robertshaw SP 735 A ingition control

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  #1  
Old 01-05-04, 08:45 PM
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robertshaw SP 735 A ingition control

Hi, l wonder if I could run this by you.
We have a furnace made by Armstrong ,its an ultra II ,EG6E-and EG7E ,it is about 10 0r 11 years old.
Last winter the furnace went out, and when the T/Stat was turned up, the blower would kick on, but the burner would not
come on. I found the combustion blower to be real hot to the touch. Also I could see it was not running.
If I supplied a vacuum to the pressure switch, the ignition control would become energized, and would light the pilot, the burner would fire. So I replaced The combustion blower .
It was fine all last winter and all this winter, till today, I was roasting, so I set the T/Stat down to about 65 degrees.
I came home and noticed it was getting cold , recalling I had lowered the T stat, turned it up to about 70 degrees.
Having had only a few hour's sleep the night before, I laid down for a nap about 7:00 PM.
About 2 hours later The wife awakened me, stating the house was cold, and the furnace was not kicking on. She had removed the cover and said the burner was not firing.
I shut the T/S down to it's lowest temp, and went to the basement, removed the cover, and had her kick up the t stat.
The combustion blower was running just fine, and tested the pressure switch(SWITCH SIDE) with an ohm meter, and it was closed.
The pilot was not lighting at this point, so I took a Light stick and taped the ignition control lightly and the "electric spark" (?)started ticking, and the pilot lit and soon the burner fired.
We kept the t/Stat up to 80 degrees for a while, but the thermometer was only reading about 68 degrees. Well I had to tap on the ignition control a few more times tonight,but I am roasting again, so it seems to be working"for Now"
I just lowered the T/stat to 68 degrees about 15 minutes ago, and just now set it up to 71 0r 72 degrees, and went down, and the "burner is burning".
So, If I start having problems again, Replace the robertshaw SP 735 A ignition control ??
About how costly is this Item.

Thank you, Tony D.
 
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  #2  
Old 01-06-04, 03:56 AM
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1st try snipping about 1/2" of the ignition wire off and reinserting it if this dont help it's time for a new module and usually cost at a supply store somewhere around $100.00.
 
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Old 01-06-04, 07:36 AM
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mattison,
Thanks for you reply.So far the unit is working on it's own.Seemed to work OK all night and this AM.
Maybe I "knocked some sense" into the board. LOL

I would like to try your suggestion, but am a little unclear as to the procedure.
You Say; "1st try snipping about 1/2" of the ignition wire off and reinserting it "
I assume you are referring to down at the (I am at a loss for the proper terms,)IOW where the wire comes out of the control box or "ignition control " and goes down to the "pilot valve"(proper term??)
Do I just pull the wire out and snip a little off and put it back in.
Hope this makes sense.
Tony D.
 
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Old 01-07-04, 07:27 AM
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The spark wire (thicker than the others, perhaps orange in color), may be covered by a small rubber insulating hood at the ignition control. After disconnecting power to the furnace, slide the rubber hood up the wire, then pull the wire off of the control (it is probably connected to the tab on the control board by a 1/4" female spade connector; grasp the connector when removing). Snip the wire off around 1/2 to 1" above the connector. Restrip the wire, crimp it onto a new spade connector, securely attach the connector to the tab on the control board, slide the insulating hood back over it, then restore power.
 
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Old 01-07-04, 07:50 AM
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I see, So IOW I will install a new connector to the spark wire ,at the ignition control end of the wire.

Thanks for clearing that up.

Just curious, Is the ignition control's only job is to create the spark for the pilot, or does it serve other functions / monitor the operation of the whole system/send other commands to the system,like a computer in a car, or a CPU in a PC.
Thanks, Tony D.
 
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Old 01-07-04, 08:07 AM
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Once the draft motor provides a vacuuum to the pressure switch, the pressure switch will close, allowing the ignition sequence to begin. (This prevents ignition in the absence of a sufficient exhaust path.) The control then simultaneously energizes the pilot solenoid in the gas valve (releasing gas to the pilot) and provides a spark to light the pilot. Once the pilot is lit, the flame sensor (a metal rod in the path of the pilot) senses that the pilot is lit, which allows the ignition control to apply voltage to the main burner solenoid on the gas valve, releasing gas to the main burners. This sequence prevents release of gas to the main burners in the absence of a pilot flame to ignite the gas, for obvious reasons.
 
