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Proper ignition/blower sequence in older gas furnace?

8K views 39 replies 6 participants last post by  how 
#1 · (Edited)
Hello,

I'm trying to come to some base line for the proper sequence of operation on this old girl. It's hard to get info on the net as to proper sequence timing. It varies so much from furnace to furnace depending on too many factors. So I decided to come here and describe the furnace and run the numbers.
(images are pretty large so I just linked them)

http://i802.photobucket.com/albums/yy303/svejkovat/SDC13529.jpg (furnace)

http://i802.photobucket.com/albums/yy303/svejkovat/SDC13531.jpg (blower)

http://i802.photobucket.com/albums/yy303/svejkovat/SDC13530.jpg (gas conversion)

BARD. Bard Mfg, Bryan, OH.
Model. NB 115
Ser. 21251

Natural Gas Conversion Unit
Magic Servant
Model. 761
Ser. 9322

Furnace probably about 60 yrs old. Oil burner converted to nat gas 40 years ago. Still seems to do the job quite well.

First things first. Think i need to replace the blower motor. Been popping it's circuit breaker lately, unable to start. Hums for ten seconds, quits, tries again, quits, tries again and takes off. Motor is a bit hot to the touch after this. I installed the motor brand new about seven years ago.

Dayton 1/4 hp split phase sleeve bearing. Appears to be still in brand new condition. Disassembled the starter contact and contacts were really badly corroded with flash. Removed this flash down to bare metal. Ran perfectly for a week and started occasionally doing it's humming on start and circuit breaker mischief. Don't know what's the problem. Is 1/4 hp too small for a blower this size? Perhaps the starter windings are a bit too fatigued anymore after all the humming and circuit breaking and I should just break down and get a new motor.

This looks like a realllly good candidate if I can get it locally for about 140 dollars (avg list on the net).
http://attachments.temcoindustrialpower.com/product_info/Leeson-M090602.00.pdf
Twice as efficient as this Dayton. Capacitor start. Ball bearings. What's not to like? Should I trade up to the 1/3hp version?

Now to the gist of my thread title. I sat down with a watch and timed the sequence through four cycles. Averaged them.

It's 32F outdoors.
It's 65F indoors.
Thermostat is set to 60F.

Ignition of burner begins and continues for 2:30
After burner goes out there is a pause for 2:25
Blower kicks in and runs for 3:45
Blower stops and system rests for 5:05 before burner again ignites.

Approx 14min total cycle time.

The burner is never on during blower operation.

Any of this sound amiss?


Thanks so much for reading any part of this. And hearty thanks and congratulations to anyone steadfast enough to have plodded on to the end. Hope I can get this baby tuned up.
 
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#13 ·
http://i802.photobucket.com/albums/yy303/svejkovat/SDC13546.jpg
3.5 inch and 8.5 inch pulley.

http://i802.photobucket.com/albums/yy303/svejkovat/SDC13550.jpg
1725 rpm motor.

710 rpm at the blower shaft.

I'll back off here for now and just keep a close eye on things over the weekend. I've been in the basement for eight hours today and the motor has not failed to start once. Monday I'll purchase and install that Leeson 1/3hp and then write down the interval numbers again. I'll return and repost them here.

Thank you all very much for the assistance.
 
#22 ·
I've read a dozen guides now on sequencing these old controls. But still have not found a straightforward plain and simple description of the cycle.

Do I have the following almost correct?

Room thermostat tells burner to ignite.

Burner heats up the plenum and fan limit switch to the set point that turns the blower on.


at this point the burner continues to burn


The blower cools the plenum and fan limit switch to the point where
first the burner shuts off, and then second, the blower shuts off at the low set point on the limit switch.

Eventually the room thermostat reinitiates the cycle.


I'm still having trouble with the burner turning off before the fan kicks in. My high set point on the fan limit is 135. I can watch the dial slowly creep up to 135 well after the burner has extinguished. So the burner can't be overheating the plenum. It's shutting off at about 120 degrees.

