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Glow Brand · Boiler troubleshooting

0-10 VDC input to modulate firing rate (Mode 5)

C95 & C140

What is going on

Mode 5 on the C95 and C140 combis, a 0-10 VDC input driving the firing rate. It works like Mode 4. The boiler needs at least 2 VDC to wake up, and at 2 VDC the display reads 1%, which is minimum fan RPM. At 10 VDC you get 100%, maximum fan RPM, and everything between follows the signal proportionally. When the rate looks wrong, I put a meter on the signal before blaming the boiler.

Likely causes

  • Control signal below 2 VDC, so the boiler never activates
  • Wrong polarity or loose terminal on the 0-10 VDC input
  • External controller output not scaled to the full 0-10 VDC range

Check in this order

  1. Apply at least 2 VDC to activate the boiler.
  2. Confirm the display reads 1% at 2 VDC (minimum fan RPM).
  3. Confirm the display reads 100% at 10 VDC (maximum fan RPM).
  4. Measure the signal at the boiler terminals with a DC meter, not at the controller.
  5. Check that the firing rate follows the signal between those points.

Applies to: C95 & C140 combi boilers

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Source: Glow Brand C95 & C140 manual, p. 64. Written in our own words; check the manual for your exact model.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.