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?
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.
Anyone have a part number for for the resistor? Thanks
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 (http://hvactechgroup.com/files/resistor.pdf)