Stack Limit mayhem--oil FHA

[JTB]

Hey all...

Here's the scene--old old house, with an FHA oil furnace from yonder. Make I believe is 'Mayflower'. It is serviced regularly, and has newer components. Late last year the Stack Limit Switch (Honeywell RA117a Protectorelay) decided to trip frequently. In the offseason I had the unit completely cleaned and checked up. It has run great until about 2 weeks ago. The STS has commenced cycling the burner. It will signal the burner to ignite, and then after a few seconds the burner shuts back down. It appears to do this mainly when attempting to maintain the existing temperature. It is aggravating.

My goal tomorrow is to pull the device from the exhaust stack and examine the internal component--the Pyrostat flame detector--to assess it's condition. The Protectorelay itself looks relatively new (only lived here for 4 years), but that doesn't mean much.

The switch is $166 and the detector $24--I'd prefer to not have to spend it--but whatever it takes I guess...

Any thoughts from any pros?

Many thanks. [xx(]

[JTB]

I'm adding this as this will be my checklist:

Follow These Steps When Servicing Stack Mounted Primary Controls:
1. Clean the bimetal element (helix) because a build-up of soot will insulate it. This will
prevent the hot flue gas from heating the helix so that the hot contacts will close, which
will lead to a safety lockout.
2. Clean the cold and hot contacts. Dirt on the cold contacts will keep the relay deenergized
when there is a call for heat or hot water. Dirt on the hot contacts could result
in a safety lockout. Run the burner and check the action of the helix and contacts. The
hot contacts must close before the cold contacts open when the burner starts. If this
does not happen, the burner will shut off and start a series of short cycles resulting
in no heat in the building.
3. Clean the safety switch contacts. Some stack mounted primary controls have open
safety switches that may get dirty. Dirty contacts can keep the relay from energizing
when there is a call for heat.
4. Check for power at the #1 and #2 terminals. #1 is the hot connection coming from
the limit controls; #2 is the neutral connection. A test light connected to #1 and #2
should light up. If there is no light, check from #1 to a grounded metal surface. If the
light comes on, that would indicate a broken ground wire to the control that would
have to be repaired.
5. Check the low voltage circuit by installing a jumper—a short piece of wire—between
the T-T terminals of the control. This will complete the circuit to the relay coil and
the control should energize.

Follow These Steps to Check Out a Stack Mounted Primary Control:
1. Check for power, 120 volts, at terminals #1 and #2.
2. Check to see if the safety switch contacts are closed.
3. Check to see if the cold contacts are closed.
4. Jumper the T–T terminals. The control should energize. If it does not, it is defective
and will have to be replaced.
5. Check the safety switch timing by disconnecting the #3 terminal wire to the oil burner
motor. Energize the control and see how long it takes for the safety switch to lock
out—30 to 45 seconds is normal for this type of control.

[JTB]

Okay...got the price down to $112 at Grainger...$106 at this other place...

[Hube]

Jtb; The protector relay could be doing the job it was intended to do, that is start or stop the burner. Best to investigate elsewhere. Have you had the electrodes properly cleaned/gapped? Also, a cracked insulator will also cause "lock out", as will a weak transformer. Hube

[JTB]

This I understand, thanks Hube.

My initial guess was thatthe Pyrostat was mucked up because in spite of the age of the unit, it is in very good condition and has been maintained regularly. When recently cleaned it was inspected and tuned, but that flame detector may have slipped them by. Regardless, since I started this thread, it has been playing along wonderfully. When the problem last arose, the Protectorelay was buzzing and appeared as though the two relays were out of sync on every second or third attempt.

Thanks again--I'll keep plugging away and if I have a conclusion to this I shall post it here.

[JTB]

Just wanted to post an update--never know if this helps someone in the end.

The furnace ran problem free for the last day or so--and then this AM it start short-cycling again. I spent an hour going over obvious points before I pull the stack limit switch with attached flame detector from the exhaust stack. I scrubbed the gum and soot from the flame detector, and while I'm sure this helped, it wasn't the root of the problem. After staring at the switch as it cycled, I noted that it was arc'ing and buzzing--so I shut the breaker off and set about cleaning the contacts--as they were in an awkward spot I used a long narrow flathead screwdriver to gently scrape the contacts clean.

Done deal. Running like a champ. And once again--not a penny spent.

Someday maybe I'll regale you with the story of how I rebuilt the squirrel fan and used shims cut from a can of Foster's...

Thanks all and good luck--it's cold out there!

[Troy]

Thanks for keeping us updated. And you are right, it will help someone else. These forums get searched and viewed much more often than people post. We may never know it, but I'll bet many people learn from your experiences.

Thanks, and keep coming back!

quote:Originally posted by JTB

Just wanted to post an update--never know if this helps someone in the end.

The furnace ran problem free for the last day or so--and then this AM it start short-cycling again. I spent an hour going over obvious points before I pull the stack limit switch with attached flame detector from the exhaust stack. I scrubbed the gum and soot from the flame detector, and while I'm sure this helped, it wasn't the root of the problem. After staring at the switch as it cycled, I noted that it was arc'ing and buzzing--so I shut the breaker off and set about cleaning the contacts--as they were in an awkward spot I used a long narrow flathead screwdriver to gently scrape the contacts clean.

Done deal. Running like a champ. And once again--not a penny spent.

Someday maybe I'll regale you with the story of how I rebuilt the squirrel fan and used shims cut from a can of Foster's...

Thanks all and good luck--it's cold out there!

[jshriver]

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Just wanted to second the experience. My oil unit did exactly the same thing one diagnostic I noticed was useful is the protector unit has only two controls as far as I can tell the thermostat and the protector relay. If you jump the thermostat it seems like it is probably the protector.

I have not fixed mine yet, but I think the instructions for stepping the contacts should help. I tried to do it with the system warm and it worked for a short time I am letting things cool and will try again.

Also I found a little manual for the part
http://customer.honeywell.com/Honeyw...tegory=Product Data_7495&Product=68-0119&ChannelID=%7b2EB2F178-20ED-44E0-97FB-CCFB4218DD64%7d br /
The more general list of Honeywell parts is
http://customer.honeywell.com/Honeyw...495&ChannelID= br / " target="_blank"> br / http://customer.honeywell.com/Honeyw...495&ChannelID= br /




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