- HVAC
- Smart thermostats
H155
Google Nest H155 on thermostat: Heat Link offline on low battery
Risk level
Low
Usually safe to check without specialist tools.
Urgency
Medium
Quick answer
Nest code H155 means the Heat Link dropped its connection to the thermostat because its batteries ran low. Fit new Energizer Lithium L91 AA batteries, not alkaline ones, and let it reconnect on its own; if it does not, contact a Nest Pro.
Safety first
Stop and check this first
- Moving the Heat Link means switching off all power to the home at the fuse box, because it is wired to the boiler: leave that job to an installer.
Safe checks you can perform
- Replace the Heat Link batteries with Energizer Lithium L91
- Watch the Heat Link reconnect by itself
- Turn the heating on by hand if you are cold
Leave these tasks to a qualified professional
- Do not attempt this professional task: Nest Pro check of the Heat Link and thermostat link
- Do not attempt this professional task: Google Nest support case with the battery history
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.
Nest Pro check of the Heat Link and thermostat link
What they doA Nest Pro or heating installer checks the Heat Link, its position near the boiler and its communication with the thermostat.
They must show youthe Heat Link test result from the thermostat Equipment menu and the colour of its status light.
In writingwhat was checked, whether the Heat Link had to be moved or reset and how the connection ended up.
Google Nest support case with the battery history
What they doIf the alert returns with freshly fitted batteries, open a case with Google Nest and give them the code and the date of each battery change.
They must show youthe code logged in the diagnostics and the thermostat software version.
In writingthe case number and the warranty or replacement decision.
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.
Replace the Heat Link batteries with Energizer Lithium L91
- Who:
- you, removing only the Heat Link cover and leaving all wiring alone.
- How:
- pull off the Heat Link cover, take out the old cells and fit new Energizer Lithium L91 AA batteries the way the compartment shows; Google warns against regular alkaline batteries.
- Result that matters:
- powering the Heat Link again so it can rebuild its link with the thermostat.
- If the alert clears after a while:
- write down the date so you know how long the batteries last.
- If H155 is still on screen:
- use the next step to see whether the Heat Link has reconnected.
- Evidence to keep:
- a photo of the H155 alert and of the batteries you removed.
Evidence: Troubleshoot Nest thermostat help codes
Watch the Heat Link reconnect by itself
- Who:
- you, looking at the thermostat display and the Heat Link status light.
- How:
- give it a few minutes after the battery change; the page explains that the Heat Link should reconnect automatically and that its light shows yellow while it is disconnected from the thermostat.
- Result that matters:
- confirming that the thermostat is back in control of the boiler.
- If the light is no longer yellow and the code is gone:
- raise the temperature to check that the heating responds.
- If the Heat Link is still yellow after that wait:
- do not open anything else and move on to professional help.
- Evidence to keep:
- a note of when you swapped the batteries and the light colour afterwards.
Evidence: Troubleshoot Nest thermostat help codes
Turn the heating on by hand if you are cold
- Who:
- you, using the button on the front of the Heat Link.
- How:
- while you sort it out, press the Heat Link button to switch the heating on (twice with the Thermostat E, according to Google); the heating light turns green when heat is on.
- Result that matters:
- keeping the home warm even though the thermostat cannot send commands.
- If the boiler fires:
- remember to switch it off the same way, because no schedule will run.
- If it will not start by hand either:
- tell the professional, as the fault may not be limited to the link.
- Evidence to keep:
- the time you turned manual heating on and whether it worked.
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
- código H155 Nest
- Heat Link se ha desconectado porque las pilas estaban bajas
- Nest H155
- Nest Heat Link low batteries
- Nest Heat Link pilas bajas
- Nest help code H155
- Nest Thermostat E H155
- Nest Thermostat E H155 code
- termostato Nest error H155
- Your Heat Link went offline when the batteries were low
What it means
On Google's 'Troubleshoot Nest thermostat help codes' page, H155 sits among the H codes, which apply to homes with a condensing boiler, OpenTherm, a hot water tank or district heating, and reads 'Your Heat Link went offline when the batteries were low'. While the Heat Link is offline the thermostat cannot send commands to the boiler. Google asks for new Energizer L91 lithium AA batteries and warns against alkaline ones. The code alone never proves that the Heat Link or the thermostat is faulty, and other models or software versions may show different messages.
Warnings and stop conditions
- Lowsource verified
Moving the Heat Link means switching off all power to the home at the fuse box, because it is wired to the boiler: leave that job to an installer.
Evidence: Troubleshoot Nest thermostat help codes
Symptoms and causes
Probable causes · in order
- Heat Link batteries depletedCommon
Google links H155 to the Heat Link running too low on battery power and losing its wireless connection to the Nest thermostat.
Evidence: Troubleshoot Nest thermostat help codes
- Alkaline cells instead of lithiumPossible
The page says plainly not to use regular alkaline batteries in the Heat Link, so alkaline cells may drain sooner and bring the alert back.
Evidence: Troubleshoot Nest thermostat help codes
- Link problem that survives the swapLess common
If the Heat Link still will not reconnect with fresh batteries, Google points you to a professional who can look at the heating system and the thermostat.
Evidence: Troubleshoot Nest thermostat help codes
Virtual technician
Does H155 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
- 2
Códigos de error de los termostatos Nest y solución de problemas
Google · 2026 · official_doc
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