HVACopediaGlow Brand · Boiler
Err 156 Blocked flue switch open
What the control is telling you
The blocked flue pressure switch (normally closed) is open.
Applies to: B85, B120, B160 (NG and LP)
Likely causes
- Blocked vent termination
- Blocked heat exchanger
- Kinked or blocked tubing to the switch
- Switch wiring harness
- Failed switch
Test and repair steps
- Check the vent termination for blockage and measure pressure at the exhaust port.
- Check the heat exchanger for blockage.
- Make sure the tubing to the switch is clear.
- Check the harness to the switch.
- Check continuity across switch terminals 1 and 2 (should be closed).
Settings for these units
Maintenance intervals · Glow B85/B120/B160
| Item | When |
|---|---|
| Annual inspection by a technician | Every year (warranty) |
| Relief valve try-lever test | Annually |
| LWCO test | Annually |
| Condensate trap clean | Annually (14.3) |
| Heat exchanger combustion-side wash | Annually or when combustion drifts (14.4) |
| Burner door silicone gasket | Replace if damaged or after 2 years |
| Service reminder | 2.203 on, interval 2.204 (default 1000 days) |
Operating pressure should sit 5 psi under the relief valve rating (max 25 psig on a 30 psi valve); water never above 180 F.
Models that show this code
3 models in Glow B85/B120/B160
Other codes on the same units
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Source: Glow Brand B85/B120/B160 Installation & Operation Manual V20231025, Table 12.2 p.65. Summarized in our own words; check the original manual for your model.
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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.