How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Phil Your Own Question
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
Type Your HVAC Question Here...
Phil is online now
A new question is answered every 9 seconds

I have a monitor GF 500 that comes on OK but doesn't seem to

Customer Question

I have a monitor GF 500 that comes on OK but doesn't seem to stay on, goes to 1st stage then 2nd stage then a short time later goes off with the screen flashing.
Submitted: 1 year ago.
Category: HVAC
Expert:  Phil replied 1 year ago.


Tell me if that looks like the service manual or not, I will be referring to it as we go along.

tell me if you are getting any of these error codes,

We can go from there.

see page

Customer: replied 1 year ago.
Don't have the manual in front of me but the unit on the front looks just like my unit. No error codes per se but flashing 88 88 and heat level indicator (the different Height bars) once it's been on a while.
Expert:  Phil replied 1 year ago.


Tell me a lot about your technical skills sets so that I can coach you most effectively... that type of heater can be a dangerous producer of carbon monoxide, you should have 2 of those detectors in the home as well.

We can go from there.

Customer: replied 1 year ago.
I work at a water treatment plant and do some maintenance. I've had the front of the heater unit off, and a couple years ago replaced a diaphragm that had a tear in.
Expert:  Phil replied 1 year ago.

Hello again,

Do you have the skills to follow these diagrams and test the electronics components as shown?


If not, you can try this heat stress test on the printed circuit board.

Use a hair drier to slowly and gently warm the card to 120F(warm to the touch but not hot and not over140F)... if you get any change in behavior that means the card is bad...

A micro crack has opened in the printed circuit at a solder joint, or inside its CPU in that case .... or a short circuit has developed as the card expanded when you heated it.

Constant expansion and contraction as these electronics heat and during their duty cycles over the years is what wears these cards out.

It is also a good idea to replug all of the connections in the unit,that burnishes any oxidized terminal connections and will sometimes solve the problem, especially at the printed circuit card and in any sensor molex connectors. Do that with the power off to the unit of course.

Let me know how that goes, we can proceed from there.