What shuts off the burner? Does the flame sensor need cleaning or replacing? Is the flame sensor adjustable?
 
#26 · (Edited)


The fan limit control is there above the right hand side of the furnace, pretty much where it is positioned in this image....


I'm off now to check out, and post a pic of the room thermostat. It's another 60 year old girl with a mercury switch inside of a coil. If I recall it does have a duration adjustment. I'll post a pic.

Meanwhile I removed the gas burner to see if anything needing cleaning or adjustment.
It's a good thing anyway that I looked into the burner since every nut and bolt in the thing was finger loose and worse.

Everything looks good on it.
Here's the room thermostatinternal adjustment...


Here is an image grab from the user's manual on this thermostat....



My heating type is "Standard warm air" so it would appear that the setting is pretty much as intended. Should I raise it in .1 ampere increments to see what happens?

I really hate it when directions aren't more explicit. When they say "furnace shuts off before" I wish they would have merely included the "furnace burner" qualifier for the sake of unambiguous clarity.
 
#28 ·
Thank you guys very very much for sharing your skills so unselfishly. Great forum.

I think I'm ok for the time being. I had my doubts but that "heat anticipator" setting is pretty damn sensitive. I moved it from .4 to .5 and the burner ran much longer.

I've had the power and gas shut off now for about four hours on a winter day so it's going to take a little while for things to settle down again. Thermostat is set on 60 and the burner ignited as soon as I turned everything back on, the blower started about two minutes later, the burner ran for about 10 and the fan ran for another five after that. This is the first time I've gotten the burner to stay lit for more than three minutes under any circumstance.

I do wonder if I might go from a 3.5 to a 5in pulley on that blower when I replace the motor. It does make sense that the CFM might not be cooling this big furnace down rapidly enough and screwing up the sequencing that it was designed for.

Thanks again.
 
#32 · (Edited)
Very good suggestion. I do have one in the living room that I installed when I put a woodburning hearth stove in my fireplace slot. I ought to get another for the other end of the house.

Incidentally, this little digital kitchen thermometer has come in so handy since I bought it three years ago for about 10 bucks. Nicely built and seems very very accurate.

I was pleasantly surprised at how accurate the markings on that fan switch dial turned out to be. The digital unit's probe extends into the furnace cavity about four inches through an empty former screw hole.


It's interesting to watch the readout as the furnace cycles. As soon as the blower starts the temp begins to drop but the fan switch dial continues very slowly upward for another minute before ever so slowly going down again. The bulk of the bimetal coil and it's metal housing stores enough heat to make it pretty slow to responsive. For the purpose it's doing its job just fine though.
 
#34 · (Edited)
No, the furnace, according to my digital thermometer never exceeds 135 degrees. I'll leave the pulley in place. I don't believe that overheating was ever actually the issue here. That upstairs thermostat "heat anticipation" setting was always too low and the burner never stayed on long enough to overheat.

However, since I posted last, and for the first time in two days, the motor balked and hummed and refused to start. Guess the starter winding has just had too much. I was resigned to replacing it, but was hoping against hope that all my adjustments (knowing full well that they had nothing at all to do with the starter switch and windings on this motor) might have put me in good graces with this old furnace sufficient to have it pay me back. Of course that was misplaced hope. Of course I'm going to replace the motor. Just sayin.

So tomorrow it's off to Grainger or Holland Electric Motor to get a 1/4hp replacement.

While I'm here, let me pick the brains a bit more. That Leeson 1/4hp high efficiency motor I listed ...
http://attachments.temcoindustrialpower.com/product_info/Leeson-M090602.00.pdf
...is spec'd to run at half of the amperage of the motor I presently have. It's described as split phase start, capacitor run. I cannot find a good description of this in the online tutorials. I had thought that the presence of a capacitor hump on the body of the motor indicated an improved starting torque, but it is for 'run' only and not for start. I'm learning more about electric motors this week than I ever thought I would. Apparently, the capacitor on a capacitor start motor does not supply "oomph" from stored energy so much (as I'd always supposed it did) as it merely induces the motor to the optimal phase angle. That's beyond my ken, but I'll accept it. In other words, adding a start capacitor to a split phase motor would accomplish nothing since the windings are not set up for it in the first place.

