Nest thermostat: H156: OpenTherm available but not selected

Our drawing: “H156”, on a round thermostat.

Your system is wired for and compatible with OpenTherm, but the OpenTherm option wasn’t selected during setup.

Applies to: Nest Thermostat E with a Heat Link. What you see: a code.

What Google says to do

On the thermostat, go to Settings, Equipment and change the boiler control to OpenTherm.
If the message stays, contact Nest support.

Google ties this to the boiler or heating system. What the symbols on a boiler’s own panel mean is on the boiler symbols page.

Google’s own help page, read on the date shown.

Google Nest Help: Troubleshoot Nest thermostat help codes — Nest Learning Thermostat (1st to 4th gen), Nest Thermostat E and Nest Thermostat; H codes only where a Heat Link is installed. support.google.com Checked 2026-09-29.