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Proper ignition/blower sequence in older gas furnace?

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#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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#2 · (Edited)
....postscript.....

I'm looking at those photos and for some reason the thought just occurred to me that this might be a good time to, at a minimum (not ready to invest in a brand new entire unit), replace that entire blower with a modern unit that is likely to be three times as energy efficient per CFM. Or do you think this massive old belt driver is not as inefficient as it looks?

Is it possible to convert these old blower frames into direct drive units with ultralight blower wheels?
 
#3 ·
above the burner/gas valve is there a dual dial controller with temperatures settings.the furnace is going off on HI-Limit within the furnace without the fan coming right on thru th dual temp controller.....need to see a pix of the temperture controller within the furnace. might be stamped FAN/LIMIT...HI LIMIT fan is usually 120F-140F range and the HI 180F-200F.it's not the stat knocking the burner off it's that hi limit. http://www.bestbuyheatingandaircond...c5&Store_Code=BB&Screen=CTGY&Category_Code=FL
 
#4 · (Edited)
First the obvious.. Make sure the bower bearings and shaft are not wearing, binding up and causing the motor to struggle.

Would also have expected a 1/3 hp motor with a blower of that size.

As Biggles pointed out..Without that motor cooling down your unit, the safety limit will shut down the main burners. This limit will allow the burner to start up again as soon as the exchanger drops back down in temp by 10 -20 degrees.

This old safety control which is only made for occaisional use is the only thing now preventing a house fire.

Shut OFF your heater until you swap out the motor.

You might also want to check out the heat exchanger before you buy a new motor in case it has been cracked from the overheating that's been going on every time the motor hasn't worked.
 
#7 · (Edited)
The blower sleeve bearings are in excellent shape. They've been inspected oiled and maintained every couple of years.

Here's the dial controller...
http://i802.photobucket.com/albums/yy303/svejkovat/SDC13541.jpg

Closer...
http://i802.photobucket.com/albums/yy303/svejkovat/SDC13539.jpg

Given the numbers/intervals I've mentioned, where would you suggest I begin with each of those three settings?

And then once the sequence begins again, very roughly what intervals am I shooting for?
How long should the burner run before the blower starts? How long should the burner continue after the blower starts?
How long should the blower continue after that point?

This is the best shot Could get of the plenum through a 2.5in diam hole.
http://i802.photobucket.com/albums/yy303/svejkovat/SDC13543.jpg
There is no really good access panel for inspecting this area without removing rivets and large sections of the furnace. If necessary I can remove a 10" X 10" panel from roughly the same perspective as the above photo for a clearer shot.
 
#9 · (Edited)
Wow. I'm an idiot. Officially. I installed that control 10 years ago and don't remember, but probably just assumed that I was smart enough to adjust it seat-o-the-pants style. So it ended up at 100/150/200 and it stayed there for the last decade.

Next thing I know the motor burns out. I replace it. Six years later this motor starts failing. Hmmmmmm.

Finally I get on this forum and at the same time start doing some responsible research on this device and discover that my "fan on" setting was probably about 35 to 40 degrees too high this whole time. What I suspect was happening is that the intervals I've described above have been overheating the plenum, and cycling the poor motor on and off at shorter and more frequent intervals than was ever healthy for it.

I've just reset it at 85/115/200.

Sure enough the fan kicked in about 1:20 after the gas ignited. It's running beautifully. I'll tweak as needed.

What do you think about the replacement motor I linked above? And should I get the 1/3hp?
 
#10 ·
Damn it now I'm confused again. I posted that last before I had a chance to see hvac5646's post.

This isn't good.

So I didn't figure this out after all? You're saying that rather than drop the cut-in from 150 to 115 and lower the cut-out from 100 to 85... that instead I should have raised the cut-in from 150 to 155 and raised the cut-out from 100 to 125?

The cut-in at 150 was what was causing the fan to wait too long to start. Thereby letting the plenum overheat and automatically shut down the burner. Correct? Won't raising that make the matter worse?
 
#12 · (Edited)
The motor is the problem. Buzzing, not running & tripping breakers?

Your lower adjustments to the fan control will make the furnace more efficient but will not deal with the motor problem.

If you are not sure if the motor is the problem, then just press in the summer fan switch on the fan/limit control and run the motor for an hour.
If it stops on it own while the summer fan switch is pressed in, then you can go the extra step of confirming the motor is getting 120V and not running which is what a tech would do. Remember to turn off all furnace power to set up your meter for such a test.

What is the operating temp on that fan/limit switch when the furnace is running?
I ask this because the pulley on your motor looks too small to push enough air through the furnace.

The exchanger check involves removing the upshot conversion to access the burner compartment which is usually more than a DIY should tackle.
 
#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.
 
