No Hot Water but Heating Works? Fixes

It's a strange and frustrating fault: your radiators are warm, the house is heating up nicely — but turn on the hot tap and you get nothing but cold or lukewarm water. If your heating works but your hot water doesn't, the problem isn't your whole boiler. It's one specific component, and on a combi boiler it's usually the same one every time.

In this guide we'll explain what's almost certainly going wrong, which checks you can safely do yourself, and which parts of the job need a Gas Safe registered engineer. We'll also give realistic Milton Keynes repair costs so you know what to expect before you call anyone.

The Most Likely Cause: A Faulty Diverter Valve

If you have a combi boiler (the most common type in Milton Keynes homes) and your heating works but hot water doesn't, the prime suspect is the diverter valve.

A combi boiler heats water on demand — there's no hot water cylinder. The diverter valve is the component that decides where that heat goes. When you turn on a hot tap, the valve diverts heat to the water flowing to your tap. When you don't need hot water, it sends heat to your radiators instead. It's constantly switching between the two.

Over time, diverter valves wear out or seize up. When the valve sticks in the "heating" position, the result is exactly what you're seeing: your radiators heat fine, but the boiler can't divert heat to your hot water, so the taps run cold or only slightly warm. It's the single most common reason for this specific fault — heating yes, hot water no.

Why does it stick? General wear, but also the hard water in the MK area plays a part. Limescale build-up around the valve mechanism makes it stiffer and more prone to seizing. It's the same mineral problem that affects boilers across Milton Keynes.

Other Possible Causes

The diverter valve is the usual answer, but it's not the only one. Depending on your boiler type and symptoms, it could be:

• A faulty flow sensor (or flow switch). This tells the boiler when a hot tap has been opened. If it fails, the boiler never gets the signal to fire for hot water — even though heating still works on its own circuit.
• An airlock. Trapped air in the hot water pipework can block flow, particularly after work on the plumbing or after the mains has been off.
• A cylinder thermostat (system and regular boilers). If you have a hot water cylinder rather than a combi, a faulty cylinder thermostat or a programmer set to "heating only" can stop the water heating while the radiators stay on.
• A faulty thermistor or PCB fault. Less common, but a sensor or control board fault can mislead the boiler about water temperature.

Symptom Checker: What's Likely Wrong

Symptom	Likely cause	DIY or engineer?
Combi: heating fine, hot water cold	Stuck diverter valve	Engineer
Combi: hot water lukewarm, never hot	Partially stuck diverter valve or worn thermistor	Engineer
Boiler won't fire when hot tap opens	Faulty flow sensor	Engineer
No hot water and pressure gauge low	Low boiler pressure	DIY — repressurise
System boiler: radiators on, cylinder cold	Cylinder thermostat or programmer setting	Check settings first, then engineer
Hot water spluttering then cold	Airlock in hot water pipework	Engineer (sometimes DIY)

Safe Checks You Can Do First

Before booking an engineer, run through these — they take five minutes and might save a callout fee.

1. Check the boiler pressure

Look at the pressure gauge on the front of the boiler. With the system cold it should sit between 1 and 1.5 bar. If it's below 1 bar, low pressure could be affecting performance — repressurise using the filling loop as described in your boiler manual.

2. Check your controls and programmer

Make sure the hot water hasn't simply been switched off at the programmer. On systems with a hot water cylinder, the programmer has separate settings for heating and hot water — it's easy for the hot water schedule to be off while heating is on.

3. Check the cylinder thermostat (if you have a cylinder)

If you have a hot water tank, find the cylinder thermostat strapped to it and check it's set to a sensible temperature (around 60°C). A knocked or failed thermostat can leave the tank cold.

4. Try other hot taps

If only one tap runs cold, the problem may be that tap or a local airlock — not the boiler. If every hot tap in the house is cold, the fault is at the boiler.

Why the Diverter Valve Is an Engineer Job

Replacing or freeing a diverter valve means opening up the boiler and working on a sealed, pressurised gas appliance. By law, that work must be done by a Gas Safe registered engineer. An incorrectly refitted valve can cause leaks, poor performance, or a dangerous fault.

A good engineer will first confirm the diagnosis — it's important to be sure it's the diverter valve and not the flow sensor or thermistor before replacing parts. At Plumbline MK we diagnose properly first, then quote a fixed price, so you only pay for the part you actually need. You can learn more about our approach and our Gas Safe credentials.

Milton Keynes Repair Costs

As a guide for the MK area:

Job	Typical MK cost
Diagnostic visit / callout	£60-£90
Diverter valve replacement (parts and labour)	£150-£300
Flow sensor replacement	£100-£200
Cylinder thermostat replacement	£90-£150
Clearing an airlock	£70-£120

Costs vary with boiler make and model — some manufacturers' parts are pricier than others. For a confirmed fixed price, the engineer needs to diagnose the fault first. If you've also lost heating, our guide to central heating not working in Milton Keynes covers the opposite fault.

Need expert help? Contact Plumbline MK for a free, no-obligation quote. Call 07805 844 016 for same-day boiler repair across Milton Keynes — including Wolverton and surrounding areas. For urgent loss of hot water, our emergency plumbing service can help fast.

Frequently Asked Questions

What is a diverter valve?

A diverter valve is the component inside a combi boiler that decides where heat goes. When you turn on a hot tap, it diverts heat to your hot water; the rest of the time it sends heat to your radiators. It switches constantly between the two. When it sticks or fails, you typically lose hot water while heating keeps working — or, less commonly, the reverse.

Can I fix a diverter valve myself?

No. Replacing or freeing a diverter valve means working inside a sealed, pressurised gas appliance, which by law must be done by a Gas Safe registered engineer. You can safely check boiler pressure, programmer settings, and the cylinder thermostat, but the valve itself is an engineer job. Attempting it yourself is unsafe and may invalidate your boiler warranty.

How much does diverter valve replacement cost?

In the Milton Keynes area, replacing a diverter valve typically costs between £150 and £300, including parts and labour. The exact price depends on your boiler make and model, as some manufacturers' parts cost more than others. A reputable engineer will diagnose the fault first and then give you a fixed quote before starting work.

Why is my water lukewarm and not hot?

Lukewarm rather than fully cold water often means a diverter valve that is partially stuck, or a worn thermistor giving the boiler a false temperature reading. It can also indicate the boiler is not firing fully when hot water is drawn. Because it points to an internal component, it needs a Gas Safe engineer to diagnose and fix.