I have an older Bunn-o-matic with 2 extra warmers on each side. The water is supplied automatically to the machine but is not heating up, despite my turning the heat knob all the way up. The warmers are heating fine.

You will need to check the heating element wiring to see that isn't a short on the neutral side. This could cause them to continue to heat even if the thermostat is working. If all wiring is ok check the function and resistance given by the thermostat.

Sounds like the fill solenoid is damaged and needs to be replaced.

Sounds like you need a new grommet for the machine.

YES.

The unit has a tank used to heat the water for brewing.
When you add more water, the unit starts the brew cycle.

You may have a thermostat problem.

Contact a Bunn service center and have the unit checked.

The pressure stat is defective and needs to be replaced.

open the cover on the brewer- depending on your model, it could be the front or the back cover. look for a brew timer dial. turn this up slowly, running a brew each time to see if you now have a full pot

You could have scale building up on the heating element and thermostat probe. try running a 1/3 white vinigar - 2/3 water mix into the brewer, let it heat up completely, then run several pots of clear water through the unit. keep pouring water through it until the vinigar smell is out of the water,

Hello. It sounds like your main heater is blown, or the hi limit is bad, or a wire is burnt off.You will need to find out your voltage at least to make a guess at the part, it sounds like you have a RL unit.

http://bunnomatic.com/pages/techdocs/manuals/autobrew_warm.html

Look at the RL manuals, and try to tell which one is yours.

Hope this helped, and thank you.

your tank is leaking
Please Follow LINK for parts etc