Friday, April 26, 2013

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.

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.