Intermittent furnace problem

[harry_and_thea]

Suburban NT-30SP. Posting this same problem again to see if I can get some more responses. First the problem was that the fan would only work intermittently but when it came on I would get ignition. Now the fan comes on but the ignition is intermittent. Before I take this thing out and bring it to a shop to get serviced I want to make sure it is not the thermostat. When I turn the thermostat up I get immediate fan response. Should there be a delay with this particular model of furnace?

[Mortimer_Brewst]

Hot, then cold. Hot, then cold. Are you sure it’s not the dreaded hot flashes?

[darsben1]

Yes there is a delay. The fan runs to purge gases from the fire box AND to make sure there is circulation before igniting. The air circulation monitoring is via a sail switch. If the sail switch does not close no ignition will take place.
If your battery is low there will not be enough energy to spin the fan fast enough to make the sail switch contact. Also sail switch can get some scmuts on them which will prevent closure of the switch or the sail switch could be bad

[Harvard]

Possibly a low battery OR intermittent sail switch if the battery is full.