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Bill, HVAC Technician
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I have a Ruud urka-a036jk10e option 594 built in 10/98. Basically

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I have a Ruud urka-a036jk10e option 594 built in 10/98. Basically a 100,000 btu natural gas/ac outside unit. The heat quit with over temperature switch open. a few days later it happened again. I bought a new switch and compared the two by heating them on our electric stove. the old switch poped long before the new one did. the new switch has worked for a month and about 100 therms of gas. It went out again what would you do now?

Thank you, Peter
Submitted: 6 years ago.
Category: HVAC
Expert:  Bill replied 6 years ago.
hello, you missed your calling ,you should be an hvac tech.....i would check your air filter and than your ductwork, when the high limit switch is tripping out, there are some things to consider....air filter: if dirty will not let enough air flow....ducting collapsed :also causing reduced air flow....indoor blower fan failing: causing low air flow when it is overheating...last but not least, dirty components : blower wheel covered with dirt/ dust or air conditioning coil plugged with dirt/dust..........
Customer: replied 6 years ago.

Thanks for youre answer. My unit has been kleened like new. I also have increased the filter area by 30%. It buggs me that it can run 28 days 3to4 hours each and then crap out. I will check the filter tomorrow and let you know.

Expert:  Bill replied 6 years ago.
you have done the normal improvements, air filter size and total cleaning, has this happened since you have owned the home,? it could be the ducting is a little undersized, only causing the high limit to trip durring long heating cycles when it is very cold outside.....

Edited by Bill on 12/10/2009 at 5:43 AM EST
Customer: replied 6 years ago.
Bill this unit is 11 yrs old. I have lived here 4 yrs. The problem is this fall only. the temp outside has been above 40 at night. I checked the air temp at the closest register the temp after 15 min was 125.6 deg f. I removed the filters and ran 15 min more the temp was less than 1 deg lower. Thanks Peter
Expert:  Bill replied 6 years ago.
wow, this is a tough one, if it were me i would see if i could raise the blower speed one setting and get another limit switch, all you describe seems normal, i really cannot think of anther solution
Customer: replied 6 years ago.

i have already raised the blower speeds. New clue-- the failure is the first 5 min after sitting all night on set back. Peter


ps maybe the blower is lazy out of the gate. maybe the high limit is not working causing the over temp to fail????

Customer: replied 6 years ago.
I wired the blower to run on hi and the output temp dropped 6 deg. The hi limit checks out ok. The unit failed on overtemp this am in hi 20's temp.It ran just long enough to warm the registers about 3 min. Peter
Expert:  Bill replied 6 years ago.
your new clue may be a good one , if the blower waits too long befor it comes on it will almost always trip the high limit safety, do you know at what temp the blower is kicking in, or how many seconds go by after the burners have lit and when the blower starts..should be 50 seconds to fan "on" and than 90 second run after flames go out on normal cycle., your control board may be having trouble .

Edited by Bill on 12/12/2009 at 12:52 AM EST
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