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where in the furnace does it say if it uses a 3 amp or 5 amp fuse for the circuit board? Or how would I know which is correct

  • Do you have a manual and/or wiring diagram, or even a make and model number? – ThreePhaseEel Feb 8 at 23:37
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    the rating should be printed on the fuse itself – jsotola Feb 9 at 1:19
  • If it's a blade fuse like the ones below, there's a color code for each fuse rating just in case the rating is not visible. – Johnny Mar 12 at 0:20
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If the thermostat is relatively close a 3 should work. Further away use the 5. If the 3 blows try the 5. If the 5 blows, you have a problem in the furnace.

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It's usually indicated on the board itself as a consequence of the manufacturing process

look closely just behind the fuse 3A [enter image description here][enter image description here]2www.amazon.ca

It should be documented in the service manual but this is a easier way to confirm. If you can't determine the size, always go with a 3A fuse. The difference between 3A and 5A on a circuit board is very significant. And a short on 3A board using a 5A fuse is very likely to blow a component.

second method

The board ID number is often specified and Googling the numbers on the board often takes you a valuable information source. This image shows a alternate location for the fuse size. enter image description herewww.amazon.ca

  • Agree to play it safe. But the catch is that if it really should be 5A and the normal draw is, for example, 4A, then if a 3A replacement blows quickly you won't know whether it is the wrong size fuse or some other damaged component (or short circuit) causing a problem. – manassehkatz Mar 12 at 0:02
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    True but if the fuse continues to blows, you are officially in the trouble shooting mode. In all honesty the current draw of the board are minimal. I think the fuses are there more to stop dead shorts from techs and homeowners working on the stats. – Joe Fala Mar 12 at 0:14

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