Monday, December 26, 2011

RobertShaw HV SP715A furnace ignition module

House call today, a furnace wasn't heating, seems this usually happens on a bitter cold weekend, that's a holiday, and usually after any store that might have be open earlier is closed. Today's wasn't quite that bad, but I have to say it. Anyway, here's the run down from the phone conversation:
  • Pilot was lit
  • Thermostat was calling for heat
  • Breakers were on
  • Main burner wasn't coming on
I was thinking a bad limit, transformer, or gas valve (not much else on a simple pilot furnace)., But when I arrived I found the furnace was actually a HV (Spark) ignited furnace.

Sorry about the quality, cell phone in basement, at least I took a picture of it.
The  Robertshaw sp715a control box:
  1. Turns on the pilot gas
  2. Jumps a spark until it senses a flame
  3. Turns on the main burner
Since it wasn't trying to light the pilot (audible clicking and a visible spark in the pilot area) it had detected that the pilot was lit, but for some reason the main burner was off. A quick check with a multimeter verified 28vac at the pilot terminal, and about 2vac at the main burner terminal. I was trying to pull the thermostat wire off the control when the main burner suddenly lit! Surprised, I let go and pulled my hand out as the burner shut off. I tried it again, confirming when I pulled on the wire (and flexed the PCB it was connected to) the burner came on.

You can't really see them in the picture, but 2 of the solder joints on the big black relay that controls the main burner were cracked. I had to run to a store to buy a soldering iron (a $6 firestarter), but it was quicker than driving home to solder them. I resoldered and reassembled the furnace. They've got heat and didn't have to buy a $100 module (or wait in a cold house until I could get one either).
Steve

15 comments:

  1. Hi. We have this same model and are being told the ignition control module is bad and needs to be replaced. I don't know if the HVAC guy is telling the truth or just wants a job to sell us a new module. The unit is working intermittently.
    How can you tell if the solder joints are the problem?
    My house is cold, its the Xmas holidays, just as you mentioned in your blog.
    And we are in California!!

    youngreps@sbcglobal.net please reply1!!

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    1. Even if the solder joints are bad you're furnace guy is telling the truth (unless it's not this box at all). Most tech's aren't going to attempt to repair something like this, they are a "Black Box", they're either good or replace 'em. If they fix it and it fails again they could be liable.

      If you want to examine it you would have to remove all the wires (throw the circuit breaker first of course), mark where they go, disassemble the control (either screws or rivets, I don't remember) and closely examine all the solder joints (especially the ones circled in the picture on this post).
      The one I fixed was very obvious on a visual inspection. Look for discoloration or gently try to move components and see if the leads are loose from the board. if you find something solder it up and try it again.

      If you don't feel comfortable doing this just replace the box and it will last a long time. Sometimes things just aren't worth messing with. It's up to you.

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  2. and p.s. -- it is exactly 1 year from your blog on this subject! cold in California!

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  3. I have the same ignition control module on my Ruud system with the same problem. Turn the heater on, pilot light clicks and ignites, but no gas to main burner. Since there is not a continuous clicking for the pilot, does that mean the sensor is working? Next, since no gas to the main burner should my focus be on the main gas control valve or the ignition control unit? I'm leaning more to the ignition control unit thinking the circuitry controlling the main gas valve is faulty.

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    1. I think you have the exact same symptoms I encountered. You're on the right track. It could be the gas valve, but the (ignition control) problem described above is more likely.

      Make sure the switch on the gas valve is on and no wires have pulled off before going any further, I've spent hours pulling out my hair because I missed something simple before. :)

      There are two easy ways to test this.
      1. With it trying to light (pilot should be lit). Check the voltage across the main gas valve (MV to Ground)with a multimeter. It should show about 24-28VAC. Compare with the pilot valve reading(PV to Ground) to catch a wrong setting on your meter or in my information (I'm sure it's 24VAC, but just in case).

      If they are both about the same (around 24-28VAC) it's probably your gas valve. make sure you use the PV, MV, and GND terminals on the gas valve! There could be 3 or 4 depending on the valve. If 4 there are 2 grounds, one is for MV and one is for PV.

