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

Published October 2026 | Boiler Repair

This checklist covers every practical check for the situation where your radiators heat up fine but your taps and shower run cold - one of the most common boiler complaints our engineers see across Bath and the wider Somerset area. Working through these checks regularly means you'll spot faults early, and that typically saves you from a much more expensive emergency call-out when a small problem develops into a serious one.

Quick Visual Checks Anyone Can Do

When heating works but hot water doesn't, the fault is almost always in the part of your boiler that handles hot water on demand, not the heating side. On a combi boiler - which is what most Bath properties run on - that typically points to a stuck diverter valve, a blocked plate heat exchanger, a faulty flow sensor, or a pressure issue. Before calling anyone, work through these checks first.

1. Look at your boiler display for fault codes. Most modern boilers - whether you have a Worcester Bosch, Ideal, or Vaillant - will show an error code when something's wrong. Check your manual or search the code online to see what it's pointing to.
2. Check the pressure gauge. Boiler pressure should sit between 1 and 1.5 bar when the system is cold. If it's dropped below 0.5 bar, low pressure can prevent hot water production even while heating carries on working normally.
3. Check your programmer or thermostat settings. The hot water schedule may have been switched off accidentally - particularly after a power cut, which can reset the boiler's internal clock. Make sure hot water is programmed on for the times you need it.
4. Check the hot water temperature dial. Many combi boilers have separate dials for heating and hot water. Check the hot water dial hasn't been turned right down, especially if anyone has been adjusting settings recently.
5. Check the pilot light on older boilers. If you have an older floor-standing boiler in a Bath period property, a blown pilot light will cut hot water production. You can usually relight it following the instructions on the casing, but if it won't stay lit, that's a job for a Gas Safe engineer.
6. Listen at the boiler when you open a hot tap. You should hear the boiler fire within a few seconds. If it fires for heating but stays completely silent when you run a hot tap, the diverter valve is likely stuck in heating mode.
7. Check whether the problem affects one tap or all of them. If only one tap is affected, the fault may be local - a worn mixer cartridge or faulty thermostatic bar valve rather than the boiler itself.

Monthly Maintenance Tasks

A few quick checks each month can catch problems before they turn into failures. None of these require tools or specialist knowledge - just a few minutes of attention.

• Top up boiler pressure if it's dropped. Check the gauge when the system is cold. If it's below 1 bar, use the filling loop to bring it back to around 1.2 bar. Pressure that keeps dropping month after month suggests a leak somewhere in the system and needs an engineer to investigate.
• Run your hot taps and check the flow rate. A gradual drop in hot water flow - particularly in older Bath properties with legacy pipework - can indicate limescale building up in the plate heat exchanger. Catching this early typically means a descale rather than a full component replacement.
• Check around the boiler for drips or moisture. Even a slow weep from a joint causes corrosion over time. A dry boiler casing is what you're looking for.
• In winter, check the condensate pipe. Condensing boilers have a plastic pipe that runs to an outside drain. In cold weather it can freeze and cause a lockout. You can carefully pour warm - not boiling - water over the pipe to thaw it without calling anyone out.
• Bleed radiators if any feel cold at the top. Airlocks can affect overall system performance, particularly on older open-vented setups still found in some Somerset homes. It takes about five minutes per radiator and can make a noticeable difference to how quickly your hot water comes through.

Annual Professional Checks You Should Book

Some checks can only be done properly by a Gas Safe registered engineer - and in the UK, Gas Safe registration is a legal requirement for anyone working on gas appliances. An annual boiler service typically costs between 70 and 120 pounds and is the single most important thing you can do to keep your hot water running reliably year-round.

During a full service, a Gas Safe engineer will typically carry out the following:

1. Visual inspection of the boiler and flue. Checking for corrosion, leaks, and confirming flue terminals are clear and correctly positioned.
2. Flue gas analysis. Using specialist equipment to check combustion efficiency and confirm the boiler is burning gas safely and cleanly.
3. Inspection and cleaning of the heat exchanger. Limescale and sludge build-up in the heat exchanger is one of the most common reasons for no hot water in Bath homes, particularly given the relatively hard water supply across parts of Somerset. Cleaning it at service time prevents a 150 pound descale from becoming a 400 to 600 pound replacement.
4. Testing diverter valve operation. A valve that's beginning to stick will often show up during testing before it fails completely. Catching it at this stage is considerably cheaper than an emergency call-out.
5. Checking the expansion vessel and pressure relief valve. A failed expansion vessel is a common reason for boiler pressure repeatedly dropping - something many homeowners mistake for a leak.
6. Inspecting the condensate trap and pipe. Clearing any partial blockages before they cause a full lockout in cold weather.
7. Testing all safety controls. Overheat stats, pressure switches, and gas valve operation are all checked to confirm the boiler will shut down safely if something goes wrong.

