Proper ignition/blower sequence in older gas furnace?

hvac5646 · 03-04-2012, 10:53 AM

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.

yuri · 03-04-2012, 11:13 AM

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.

hvac5646 · 03-04-2012, 11:25 AM

Quote:

Originally Posted by yuri

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.

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

yuri · 03-04-2012, 11:45 AM

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.

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.

hvac5646 · 03-04-2012, 12:21 PM

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

.

yuri · 03-04-2012, 12:39 PM

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.

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.

frascati · 03-04-2012, 12:52 PM

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?

yuri · 03-04-2012, 01:22 PM

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.

how · 03-04-2012, 01:32 PM

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.

biggles · 03-04-2012, 02:13 PM

if the stat is calling heat the burner always runs...the supply fan is the last thing to come on and the last thing to go off....that Honeywell limit controller should be in the heat exchanger area not the plenium.it need to see the heat asap to react either it be fan ON or Hi limit for burner

frascati · 03-04-2012, 03:02 PM

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.

yuri · 03-04-2012, 03:41 PM

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.

frascati · 03-04-2012, 04:04 PM

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.

yuri · 03-04-2012, 04:27 PM

No, no no. Stay with that sized pulley or you will end up putting a 1/2 hp motor on and it will be noisy as heck. That belt drive fan is designed for 2 pulleys of the size you have and we only speed it up if AC is added or freezing up.

frascati · 03-04-2012, 04:49 PM

Good'nuff then! And thanks again.