Hey all - I've got a Tempstar DC90 with a Honeywell Wifi thermostat that doesn't run on batteries. In the middle of the night, I went to change our temp but the thermostat was completely blank / off. FYI: my wife had changed the temp about an hour before that. Realized I had a problem.

Here are my symptoms:
1. Blank thermostat - i.e. it's not getting power - verified with multimeter
2. Furnace blowing cool air - not cold, not warm but cool - even though it would normally blow warm air.

Here is what I've done so far:
1. Checked AC voltage at the thermostat - It came back 0 - which made sense based on no power to thermostat

2. Checked if there was voltage on the board in the furnace that connects directly to the thermostat. I was getting 0 volts AC. I made sure the door button was pushed and the furnace was getting power. I checked from "C" to "R" and from "C" to several other points. Here's a picture of my terminals:



3. Next thing I checked was that the transformer was getting 120v in and putting out around 24v. I got 120v AC coming in and 21 to 28v AC coming out of the transformer.

4. I replaced the 5amp fuse on the board - the old one looked fine and I verified that it was with a continuity check, but I replaced with a brand new one anyway.

Nothing is obviously burnt / malformed on the board itself. I'm guessing I'm looking at a board replacement? Is my line of thinking correct here. My only other thought was the large capacitor next to the board - I'm not sure what that does or if it would have any impact on my symptoms.

Thanks for the help. Here are a few extra pics for reference:

 

Any ideas? I was hoping PJMax would chime in...

Bad news: I got a replacement board overnighted. Put it in and assumed it would work. Doh! I'm still having the same symptoms. Getting 120 volts on the primary side of the transformer and exactly 28 volts AC coming out of the transformer and hitting the board.

When I go to check voltage across "C" to "R" and "C" to the other terminals, I'm getting incredibly low voltage - like - 0.3 VAC at most...I did double check the new fuse and it's intact...

Also, I did replace the large start/run capacitor next to the board just to be safe. With the furnace on, I'm getting cold air and no thermostat power.

 

Sorry.... PJ has been working overtime.

Your furnace is telling you that you have an open safety limit switch. The loop carries the 24v to the R terminal. The blower runs continuously to let you know there's an open switch. It's going to be one of the switches in the red loop. The diagram shows some are manually resettable. That means there will be a little red button that needs to be pushed in. Once you've determined which one has tripped...you need to address the reason for the overheat as those switches are life safety items.