there are 4 safety switches a limit ,2 roll out and rds (don't know what it is). If theres a open follow the red wires check each safety.
On diagram four flashes limit open. Doesn't make sense that the heat runs with limit open. If that is true pull board has to have burn marks on the back side.

I checked the red wires on the 9 plug and one of them have a 25 vac reading and the other red wire gave 1.3 vac.

I don’t understand the other directions you gave. Can you explain? Thank you
 

Fitter30

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If there is 25 volts between the red wires there is a limit open there should be 0 not move than 2 volts. A closed switch will have 0 volts across contacts full voltage if open.
 
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