Hi I have a Intertherm Model MMHA 070ABHr-01 gas. had a tech out replaced the ignitor/and adjusted it. Problem still is furnace calls for heat prepurge fan motor kicks ??? in, ignites runs for 2-3 minutes kicks out fan motor starts runs for 30 sec shuts off. in the mean time the furnace has restarted only this time it will finish the whole heating cycle. ????????

Try setting the thermostat higher to see if that helps. If not post back.

Ok that seems to work ,but I can`t keep setting it higher each time. Bad thermostat??

You might have to adjust your thermostats heat anticipator. look in the sticky notes at the beginning of this board for info.

Ok I`ve the ajusted anticipator it to short cycles ,seems to work the best but furnace runs every 15 minutes. Larger cycles seem to produce the same problem as first stated. Programable thermostat the answer? lower temp to kick in higher temp to shut off/. Seems if a manually set the temp higher 4-5 degrees it runs perfect.

Yep sounds like a thermostat or thermostat location problem.

do this before replacing the t-stat,( ive had 8 spiders nest in 3 days)this is the same situation
measure the air in and air out,,, if you temperature difference is too low ,you may have a small flame, this will cause the fan to short cycle. possably a spiders web in the gas orifice?

again,, just a guess