"AC/DCdude17" wrote in message
.. .
I've a gas furnace of unknown make, model and have an AC installed as
well. It connects to a thermostat through a four wire configuration.
The whole setup is about 13 years old. The thermostat have three settings
like the average thermostat. A/C(Red, yellow and green shorted),
heat(red, white and green shorted) and there's a switch labeled auto/on
that let's you turn on the blower manually(red and green shorted).

Every once in a while, furnace blower keeps running after turning off
after using fan only and only when I use fan only. I thought it was a
problem in the thermostat, so I pulled it out of the socket. Fan
continued to whir.

I measured the voltage across green and red and read 24V as the fan
continued to run, so it's clear that there isn't a short in control
wiring.

To shut it off, I'll have to short red and green a few times with a
paperclip at the thermostat socket and fan eventually turns off.

If I don't do this, it contiues to run forever. I can also do the same
shorting technique to get the blower stuck running. My assumption is that
pulsating the control terminals turns it off by jiggling the stuck
terminals apart.

Is there anything I should suspect beside the mechanical relay?

You should suspect that we can't tell you anything about your system without
seeing it. Make/model/serial numbers help, but still not enough information.
You shou should also suspect that you will find a local, *competent*,
licensed, professionally trained, HVAC technician in your Yellow Pages.