High low limits adjustment!

[Barelohim]

Have an older Bryant 80% single speed gas furnace. Would like to set the low limit slightly higher so the blower would shut down a little sooner.

Is there a schematic as to where these limits are located and if they are adjustable, on an older furnace?

Thanks

Bar.

[Barelohim]

the blower fan keeps blowing until the heat exchanger isn't hot anymore, so it is cooler air. Same when the furnace blower first starts up, the cooler air in the plenum comes up first, and it is cool as well, but not for long. After the heat is down to the low limit, like I said, it is cooler air, which, if the limit was set just a tad higher, the blower would quit sooner, thus not continue to move much cooler air..

[nomadpeo]

some older models had a fan/limit control utilizing one sensor, a helical coil comprised of two strips of dissimilar metal laminated together. the sensor protrudes into the cabinet and resides next to the heat exchanger so it is affected when the heat exchanger heats up. as it heats up, it tightenens, causing movement on a wheel with cams. if you have this, the part you see is a rectangular control with a metal cover. two thermal switches are inside, one to control the fan and the other to control the gas valve. when you remove the cover, you see four or more wires going into it and a silver metal dial. this dial contains three setpoints that are adjustable; fan off, fan on and limit. the max limit setting is 200 degrees and will disable the gas valve if the temperature exceeds the setting. the fan settings are adjustable to compensate for the fact that the coiled sensor becomes weaker over time. back in the day, many service calls were generated because the fan would not shut down. a slight adjustment of the 'fan off' setting would remedy the problem. we would also adjust the 'fan on' slightly lower too. as pointed out, air moving across the heat exchanger is essential, but typically, the heat comes on first, then the fan. the fan goes off last to finish cooling down the heat exchanger.

note - if you have this type of control, you have a very old furnace and you should thoroughly inspect and test the heat exchanger for cracks and holes.

question - do you have a metal box hanging off your furnace that your flue pipe connects to ?

[Barelohim]

It is a Bryant gas furnace. Series A Model 394HAD042070GAJA Serial # 0592A05640 Where the first four numbers of the serial are the week and year of production, which would be fifth week of 1992...so app 18 years old. Running app 6 month of the year here in Saskatchewan, Canada.

There is a rectangual box on the side of the vent pipe leading out the top of the furnace, with two brown wires hooked to a sensor. There is also a tube attached to a vaccum device that appears to be attached to where the heat exchanger resides. But I can't see any markings that would indicate adjustment!

Does this help with the diagnosis?

Mr.B.

[nomadpeo]

the tube is connected to a diaphram mechanism that serves to prove air flow through the flue pipe. manufacturers had just started adding these to residential units. it probably also has a spark type ignitor. it may not have the adjustable type fan/limit switch i described. if the thermal fan switch is hanging up and causing a cool draft before it shuts off, it's weak. it's likely that a timer turns on the fan and the thermal switch turns it off. in any case, the best thing to do is to replace it. high speed fan is used for cooling typically and that wire going into the motor is black. heat will usually use the low speed (red) or medium speed (blue). unused speeds will be isolated with tape or some other means. follow the low speed wire being used until it takes you to the thermal fan control. the wiring diagram on the inside of the panel should be helpful also. once you find it, write down any information on it. a carrier suppllier should be able to match it with the unit model and serial number, but given its age, sometimes manufacturer part numbers and description helps. to be safe, it is best to replace it with a carrier specified part.

to reiterate my earlier comment, i think you should have the heat exchanger checked for cracks. a cummulative 9 years of running time warrants annual inspections to avoid carbon monoxide and fire danger. please do not disregard that issue. good luck.

[Barelohim]

thanks for the info.... No spark ignition, only the good ole pilot light and thermocouple.. I will do the inspection myself, since it is an easy inspection once the plenum is removed. The wiring schematic is on the bottom door, wherein the blower lies.

TAL Mr.B.

[nomadpeo]

does this furnace have a cast iron heat exchanger with the burner rack slid from the front underneath ? although it is a lot of work, it might be worth the time to heat up the furnace, shut it down, remove the burner rack, pull the blower and shine a light to detect cracks. also, inspect from the burner side with a light and mirror. while the burner rack is out, dump any rust out into newspaper. note - may take a while to relight the pilot........air has to purge through the pilot line.

[Barelohim]

Good idea! I will get on it, as soon as it warms up outside! This is the only heat source in the house, and it would be down for a spell, especially, like you said, pulling the blower! I'll get some pics on the procedure, and enclose it, when I hit the checkout!

thanks
Mr.B.