[marksind]

The furnace guy was out doing the annual furnace check and said the control board he replaced last year showed signs of excess heat that suggest a surge protector should be installed. He wanted $250, which seems awfully high.

I bought one online from Johnstone Supply (a ICM515) that from the online description appeared appropriate. Got it today and it suggests installing it at the service disconnect for the A/C Condenser. Two questions:

Does it matter that it's a furnace circuit instead of A/C?

There's no "service disconnect," just a wall switch to turn off the furnace. Would that be where I'd install it, or could I install it where the external wiring is joined to the furnace wiring in the furnace electrical box?

Thanks for the help.

[KISS]

This is, in essence, a whole house surge suppressor.

Meaning, the larger the breaker you install it on, the larger surges it can handle.

So, it's better to install it at the service panel and put the surge suppressor's ground as close to the neutral/ground bond as you can.

The AC disconnect is just a quick and easy place to install it. It has the benefit of using thick wires.

You could install it at the heater, but it will be more effective if installed at the panel.

I question this statement:

signs of excess heat that suggest a surge protector should be installed

Heat suggests loose connections, poor contact resistance or bad airflow. It usually isn't the sign of a surge when things are still working.