RobertShaw HS780 Teardown

I recently replaced a old furnace with a 90+ for my father, I don't recall the make or model of the old furnace, but I salvaged some parts from it to look at. Today we have a Robertshaw HS780 Hot Surface ignition control. The furnace was working, so there wasn't a failure to fix, but it may be of some interest.

RobertShaw HS780

If you watch the video be warned, It's my first attempt at an ad-lib teardown. I kept talking as I figured out the board and was a bit off. It's not bad, hope to do better next time. I think the left small relay is the gas valve, the right one powers up this board when the thermostat closes.

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By far the most common failure on HSI ignition systems is the ignitor (not this control). They are fairly fragile. Never touch the heating part, it could cause early failure.

Diagnosis would go like this:

• 120VAC present at terminals labeled 120 and NEUT?
• 24VAC present at TH and TR? (thermostat calling for heat?)
• When calling for heat is 120V present at  IGN terminals AND HSI not glowing? (if yes replace HSI, if no suspect large relay)
• HSI Glows and 24VAC present between VALVE and GND? (if yes, suspect gas valve / safety switches/wiring, if no suspect outer small relay)
• Failing all that test the inner small relay, inspect all solder joints for failure and caps for leakage. (if you can't find anything at this point consider replacing the control.)

Sometimes a relay will actually fail (shorted coil, open coil, or bad contacts) and they can often be sourced for $5-10 from digikey or a similar company.

Video Notes:

1. The fan/limit control won't come apart easily.
2. Typical setting are ~100 fan off, ~120 fan on, ~180 high limit.
3. You shouldn't be bouncing off the high limit, if you are you have an airflow problem, oversized furnace, etc.
4. Flame sense is through the ignitor.

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

UPDATE: A Few Links

http://www.robertshawtstats.com/spaw2/SiteContent/Files/FAQ/ControlTips-TroubleshootingPilotIgnition.pdf

http://s3.supplyhouse.com/product_files/780-Install.pdf

http://www.forwardthinking.honeywell.com/related_links/combustion/s8610u3009/install/69_1955.pdf