No Hot Water But Heating Works - What to Check in Addlestone

Published September 2026 | No Hot Water But Heating Works What to Check

Why This Time of Year Matters for Boiler Repair in Addlestone

September and October are the months when most Addlestone households switch their heating back on properly for the first time since spring. After sitting largely unused through the warmer months, boilers get a sudden shock to the system. That extended rest period is when small problems that were quietly developing over summer finally make themselves known.

The issue of heating working while hot water doesn't is particularly common at this time of year. During summer, your combi boiler has been working almost exclusively in domestic hot water mode - heating water on demand whenever you turned on a tap. The central heating side of things hasn't been asked to do much. Come autumn, when you flick the heating on properly, the system has to start switching between two jobs again. If any component involved in managing that switch has worn or partially seized during the quieter months, you'll notice it quickly.

In Addlestone specifically, the local water supply sits in a moderately hard water area. Over time, limescale builds up inside heat exchangers and around components like sensors and valves. A summer of lower boiler activity lets that scale settle and harden, which is one reason why the start of autumn is peak time for hot water faults across this part of Surrey.

The good news is that this is also the best time to get the issue sorted. By November and December, demand for boiler repair engineers across Surrey rises sharply and waiting times for non-emergency appointments can stretch to several days. Dealing with it now, while the system has just started playing up, is always the smarter move.

The Problems We See Most Often Right Now

When our engineers visit Addlestone homes with this exact fault - heating on, hot water off - there are a handful of causes that come up time and again. Understanding what they are helps you give useful information to an engineer and, in some cases, rule out the simplest fixes yourself.

The Diverter Valve

On a combination boiler, the diverter valve is the component that decides where heated water flows - to your radiators or to your hot taps. When it fails partially, it commonly gets stuck in one position. If it sticks open towards the central heating circuit, your radiators stay warm while nothing comes out of your hot taps at any useful temperature.

This is the single most common cause of this particular fault pattern. Manufacturers like Worcester Bosch, Ideal, and Baxi all use diverter valves in their combi boilers, and they do wear out over time - typically after 8 to 12 years on a heavily used unit. Replacing a diverter valve typically costs between 150 and 300 pounds depending on the boiler make and model, though on some older units the additional labour time can push this figure higher.

The Plate Heat Exchanger

Combi boilers use a plate heat exchanger (PHE) to heat domestic hot water rapidly on demand. In hard water areas like Addlestone, limescale builds up inside it to the point where water struggles to flow through properly. You might notice lukewarm water rather than no hot water at all, or water that heats at the start of the flow and then cools off. A blocked PHE typically costs between 200 and 400 pounds to replace, though some can be descaled successfully if the build-up is caught early enough.

The DHW Thermistor

The domestic hot water thermistor is a small temperature sensor that tells the boiler how hot the water coming out of the heat exchanger is. If it gives a false reading, the boiler may think the water is already at the right temperature and stop firing when you turn a tap on. On older boilers this doesn't always trigger an error code, which makes it trickier to spot without the right diagnostic equipment. Thermistor replacement typically costs between 100 and 200 pounds including parts and labour.

Programmer and Timer Settings

It sounds too simple to mention, but our engineers regularly arrive at Addlestone properties only to find that the hot water schedule on the programmer has been accidentally reset - or was never correctly configured after a boiler replacement. If you have a system boiler with a hot water cylinder rather than a combi, check that your programmer is set to heat the cylinder at the right times of day. Your cylinder thermostat should be set to between 60 and 65 degrees Celsius - any lower and you risk Legionella, any higher and you're wasting energy.

Motorised Zone Valves

If you have a system or conventional boiler with a separate hot water cylinder, a motorised zone valve controls flow to the cylinder. These valves contain a small motor and a set of microswitches that can fail with age. When a zone valve fails on the hot water circuit, the boiler won't fire for hot water even though the central heating continues to work through its own separate valve. Zone valve replacement typically costs between 100 and 200 pounds.

Preventive Steps You Can Take This Week

There are several checks worth working through before calling an engineer. Some of these can resolve the fault entirely. Others will at least give the engineer useful information when they arrive, which can shorten the diagnosis time and reduce your bill.

1. Check your programmer or thermostat settings. On a combi boiler, confirm that hot water isn't accidentally set to "off" or to a timer schedule that doesn't cover the times you're trying to use it. On a system with a cylinder, make sure the hot water programme is active and that the immersion or cylinder thermostat is set correctly.

2. Use the Voltrade GoFIX diagnostic tool. If you're not sure what fault you have, GoFIX walks you through your symptoms and boiler type to help identify whether you're looking at a simple settings issue or something that needs a Gas Safe engineer on-site. It can save you the cost of an unnecessary call-out.

3. Check for error codes. Most modern boilers from manufacturers like Vaillant, Worcester Bosch, and Glow-worm display a fault code on the front panel when something goes wrong. Note the code and look it up in your boiler manual - it often points directly at the component that's failing.

4. Check your boiler pressure. Low pressure won't usually cause heating to work and hot water to fail independently, but it can cause unpredictable behaviour. The pressure gauge should read between 1 and 1.5 bar when the system is cold. If it's reading below 1 bar, repressurise using the filling loop and check whether anything changes.