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Old 01-07-04, 08:40 AM
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Thats Intresting. I guess it sort-of takes the place of like a safety valve in a gas oven.

Do these ignition control themself go bad very often.
I am going to replace the connector at the ignition wire, and maybe clean and check the other wires.

I guess I can put a piece of shrink tube in place of the rubber sleeve on the connector. Looks like thats what's on ther now, and it will not be reuseable.
Is that wire subject to possible damage,more than the other wires?

I just had to tap the module again, I'm going to go replace that connector in a little while.

Do you know if this replacement is a good unit should I need one,
780-002

Thank You, Tony D.
 

Last edited by bubinga1; 01-07-04 at 09:07 AM.
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Old 01-07-04, 09:11 AM
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My furnace has a 715, which is the same as the 735 except that the 715 will continuously try to ignite rather than locking out after a few unsuccessful tries, which is how the 735 operates. If you use propane, you must have a lockout-type control. The unit your post links to is the exact replacement for the 735. I can't say if the units are prone to failure, but I've had to replace my 715 once. If it goes bad again, I'd be inclined to replace it with a Honeywell.
 
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Old 01-07-04, 09:18 AM
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I missed part of your post. I would not use shrink tubing because of the heat in the area. The purpose of the rubber hood is to completely insulate the spark voltage (which I believe is several thousand volts, but at a very low current) from the surrounding area. I don't know whether the spark wire is more prone to failure than the others, but I have seen many times the suggestion that its connection at the ignition control be re-done to ensure a good connection.
 
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Old 01-07-04, 09:36 AM
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Thanks, No I have a gas unit. I should have ststed that.
Can I replace it with the 715 type, (non lockout).
Thanks, Tony
 
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Old 01-07-04, 10:06 AM
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It appears the one on the link will work and since your furnace seems to kick on with a tap to the module sounds more like it needs replaced anyway.
 
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Old 01-07-04, 10:35 AM
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Ok I replaced the connector
The purpose of the rubber hood is to completely insulate the spark voltage (which I believe is several thousand volts, but at a very low current)
Sort of like a spark plug wire boot.
Looked like shrink tube they had on there, I crimped on a new connector, and then heated up a piece of rubber vacuum line,
(slid it over the wire 1st.) and pressed it firmly over the spade connector. I also made sure the wire was crimped firmly.

So far its working, *but that don't mean it's fixed*; But we'll see.

Did any of you ever open one of these units and find a "cold"solder joint. I thought of that.
Thanks 2 the 2 of you for all of your help.
Tony D.
 
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Old 01-07-04, 11:03 AM
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I don't believe these units are serviceable. They are sealed, and the prudent course for dealing with a bad one is the same as for bad gas valves -- replace.
 
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Old 01-07-04, 11:58 AM
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Another Thing Please

I appreciate all your patience.
Let me ask Another Thing.

Is the burner supposed to cycle on and off at all during the heating cycle? WHILE THE BLOWER FAN IS RUNNING?
IE. I Set the T-Stat to ~72 Degrees. Ran down stairs, blower was running, The combustion blower was running, Tic-Tic-Tic-Tic-Tic-Tic- the box is lighting the pilot. Click-Click goes the gas valve, Whoosh the burner lights.

I come up stairs for a while, Still hear the blower running, wait a while, Thermometer in T-Stat is reading~70 degrees, feel the vent, (luke warm) run down stairs(blower still running !!!) ,remove panel, look in furnace, burner is out, combustion blower is off,(no sense taping module) Put panel back on , stand there a while, combustion blower kicks on, (you can hear that turbine sound); Tic-Tic-Tic-Tic-Tic-Tic- the box is lighting the pilot. Click-Click goes the gas valve, Whoosh the burner lights. Keep in mind the blower is running the whole time. In the time it took me to type this, the Thermometer in T-Stat is reading ~72, (I'm a little slow typing,)

Maybe this is a clue too, as long as I have known this girl who's house it is, about a year, well this and last winter, the furnace will kick on, run a normal legenth of time, you hear it kick off, blower and all, maybe~20 seconds later furnace kicks back on for about 1 minuet, then cuts back off. (don't know if its just the blower fan. or the fan and burner. I asked her, she said "it always has done that". Is this also normal?
It was her mothers house, but she has been here about ~3 years.
Thanks, Again, Hope this makes sense.
 
