No Hot Water But Heating Works in Barnet - What to Check and When to Call

Published October 2026 | No hot water but heating works what to check

In the First 10 Minutes

When your heating is running but every hot tap in the house is cold, the first instinct is panic. Don't. This is one of the most common boiler faults our engineers attend across Barnet and the wider Greater London area, and in most cases it has a clear diagnosis. The key is working through a short checklist before picking up the phone.

Start here:

1. Check the programmer or timer. Your boiler's programmer controls heating and hot water separately. It's surprisingly common to find the hot water schedule has been knocked off after a power cut or when the clocks change. Make sure the hot water is set to "on" for the current time of day.
2. Look at the boiler pressure gauge. It should read between 1 and 1.5 bar when the system is cold. If it's reading under 1 bar, top it up using the filling loop before doing anything else. Bear in mind, low pressure usually affects both heating and hot water, so if your radiators are fine, pressure probably isn't the culprit.
3. Note any error codes on the display. Modern boilers from Worcester Bosch, Vaillant, Baxi, and Ideal all display fault codes when something's wrong. Write the code down before you reset anything - it's exactly what your engineer will want to see.
4. Run the hot tap for 30 seconds. Occasionally a minor airlock in the hot water circuit clears itself with a short burst of flow. Worth trying once before you call anyone.

What you should not do in the first 10 minutes: don't open the boiler casing, don't reset the boiler more than twice, and don't assume the problem will resolve itself overnight.

Within the First Hour

If the quick checks haven't resolved the issue, it's time for a proper assessment. The most likely cause on a combi boiler is a faulty diverter valve - a component that controls where heated water flows inside the boiler. When it's working correctly, opening a hot tap diverts water away from the radiator circuit and towards your taps. When it fails or sticks, the boiler carries on heating the radiators while your taps stay cold.

Other faults to consider at this stage:

• Plate heat exchanger blockage. On combi boilers, the plate heat exchanger heats tap water directly. In hard water areas - and Barnet is firmly in that category - limescale builds up inside the exchanger over time, eventually restricting flow to the point where hot water fails entirely. The heating circuit is not affected because it runs through a separate loop.
• Motorised valve failure. On system or regular boilers with a hot water cylinder, a motorised valve controls the flow of hot water to the cylinder. If the valve motor burns out, the cylinder stops heating up even while radiators remain warm.
• Immersion heater fault. If your property has a hot water cylinder with an immersion heater - common in older homes with system boilers - the immersion element or its thermostat may have failed independently of the boiler.

This is a good point to run the Voltrade GoFIX diagnostic tool if you're using the Voltrade app. It walks you through a structured fault check and gives you a clearer read on whether you're dealing with something user-addressable or a job that needs a Gas Safe engineer on site.

While you're assessing, note your boiler's make, model, approximate age, and when it was last serviced. Your engineer will ask all of this when you call, and having the answers ready speeds things up.

Same Day

Once you've established that the fault is inside the boiler itself - which covers the diverter valve, plate heat exchanger, and motorised valve in most cases - the next step is booking a Gas Safe registered engineer. This isn't optional: only a Gas Safe registered engineer is legally authorised to carry out work on gas appliances in the UK. Gas Safe registration is a legal requirement, not a quality mark you can choose to prioritise or skip over.

When you call to book, give the engineer:

1. Your boiler make, model, and age
2. Any error code currently showing on the display
3. A clear one-line description - "heating works fine, no hot water from any tap, started this morning"
4. Whether you have a combi boiler or a system/regular boiler with a separate cylinder

This lets the engineer come prepared with likely parts. In Greater London, same-day availability for boiler repairs is common in autumn and winter - it's worth asking specifically when you call rather than assuming the next available slot is days away.

While you wait for the appointment:

• Use a kettle for washing up and hand washing
• If you're vulnerable, elderly, or have young children in the house, say so when booking - most engineers in Barnet will prioritise accordingly

The Repair Visit

A boiler repair visit for this type of fault typically takes between 1 and 3 hours. Here's what to expect from start to finish.

The engineer starts with a full diagnostic - checking flow rates, temperature differentials, pressure readings, and boiler operation. They confirm the fault before fitting any parts, because replacing the wrong component wastes your money and their time.

