I look after an elderly widow (84) who has had a lot of trouble with her 1990 Ruud UGDA- 10ECBEA HIS furnace. I am not in the HVAC trade, neither am I a DIY or a DYI I reside near Vancouver British Columbia Canada. I have hired pro contractors to do the job, but after three different contractors, the latest solution suggested is to replace the furnace. Considerable dollars have already been spent $1700 in repairs with reputable companies.
Parts Replaced:
Nordyne HIS 903758 Mar. 31, 2004
Millivolt thermostat Oct. 01, 2004
Main Gas Valve Honeywell VR8Z45M2530 Dec. 30, 2004
Relay switch for control Dec. 30, 2004
Time Delay control Jan. 31, 2005
Control module (board) Feb. 2005
Control module (board) Mar. 2005

After the control module has been reset (by power off) the furnace will cycle through the start up sequence and fire the burners, blow heat, burners shut down, relight and blow heat and shut down and fault with no restart unless the power to the furnace is cut and the control module reset. This has stumped the contractors who have tried diligently to fix the problem.

In desperation I brought over a electronic technician friend of mine to take some measurements, he has 25yrs in circuit boards. When the control module faulted it was intermittent, sometimes it would kick 24v to the gas valve but not open it, or at other times no volts were sent to the gas valve. This control module has been replaced twice in two months and it appears to be faulty again. Should I have this replaced again? Any suggestions other than a new furnace.

Control Module: Robertshaw HS780 34NL-306A Part # 100-00812-31