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Old 01-07-04, 01:02 PM
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First, make sure you have adequate airflow through the furnace (all registers and returns open and unobstructed, and filter clean). If that isn't the problem, the limit control may be bad or improperly set. The limit control is a small rectangular box in the furnace which has a temperature probe that juts into the plenum. If you remove the cover, you will see three pointers on a circular temperature dial. The lowest point is the temperature at which the limit control shuts off the fan after the call for heat is completed. The middle point is the temperature at which the fan comes on once the burners have warmed up the plenum upon a call for heat. The highest point is a safety, which shuts the system down if temperatures get too high (for example, if the blower motor fails or if there is restricted airflow). I don't know about yours (consult the manual), but the manual for my furnace calls for the fan-off (low) point to be 90 degrees and the fan-on (middle point) to be 130 degrees. It is possible, but unlikely, that the limit is not set to the right fan-on/fan-off temperatures. If you decide to try adjusting this (which I do not recommend), make sure you hold the circular dial firmly in place while moving the tabs. Do not adjust the high limit. The problem you describe also can be caused by insufficient gas pressure, again unlikely unless someone has been messing with the gas valve.
 
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Old 01-07-04, 03:56 PM
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This is intresting.
The middle point is the temperature at which the fan comes on once the burners have warmed up the plenum upon a call for heat.
So IOW are you saying the blower is not supossed to start till after The burner has come on?

Because on this one when the T-Stat calls for heat, The combustion blower comes on, and closes the pressure switch,witch fires the Pilot (hopefully :>) , But the Blower motor/fan are allready running.

IOW,the combustion blower and the Blower motor/fan
come on AT THE SAME TIME!

I think you may have pointed me in the right Direction.
I did not look in my book yet to see what temp they call for,
But I did remove the cover to get a better Idea, and find one of the two heavy wires, (They look like # 8 0r #10,) is melted about 1/4" from where it goes into the switch, and the connection looks
"Crispy" (Burnt)
That could very well be the problem.
I just have to see now if the conection looks like It can be repaired, of if the limit control needs to be replaced.
Is the limit control a costly item?
Thanks, Again, Tony D.
 
  #17  
Old 01-08-04, 06:17 AM
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On furnaces with electric heating elements, the blower motor will start immediately upon the call for heat, but it's unlikely that your gas-fueled system was designed to do this. You might want to try repairing the bad connection at the limit. If that doesn't work, you may need a new fan/limit control. They are around @$60-80 to a DIYer, and you might find a local supply house that will sell one to you. The website that your earlier post had a link to also probably carries them. If you do decide to replace it, make sure that you get one with the right probe length (which you can determine by removing yours and measuring it), as they come in two or three different lengths.
 
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Old 01-08-04, 07:47 AM
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THANKS!

Thanks A million!
I'll let you know.
Tony
 
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Old 01-09-04, 04:46 PM
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better but still

Oh We got a new board, and a new limit control.
The old limit control was bad for sure, because I opened it up and on the side that had the blue wire burnt, The contact points were burnt, and also the Arm that gets contacted by the offsets or cam's in the switch were not springing back on that side.
Good I say. Get the new parts, Put it all back together.Had someone Set the t-Stat,to ~72 degrees, while I was down next to the furnace. Right away the blower starts, as soon as the t-Stat is kiched on. There is a switch on the new limit control for manual or automatic, It is set to automatic.
And still before the t-stat reaches the set temp or close to it, the burner kicks off. All the while the blower is still running. Few minuits later, (blower still running) burner kicks back on.
Does this a few times, then burner kicks off.
Temp in house is close to set point at this time.
So I am watching for fan to kick off, almost to Off point(have it set to 90 degrees, as per manual) kicks off, few (~30 seconds max) burner and fan back on. It's like this damm thing is running too much.
It is heating Ok and the board is firing the pilot all the time, its just this cycleing thing, and the fact that the blower starts at the same time as the T-stat calls for Heat.
This thing is driving me nuts. We really can't afford a repair tech at this point till I start working again.
BTY we got a pretty good price on the board locally, something like $113.00 &Tax, and Like $42.25 for the limit control.
Could the wires on the limit be on backwards from the get go?
I wired it the same as I took it off, blue on the left (as your looking at it) Red on the right, black on the bottom.
The low voltage break-a-way tab I left connected, as thats the way it was on the old one.
I dont know what else to do.
The girlfriend says maybe thats the way it's supossed to be, but I say NO.
Any more Thoughts.
Thank You, Tony D.
 

