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.

 

[Megawatt]

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.
View attachment 64434

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.

I’ve had a relay blow on my unit and with no secondary fuse it fried all the wires in the control box and I didn’t even know it until I started smelling smoke, I looked and my kitchen was full of smoke. Had to replace the unit. That’s why I tell you to add a secondary fuse. If mine would have had a secondary fuse my unit could have saved.

 

[brianmoooore]

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

Don't agree. The Tx comes with primary side protection in the form of the thermal fuse.
The fuse in this case is required because it connects to wiring outside the boiler, which is not under the control of the manufacturer, so should be fitted to the secondary.

 

[ktnwin]

I will add the fuse to the secondary then as it seems to make sense to me. When the 24 VAC line get shorted somehow (by accident), then the fuse inline on the secondary will blow and the two wires of the secondary are no longer connected, the primary remains intact in this case.
If I know the max amp rating of the primary, I can add one inline here too (1, 2 or 3 Amp ???). This 24 Volt transformer is connected to a 15 Amp circuit along with 3 other 18 Volt transformers. These four transformer can hardly draw 15 amps on 120 Volt line.
The fuse kit I bought has 5 fuses for each amp rating, plenty of fuse holders too.
(1A 2A 3A 4A 5A 7.5A 10A 15A 20A 25A 30A 35A 40A)
The transformer and fuses will be delivered tomorrow, I can't wait to get the heater to work again.
I will be very careful this time, connect 24 volts to the thermostat (only 2 wires), then change system (heat, cold), FAN settings to verify that the 24 Volts appear at the other 3 wires correctly based on the settings.
Only when the thermostat behaves properly, I will connect the other 3 wires.

 

[Megawatt]

I will add the fuse to the secondary then as it seems to make sense to me. When the 24 VAC line get shorted somehow (by accident), then the fuse inline on the secondary will blow and the two wires of the secondary are no longer connected, the primary remains intact in this case.
If I know the max amp rating of the primary, I can add one inline here too (1, 2 or 3 Amp ???). This 24 Volt transformer is connected to a 15 Amp circuit along with 3 other 18 Volt transformers. These four transformer can hardly draw 15 amps on 120 Volt line.
The fuse kit I bought has 5 fuses for each amp rating, plenty of fuse holders too.
(1A 2A 3A 4A 5A 7.5A 10A 15A 20A 25A 30A 35A 40A)

If I was you I would go with the 3 amp and remember we are not worried about overcurrent, we are worried a short circuit.

 

[ktnwin]

If I was you I would go with the 3 amp and remember we are not worried about overcurrent, we are worried a short circuit.

Correct, these transformers cannot draw up to 15A. I will also add a 3A fuse to the primary. So, two fuses for this transformer to protect it. It is very annoying when it is toasted.

 

[ktnwin]

Correct, these transformers cannot draw up to 15A. I will also add a 3A fuse to the primary. So, two fuses for this transformer to protect it. It is very annoying when it is toasted.

Correct, these transformers cannot draw up to 15A. I will also add a 3A fuse to the primary. So, two fuses for this transformer to protect it. It is very annoying when it is toasted.

The new transformer cannot fit in the place where the old one is. So, I left the old one at the same place after disconnecting it from the 120V input, its outputs also disconnected. The new one is installed on the side of a metal switch box (where other transformers are), I ensure that its hot is only hot when the switch is ON. The 24VAC output goes thru a cable with its 2 wires connected to where the old transformer outputs are connected to. The hot 24V also has a 3A fuse inline. I have no room to add fuse for its 120V input.
Turn ON the breaker and test, my new Honeywell thermostat did control the heat as expected. But the fan is ONLY on if I set it to ON. It does not turn ON in AUTO mode even if the heat is ON (a new post will be created for this).
For now, this topic ends here.