- HVAC
- Smart thermostats
H73
Google Nest H73 on smart thermostat: OpenTherm not detected
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
High
Quick answer
H73 on a Nest (or H153) means it can't detect OpenTherm on the boiler. If it shows during installation, check whether your boiler supports OpenTherm; if it shows later, check the boiler has power. Heat Link wiring is for the installer.
Safety first
Stop and check this first
- Heat Link and boiler wiring is 230 V: don't open it yourself; a qualified installer must do it with power off.
Safe checks you can perform
- Boiler power if H73 shows after installation
- OpenTherm compatibility of the boiler
- Boiler control setting on the thermostat
Leave these tasks to a qualified professional
- Do not attempt this professional task: Check Heat Link wiring against the OpenTherm diagram
- Do not attempt this professional task: Diagnose boiler power after H73
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Check Heat Link wiring against the OpenTherm diagram
What they doThe installer, with power to the Heat Link and home off, compares the Heat Link connections with the installation guide diagrams and the boiler's OpenTherm terminals, and checks continuity with a multimeter.
They must show youthe Heat Link and boiler connections against the diagram and the continuity reading.
In writingwhich connection was wrong and how it was left.
Diagnose boiler power after H73
What they doIf the switch is on but the boiler gets no power, the technician checks the boiler supply and its board.
They must show youwhere the supply was being lost and the reading after the repair.
In writingthe cause and the boiler repair.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Boiler power if H73 shows after installation
- Who:
- you, at the fuse box and the boiler switch, without opening it.
- How:
- check the switch feeding the boiler is on and the boiler has power (display or lights on).
- Result that matters:
- whether the boiler is powered.
- If it had no power and restoring it clears H73:
- it was a supply cut.
- If the switch is on but the boiler gets no power:
- Google asks you to call a professional.
- Evidence to keep:
- photo of the switch and the boiler.
Evidence: Troubleshoot Nest thermostat help codes
OpenTherm compatibility of the boiler
- Who:
- you, with the boiler documentation.
- How:
- look in the manual, on the manufacturer's site or on the boiler for the OpenTherm logo.
- Result that matters:
- whether the boiler supports OpenTherm.
- If it doesn't:
- set On/Off boiler control instead of OpenTherm (Settings, Equipment, Pro Setup).
- If it does and H73 stays:
- the installer should check the Heat Link wiring.
- Evidence to keep:
- photo of the rating plate or manual page.
Evidence: Troubleshoot Nest thermostat help codes
Boiler control setting on the thermostat
- Who:
- you or the installer.
- How:
- in Settings, Equipment, Pro Setup, check whether boiler control is set to OpenTherm or On/Off, to match what your boiler supports.
- Result that matters:
- whether the setting matches the boiler.
- If it didn't match and correcting it clears H73:
- it was a setting.
- If it matches and H73 stays:
- the installer checks the wiring.
- Evidence to keep:
- photo of the settings screen.
Evidence: Troubleshoot Nest thermostat help codes
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- HVAC
- Brand
- Google Nest
- Product type
- Smart thermostats
Known code variants
- Heat Link OpenTherm no detectado
- Heat Link OpenTherm not detected
- Nest H153
- Nest H73
- Nest H73 OpenTherm
- Nest thermostat H73
- termostato Nest H73
What it means
Google explains that H73 and H153 appear if the thermostat can't detect OpenTherm, the protocol the Heat Link uses to talk to some boilers. During installation it points to an incompatible boiler, Heat Link wiring that doesn't match the diagram, or a setting that must change to On/Off control. After installation it mainly points to a boiler without power. The code doesn't prove a boiler fault.
Warnings and stop conditions
- Higheditorial warning
Heat Link and boiler wiring is 230 V: don't open it yourself; a qualified installer must do it with power off.
Evidence: Troubleshoot Nest thermostat help codes
Symptoms and causes
Probable causes · in order
- Boiler without OpenThermCommon
Shows during installation. Checked in the boiler documentation, with the manufacturer or by looking for the OpenTherm logo.
Evidence: Troubleshoot Nest thermostat help codes
- Heat Link wired incorrectlyPossible
The Heat Link connections don't match the diagrams in the installation guide.
Evidence: Troubleshoot Nest thermostat help codes
- Boiler without powerPossible
Shows after a while in use. Google asks you to check power to the boiler hasn't been turned off.
Evidence: Troubleshoot Nest thermostat help codes
Virtual technician
Does H73 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Troubleshoot Nest thermostat help codes
Google · 2026 · official_doc
Was this entry useful?