Last edited by bubinga1; 01-10-04 at 12:32 PM.
  #20  
Old 01-10-04, 11:35 AM
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Here is another thing I find.
After looking at the diagram that came with the limit control, I see the black wire (bottom) is power in.
Red, (upper right) is limit control,
Blue(upper left) is to fan.
Here is the funny thing!
Even if I disconnect the blue wire from the switch,(limit control) and turn furnace back on, The fan continue's to run.
And also there is 120 volts at the Blue wire. (wire disconnected)
Seems the blower is getting power from somewhere else ,before it gets it from the limit control.
Tony .
 
  #21  
Old 01-12-04, 01:09 PM
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It sounds like something may previously have been wired incorrectly. Based on your posts, I assume that you can follow a schematic. If you can verify that everything is wired correctly (paying particular attention to the thermostat and fan relay), I would suggest having a tech take a look.
 
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Old 01-12-04, 02:06 PM
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Thanks,
I can a follow a schematic pretty well.
I opened the junction box and according to the schematic and the color of the wires the limit control IS wired properly.
Here's what else I found.
The manuall calls for the anticipator setting at the t-stat of
.9, and it was set to .35, I kicked it up to .9 and it made a world of difference.
The burner is no longer kicking on and off, and the house is warmer than ever.
It seems to cycle better now. It stays on for about 7-10 minuits, then off for about 7-10 minuits. It's still pretty cold here Outside.
I noticed before, the burner would kick out at a pleteum temp of about 120,(according to the limit control),about where the the fan should have kicked on. but now the burner is staying on a proper legenth of time.
Looks like around 150 degrees at the limit control, when the burner kicks OFF.
Then the blower fan continues to run till the off setting of 90 degrees, then shuts off, and there is no more voltage at the fan side of the limit switch.
The fan is still kicking on from get go, but I may be able to live with that for now, although I know it is not 100% correct.
The only other thing I notice now, is the t stat is set for 70 degrees, and when it kicks off, the thermometer is reading about ~72 degrees.
Again I can live with that, as long as the unit is not kicking on and off on and off on and off all day like it was this weekend.
Thanks For all your help.
BTY there is a relay,and the coils of the relay are running to the transormer , marked G C.the relay has no power. but they have power if I switch the AC on.
I am guessing this is normal?
This relay appers to be tied in to the blue wire of the Limit control. (after the control though)
Tony
 
  #23  
Old 01-13-04, 05:06 AM
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I can't tell without knowing how your unit is wired. The only reason I was able to provide some knowledge earlier is that I have a 715 control, which is nearly identical to mine. But it sounds like you've taken care of most of the issues.
 
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Old 01-13-04, 06:29 AM
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But it sounds like you've taken care of most of the issues.
Yes, I am pretty happy with the way it is working now.
It's funny how with the old board and limit control, the furnace never did all that on off on off all day routine.
My guess is the new limit control and board are drawing slightly different amprage from the t-stat.

It would be nice to get the blower fan delay to work as it should, but all in all I'm pretty happy with it.
bubinga1 said ;BTY there is a relay,and the coils of the relay are running to the transormer , marked G C.
All refference to t-stat wiring I'v seen says "G" is fan relay.

The only reason I was able to provide some knowledge earlier is that I have a 715 control, which is nearly identical to mine.
You seem to have learned a good bit about it though.
Thanks again for your help.
Tony D.
 
