Furnace Info - RUDD Model UGED - 10ERAJS SERIAL BU5D707 M5092 4609
BTU 105,000 Natural Gas , It's an Upflow Furnance , 90 Plus High Effieciency.
Thermostat in house - Honeywell , Small rectangular, manual (no high tech digital or programmable) , one side has Heat / Cool / Off switch , other side has Fan switch ON or Auto. Just basic thermostat.
Furnace in basement of house. Uses outside air for combustion , Exhausts to outside as well. Furnace is 15 years old, installed new when house was built. NO modifications to furnace in any way such as lengthening or shortening of air inlet or exhaust.
2 years ago a furnace control board (printed circuit board) was replaced , 1 year ago the Hot surface ingniter was replaced. Last Friday the gas valve was replaced , and the pressure or vacuum (don't know correct name) switch was changed to no avail. This pressure / vacuum switch senses difference in pressure / vacuum of the main combustion chamber where gas burner is and the secondary (exhaust) heat exchanger. Checking this switch with a Digital Voltmeter , and visual inspection it is working, lines (clear tubing) not clogged.
Symptoms - Thermostat calls for heat , Secondary heat exchanger (exhaust) motor comes on and runs, after about 20-30seconds the Hot surface ingniter comes on and glows fire red , Gas solenoid valve energizes and gas flows to combustion chamber , Gas ingnites , BUT Gas only stays lit maybe 2 seconds, or a brief flash, and extinguishes , All of the above is a normal sequence of events except flame is not maintained , operating temp is never reached and main blower motor is never called upon to run , which it should not as there is no heat to distribute. Given about 1 1/2 minutes the sequence will start again with same results. Furnace will not stay lit. The Furnace Control Board does not flash any error codes on the LEDS ( 2 green , 1 Yellow ) whatsoever.
The Main Gas valve has a vent tube (clear tubing) that runs to the gas inlet on the combustion chamber , which is common to the outside combustion air inlet. Thru experimentation if this vent tube is removed from the Gas/Outside combustion air inlet and a slight pressure is applied (blowing by mouth on tubing) I can get the burner to light and stay lit with a little careful manipulation. This tube I am blowing on is still connected to the main gas valve (which is new) and disconnected from the combustion air inlet. Flame will be maintained , main blower motor comes on , Heat is distributed thru house until Thermostat setting is satisfied , and then furnace is shut down. If while furnace is operating , blowing heat , and I try to reconnect the vent tube from the main gas valve to the Gas / Outside combustion air that leads directly to the main burner the flame in combustion chamber immediately goes out , and back to original problem ....
As I said previously the main gas valve , and the pressure/vacuum switch between the combustion chamber and the secondary (exhaust) chamber are new. Same results/symptoms with new parts as with old parts ??
I have not physically checked the air inlet pipe , not the exhaust pipe for obstructions. I believe both are 3" or 4" PVC. These pipes run up inside the basement at about a 45 deg. angle with some bends so I can't SEE thru them to determine if blocked. From outside where both pipes are and both intake and exhaust running on the furnace there is good suction on the inlet , and good pressure on the exhaust. On Monday morning I shall rig some device that I can pull / push thru these pipes to insure they are not blocked.
If anybody needs clarification please ask. If you need pictures just ask. If you prefer I call you or send you direct email just PM me at this site. Long time TBN'er... THANKS in advance for any help..
