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.

[nvr2old]

forget timer setting. the second stage board is set for what operation, one/two/timed?

[grovecreek]

w2 timed

[nvr2old]

did you remove stat from circuit and operate directly at circuit board?

[grovecreek]

No I did think of that later that I should have taken the stat out of the circuit and just jumpered it at the board.

[nvr2old]

thats the place to start. could be stat,wire between stat and furnace. if something chatters its usually because of low voltage,short or conflicting signals.

[grovecreek]

High fire pressure switch is cutting out after about a min.
I checked pressures with a monometer it does go down after the furnace heats up for awhile. I checked all over the heat exchangar for cracks but had no luck. I would like to go back and take apart the collector box area to see if there are any problems in that area.

[Wirenut]

Grovecreek
When the G32's first came on the market I had one do almost exactly the same thing. I changed the circuit board and it still had problems. The problem with that furnace was a improper ground, just thought I would throw that out there, it is an easy thing to eliminate as the problem or correct if it is.