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Old 01-15-04, 05:48 AM
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It would be nice to get the blower fan delay to work as it should, but all in all I'm pretty happy with it.
Hello again.
This might sound dumb, but.
I wonder If I am either missing something or Is /was there ever a heating system designed to operate in the manner this one is.
That is designed to operate the blower motor on low as soon as the t-stat calls for heat, instead of allowing the burner to burn for a few minuets before starting the blower.
All the looking at the schematic that I have done, and testing voltage at the relays and testing the relays for proper operation, according to the schematic, as far as NO/ NC/ and the opposite continuity/or lack of when the coil is energized. being confirmed, and again studying the schematic, leads me to believe this system is working as designed. I'll try to explain this the best I can.
It appears to me, + power comes in, (goes through the interlock)
to a 3 way connection; 1st to the power in side of the fanlimit, 2nd to the 1st pole of the NO combustion blower relay,
a STDP relay (power is then bridged to the 2nd pole of the NO combustion blower relay, but thatís for the combustion blower) and then to the fan relay,( NO terminal (R2,S2 of the relay goes to the High of the blower motor, but thatís only used for the A/C, the relay is not energized in the heat mode.)
Now out of the Combustion blower relay, comes a red wire, and (when the t-stat calls for heat this NO relay is energized, and in now closed) sends power to the Red wire, that runs all the way up to the fan side of the limit control, and also taps off before limit control, to the NC side of the fan relay (R2, through the NC side, (witch is closed because the relay is not energized), out of there and to the low connection on the blower motor.
So IOW the power is fed out of the combustion relay, into the NC side of the fan relay, out of the NC side of the fan relay to the motor. After the burner shuts off the combustion blower relay drops the voltage to the NC side of the fan relay and the NC fan relay now gets its power from the Limit control.
See if I remove the red wire from the combustion relay, the blower does not run at the start of the cycle, but only comes on when the NC (side) of the fan relay gets power from the limit control.
Well I'm sure I made this "clear as mud" I wish I had a scanner to post the schematic, so You could see what the hell I'm talking about, If I am correct, or if I am missing something.
I may try to get someone to scan it and put it on a floppy so that I can post it.
I am also going to try to call the company.
If your still reading and I didnít loose you, hope this makes sense.
Thanks, Tony D.
 
  #26  
Old 01-15-04, 12:01 PM
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Because I'm not familiar with your particular furnace, I can't really comment. What I can say is that although furnaces with electric heating elements energize the blower motor immediately upon the call for heat (because the heating elements are also energized at that time), I would be surprised if a forced air natural gas furnace was designed to do so.
 
  #27  
Old 01-15-04, 01:27 PM
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Thanks, I know what you mean.
FWIW I had a guy look at the schematic, thatís works as head of a heating/plumbing parts store. And he agreed with me on my thoughts of the schematic.
There was another fellow there, that he said was a plumber or heating guy or something, and he said they did that for efficiency.
and that some furnaces were like that.
Like I said unless I am missing something, it seems to me that the way this thing is set up and wired, this system can "not help" operating the way that it is.
I'll try to get a schematic posted so you all out there can get a better Idea.
I have to see if someone can scan it for me.

Thanks, Tony D.
 
  #28  
Old 01-17-04, 02:06 PM
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Got a (small) schematic

See if this schematic helps any.

Unless I am missing something, this thing is wired to where the blower fan can "not help" but come on once the STDP NO Combustion blower relay is energized.

I have highlighted the "path" I suspect the voltage is taking.

Like I said, once the burner kicks off, the Blower fan is then receiving voltage from the fan limit control. Limit control then cools down to fan off temp of 90 degrees, and turns fan off as it should.

Hope this makes sense, and the schematic is large/clear enough to see.

Fan limit control is new last week
You May have to save it,and open picture view to enlarge it.
Thanks, Tony D.
I can also E mail the attachment if needed, please provide E mail address.
I could not get attachment feature to work.
ThanksTony D.



 

Last edited by bubinga1; 01-17-04 at 08:09 PM.
  #29  
Old 01-20-04, 03:39 PM
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Better attachment scematic

See if this one is better.
I have highlighted in red the path I supect the voltage is taking at start-up.

I did not highlight the second path, but you can see, after the burner shuts off, the combustion blower relay is no longer energized, and the blower then recives voltage through the Fan limit control.
Thanks for looking, Tony D.
 
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Old 01-20-04, 08:30 PM
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Have attached the schematic, as the link may not work to the public.
Thanks,Tony D,
Does not seem to be working.
 
  #31  
Old 09-25-04, 07:50 PM
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Robershaw Ignition Boards

If you wind up needing a robertshaw ignition board I purchased one of a web site last week. My contractor told me the board would be $400.00 and I purchased it for $135.00. It was a robertshaw SP790-320. Good lUck
 

Last edited by hvac4u; 09-26-04 at 07:35 AM.
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