Sears Wall Heater - Short Run Cycle - http://www.hvacmechanic.com/ Forums
Original message
| dbrown3611
 |  "Sears Wall Heater - Short Run Cycle"
, posted Sat 18 Oct 10:56  
Sears model 867.736212
Unit has not been run for at least 12 years, has original external mercury thermostat. Lit pilot and powered up unit. It runs for about 2-3 minutes then blower shuts off. After about 15 minutes blower runs for another 2-3 minutes. Desired temperature is never reached.
What could be causing this short run cycle?
Kind regards,
Brownie
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Replies:
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| techshawn
| "Re(1):Sears Wall Heater - Short Run Cycle" , posted Sat 18 Oct 15:37
check the fan control and make sure its setting is right(about 90-120F).If the setting is too low, it'll act like what you post.
I like fixing everything.
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| | Freon
| "Re(2):Sears Wall Heater - Short Run Cycle" , posted Sat 18 Oct 15:55
Call for heat and wtch the furnace. Does the main burner remain lit all the time even though the fan does its cycling? If yes, then as Shawn suggested it may be the fan control.
See if you have a small 3x5 rectangular box mounted on the front of the furnace. It usually has an aluminum cover and may say Honeywell. See if you can remove the cover by squeezing in the sides. Is there a round dial? Let us know what you find.
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| | dbrown3611
| "Re(3):Sears Wall Heater - Short Run Cycle" , posted Sat 18 Oct 18:33
The main burner remains lit when the fan cycles. Burner only shuts off if I lower the thermostat setting to below ambient.
No such box as you describe. There is a box approx 3x3 that has connections for gas inlet, pilot feed (with an adjusting screw) and thermocouple. Also appears to have a transformer mounted on the back.
I have a wiring diagram pasted inside of unit. It shows 2 switches labeled "fan" and "limit". The "fan" switch is in series with the fan motor and shown normally open. The "limit" switch is in series with the transformer primary and shown normally closed.
Both switches are mounted on a panel about midway up the chassis. Both have 2 wiring terminals. Both are silver and about 1/2" in diameter with a mounting flange. The limit switch rattles when I shake it.
At this point I am ready to try replacing these switches. I have a local Johnstone Supply, they are not open until Monday.
Thoughts and comments are welcome.
Regards,
Brownie
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| | techshawn
| "Re(4):Sears Wall Heater - Short Run Cycle" , posted Sat 18 Oct 19:04
wait, you mentioned 'fan' and 'limit'. that's what we need to fix. choose the fan, watch for any lever on its panel whatever. Make sure that the present setting is respectively 90-120-200F for fan-off-on-limit tripping temperature.
I like fixing everything.
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| | techshawn
| "Re(3):Sears Wall Heater - Short Run Cycle" , posted Sat 18 Oct 16:23
thank you for your diligent and detailed response here.I just drop by.
I like fixing everything.
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