A model number may be helpful especially if you need parts so try to find the plate.

You said "specialist" but I do not know what you mean by that.

" he found there to be an electrical fault.. that the output was too low. it was relaying 1/2 the output it should be. the fan works fine."
I am assuming that this is an electric furnace.
Also that the unit is only getting 120 volts instead of the required 240 volts.
In other words you are missing half the line.

I do not know what this specialist did.
Did they use a meter to check the voltage?
Did they flip the breaker for the unit off/on slowly a couple times. This should be a double breaker.
If your house panel uses fuses did they check the fuses.
Did they disconnect the furnace and check that the line was still no good.

If the unit is only getting 120 volts to it then an electrician should be able to fix it.
Note: There may be something in the furnace that is blowing half the line and I do not know if your specialist checked for this.