Firstly, just so you know, whilst you troubleshoot, you can press the button on your Heat Link to turn heating on or off. If you have Nest Thermostat E, press the button twice. If you have Nest Learning Thermostat, press the button once. The heating light will turn green to let you know the heat is on. The H72 error code means your thermostat has lost power. Is your thermostat on a stand or wall mounted?
If your thermostat is on a stand
I know it may sound obvious, but make sure that the power cable is fully inserted into the power plug and that the plug has been pushed all the way into the wall socket. I would also check that the wall socket is switched on. You can also double-check that the socket is delivering power by plugging in a lamp or other electronic device and turning it on.
If your thermostat is on the wall
Make sure that the thermostat display is completely attached to the base. If it’s not, push it down on the base as you should hear it click. You can also try moving the Heat Link.
- Before you move the Heat Link, turn off all power to your home at the fuse box.
- Move the Heat Link so that it is at least 30 cm away from the boiler or other large metal objects.
- Also, check that there isn’t anything directly between your Heat Link and thermostat, like a large mirror or refrigerator.
- After you move the Heat Link, turn the power to your system back on.
Once you've checked those things depending on if it's on a stand or on the wall:
- Check your system's fuse. It’s possible that the fuse controlling power to your system isn’t properly seated or is blown. Turn off the power to your system, then check the fuses. Re-seat or replace it as necessary. Refer to the manufacturer's manual or website for exact instructions and safety precautions.
- Make sure that the switch is set to on. Sometimes, fuse switches can look like they’re on, but haven’t been flipped on all the way. Try flipping the switches that control power to the Heat Link and your thermostat off and then on again.
If you've checked all those things and the issue persists, I'd recommend contacting a local Nest Pro or a local HVAC technician, who can properly diagnose your system.