Our engineers use the Voltrade GoFIX diagnostic tool during service visits, which pinpoints faults in the control electronics quickly - particularly useful on modern combi boilers where fault codes can be cryptic without the right diagnostic equipment.

Warning Signs That Need Immediate Attention

Some symptoms mean you should stop using the boiler and call an engineer the same day. Don't put these off.

• A smell of gas near the boiler. Turn off the gas supply at the meter, open windows, don't use any electrical switches, and call the National Gas Emergency Service on 0800 111 999. Don't attempt any checks yourself - get out first.
• A yellow or orange flame instead of a blue one. If you can see the burner and it's not burning blue, the boiler may be producing carbon monoxide. Turn it off and call a Gas Safe engineer immediately.
• Banging, kettling, or clunking sounds. Kettling - that sound like a boiling kettle coming from inside the heat exchanger - commonly indicates limescale build-up. Left untreated it causes the heat exchanger to overheat and crack, turning a 150 pound descale into a 500 to 800 pound component replacement.
• Water pooling under or around the boiler. Any active leak from the boiler casing needs investigating. Turn off the boiler rather than leaving it running and dripping.
• The boiler keeps locking out. If your boiler repeatedly shuts itself off and displays the same error code, something is consistently tripping a safety switch. Resetting it each time without finding the root cause isn't safe and typically makes the underlying fault harder to fix.
• Your carbon monoxide alarm activates. Treat a CO alarm the same as a gas smell. Get out, open windows, and call the emergency line before calling anyone else.

Your Maintenance Schedule

Use this as a working calendar to keep your boiler and hot water system in good shape throughout the year.

Weekly

• Check the boiler display for fault codes or warning lights
• Run a hot tap briefly and check the temperature feels normal

Monthly

• Check boiler pressure gauge - should read 1 to 1.5 bar when cold
• Check for drips or moisture around the boiler casing
• Run hot water at each tap and note whether the flow rate has changed

September and October - before winter

• Book your annual boiler service if you haven't had one this year
• Check the condensate pipe route and insulate any exposed outdoor sections before the first frost
• Test your carbon monoxide alarm and replace the battery if it's been over a year
• Check your programmer is set correctly for the colder months - it's common to find the hot water timing hasn't been updated since spring

Annually

• Gas Safe boiler service, typically costing between 70 and 120 pounds
• Check the inhibitor level in the central heating system - a corrosion inhibitor such as Fernox F1 protects the heat exchanger and pump from sludge damage
• Consider a powerflush if radiators are slow to heat or have cold spots - typically costs between 300 and 600 pounds for an average Somerset property

Every 10 to 15 years

• Have the expansion vessel re-pressurised or replaced if the engineer advises it
• Discuss boiler replacement if repair costs are becoming a regular expense - most boilers have a working life of around 10 to 15 years, and a new A-rated unit will typically pay back its cost in lower running bills within a few years

Checklist Questions

Why does my combi boiler heat the radiators but not produce hot water?

On a combi boiler, the diverter valve switches the boiler's output between the central heating circuit and the domestic hot water circuit. If the fault is in the component that handles hot water on demand, heating can continue to work normally while hot water fails entirely. A stuck or worn diverter valve is the most common cause - it stays locked in heating mode and never redirects heat to your hot water. A Gas Safe engineer can test and replace it, which typically costs between 150 and 300 pounds including parts and labour in the Bath area.

Can I check the diverter valve myself?

How much does it cost to fix no hot water on a combi boiler in Bath?

Costs vary depending on the fault. A diverter valve replacement typically costs between 150 and 300 pounds. A new plate heat exchanger - needed when limescale damage means cleaning isn't enough - typically costs between 200 and 500 pounds including labour. A flow sensor replacement is usually at the lower end, around 80 to 150 pounds. Our engineers use the Voltrade GoFIX tool to pinpoint the fault accurately before recommending any repairs, so you're not paying for parts that don't need replacing.