Your Baxi boiler is making hot water fine, but the radiators are staying cold, that’s frustrating, especially in winter.
Below are the real causes I see in UK homes all the time.
1) Diverter valve stuck (very common on combi boilers)
Is this your problem?
You get perfectly hot taps and showers.
The boiler fires up.
But when heating is on, the radiators stay cold.
What to do
This usually means the diverter valve is stuck.
On a combi boiler, that valve switches between hot water and heating. If it sticks, it stays in hot water mode.
Safe DIY checks:
- Turn heating on.
- Listen near the boiler.
- If it fires but pipes to radiators stay cool, suspect diverter valve.
You cannot safely repair this yourself.
Stop here and call a Gas Safe engineer.
This is an internal boiler part.
2) Heating is on but programmer or thermostat isn’t actually calling for heat
Is this your problem?
Hot water works.
Heating shows “on” on the programmer.
But the boiler doesn’t fire for heating.
What to do
Check the basics properly:
- Turn room thermostat up to 25°C.
- Make sure programmer is on “Heating ON”, not just hot water.
- Check batteries in wireless thermostat (very common issue).
- If you have a Hive/Nest, check signal connection.
If the boiler doesn’t respond at all, it could be:
- Faulty thermostat
- Faulty motorised valve (on system boilers)
You can replace thermostat batteries yourself.
If wiring or valves are involved → call an engineer.
3) Low system pressure
Is this your problem?
Hot water works.
Heating tries to start but shuts down.
Pressure gauge is below 1 bar.
What to do
Check pressure on the boiler display or gauge.
For most Baxi combis:
- Cold pressure should be 1–1.5 bar.
If it’s low:
- Locate the filling loop (silver braided hose under boiler).
- Slowly open both valves.
- Watch pressure rise to 1.3–1.5 bar.
- Close both valves firmly.
Do NOT go above 2 bar.
If pressure keeps dropping repeatedly → possible leak → call a professional.
4) Motorised valve stuck (common in system boilers)
Is this your problem?
You have a hot water cylinder (not a combi).
Hot water works.
Radiators cold.
What to do
You likely have a 2-port or 3-port motorised valve near the cylinder.
When heating is turned on:
- Touch the valve body carefully.
- Do you hear a faint whirring?
- Is the pipe after the valve still cold?
If yes, valve may be stuck.
You can:
- Gently move the manual lever on the valve (if accessible).
If that temporarily fixes heating → valve head likely faulty.
This is electrical — call a qualified heating engineer.
5) Airlocked or blocked system
Is this your problem?
Boiler fires.
Some radiators slightly warm at bottom only.
Top completely cold.
What to do
Bleed the radiators:
- Turn heating off.
- Use radiator key.
- Open valve slowly until air hisses out.
- Close once water appears.
- Recheck boiler pressure.
If multiple radiators stay cold or sludge suspected → powerflush may be needed.
That’s not DIY.
6) Circulation pump not running
Is this your problem?
Boiler fires.
Hot water works.
Heating pipes near boiler get hot — but radiators don’t.
What to do
You may have a faulty pump.
Carefully place hand on pump housing (if accessible).
You should feel vibration when heating is on.
No vibration = pump not running.
Do not attempt to dismantle.
Call an engineer.
When to stop and call a professional
Call a Gas Safe engineer if:
- Diverter valve suspected
- Motorised valve suspected
- Pump suspected
- Boiler showing fault code
- Pressure keeps dropping
DIY is safe for:
- Repressurising
- Bleeding radiators
- Thermostat battery checks
Anything inside the boiler casing → leave it.