I have a Rheem model RGEB gas furnace this is a 90+ about 18 yrs old, the problem I am have is that the hot surface ignitor will come on and glow, but the gas valve will not turn on and after a few seconds the ignitor will cycle off, this will happen a few times and then the furnace may start. It can take up to 15 min before the furnace will start. Sometimes I will need to shut off the power to the furnace and then turn it back on, after doing this it seems to start right away. This problem seems to be worse when the furnace is running more. It also seems to get better if I leave the front panel of the furnace off. Thanks for any help.

With a unit that old when was the last time you had it cleaned and checked by a professional. Rheem had a series of bad heat Exchangers so I would also have that checked during the service.

I have had a service man look at it and the heat exchanger has been replaced. The problem is intermittent and seems to work fine when the serviceman has looked at it.

Wow if the service man could not find the problem when he was there I doupt if anyone else is going to be able to fix it on this venue. You might try to call a different company to see if they have a smarter service person on staff. Intermittent problems are the worst to find and you almost have to be standing there when it happens.