Why does the furnace blow 5 amp fuses?

The problem is in the thermostat wiring, likely it is grounding somewhere between the thermostat and the air handler/furnace unit.

You have a short some where. Look for a nick on the thermostat wire. And it could be anywhere. at the furnace or Ac. Take all of the wires off of the thermostat and try it. if it blows the stat is ok now take one wire off at a time from the terminal strip on the furnace and try the fuse each time. If you can get your hands on a little popper and put it in place of the fuse it is a mini circuit breaker 3 or 5 amp. Saves on fuses It will be a matter of tracing out which wire hooked up blows the fuse or popper. and then what is being energized a contactor, relay, gas valve etc....It takes some time

The control fuses keep blowing tells me one or more of the control that operate the control on 24 volts is malfuctioning. Most controls operate at about 5 watts and on a clamp on amp meter normally pull about 0.2 amp (p = current x voltage, 5 watts = 0.2 amps x 25 volts) I install 75 watt 24 volt transformer with a resettable overload to save on fuses. I reset the transformer and quickly go to the controls and determine the amp draw. When I find the one >0.2 amps I found the problem. The things to check is the emergency heat sequencer, fan control sequencer, reversing valve for heat pumps, main contactor on the condensing unit and any other 24 volt control loads. I have had to replace the themostat wire because the increase current for the thermostat wire can cause the entire length of wire to form a long heater and cause insulation degradationa and cause the thermostat wire to short out and blow the fuses.
Hope this helps

check for breaks in the thermostat wiring.

Control fuses only blow with a 24 volt problem. Make sure that you replace it with a fuse NO LARGER than 5 amps.You could have a thermostat wire pinched or some wire touching the cabinet inside the unit, but it is a 24 volt problem.

Check the 3-5 amp fuse on the control board of the Furnace, tell me if the fuse has a burn spot on it.

There is a 3 or 5 amp fuse on the control board inside the air handler.
Disconnect power to the air handler,locate the fuse and replace it with a good fuse.
The fuse is the same type used in automobiles.
You can purchase them at a auto parts store.
After replacing and turning on the power there will be a 5 minute delay before the board and thermostat energize.
After 5 minutes set the thermostat for you heat or ac function.
If the new fuse blows a complete diagnostic must be done to the system.

A fuse is rated for it's current carrying capacity so yes you can use a 240 volt 5 Amp fuse in a 12 Volt circuit.
The Voltage is given to indicate the insulation of the fuse, i.e. if you used a 12 volt fuse in a 240 Volt circuit it may arc and therefore is not suitable. You can always use a fuse with a higher voltage rating in a lower voltage rated circuit.
A 5 Amp fuse will carry 5 Amps indefinately and will only blow at 2 or three times it's rated current.

Hi,

when a fuse blows, its telling you that theres a fault. this could be a loose or broken wire.
or alternatively, the fuse rating is too low. you could check the wires are tight. and also possibly try fitting a 5 amp fuse. check first that the fuse is rated correctly for the appliance. usually 3-5 or 13 Amps.

Mike @ compurepair.

3 amp is standard.the fuse is blowing to protect the transformer.It is telling you that there is a short in the low voltage wiring.Goodman also had an issue with the coil shorting on the contactor.To prevent damage.....i'd call a pro.It should be a fairly simple repair.