"Eigenvector" writes:
Having never owned a home before, and certainly never having had to service
a furnace before, what signs should I look for that my furnace is past the
point of serviceability and simply needs replacement? That giant cloud of
gas that forms in the basement shortly before detonation is an obvious one -
what others?

Some facts about my situation, but I guess I'm more interested about the
general case.

My particular gas furnace is approx 25 years old. It seems to heat just
fine, however its startup behavior seems weird to me.

That's pretty darned old. Well past the age for which you should be
seeking annual cleaning/inspection/servicing. They should check for
CO emission, clean the burners, do a visual inspction of the heat
exchanger, and perhaps other tests on the HX if visual and CO tests
warrant further investigation.

At what point should I just call in the HVAC dude and have
him/her/it repair it?

Now. :-)

Just ask the question about the long duration of ignitor ticks prior
to turnon during the annual servicing you should already be doing.

And get yourself some Kiddie Nighthawk plug in CO detectors for your
bedrooms if you don't have em already. No way I'd be without em with
a furnace of that age.

Best Regards, --
Todd H.
http://www.toddh.net/