#15 · (Edited)
That direct drive fan has a 230 volt motor and you have 115 in the house. If you can find one that fits and cobble it together mounting wise then go 4 it. Not easy but we used to do that temporary when major repairs were needed. I would set the fan limit control to 130 cut in 90 off and limit at 200. Those controls are not and have never been accurate so it is a matter of finding a setting that works. They also stiffen up with age and slow down. That industrial motor is 15 lbs and a lot heavier than the original so it may vibrate the fan or be top heavy. .25 hp works due to the small pulley and I saw hundreds of them like that back in the day. As long as the belt has at least 1 inch deflection and is not too tight it will work. .33 hp may be better. Graingers has 115 volt dd fans and I would buy a 4 speed one so you can choose the best speed. 1 speed may be too high or slow and then U are stuck with it.
 
#16 ·
90 and 130 are great when considering energy efficiency but lousy for comfort. Air blowing at 90* will get you complaints.

That F&L control will short cycle like crazy off the stored heat of that monster ht ex. Bard's had a second Kidney in the the flue..judging by the length of OPs furnace he has exactly that.

That blower will cool down the fan helix so fast it will kill fan before all the heat is dissipated off the exchanger and the fan will cycle on and off with the burner coming on in between on and off periods on a call for heat.

I loved to change those beasts. Had to use torches to cut up exchanger.
 
#17 ·
At the end of the day the setting has to be done in house and the fan speed and size of ductwork etc is the issue. I would set them so that when the fan shut off it stayed off for at least 4-5 mins B4 cutting in again. Whether that was 90 or 100 deg depended on the age of the control and how sensitive it was etc. Running it up to 150 just wastes heat but on a modern unit 130 would cause short cycling so 150 prevented that. No way that old buzzard will short cycle once that lump of cast iron heated up.:no:
 
#18 ·
talking about fan short cycling Yuri, not heating cycle.
Running them at 150 was a necessity..the HOs were all very elderly..they liked it hot.
And all the system I have worked on were low pressure low velocity so the duct work was humongous by today's standards. Was great putting in a new air though..bigger pulley and ba-bam..instant cooling comfort:thumbup:
 
#19 ·
And now you have this 2 foot square plenum and have to block 2/3 of it off to get the air to move thru these small coils and ain't that a beauty to do and deal with.:yes:

I think a lot of furnace overheating is due to the plenums being reduced for the ac coil and nobody putting in bypass dampers or it is impossible and the fact the blowers are minimally sized and the extra blockage overloads them.
 
#20 ·
I would mention that I was a kid and under Uncle's wing when we did an ac.

There were still an abundance of slab coils available that fit in the old low boys. So there was no undersized coils to restrict air flow. Uncle would always put a 1/3 hp in with a five inch pulley. Moved a lot of air.

I have few ten SEER ac's---- mostly 2-1/2 to 3 ton--- and we had no problem finding evaps for them that were large enough to work with the furnace supply opening. Don't forget those furnaces were not all 2 ft by 2ft...they varied from 14" FD to 16" FD as well. But the funny thing was they all were 100k BTU:laughing:.
 
#21 ·
Come up where I am. We had ICG 250 and 275,000 BTU lowboy furnaces so your 100 is a cigarette lighter compared to what we used/had.:laughing:

And yeah, the old rule of thumb/bigger is better biz was very common. Pretty well every house we have got/used a 2 ton or bigger ac even though 18,000 was available but it was a lot easier to buy a couple hundred 2 tons at a time instead of stocking some smaller units with them.:yes:
 
#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?
 
#23 · (Edited)
Your description is almost right. Your room thermostat is faulty/short cycling. Post a pic of it. It may have an adjustible heat anticipator inside it to give it longer cycles. If the tstat is getting a draft or there is a hole in the wall behind it then it can short cycle. The burner should run continuosly for at least 10 mins to heat the house. The fan control only turns the fan on/off with air temp and the limit is there as a safety in case the motor stops or belt breaks or the ducts are too small/poor airflow. The burner is operated by the tstat only except for an overheating situation where the limit is used.
 
#24 ·
Normal sequence should be
-Stat calls for heat.
-Burner comes on.
-When temp gets to the middle nib on the fan/limit control - fan turns on.
-When stat is eventually satisfied the burner turns off.
-When the fan/limit control temp drops below the first nib -fan turns off.

Limit nib is only acheived if there is an air flow problem.

If your burners are turning off while the thermostat is still calling for heat and the fan/limit control has not limited, then something is not working properly.

Thermostat, fan limit control, gas valve, unfound aux limit are the usual things to check.
 
#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.
 
#27 · (Edited)
Normally the tstat does not develop a life of its own and that setting has been there for a long time but you should try move it at .1 increments higher and it will improve. I suspect a draft. That chart is for reference only and is not always the best settings. The higher the # the longer the cycle of the burner. Your gas valve may be dying and drawing too much current or the wiring is loose, old and corroded and causing that problem too. I would go buy a decent Honeywell Focus Pro electronic tstat from HDepot and use it instead to eliminate that problem.
 
#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.
 
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