Proper ignition/blower sequence in older gas furnace?
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/y...t/SDC13530.jpg (gas conversion)
BARD. Bard Mfg, Bryan, OH.
Model. NB 115
Natural Gas Conversion Unit
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).
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.
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?
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.bestbuyheatingandaircondi...tegory_Code=FL
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.
Burners is knocking off on limit before fan comes off....motor should come on and stay on even with limit killing burners.
Motor is stalling and not coming on...or the fan side of the control is intermittent in operation.
Seen way too many of those beasts in my training years...had one in my childhood home.
The blower sleeve bearings are in excellent shape. They've been inspected oiled and maintained every couple of years.
Here's the dial controller...
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.
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.
set them for 155* cut-in and 125* cut out.
no set time for anything you asked...unpredictable due to condition and age of equipment.
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?
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?
If that control cuts off the burners the fan is supposed to be running.
If the setting causes the the limit to trip then the control is weak and needs to be changed.
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.
3.5 inch and 8.5 inch pulley.
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.
Here is a direct drive furnace blower with the same size blower wheel as mine.
RPM is 1075.
Perhaps the 1/3 hp motor with a 5.3 (5.5 nom) pulley driving the 8.5 is more in order here to arrive at approx 1075?
(i'm using an online pulley/rpm calculator)
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.
|All times are GMT -5. The time now is 05:53 AM.|