Hi all, new to the forum here but I've been having a heck of a time trying to get my furnace back online. The 3 amp fuse blew a couple times about a month ago but then was running fine. Now it blows immediately when I replace it. I've tried everything I could find online about what to try. I've looked at all the wires and can't find a short anywhere. I wouldn't say I really know what I'm doing but I did check the voltage across the connection points for the thermostat wires and get 27.7 volts. Is this high enough that it would cause the fuse to blow. Again, not exactly knowing what I was doing I was trying to check continuity between the R and W terminals on the furnace and the unit started up while I was doing that. If I use a plain wire as a jumper it just blows the fuse but with the multimeter it must have enough resistance in the meter to keep from blowing the fuse. Do I need to replace the transformer because the voltage is too high? If so I haven't been able to find a local place in Northern Virginia that will sell to someone not a licensed contractor. Would it be okay to put in a temporary 4 or 5 amp fuse until I can get the new transformer from somewhere online? Don't want to blow anything by allowing too many amps to flow.

Oh yeah, also if I have the heat off but switch the fan on using the thermostat, then the fan comes on fine and does not blow the fuse.

It is a Bryant Plus 90 Gas Furnace 350mav048080.

I am not a heat tech. I cannot tell you exactly what and where the problem is but I can tell you that it is not the transformer. Transformers can fail like anything else, but they do not fail such that you have too high voltage. If a transformer fails, you no voltage on the output side.
I can also tell you that voltage is not the problem. Fuses do not blow because of voltage. Fuses blow because of too much amperage. You could replace the fuse with 4 or 5 amp fuse but would only blow that. You would probably blow a 20 amp fuse.

You have a short circuit somewhere and that is causing too much amperage to go through the fuse. The fuse protects the wiring and devices in the low voltage system.

Base on what you have said I would look for something around the thermostat and its wiring. When you do a voltage check, the meter compares the voltage on one conductor or terminal and the voltage on the other conductor or terminal. It does not make a connection between the two conductors. So when you did the voltage check, you changed something in or around the thermostat that completed the circuit to the furnace. You also know that the short circuit is not in the fan control wiring. When you switch the thermostat to fan "ON" setting you complete the low voltage circuit to the fan relay which turns the fan on. When on the heat setting, the thermostat activates the burner circuity. When the burner gets the heat chamber hot, a sensor turns the fan on. That way you don't get cold air blowing out when the furnace first starts.
When you put the jumper wire on you probably connected the wiring that has the short circuit.

The reason I believe that the short circuit is in or around the thermostat is that you said that it has been intermittent. That is probably because as you adjusted the thermostat, you moved something, that made or broke the short circuit. In the process of doing the voltage check, you did the same thing.

At least that is where I would start.