Lennox furnace problem

[grovecreek]

I have been working on a G27M5-120A-2 serial#6399b12127
the furnace will not stay on high fire. Low fire runs fine gas pressure is 1.70 inducer pressure is .70'w/c on low. Furnace trys to ramp up to high fire pressure goes up to .84'w/c then drops out second stage circuit board relays start to chatter. This all happens in about 10 seconds. I have changed out circuit board on second stage and gas valve and no luck. Has anyone had this problem thanks Grove

[airconman]

are you sure the pressure switch isn't causing board to chatter?

[grovecreek]

It seams that the Inducer does not want to stay at the higher speed for very long. That would cause the pressure switch to not make. So I wonder what would cause the motor to not stay in the higher speed?

[docholiday]

Capacitor, motor winding. What's the high fire pressure switch setting?

Also check vent sizing and HX for blockage and while your at it, make sure the drain is proper (assuming 90+).

[grovecreek]

High fire pressure setting is .68'w/c furnace is 80% flue size is sized correct.

[pipedope]

is the furnace sealed combustion

[grovecreek]

The customer has had this problem for three years. Another company that has finally gave up had us see if we can figure it out. I really want to help this family out because when it gets cold they freeze because the furnace only runs on low fire.

[nvr2old]

what position is the furnace? upflow,down,horizontal? Stat-single or two stage? Two stage board jumper set for one,two or timed stage?

[grovecreek]

Furnace is a upflow with a single stage stat T87. Circuit board is on timer for 8min.

[dave82323]

ever consider calling Lennox technical support and have a service rep come out and assist you.

[grovecreek]

That is where I am headed towards on Mon I thought I would see if anybody had any ideas over the weekend.

[nvr2old]

Ok you've got a single stage stat. Is the 2 stage board set for single stage? On single stage setting furnace will always fire high heat only, the 8/12/15 timer is not active in this setting. The 8/12/15 timer is only active if the board is set to timed operation while using 1 stage stat.

[Edited by nvr2old on 02-06-2005 at 12:00 PM]

[grovecreek]

The board is set on the 8 min setting for 1 stage. It does time out on the 8 min setting ramps up on the inducer for high fire gas pressure drops and second stage board starts to chatter and then drops out. Starts all over again on low fire same thing happens.