Limit switch-Can't find it if I have one

rgar98

I have a 20 year old AFG forced air system (I just replaced the old fuel line/valves/filters with your help) with the original R8184G primary and it works well. I got a R7284U primary to replace the original one if I need to, but I cannot find the limit switch on my system. There are 2 sensors in the plenum at 2 different places and a wire (red) from them that plugs into a Honeywell circuit board in the lower compartment, but no fan switch. Any suggestions? Thanks again for your previous help.
Richard

EdTheHeaterMan

Not enough info to answer your query.

20-year-old AFG may refer to the oil burner. The RW Beckett company made a burner with that designation. The oil burner is only a component of the furnace. A picture of the furnace with doors off (if any) and a picture of the rating plate with the model and serial number will help in locating the limit controls.

The model number of the Honeywell circuit board in the blower compartment will also help.

The fan control may not be temperature-activated. It may be a timed function of the Honeywell control in the blower compartment.

Also, look for a wiring diagram on the blower or the blower door that looks like this:



This is from a Honeywell control that was popular on oil-fired furnaces 20 years ago. The red wire with 2 sensors are most likely the high limit control switches. The red wire connects to the #1 and the #9 pin on the Molex plug. (upper right and lower left in the diagram) if you have this control notice that the burner motor does not connect directly to the orange wire of the R8184 Primary Control. The orange wire connects to the number 1 pin on the Molex plug (upper left). The motor then is powered by the #2 pin (upper middle)

EdTheHeaterMan

All this info is assuming that you have the S9103 or ST9103A control.

Now replacing the R8184 with an R7284 primary control is similar in reference to the motor wire.

But I might guess that your problem is in the control in the blower compartment. Sometimes the solder connection on the printed circuit board that turns the motor on goes bad. Sometimes this lets the smoke out of the board and sometimes it just disconnects, leaving the board functional but the burner motor with no power. Hence the primary control goes off by safety lockout.

In that case, you can get heat by connecting the motor and ignition directly to the orange wire from the R8184 Primary control directly. keep the orange wire from the Moles plug pin #1 connected also. this will allow the control logic to continue to function for the blower. This is a temporary fix and the Honeywell ST9103 should be replaced promptly.

Hope this info helps.

rgar98

Thanks Ed. I'll get the info organized and get back to you. As always, I appreciate your help.
RG

rgar98

Ed,
You're right. It turns out that I have the Carrier manual for my oil furnace model 58CMA. It shows that I have a circuit board, ST9103, with two limit controls on the board that are wired (red wire) via the two pins, #1 & #9. Now I can at least try to get the wiring figured out without driving myself crazy trying to find a non-existing analog fan limit control switch. I'll try to figure out the wiring to transfer from the R8184 to the R7284 and if I have questions, which is likely, I'll ask for your assistance again.
Thank you very much for your thorough response to my query.
Richard

rgar98

rgar98

Ed, This diagram is from the instructions that came with the R7284U primary. I guess my ST9103A limit control is the "other limit" shown on the diagram. I have 2 temp. sensors in the plenum. The lower one is near where air from the house enters (through the filter) and the upper one is near the heat exchanger. Each has 2 Red wires attached. One wire connects the sensors to each other and the second wire from each connect via the Molex plug to the two pins (upper right (#3 & lower left #7) on the ST9103.
How does this work with the diagram above? How do my "Red" wires work with the diagram? There are NO red wires in my current primary (R8184).
I hope you can make some sense of what I'm asking and thanks again for your time.
Richard.

rgar98

This is the ST9103 diagram from inside the blower compartment. It is almost identical to the one you sent me. You can see the two limit controls on the right middle edge.

neilc

withdrawn

rgar98

@neilc
What happened? I was looking forward to some feedback about my "problem."

neilc

I read that last post and thought you couldn't find the 2 limits, then I read it again,
honestly, I'm not sure what you're trying to fix here

rgar98

Thanks for trying.
I'm not trying to "fix" something. I'm trying to wire a Honeywell R7284U Oil Primary to my oil furnace that has a Honeywell electronic fan timer on a circuit board (ST9103A) rather than the more common situation which is to have an actual analog fan limit switch that is wired to the primary.
I thought the 7284 would be an upgrade; the original 8184 is about 20 years old and I wanted to have a backup. Plus, the newer model costs about $50 and the exact replacement for the original is about $100.

EdTheHeaterMan

This is what you are looking for


I added a wire to power the primary control to get its full functionality. Use the red wires I added to make the electronic primary control have constant power.