Diverter valve replacement is the most common fix for this exact fault. On a combi boiler, expect the total cost - parts and labour combined - to sit between 150 and 350 pounds depending on the boiler make and model. Worcester Bosch and Vaillant tend to be at the higher end; Baxi and Ideal are typically a bit lower.

Plate heat exchanger replacement is a more involved job. Costs typically run between 300 and 600 pounds. If the engineer finds the failure is limescale-related - which is particularly common in Barnet given the local water hardness - they may recommend fitting a system filter or scale inhibitor at the same time to slow future buildup.

Motorised valve replacement on a system or regular boiler is usually the quickest and least expensive repair of the three, typically costing between 100 and 250 pounds including labour.

Immersion heater replacement generally runs between 80 and 180 pounds depending on element type and how accessible the cylinder is.

Before the engineer starts work, ask to see their Gas Safe ID card. Every registered engineer carries one and you can verify the registration number at the official Gas Safe Register website. It takes 30 seconds and confirms you're covered.

The Following Week

The repair's done. Here's what to monitor over the next seven days to make sure everything has settled properly.

Hot water temperature. From a combi boiler, the water should feel hot at the tap within a few seconds of running. From a cylinder system, stored water should reach around 60 degrees Celsius. If it's coming out lukewarm, there may be a secondary issue - a hot water thermostat adjustment or a partially failed component - that needs looking at.

Boiler pressure. After any repair involving the water circuit, pressure can drop slightly as the system settles. Check the gauge 48 hours after the repair. If it's dropped below 1 bar, repressurise to 1 to 1.5 bar using the filling loop. Your boiler manual covers the exact procedure.

Radiator performance. If the engineer worked near the heating circuit, some air may have entered the system. Bleed any radiators that feel cooler at the top than the bottom.

If the same fault returns within a few weeks - no hot water, heating still running fine - call the engineer back. A quick recurrence usually means the root cause wasn't fully resolved, or there's a second underlying problem such as heavy limescale throughout the system rather than just in one component.

Long Term

A single no-hot-water episode is often the first signal that a boiler needs more regular attention. Here's what to put in place to reduce the risk of a repeat.

Annual boiler service. A service gives an engineer the chance to clean internal components, check the diverter valve operation, inspect the heat exchanger, and catch wear before it becomes a failure. In Greater London, annual boiler servicing typically costs between 80 and 150 pounds - considerably less than most repair bills. Many manufacturers, including Worcester Bosch and Vaillant, also require evidence of annual servicing to keep the manufacturer warranty valid.

System filter or magnetic filter. Fitting an inline filter such as a Fernox TF1 or Adey MagnaClean removes debris and magnetite from the system water before it can reach the boiler's sensitive components. These are particularly valuable in Barnet given the local water hardness, and a good engineer will fit one while they're already at the boiler rather than treating it as a separate visit.

Limescale inhibitor. A chemical inhibitor added to the system water slows limescale buildup inside the plate heat exchanger. It's inexpensive, takes a few minutes to add, and makes a measurable difference in hard water areas over the long run.

Consider boiler age . If your boiler is more than 12 to 15 years old, the repair-versus-replace calculation starts to shift. Older units are less energy-efficient and increasingly difficult to source parts for, and a pattern of faults often signals the end of a boiler's reliable working life. A modern A-rated condensing boiler typically reduces gas bills meaningfully and comes with a manufacturer warranty of up to 10 years on some models.

Timeline Questions

How long does it typically take to get hot water back after a diverter valve repair?

Most diverter valve replacements are completed within 1 to 2 hours on the day of the engineer's visit, assuming they have the right part on the van. For popular brands like Worcester Bosch, Vaillant, and Ideal, parts are widely stocked across Barnet and the surrounding area, so same-day completion is the norm rather than the exception. If the part needs ordering, you might wait 24 to 48 hours.

Is it safe to keep using my heating while I wait for the repair?

In most cases, yes. If your heating is running normally, there are no unusual noises, no gas smell, and no error codes pointing to a safety issue, it's fine to continue using the radiators while you wait for an engineer. If you smell gas at any point, stop using the boiler immediately, leave the property, and call the National Gas Emergency Service on 0800 111 999.

Can I carry out any of this repair work myself to save money?

How much should I expect to pay for a full boiler diagnostic in Barnet?