      2. If the main valve is low (or you don't have a multimeter handy) give the ignition module a(several) light thump(s) (don't break anything). We want the relay's solder joints to move and hopefully make contact. The main burner may light, flash and go out, the gas valve may click, or nothing may happen. If something happens it will pretty much confirm your problem lies with the ignition module. If it is the module either repair as described in this post or replace it.

      You could also pull the module apart and visually inspect the solder joints. Depending on your skill level it may be quicker to just pop it out and check them.

      Link to a replacement unit manual, may be useful for wiring diagrams:
      http://s3.pexsupply.com/manuals/1297447906847/47273_PROD_FILE.pdf

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  4. I have a RobertShaw HV SP715 in my Magic Chef Altra II 90 furnace model EC6F100DC16-2 that loses the 24 VDC feeding my Carrier thermostat (I have a Carrier heat pump) for about 45 seconds every time the burners turn off. The loss of 24 volts causes my thermostat’s display to go blank. I have measured the loss of voltage at the transformer. This may have been happening for a while because unless I look at the thermostat during those 45 seconds, I usually do not know anything is wrong. I do notice sometimes in the morning that the heat pump is on when it should not be and it runs for a long time trying to bring my house up to the daytime temperature from the cooler night time setting, then I notice the thermostat display is off.
    It sounds like a really strange problem and I was hoping you could tell me what could be causing this to happen. Thanks in advance for your help.

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  5. Great post! Saved me several hundreds on the repair. Two notes:
    1. In my case another relay joint required soldering, the one corresponding to contact 1 on the relay;
    2. How do yo extract the board from the case? It appears to be glued at the holes in the corners. Is there a recommended method for separating it without breaking? (I didn't have to remove the board for soldering thought.)

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  6. Hi Phil and Sergey, I'm 600 miles from my computer and reply doesn't want to work. I'll try to separate this in a few days when I get back.
    @ Sergey - glad you fixed it. I noted the bad joints on the one I fixed, but probably failed to mention that all the joints should be inspected. So yea, check them all. I think the board I fixed was loose. Sometimes a little heat from a heat gun or hair dryer will soften that glue, sometimes a screwdriver will pop them loose. This particular board is pretty rugged. Of course if you can fix it without removing it that's probably the way to go.
    @ Phil - that is weird. I will ponder on that for a while. The only things I can think of at the moment are:
    Is it just the 24vac or the 120v that disappears?
    1. If the 120 stays you probably have some sort of short. You could measure the current on the 24v side with a multimeter. I suspect the current will spike when the voltage drops. Perhaps you can locate it.
    2. I just found a wiring diagram for a magic chief furnace with a sp715. It shows the high limit will break the 120v line going to the transformer. That would explain your problem. It could be a bad limit switch or an airflow problem. It would automatically reset after the furnace temp drops. It may be one of the rotating fan control limit switches in which case you can pull the cover off and watch it. It may be a snap disk limit that you can't. Either way make sure the blower fan is working properly, the filter is clean and the vents are clear.
    I'll let you know if I think of anything else.

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  7. Hi Steve and thanks for your reply. I did a little more trouble shooting yesterday and it looks like the problem is in my Carrier Model 38YRA030300 heat pump. I disconnected all the leads from the furnace to the heat pump leaving only the 24 volts from the transformer to the t-stat (red and blue) and the white lead from the t-stat to the furnace. The furnace operated correctly without the loss of 24 volts and only a ‘trouble’ indication on the t-stat.

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    1. Hmm, very interesting. I still like the limit switch idea, but that seems somewhat less likely. Lets see if we can eliminate that as an option. I have found some diagrams and had some more thoughts.

      1. Do the heat pump and the furnace run at the same time when it acts up? (could make things a little hotter and help support my limit switch idea)

      2. Have you checked if it looses 120v at the thermostat or how high the fan control / high limit dial goes (if equipped)? (again could support my limit switch idea, perhaps the indoor coils need cleaning?)
      We know we loose 24vac at the transformer, we need to determine if it's a short on the 24vac side or if we are loosing the 120v to the transformer. (possible test - it may buzz louder if it's a short and be completely silent if it looses power - not as good as a multimeter, but it will do in a pinch.)

