After the last response I guess I'm taking my chances on getting an answer
to this one- but what the heck.
Forced air 119,00 BTU Nat gas furnace
Mod # 397HAW036100
Prod # 397HAW036100-ACBA
old - as in 1984 installation - new main control board two or three years
Problem- Noticed today that it is doing some rapid uneven cycling.
Burner on for anything from 1 second to 2 minutes,
Off for 30 seconds to 2 minutes until pilot re-ignites,
then 20 to 40 seconds to start of next burn.
Worse on start up from no run periods and gets somewhat better as it cycles
a few times though never evens out - even when thermostat jumped to constant
Eliminated limit and exhaust purge switches as problem
Burners clean with good even blue green flame
Air flow and filters clean and clear
I'm guessing it must be in boards or main valve, but would like to know if
I'm missing anything adjustable or minor replace (main board over $100)
before I make major decision.
Also noticed occasionally (once every eight or nine starts) takes up to 10
igniter "sparks" for pilot to stay on though normally only takes 1 to 3
The cycling problem is burner only- the exhaust purge fan and the blower
continue to run.
Thermostat checks okay
"blower delay time" adjustment advanced and retarded slightly with no effect
on this problem.
Getting by as is for now (still heating house okay) but uneasy about odd (I
Thanking in advance for any comments. BTW- if it matters I worked for
heating and plumbing contractor years ago running black pipe gas lines
etc.and have installed several new furnaces on my own and done regular
maintenance on my own for years. Probably end up getting new soon anyway,
but always try to learn as I go.