I have a furnace that seems to work fine except...

It's a gas furnace from the early 1980's, Heil brand.

When i'm downstairs at the furnace and if i use the manual switch and turn it off then on the furnace will start and run. but once it turns off it won't turn on again from the thermostat. When i change the thermostat i can hear the furnace click in the basement but it doesn't come on.

I've been chasing this problem awhile now.

There is another glitch i've noticed and that's the igniter keeps clicking even while the furnace is running. That seems bad. i can hear it just ticking away. so i thot the automatic igniter was goofy and replaced it w a salvage item from ebay that was said to work. no change in clicking or furnace behavior. actually, the new igniter now sometimes stops clicking after 5 seconds -- and also the gas shuts off. then i turn the manual switch off and on again and it reignites and stays lit. and keeps 'tick tick tick'-ing.

I then thought something was up in the gas valve solenoid. the solenoid is a nice separate little part on the valve but doesn't seem separately available. so i bought a salvage gas valve off ebay, claimed to work fine. no change in furnace behavior.

the furnace now has 4 other parts i could replace. one is called a fan limiter and the other two are mounted on a box at the side of the compartment. then there's the igniter itself.

my brother told me to look for some sensors near the flame area that might be failing but i see no such devices.

i was thinking this might be an easy fix in the end and so i then installed a nice new programmable thermostat -- no change.

Any ideas? thanks!