what causes loss of 24 VAC at the thermostat (and furnace) ?

[ktnwin]

I replaced a very old thermostat, that still uses mercury for ON / OFF with a Honeywell digital thermostat.
The only issue with this old thermostat is it does not turn off the fan automatically after the gas furnace is OFF when room temperature reaches the setpoint.
The connection appears to be straightforward as there are 5 wires ( Red, Cyan, Green, Yellow and White )
These are at the thermostat (new and old) AND also at the furnace circuitry.
Red is the 24 Volts AC and Cyan is common. Green for the fan, Yellow for AC and White for heater.
After installation, the new thermostat does not turn on the furnace. It turns ON the fan even set to OFF.
I play around a few minutes, then nothing work, even the fan.
So I check the voltage between the RED and CYAN wires at the thermostat, which shows 24 Volts AC before, now shows 0 Volt. I go and check these same two wires at the furnace and also get 0 Volt,
Here are the pictures at the furnace

The picture may show the top two terminals shorted but that is not the case if looking from top.
I did verify that there are 120V AC going the back of the transformer in this photo. There is NO fuse, no controller board.
What could cause the lost of 24V AC between these two RED and CYAN wires ?

I appreciate all help I could get.
Many thanks.

 

[telectrix]

transformer burnt out?

 

[Lucien Nunes]

There is probably a thermal fuse or cutout within the transformer that has operated due to a short-circuit on the 24V side, or the transformer has failed. Disconnect all outgoing 24V wires from the terminals. If there is still 120V going in but no 24V coming out, disconnect the 120V input wires and check across the transformer terminals for continuity of the primary winding. It will probably show open-circuit, in which case you will need to either replace the transformer or disassemble it in case it is quicker and easier to replace the thermal fuse. Sometimes it is embedded within the winding and cannot be replaced, as the complete replacement transformer is unlikely to cost much.

 

[brianmoooore]

Seems a bit odd for a Tx intended to supply external wiring not to have a fuse on its output, and to just rely an internal thermal fuse.

 

[Lucien Nunes]

Yes, I wonder actually whether something has been bypassed?

 

[Megawatt]

I replaced a very old thermostat, that still uses mercury for ON / OFF with a Honeywell digital thermostat.
The only issue with this old thermostat is it does not turn off the fan automatically after the gas furnace is OFF when room temperature reaches the setpoint.
The connection appears to be straightforward as there are 5 wires ( Red, Cyan, Green, Yellow and White )
These are at the thermostat (new and old) AND also at the furnace circuitry.
Red is the 24 Volts AC and Cyan is common. Green for the fan, Yellow for AC and White for heater.
After installation, the new thermostat does not turn on the furnace. It turns ON the fan even set to OFF.
I play around a few minutes, then nothing work, even the fan.
So I check the voltage between the RED and CYAN wires at the thermostat, which shows 24 Volts AC before, now shows 0 Volt. I go and check these same two wires at the furnace and also get 0 Volt,
Here are the pictures at the furnace
View attachment 64398
The picture may show the top two terminals shorted but that is not the case if looking from top.
I did verify that there are 120V AC going the back of the transformer in this photo. There is NO fuse, no controller board.
What could cause the lost of 24V AC between these two RED and CYAN wires ?

I appreciate all help I could get.
Many thanks.

I replaced a very old thermostat, that still uses mercury for ON / OFF with a Honeywell digital thermostat.
The only issue with this old thermostat is it does not turn off the fan automatically after the gas furnace is OFF when room temperature reaches the setpoint.
The connection appears to be straightforward as there are 5 wires ( Red, Cyan, Green, Yellow and White )
These are at the thermostat (new and old) AND also at the furnace circuitry.
Red is the 24 Volts AC and Cyan is common. Green for the fan, Yellow for AC and White for heater.
After installation, the new thermostat does not turn on the furnace. It turns ON the fan even set to OFF.
I play around a few minutes, then nothing work, even the fan.
So I check the voltage between the RED and CYAN wires at the thermostat, which shows 24 Volts AC before, now shows 0 Volt. I go and check these same two wires at the furnace and also get 0 Volt,
Here are the pictures at the furnace
View attachment 64398
The picture may show the top two terminals shorted but that is not the case if looking from top.
I did verify that there are 120V AC going the back of the transformer in this photo. There is NO fuse, no controller board.
What could cause the lost of 24V AC between these two RED and CYAN wires ?

I appreciate all help I could get.
Many thanks.

I have to agree with @telectrix and I’ve been through this before, the transformer has NO protection on the secondary side of your transformer and somehow when you was wiring it up obviously the 24vac live wire event to ground or made contact with the neutral. The transformer is no good no more. If you replace it install a inline fuse on the secondary side of the transformer

 

[ktnwin]

First, I verified that the new thermostat setting was for electric, supposed to be gas. My fault here. That was why it does not control the furnace properly. Anyway, I lost 24 VAC. Today, I remove the wires going into the two top terminals at the transformer, shown by yellow arrows, RED on the left, and the 3 wires taped together with blue tape on the right, the large size black wire cannot be removed.

I opened the transformer and yes, the two wires from the secondary are soldered to these two terminals.
I measured the resistance between these two terminals and got approximately 5 Ohms. So the secondary is NOT opened, NOT shorted. I have no idea whether 5 Ohms is correct or not. Any advise with this new info I like to be sure it is bad before replacing it.

 

[telectrix]

check for 24V ac on the secondary with those wires disconnected.

 

[brianmoooore]

Any internal thermal fuse will be connected to the transformer primary winding. Check the resistance across that.

 

[Lucien Nunes]

The secondary winding is unlikely to fail. As I suggested, test the primary, as that is wound with much finer wire and is the more likely to fail open, and also if there is a thermal fuse that will be in the primary.

 

[ktnwin]

Still trying to remove the transformer to access the primary.
A second thought: 5 Ohms for secondary is too low as it means 24V / 5 = 4.8A current going thru it. These transformer uses 3A fuse if they have one.
I will order a new one anyway, it only costs around 20+$. Will make sure to add a fuse.

 

[brianmoooore]

DC resistance, measured with a meter, is much lower than AC "resistance".

 

[ktnwin]

DC resistance, measured with a meter, is much lower than AC "resistance".

Thanks, I will double check my understanding with one of the 3 working 18 VAC transformers. These are for door bell, light for house number in front of garage, and one for ???
Just measure the secondary of these 3 18 VAC transformers, all measure 6 Ohms. From my understanding, 5 and 6 Ohms are normal for secondary.
Will find out more after removing the transformer. I am very curious to know. This transformer has a switch to cut off power going to it, very convenient to work with.

 

[ktnwin]

Eureka, I finally remove the transformer. Carefully mark where its black and white wires were connected with black tape and white tape on the wire nuts. These are simply hot and neutral wires in any electrical box.

The primary winding is indeed OPEN. A simple test by connecting it to 120 VAC results in 0 Volt at the secondary output lines. I am going to order a new transformer and 3A fuses now.
Question: is the fuse to be inline with the primary or secondary ? My guess is secondary but I like to be absolutely sure from expert input.

 

[telectrix]

fuse should be on the primary side, but there's nothing to stop you adding a fuse to the secondary.