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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8136
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have a 28 year old Rheen 75K heating cooling unit in my

Customer Question

Customer: Hi, My name is***** have a 28 year old Rheen 75K heating cooling unit in my attic. I live in Southern Massachusetts. My unit was built in 1988 and installed in 1990. RGUC-07EA-ER. In temperatures in and above 50% my heating side functions as it should. Lowers temperatures in the day or night time cause it to run when I place the fan in the RUN/ON thermo position. In the Auto position the furnace exhaust fan spins but the pilot does not lite to start the burner. If put the fan in the run manual position it lites right and works like a charm. If I put it back in the Auto spot it will cycle and runs to thermo set temp shut off. How ever it will then revert back to the problem. If I just let the fan stay in the run on stetting it works just fine and keeps the house at the set temp. The fan just runs at a slower speed when the burner in not heating up. If I get a solution to the problem payment is ok. I do not want to pay for guesses. My heating guy has been hear and replaced my Fan Limit control already. That was supposed to solve the problem. Now he has suggested another part replacement at another $300. If you fell you can diagnose this from afar take shot at the title.
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Submitted: 1 year ago.
Category: HVAC
Expert:  Phil replied 1 year ago.


So far it sounds like the printed circuit board relay contacts are bad, I could write you a book and that would only confuse the issue.

Lets do a heat stress test on the printed circuit card.

Use a hair drier to slowly warm the printed circuit card up to120F or a bit warmer, but not over 140F... if there is any change in the error codes or other furnace behavior... the card has a cracked micro circuit, invisible a few atoms wide is all it takes, that opens and closes as the card expands and contracts... that proves a bad printed circuit card.

No change however does not prove the card is good however... but a change proves the card is definitely bad.

If that fails to show any change

Next please attach a well focused, close up photo of the wiring diagram glued inside the service panel, the picture needs to be taken from straight on, in very good light.

You can use the paper clip- icon at the right end of the tool bar at the top of this dialog box to attach it here.,Include a picture of the thermostat wire connections to the printed circuit board in the same area.

If you have an error code showing, tell me what the code read out is on the wiring diagram or a sticker next to the diagram please.

If there is no paper clip-icon, look for the '+attach files'link at left of your send button.

I can mark it up with trouble shooting instructions.

We can go from there.

Customer: replied 1 year ago.
Hi Phil, I do not have such a board. My Unit is pre board period. Rheem RGUC-07EA-ER
Customer: replied 1 year ago.
Just add to the problem there is no such wiring diagram on my unit as told to me by my HVAC guy. Go figure huh? Rheem must have forgot to put it in.
Expert:  Phil replied 1 year ago.

Thank you, ***** ***** one of the reasons I asked for the wiring diagram photo's... the industry was in the process of changing to printed circuits in that time frame.

....and unfortunately, of the several hundred thousand models ever built, I have only worked on a few thousand of them, less than 1%. Manuals on the older furnaces predate the web and are commonly not available.

Send me a photo of the wiring diagram, we can go from there if you wish.

Expert:  Phil replied 1 year ago.

If there is no diagram remove both covers on the furnace and send me a photo of the furnace from 8 feet away, and a closer shot of each compartment... I can most likely figure it out from there.

Expert:  Phil replied 1 year ago.

Photo's need to be in very good light and well focused

Customer: replied 1 year ago.
If you can point me the the right direction as I have looked for such a wiring diagram and asked Rheem for one for years to no avail. That furnace is in mt attic and there is no way to complete that task without be a contortionist. It has been suggested that I replace the Honeywell S86F Module to solve this problem But what does that module have to to do with the start up and pilot lighting start up?
Customer: replied 1 year ago.
I have looked and I can buy such a module but the several versions such as S86F1000 to S86F1058. My module only shows the S86F with a REV A 97-3656. That's it. The gas control is a Honeywell V850. Which S86F module works with the control???
Expert:  Phil replied 1 year ago.

Hello again, whomever told you that about the module was guessing, that guess would have been in the 40% accurate range, to get any closer tests need to be done, there can be numerous other problems.

It is unfortunate that some thoughtless outfit stuffed that furnace into an inaccessible location, its not the worse i have seen though, one company dug a hole under a house and put the furnace there, in the winter the hole filled with water, so you are lucky in that regard i guess.

On such an old furnace it will be easy to get the wrong part especially considering the many substitutions since then.

If you have the money it might be time to replace the entire system... let me know if you need any advice on that approach... a 28 year old furnace is beyond its normal life span and could be leaking carbon monoxide.

Be sure to have 2 CO detectors in the house for that reason.

Customer: replied 1 year ago.
Just had to replace my driveway at 9K. Money that was spare is no longer there. I will get some of the detectors though. Healthy & Happy New Year Phil.
Expert:  Phil replied 1 year ago.

Thanks Beano, have a great new year yourself,

If you choose to give me a positive rating for the scant advice I offered, the company will pay me for my time, rating will not increase your charges, but is how I earn a living.


Customer: replied 1 year ago.
Unfortunately for me he was not able to provide any help. My question remains unanswered.
Expert:  Phil replied 1 year ago.

Thats OK, I understand.