I've been doing some reading here and, I can see I'm not the only one to have this problem. When I put the next one in, I will wire in the 3amp automotive fuse I have seen referenced in several threads here.

My problem is happening when I swith the thermostat from off to heat. It first happened before this winter. As the weather started to turn, I turned the furnace on and set it a couple of degrees higher than what the house temp was. I didn't want to wait until it was frigid before I tested the furnace after sitting all summer. This turned out good because it ran for about 30 seconds then shutoff. I knew this wasn't right so I immediately went to the furnace and took the cover off. After a few seconds of looking around, the transformer popped and huffed out a puff of smoke. I ordered a new one and, had no problems all winter.

Last week the weather got quite warm and I turned the T stat to off. I was going to turn on the AC but didn't. Well it got cold again so I turned on the furnace and didn't notice until the next day that it wasn't working again. Transformer blown. I will receiver another one Wednesday the 18th.

I'm going to inspect the wiring going to the Central air unit Thursday.
My question is... can the thermostat itself be doing it? It's an Emerson EDT-193. The furnace is a Johnstone Model MBA 100 NH4R.

Thanks for any advice.

If this occurs when switching between "HEAT & "OFF", it isn't likely the air conditioner has anything to do with it. I would begin with an inspection of the thermostat and low-voltage wiring.

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Originally Posted by KC13

If this occurs when switching between "HEAT & "OFF", it isn't likely the air conditioner has anything to do with it. I would begin with an inspection of the thermostat and low-voltage wiring.

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Thanks KC, I was thinking that too but, that is the only outside wiring that could possibly be chewed on. I have don't have any pests in the house. That's why my real suspicion is the thermostat itself. I thought I'd see if anyone else here had a thermostat cause this.

I got a new transformer. I also got a new thermostat. I installed the thermostat. I just installed the transformer with a 3 amp glass fuse rated to 250V. As soon as I threw the main switch to power the furnace, the fuse blew. This was before I even set the thermostat to call for heat.

I know this transformer has an output current of 3.7 amps. I'm thinking a 3 amp fuse isn't going to be sufficient. All I keep reading here is people using a 3 amp fuse.

Any ideas?

Quote:

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Originally Posted by Rockguy

I got a new transformer. I also got a new thermostat. I installed the thermostat. I just installed the transformer with a 3 amp glass fuse rated to 250V. As soon as I threw the main switch to power the furnace, the fuse blew. This was before I even set the thermostat to call for heat.

I know this transformer has an output current of 3.7 amps. I'm thinking a 3 amp fuse isn't going to be sufficient. All I keep reading here is people using a 3 amp fuse.

Any ideas??

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Actually it appears it's only a 1.67amp output. I could have sworn they told me 3.7 last fall.

Here is the exact transformer datasheet. Hmmm
TCT40-01E07K pdf, TCT40-01E07K description, TCT40-01E07K datasheets, TCT40-01E07K view ::: ALLDATASHEET :::

Well I tried another fuse and it seems to be working. The only thing I did different was switch the red wire on the thermostat to Rc from the RH. I don' think this would make any difference because they have a jumper between them anyway. I made the switch because the directions that came with the T stat had you hooking the red to Rc. I had it hooked to RH because the previous T stat was hooked up that way. I really don't see how that can matter.

It was pretty warm here today so, I didn't want to run the heat very long. It's going to cool off next week so I can test it more extensively then.