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Last year I had to replace the integrated furnace controller…

Customer Question
Hello. Last year I...

Hello. Last year I had to replace the integrated furnace controller in my Rheem furnace. The replacement I was shipped is a Model 1012-920A. It works fine except when the power goes out and I'm trying to run from a standby generator. When the blower attempts to start, the blower load causes the generator's output voltage to sag, I think it was down to 105 VAC. When that happens, the board seems to reset. The blower runs for not quite a minute, then shutdown. Then the board restarts the ignition process but when the blower is started, the flame goes out. lather, rinse, and repeat. Before changing the board. it worked okay. Any ideas about how to resolve the problem? I have the old board, but the components on the damaged corner are too badly singed to read the color codes and device numbers, so I can't repair it.

Contractor's Assistant: Did you check the furnace's filters?

Yup, new last month

Contractor's Assistant: Do you plan on doing the work yourself?

I opened the access, jumped the safety so that the blower could pull directly from the room. No difference.

Contractor's Assistant: Anything else we should know to help you best?

I didn't measure the frequency change, only voltage, which averaged 124, I think with a low of 105.

Submitted: 2 months ago.Category: HVAC
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Customer reply replied 2 months ago
I will probably do it myself.
Answered in 1 hour by:
3/22/2018
HVAC Technician: Phil, Mechanical Engineer replied 2 months ago
Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6,938
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
Verified

Hello. what is the full load amp rating on the generator,, and its brand and model number...and tell me in great detail how its grounded. So far it sounds like the generator may be slightly undersized.

.

We can go from there.

.

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Customer reply replied 2 months ago
It's a 4 kw Generac portable that I've been using for nearly 20 years. It's always run the furnace before. Connected via 4 prong 30 amp cordset to a sub panel. Mating plug from the generator has the ground connection tied to a ground bus bar which is independent from the neutral bus. Both the neutral and ground bus bars are connected via 6 ga solid copper wire to two 8' grounding stakes set 12' apart. Ground connections are clean and tight with little measurable resistance between the plug connector and the ground stake.The generator formerly ran the furnace, both refrigerators, and some lights previous to installing the 62-24084-02 Model 1012-920A furnace control board. I shutdown every circuit in the generator's sub panel except for the furnace, which is on a dedicated circuit, and control board still turned off the gas valve when the blower motor started. The PWR LED remains lit at all times. The OK LED remains lit during the purge, ignitor heating, ignition cycle until the the blower tries to start. When the blower tries to start the gas valve closes and the OK LED blinks.
HVAC Technician: Phil, Mechanical Engineer replied 2 months ago

Thanks, just replacing the printed circuit board, even if the board was not operating correctly should not have causes the generator to stall out. ... unless the blower motor wire connections put power to both its low and high speed windings at the same time.... IF that were the case however, then the furnace would run on utility company power either.

The problem is with the furnace drawing too many amps at start up (due to a bad blower motor, or possibly just a bad run capacitor on the blower motor)... OR a problem with generator output. We should dig into those issues deeply and ignore any real or coincidental issue related to replacing the board for now... (making more than one assumption at a time can lead is in to the all weeds so to speak, so we will test these one at a time).

.

Use a clamp around amp meter to measure amperage draw at your 120v furnace power supply while on city power.. ahead of its transformer or at its breaker in the house control panel, if the motor draws its listed LRA, (locked rotor amperage) for more two seconds when it starts, then the blower motor has a problem... if you see that amperage, then check it on the Neutral wire that feeds the blower motor to be sure that its not a problem with the other furnace components.

Let me know how that goes.

--------------

If the furnace runs well on utility company power, and its blower motor draws LRA for less than 2 seconds, and does not exceed its FLA (full load amperage) the problem is with the generator itself or its wire connections... In that case we will do various load tests on the gnerator independent of the furnace connection using both volt and amperage draw meters,

.

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Customer reply replied 2 months ago
Current draw doesn't seem to exceed 15A. Just for fun I wired the 30amp connector on the generator directly to the furnace. The controller board won't start at all. Searching the internet makes it look like this is not an uncommon problem and it appears insoluble without either replacing the furnace or the generator. Yet another case of bad engineering where the design team made too many assumptions about the operating environment, in this case, line power.I might try figuring out the value of the resistor that burned on the old board and try repairing it.
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