The thermostat shows a flame icon, the app says the heating is on, and the house is cold. This is a genuinely useful fault to be able to diagnose yourself, because a call-out fee for something that turns out to be a schedule conflict is an expensive lesson.
The short answer
Work out whether the thermostat is actually calling for heat, and whether the boiler is receiving that call. The receiver unit near the boiler usually has a light or an indicator that shows the demand signal. If the thermostat says it is calling and the receiver shows nothing, the link between them is broken. If the receiver shows demand and the boiler does nothing, the problem is the boiler.
Understand the three parts
Most smart heating setups have three pieces, and knowing which is which makes the diagnosis straightforward:
- The thermostat. Measures temperature and decides when heat is needed. Often battery powered.
- The receiver or relay. Wired to the boiler. Receives the demand signal from the thermostat and closes a contact to fire the boiler.
- The hub or bridge. Connects the system to your Wi-Fi so the app works. Importantly, on most systems the heating works without it; only the app stops.
That last point matters. If the app cannot connect but the thermostat itself still controls the heating, you have a network problem, not a heating problem, and the house will stay warm on the schedule already programmed.
Turn the target temperature well above the room temperature
Set it to a number clearly higher than the current reading, such as 25 degrees, and listen. You should hear the receiver click within a minute or two. That click is the demand signal reaching the boiler, and hearing it tells you the thermostat side is working.
Check the receiver indicator
Find the receiver near the boiler and look at its status light. Most show clearly whether heating demand is active. If the thermostat says it is calling and the receiver disagrees, the wireless link between them has dropped.
Replace the thermostat batteries
Low batteries cause a thermostat to display normally while failing to transmit reliably. This is a very common cause and the batteries are usually standard AA cells. Replace rather than test them.
Check for a schedule or mode conflict
Look for holiday mode, away mode, a frost protection setting, or an open window detection feature that has triggered. Also check whether a schedule is overriding your manual change, which most systems do at the next scheduled time slot.
Check the boiler itself
Look at the boiler display for an error code, check that its pressure is in the normal range, usually between 1 and 1.5 bar when cold, and check the boiler is not in a summer or hot water only mode. A boiler with low pressure will not fire regardless of what the thermostat asks.
Check individual radiator valves
If some radiators are warm and others are not, the thermostat is doing its job and the problem is distribution. Smart radiator valves can close individually, and a valve pin stuck closed is common on radiators that have been off all summer.
Re-pair the thermostat and receiver
If the receiver never shows demand, the wireless link needs rebuilding. The procedure is model-specific and usually involves holding a button on the receiver until it enters pairing mode. Have the manual to hand.
Check the wired connections at the receiver
With the power off at the fuse spur, confirm the wiring into the receiver is secure. Do not rewire anything yourself unless you are qualified; this step is about spotting an obviously disconnected wire, not making changes.
Do not work inside the boiler or alter mains wiring. Checking indicator lights, pressure gauges and batteries is safe and sensible. Anything beyond that is work for a qualified heating engineer, and in many places it is legally required to be.
The stuck valve pin
Worth its own section because it is so common at the start of the heating season. Thermostatic radiator valves have a small pin that is pushed down to close and springs up to open. After a summer of sitting closed, the pin seizes and the radiator stays cold even with the valve fully open.
The fix: take the valve head off, and you will see the pin protruding. Free it by pressing it gently a few times with pliers, or tapping it lightly. It should spring back up on its own. Refit the head. This revives a lot of radiators that appear to have failed and costs nothing.
App works, heating does not
If the app is happy and the house is cold, check what the thermostat itself displays rather than what the app shows. The two can disagree when the connection between the hub and the thermostat has dropped: the app shows the last known state, not the current one. Walking to the thermostat and reading it directly is the check most people skip.
Will my heating still work if the Wi-Fi is down?
On most systems, yes. The thermostat and receiver communicate directly, and the internet is only needed for the app. You lose remote control, not heating.
Why does the thermostat say heating but the radiators are cold?
Either the demand signal is not reaching the boiler, the boiler is not firing, or the heat is not reaching those radiators. Check the receiver light first, because it cleanly separates the first case from the other two.
How often should I replace thermostat batteries?
Roughly once a year, and it is worth doing before the heating season rather than during it. Low batteries cause intermittent faults that are hard to diagnose.
What boiler pressure should I see?
Typically between 1 and 1.5 bar when the system is cold. Below that, the boiler will usually refuse to fire, and topping up is a routine job described in the boiler manual.
Why is one radiator cold when the rest are hot?
Most often a seized valve pin after a summer of being closed, or air in the radiator. Free the pin first, then bleed the radiator if the top stays cold while the bottom warms.
The bottom line
Turn the target temperature well up and listen for the receiver to click, because that single test tells you whether the thermostat side is working. Replace the batteries before assuming anything has failed. If the receiver shows demand and the boiler stays silent, the thermostat has done its job and the problem is the boiler or its pressure.