5. Test your hot water flow rate. Turn on a hot tap fully and note the strength of the flow. If it's weak even before the water warms up, the issue may be a blocked PHE or a failing flow sensor rather than a valve or electrical fault. A Gas Safe engineer can measure flow rates accurately with the right tools.

6. Book a service if you haven't had one this year. A boiler in Addlestone that hasn't been serviced since last winter is more likely to develop faults as demand increases through autumn. A standard service typically costs between 80 and 120 pounds and can catch diverter valve wear, scale build-up, and sensor drift before they become complete failures.

Emergency Signs - Do Not Wait on These

Most hot water faults aren't dangerous - frustrating, but not an urgent safety concern. There are some signs, however, that mean you should stop using the boiler immediately and contact a Gas Safe registered engineer without delay.

If you can smell gas near the boiler or anywhere in the property, leave immediately, don't switch any electrical items on or off, and call the National Gas Emergency Service on 0800 111 999. This is not a boiler repair situation - it's a gas leak and it requires immediate emergency response.

If the boiler is making a loud rumbling or banging noise (commonly called kettling), this typically indicates severe limescale build-up inside the heat exchanger. Left untreated, it causes progressive component damage and can eventually crack the heat exchanger itself - an expensive repair that often makes full boiler replacement the more economical option.

If you notice any water dripping or pooling near the boiler casing, call an engineer promptly. A leaking boiler can indicate a failed seal, a cracked internal component, or a pressure issue that could cause further damage to both the boiler and your property.

If your boiler is over 15 years old and developing its first serious fault, ask for an honest assessment on repair versus replacement. Older units from brands like Ideal and Potterton can sometimes be repaired cost-effectively, but parts availability varies and labour time on older boilers tends to be longer. A reputable Gas Safe engineer should give you a clear recommendation either way.

Preparing for the Next Season

Once your hot water fault has been repaired, there are a few additional steps worth taking while your engineer is still on-site - or soon afterwards - to reduce the chances of a similar issue next year.

Ask the engineer to check the inhibitor concentration in your central heating system. Inhibitor is a chemical treatment that prevents internal corrosion of your radiators and pipework. If it's been diluted or depleted, sludge (a dark, muddy residue called magnetite) builds up and accelerates wear on the pump and heat exchanger. Topping up inhibitor is a minor addition to your service bill but meaningfully extends the life of the system.

If you don't already have a magnetic filter fitted, now is a good time to add one. These devices sit on the return pipe and capture magnetite before it can circulate through the boiler. Products from Adey MagnaClean and Fernox TF1 are widely used and well-regarded. They typically cost between 80 and 150 pounds to supply and fit, and they can make a real difference to boiler longevity in older systems.

Finally, if your boiler is approaching 10 years old, consider a boiler health check this autumn rather than waiting for the next fault to emerge. This goes beyond a standard annual service and includes a detailed look at diverter valve condition, heat exchanger performance, flow rates, and flue integrity. It gives you a clearer picture of what the next couple of winters are likely to bring - useful information for Addlestone homeowners who'd rather plan ahead than deal with emergency call-outs in January.

Seasonal Questions About Hot Water Faults in Addlestone

Why does my hot water stop working when the heating comes on in autumn?

This is almost always a diverter valve issue on a combi boiler. The valve switches flow between your radiators and your hot taps, and when it partially sticks towards the heating circuit, the hot water side loses out. It's particularly common at the start of autumn when boilers resume regular heating use after a quieter summer. A Gas Safe engineer can test the valve and replace it if needed, which typically costs between 150 and 300 pounds including parts and labour.

Can I fix a diverter valve or heat exchanger fault myself?

No. Any work inside a gas boiler must be carried out by a Gas Safe registered engineer - this is a legal requirement in the UK, not a suggestion. Attempting to repair or replace internal boiler components without Gas Safe registration is illegal and will void any remaining warranty on your boiler. You can carry out useful diagnostic checks yourself - error codes, programmer settings, water pressure - but the repair itself must be done by a qualified professional.

How much does it typically cost to fix a boiler with no hot water in Surrey?

It depends on what's causing the fault. A diverter valve replacement typically costs between 150 and 300 pounds including parts and labour. A plate heat exchanger replacement is commonly between 200 and 400 pounds. A thermistor or sensor replacement is typically between 100 and 200 pounds. Gas Safe engineers in the Surrey area commonly charge between 60 and 100 pounds for a call-out, which is usually applied towards the repair cost if work goes ahead on the same visit.

Is it worth repairing an older boiler that's lost hot water, or should I replace it?

If the boiler is under 8 years old, repair is almost always the better choice financially. Between 8 and 12 years old, it depends heavily on the repair cost and the overall condition of the unit. If the repair quote comes to more than around half the cost of a comparable new boiler, or if the engineer flags several other components that are also showing wear, replacement is often more economical over a 3 to 5 year horizon. Ask your Gas Safe engineer for a clear, honest comparison - a good one will give you both options with realistic figures.