Use the black wire from Pin #2 to power the Limit terminal on the Electronic primary control.

The limits on this furnace are an integral part of the ST9103 circuitry therefore the black wire from the Limit on the Primary control is properly connected to the furnace limit when powered by Pin #2 on this diagram.

rgar98

Ed, Thanks. Let me "digest" this and I'll get back. I appreciate it.
Rich

rgar98

A few questions (thanks for your patience):
(1) red wire connects to L1 on R7284. Correct?
(2) I'm not clear on where the other end of red wire is connected to. I know it's on the board, but is that a plug-in connector?
(3) Do I just disconnect the black wire from pin #2? Is pin #2 the middle pin in the bottom row of Molex?
Thanks, again.

STEVEusaPA

Do you have an oil valve on the fuel pump? If not, make sure you properly program the control so you don't use pre/post purge.
Look at @EdTheHeaterMan drawing. There's a call out of the pins, left to right, bottom to top.

rgar98

No valve. No pre and post purge. Thanks. I cannot read the pins.
Is it:
1 2 3
4 5 6
7 8 9

In the diagram, the black wire that connects to the LIMIT (on primary) is middle pin bottom row.

rgar98

I see it now: bottom-middle.

HVACNUT

The way it was originally designed, the Black wire (L1) at the primary is from, or rather in series with the limits. If a limit opens, then the primary goes dead.
Use that black wire for your limit, and bring in an additional wire for constant 120v to L1 on the new primary. There might be an empty spade terminal if you have a crimp tool, or if there's slack to splice in to another black wire in the upper left of the board, terminals 2 and 3 (labeled S, for power and transformer).
Unfortunately, I don't see how it can be set up for interrupted ignition. It's showing Orange going from the primary into the 9 pin molex, through a path in the board, and out on Violet to the burner igniter and motor. Kind of a weird setup. Maybe Orange also starts the fan time delay as well? Still weird. I don't know if I've ever come across something like that in the field.

EdTheHeaterMan

rgar98 said:

A few questions (thanks for your patience):
(1) red wire connects to L1 on R7284. Correct?
(2) I'm not clear on where the other end of red wire is connected to. I know it's on the board, but is that a plug-in connector?
(3) Do I just disconnect the black wire from pin #2? Is pin #2 the middle pin in the bottom row of Molex?
Thanks, again.

There are 3 spade connectors soldered to the board that are all L1.

If your PC board only has 2 spade connectors in this location then you can use a piggyback connector to make the connection.
Three wires are connected here L1 power feed from the burner switch, hot to the 24 v transformer primary coil, and the red wire from my previous diagram to provide power to the primary control.

The 3 spaces on the PC Board are like using a wire nut to connect 3 wires to the same place on the board.

It appears the Molex 9 pin plug is numbered differently in the most recent diagram.
Pin #2 (bottom center) is power to the primary control after the limit switches. This means that it goes to the black wire on the old R8184G On the new control it goes to the spade terminal marked Limit.

The orange from the Primary control must feed the PC board at pin #4 to enable the logic chips in the PC board to start the blower ON timing. This is a safety feature built into the control for some reason. The burner motor then receives power from the PC Board Pin#8

I hope this answers your Query

HVACNUT

And hopefully to wrap it up, I was wrong. At least I think was. Orange from the primary will continue Orange to the molex plug, back on Violet to power the burner motor. You can still use Blue (ignition) on the primary and go directly to the igniter and program it for interrupted ignition. 
Is the primary mounted directly on the burner? In a junction box to the right of the igniter? 

SuperTech

@HVACNUT I think you were wrong too after looking through everything.  Thankfully it can be set up for interrupted ignition.  
I've seen a few boilers with the R7284U installed in place of the crappy R8184G. Usually guys just wire nut the limit wire together with L1, because power to the primary control is switched off by the aquastat.  I always wire it so the primary control has constant power.  Its nice to be able to check operation history without the burner running. 

HVACNUT

SuperTech said:

@HVACNUT I think you were wrong too after looking through everything.  Thankfully it can be set up for interrupted ignition.  
I've seen a few boilers with the R7284U installed in place of the crappy R8184G. Usually guys just wire nut the limit wire together with L1, because power to the primary control is switched off by the aquastat.  I always wire it so the primary control has constant power.  Its nice to be able to check operation history without the burner running. 

