Author Topic: Help with gas fired furnace short cycling  (Read 3896 times)

Offline joetar6

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Help with gas fired furnace short cycling
« on: November 01, 2007, 12:35:51 PM »
Have a customer with an older Heil Gas fired furnace/AC system. T-stat did not start up the furnace. Replaced a Hunter Digital Tstat witha Honeywell T8112d. Unit fired ran ok for a few minutes, but then burner started to short cycle(coustomer told me at this point "that's what it was doing before"). Jumpered the heat contacts on the board-system fired up fine and  continued running.
Heat Guy at the supply house said I need a resistor on the C & W contacts on the board to stop the short cycling. Honeywell tech. Serv. says Could be a bad Tstat and did'nt feel the need for a resistor. Since The original and the new Stat create the symptoms I think it's more than a bad Tstat. Any Suggestions?

Offline TECH X

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Re: Help with gas fired furnace short cycling
« Reply #1 on: November 01, 2007, 05:38:29 PM »
The hunter thermostat had a built in delay, that's why it didn't start.

The cycling is a well known problem with the board in that furnace when wired to a power stealing thermostat.  Your T8112d (Magic Stat) is the most popular brand that causes these symptoms.  Either install a thermostat with a common connection to the furnace, or install the 1000 Ohm resistor between W and C at the furnace.  I forget the part number for the resistor, but the instructions specifically mention the board used in that furnace.

Offline joetar6

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Re: Help with gas fired furnace short cycling
« Reply #2 on: November 02, 2007, 07:04:26 AM »
Anyone have a part number for for the resistor? Thanks

Offline TECH X

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Re: Help with gas fired furnace short cycling
« Reply #3 on: November 02, 2007, 04:04:54 PM »
If your furnace has the ST9120 or ST9141 fan timer board, then use the Honeywell 1000-ohm resistor #4074FAB.

You can read the instructions for the resistor - Here