So what is the advantage of purchasing a "capacitor run" motor like the one I listed? Other than the obvious in this case.... higher running efficiency.
 
#37 · (Edited)
Well, for the time being I think I'm ok. I'll post the following for the off chance that anyone else finds themselves here with a similar malfunction.

And incidentally, this really does seem to be a very good value for a belted furnace blower replacement motor...
http://www.electricmotorwholesale.com/LEESON-M090405/

Runs at nearly half the amps as similarly rated motors. It's about six lbs heavier than others as well. The website suggests that this is in sturdier end castings and higher copper content in order to meet efficiency goals.

Anyhoo..... before clicking on "put in basket" at that site I decided, since I'm such a cheap bastard, but just as much because I hate to give up, to remove the motor, put it on the bench, and cycled it over and over and over until I could get it to balk. Just when I was beginning to think it was only going to do so under load.. it balked. It just sat there and hummed. Peering into the vents I could make out that the centrifugal swash plate was stuck.

I took the rear end off and looked into things closer. The swash plate is driven by two stamped metal flying weights held in place by two springs. When fully extended under high rpm the whole mechanism is so extended that it puts very tiny pressure against the plastic swash plate. It is that pressure that starts the plate back on its way to closing the starter contacts when rpm drops. My mechanism was exerting almost zero pressure against the swash plate at full extent. Seriously. This was the only thing wrong. The springs are mounted on four tabs on the arm weights. I bent each tab every so slightly backward, thereby exerting a tiny bit more, but substantial, pressure on the swash plate at full extent. Anyone remotely familiar with this mechanism can picture this.

Put everything back together. It's been running without a hiccup for over 24 hrs.

There's nothing wrong with the contacts or the windings. The starter contacts were often not closed when they were needed. The humming was the motor trying to start via the run windings alone. Without the starter windings, these windings, literally, didn't know which way to turn:) These windings are much more robust than the starter windings so probably didn't suffer any damage from this. Just shut off the overload switch. A couple of such cycles were sufficient to nudge the rotor and snap the swash plate back against the starter winding contacts and presto, it would finally start. Meanwhile, when the starter contacts were suddenly brought together under current, a condition they weren't designed for, they flashed and started breaking down.

So all's well for now. I only saved 160 bucks, but the feeling of triumph is priceless.

I'll just put that 160 toward a new furnace in a few years.

Thanks again for the advice and the fun here.
 
#39 · (Edited)
Still... the supply at only 135*? on a furnaces designed for a 160* ...?
Or are you saying that I should leave the cut in at 130 and that the burner should remain lit and capable of continuing to raise the plenum temp to 160, with the blower operating, then maintaining that 160 until the room thermostat is satisfied?

So...

1 thermostat tells burner it needs heat,
2 burner lights,
3 blower starts when furnace temp hits 130,
4 blower and burner work together while continuing a climb to approx 160 degrees,
5 when room thermostat is satisfied with the room temp the burner shuts off,
6 then the blower continues until the furnace temp is reduced to approx 90 to 100 degrees.

I should raise that "anticipator' setting then to keep the burner on longer yet and see if I can get the furnace temp higher during blower operation?
If that does not suffice then I need to investigate either why the burner is not hot enough or whether the CFM is in fact too high rather than, as I feared, too low.

********postscript****** just went down and went through the sequence again for the first time since I've had the motor running correctly.
The blower starts at 135 degrees. The temp drops within about one minute to 125 and then slowly rises to 145 degrees staying there. Burner shuts off and temp goes down and blower stops at about 95 degrees.

All in all, rather than having too little blower air as I originally suspected, I may have a tad too much. That would probably limit that droop to a few degrees initially and allow the operating temp to rise closer to 160.

Not sure if it's worth a new pulley and belt. Is it?
 
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