Yeah, I've done a ton of primary upgrades and converted to interrupted ignition. I just don't think I've ever seen Orange leave the primary to go through a circuit board, then come back to the motor. I'm wondering if the primary mounts to a 1900 box on the side of the cabinet rather than on the burner. 
Oh, and thanks a bunch. It's not as if I wasn't already beating myself up about it. I fell asleep thinking about that stupid design. IMHO.😁

SuperTech

I've seen something similar with the orange wire when a power vent is being used. I do a ton of primary control upgrades too, but I don't remember doing one on a furnace like this. Usually I'm doing them on Thermoprides or Hallmarks.

I know its off topic, but whats the point of intermittent ignition? Why would it ever be used? Its like keeping the HSI energized on a gas furnace .

rgar98

"I hope this answers your Query."
Thank you Ed. you've been very generous with your time.

STEVEusaPA

I wish they would use 'constant' and 'interrupted', instead of 'interrupted' and 'intermittent'.
For those playing along at home...
Oil burners use either interrupted or intermittent ignition systems. In an interrupted ignition system, the spark is active for a brief time at the start of the operating cycle, then turns off once there's a flame. In an intermittent ignition system, the spark stays on while the burner runs.

rgar98

Setup new primary (R7284).
I'm getting TT closed limit closed when running.. Wired with L1 hot , always, via red wire and black from molex to limit.
Hot wire from burner switch and red wire a re BOTH connected to LI at the primary. Is this correct? Or is the hot from switch supposed to be connected to the S9103 board?
TT open, limit closed when in standby.

rgar98

Ohms are about 1300.
When does the limit open, if ever?

Thanks for your help.

STEVEusaPA

Not correct. Ohms may be too high, especially if burner underfired or not properly tuned.
Furnace Limit opens when temperature exceeds it's setting.

rgar98

Thanks, Steve. Before you tell me to have a "pro" look at the system, please suggest why OHMS may be high. I installed a new igniter and CAD cell and primary. Burner switch, red wire from board are BOTH connected to L1 on primary.

SuperTech

It could be a few things. Dirty flame retention head or poor combustion could be possible causes. Draft could be a factor.  Its a combustion issue of some sort, you need a technician who is familiar with oil burners and has the proper equipment to properly troubleshoot the burner. 

Jamie Hall

Having done assorted rewiring with safety limits, may I suggest that you ensure that they are still connected in such a way that they shut things down as they should?

EdTheHeaterMan

FYI the old R8184G will detect there is no flame @ 1700 ohm and above. That is why we like to see Ohm on the cadmium cell flame detector at or below 1000 ohms

Many of the electronic primary controls like the Beckett Genisis and the Honeywell R7284U will sense a good flame signal as high as 3000 ohms from the Cad Cell. Some of the Carlin controls can even be adjusted to 5400 ohms as a safe signal. This is why I like Carlin. There are some burners like the Weil Mclain QB and others that just don't play well with flame signals from Cad Cells.

The company that was/is making the furnaces for Carrier, ICP, York Lennox, and Bryant along with Armstrong used that Fan Timer Board on their residential furnace line a lot in the 1990s thru to today. As these companies swapped from one oil furnace supplier to another over the years, the control was dropped and other limit controls were implemented.

Does anyone remember the Honeywell Fan Limit that has 2 extra wires for a "heat anticipator" type of timer that would cause the fan to start within 90 seconds of the burner firing regardless of the plenum temperature?


Can't find them anymore.

Merry Christmas
Ed

EdTheHeaterMan

rgar98 said:

Thanks, Steve. Before you tell me to have a "pro" look at the system, please suggest why OHMS may be high. I installed a new igniter and CAD cell and primary. Burner switch, red wire from board are BOTH connected to L1 on primary.

Ohms are high based on the brightness of the flame. You may be able to adjust the flame with 120% excess air and see the flame get dimmer/smaller because the extra air is blowing the fire out and cooling it down. As you reduce the combustion air adjustment on the burner, you will see the OHM reading get higher. If you go too far closed the flame will turn orange and get smokey, this will also lose brightness and the OHM will go back up

There is a sweet spot where the flame is as bright as it will get and the Smoke # reading on your combustion analysis will be zero. Some say they can adjust a burner by the cad cell OHM reading. That is partially true, but only partially, you need to verify with Combustion instruments.