      3. If the 120v stays and the 24v (both measured directly at the transformer) goes we can eliminate the limit switch idea, it could open the 120v or the 24v, but should never, ever short either. Then it must be a very strange short.

      4. Another test, we will call this the "cart before the horse theory".

      Does the 24v disappear when the burner shuts off or does the burner shut off when the 24v disappears? It is important. The burner will definitely shut off if it looses power (24v), but is the thermostat still calling for heat?

      Get it so it is acting up and crank the temperature way up (like 90F) then wait and see if it shuts off after the usual run time. (should be less than 30 minutes, probably less than 10) If it does it's definitely a limit of some sort, either time (unusual, t-stat, control board, or mechanichal timer somewhere?) or temperature (probably a high limit somewhere).

      5. If it doesn't, just keeps heating forever, turn it back down and check - does the fan shut off immediately when the thermostat stops calling for heat? it should run for a few minutes after the burner shuts off. If it shuts right off it could be residual heat from the furnace is tripping the high limit (check the fan control).

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  8. Hi Steve, Thank you for the great information. I have a similar problem. I have the Robershaw SP715A on my Weil Mclain boiler. My problem is intermittent. Some times the boiler works normally for days but then it will malfunction. The T-stat will call for heat the ignitor sparks the pilot lights and thats it. No main burner. But the ignitor continues to spark with the pilot lit. I replaced the ignitor/pilot assembly but still have the same problem. I checked for 24V at Main valve and do not have it when malfunctioning. So the module is not telling Main valve to open. I may try to open module to inspect before buying new one. Maybe I'll get lucky and see a problem? Otherwise should I get another SP715A? Or the replacement 780-715 or L39-646 ? Confused about a replacement.

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    1. From what I can tell L39-646 is a kit that includes a 780-715, a mounting bracket, some wires, adapters, etc. So either should work. I think the 780-715 is a drop in replacement for the sp715a (nothing else needed), but I could be wrong.

      I think I'd go with a 780-715 over the sp715a if I couldn't fix it. I would hope they made the newer model a little better, but who knows? The sp715a's have lasted a long time. I'm not sure they make the sp715a anymore anyway.

      It sounds to me like it's not sensing the pilot flame (still ticking).

      It really sounds like the sp715a is the culprit. I would at least take the back off the control and closely inspect all the solder joints. The ignitor could easily arc over a bad joint and light, but the much lower sense voltage / current wouldn't, so it wouldn't turn on the main gas valve. Pull the sense wire off and put it back on too, a little corrosion could cause this.


      Some units have a separate flame sense (looks like an electrode, others use just one (single electrode spark and sense, this may only apply to replacements, not the actual sp715a, I'm not sure). If it has 2 electrodes and one wasn't replaced with the pilot assembly (I'm sure the sparking one was) you could try lightly cleaning it with something mildly abrasive like a red scotchbrite pad. They seem get some sort of a coating on them eventually and cause exactly the problem you describe, excluding the intermittent part.

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  9. Thank you for the reply, the new pilot sensor assembly I installed does have a seperate sense electrode and spark enlectrode just like the old one. I did try pulling the sense wire off. And wiggling all the wires.
    I plan on trying to take the SP715A apart this weekend. I think it is stuck to the inside of boiler with some sort of adhesive? probably be a PITA to get off.
    I found someone selling the SP715A on ebay who says they are newly manufactured, not old stock.
    So that's why I'm not sure which one to get?
    Note: it's been working for about 24 hours without a problem .

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  10. mire su plaqueta y es igual ala mia pero mi problema es q hay uno de los fet o no se que sera ya que solo quedaron las patas podria desime usted cuales son

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    1. Hi Luis, I'm afraid I don't know Spanish and Google translate isn't helping me much.
      I think you need to identify Q1?
      Unfortunately I don't have the board to look at, but based on the pictures I think Q1 and Q6 are running in parallel (for increased current capacity) and should be the same. If Q6 is still intact you should be able to get the numbers off it.
      I'd be worried about what took out Q1, I'd expect Q6 to be dead too (probably Q6 died and Q1 couldn't handle the current alone and blew), I'd replace the other Q's to be on the safe side.

      Hope Google translate works better for you :)

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