EdTheHeaterMan

STEVEusaPA said:

I wish they would use 'constant' and 'interrupted', instead of 'interrupted' and 'intermittent'.
For those playing along at home...
Oil burners use either interrupted or intermittent ignition systems. In an interrupted ignition system, the spark is active for a brief time at the start of the operating cycle, then turns off once there's a flame. In an intermittent ignition system, the spark stays on while the burner runs.

I grew up with oil burners that were Constant and Intermittent. Then Honeywell got smart in the 70s or 80s and decided that a standing pilot on a gas heater was constant ignition. So where does that leave the oil burner folks??? Well we have to rename everything.

I remember a time when the RA117A was labeled Interupped Ignition (formerly intermittent ignition) and the RA116A was labeled Intermittent ignition (formerly constant ignition)

For the children: Those model numbers are for Honeywell Stack Relays. Primary control that preceded the Cad Cell Relay. It was installed in the vent pipe and if there was a flame in the chamber then the vent pipe would get hot and activate the relay to move to a "Safe to operate mode" No flame... then the control would go off by the manual reset safety switch.

rgar98

ED,
The TT setting is "yes", ie, no internal jumper. Is this correct?
The system does not turn off it just runs. The CAD cell is about 1300.
Please can you tell me if the hot wire from the burner switch should be connected to the the L1 at the primary with the red wire I installed. It is NOT connected to the board but the primary is getting power when the burner is OFF.

rgar98

What I'm asking is: does the hot wire from the burner switch supply power to the board through the primary?
This is a hot air system . What should the TT setting be?

Thanks.

SuperTech

It shouldn't be a problem for the primary to be constantly powered. It's nice to check the diagnostic information without the burner firing.  

The TT setting "on" is for burners on equipment that has a circuit board or aquastat power the burner during a call for heating.  Forced air equipment that has the thermostat switch the burner on directly use TT configured off normally. 

EdTheHeaterMan

The TT CONFIGURED ON setting should be set to NO
The blue and yellow wire from the Molex plug should be connected to the TT screws on the L7284 as indicated in my wiring diagram above. When there is a call for heat to the Fan Timer PC Board, the blue and yellow wires will make the call for heat happen on the L7248 primary control.

EdTheHeaterMan

The hot wire from L1 anywhere is correct. L1 Hot is L1 Hot where ever it shows up on the diagram. If you have room on the board or if you get it from the power switch, makes no difference.

It is like saying can I get the + from the battery on a DC circuit from the battery + terminal or from the other end of the wire that is connected to the battery + terminal? Both ends of the wire are the same electrically speaking.

Steamhead

EdTheHeaterMan said:

STEVEusaPA said:

I wish they would use 'constant' and 'interrupted', instead of 'interrupted' and 'intermittent'.
For those playing along at home...
Oil burners use either interrupted or intermittent ignition systems. In an interrupted ignition system, the spark is active for a brief time at the start of the operating cycle, then turns off once there's a flame. In an intermittent ignition system, the spark stays on while the burner runs.

I grew up with oil burners that were Constant and Intermittent. Then Honeywell got smart in the 70s or 80s and decided that a standing pilot on a gas heater was constant ignition. So where does that leave the oil burner folks??? Well we have to rename everything.

I remember a time when the RA117A was labeled Interupped Ignition (formerly intermittent ignition) and the RA116A was labeled Intermittent ignition (formerly constant ignition)

For the children: Those model numbers are for Honeywell Stack Relays. Primary control that preceded the Cad Cell Relay. It was installed in the vent pipe and if there was a flame in the chamber then the vent pipe would get hot and activate the relay to move to a "Safe to operate mode" No flame... then the control would go off by the manual reset safety switch.

Those old stack-mounted primaries should be banned. Their trial for ignition ranges from 70 to 90 seconds, which is way too long for a safe lockout if the oil flame doesn't ignite.

Same goes for the R8184 and similar controls. 45 seconds is better than 70-90 seconds, but not by much. Modern primaries have 15-second trial for ignition periods. If the burner doesn't light immediately, something is wrong, and it should shut down quickly.

We've converted quite a few burners from stack controls to 15-second cad-cell ones. If the burner is a Beckett and doesn't have the box where the cad-cell control mounts, you can get one from Beckett. I keep one on the truck. Some Sunray and (I believe) Wayne burners use a standard 1900 box screwed to the side of the burner.

Regarding interrupted ignition, this results in less wear on the electrodes and ignitor, as well as a slight reduction in energy usage. I think the reason the industry went to intermittent ignition was because it was cheaper to make that type of control- figures, doesn't it?