Hot Water Not Hot Enough? Causes and Fixes for Combis

If your hot water isn't hot enough, start with the two things you can check yourself: the hot water temperature setting on the boiler, and how far the tap is open. A combi boiler heats water as it flows through, so it can only raise a certain amount of water by a certain number of degrees; open the tap too far and the water comes out warm rather than hot. If the setting is right and the water is still lukewarm at a gentle flow, the common faults are a plate heat exchanger clogged with limescale, a diverter valve that is sending some heat to the radiators, a faulty temperature sensor, or not enough gas reaching the boiler. This guide goes through them in the order that makes sense to check.

One tap or every tap?

This is the first question to answer, because it splits the problem in two.

• Only one shower or tap is lukewarm. The boiler is fine. The fault is in that fitting: a thermostatic shower cartridge that is worn, scaled or has its maximum temperature stop set too low, a mixer tap cartridge, or a thermostatic mixing valve on that outlet. In newer homes, the bath is usually fitted with a device that limits the hot water to 48°C, which Approved Document G requires in new dwellings, so a bath that runs cooler than the kitchen tap may be working as designed. Our guide to no hot water in the shower covers shower-only faults.
• Every hot tap is lukewarm. The problem is at the boiler or cylinder, or with the supply to it. Carry on below.

It also helps to notice whether the cold tap next to it is running warm. If hot water is crossing into the cold pipework through a faulty mixer, cold can also cross into hot, so both sides end up lukewarm. Our guide to hot water coming out of the cold tap explains how that happens. If one tap gives no hot water at all while the others are fine, see our guide to a hot water tap not working.

Check the boiler setting and the flow

The hot water setting. Most combis have two temperature controls: one for the radiators and one for the hot water. It's easy to turn down the wrong one, and a child or a visitor can knock a dial. Some boilers also have an eco or pre-heat mode that changes how quickly hot water arrives. Check the manual for your model and set the hot water to a sensible level. Our guide to what temperature your boiler should be set at explains the difference between the heating and hot water settings.

The flow rate. A combi heats water on demand, and its output is fixed by its size. As a rough guide, heating each litre per minute of water by 35°C takes about 2.4 kW. So a combi can only deliver so many litres per minute at a properly hot temperature before the water starts coming out cooler. In winter the mains water arrives colder, so the same boiler gives slightly cooler water at the same flow than it does in summer.

Test it: run the kitchen hot tap only half open for a minute. If it gets properly hot at half flow but goes lukewarm when fully open, or when someone runs a second tap, the boiler is doing what it can and is being asked for more than it can heat. That's a sizing issue rather than a fault. If the water is lukewarm even at half flow, something is wrong.

Hot, then cold after a few seconds

Water that goes hot, then cold, then hot again is a slightly different symptom.

• The flow is too low. A combi needs a minimum flow through it before it fires, and if the tap is only just open the boiler can switch on and off. Open the tap a little more and see if the temperature steadies. A thermostatic shower that restricts flow can cause the same thing.
• The boiler is shutting down. If it runs hot then cold at a normal flow, look at the boiler display while it happens. A fault code, a flame symbol going off or a pressure reading near zero all point to the boiler cutting out. Our guide to a boiler that keeps turning off covers the usual causes, including low pressure, a blocked condensate pipe in cold weather and overheating.

Boiler faults that cause lukewarm water

If the setting is right and the water is still lukewarm at a modest flow, one of these is the likely cause. All of them need a Gas Safe registered engineer, because they involve working inside the boiler.

Sign	Likely fault
Radiators get warm when you run a hot tap in summer	Diverter valve sticking, splitting heat between heating and hot water
Lukewarm water, boiler noisy or cutting out, hard-water area	Scaled plate heat exchanger
Temperature swings or reads wrong on the display	Faulty hot water temperature sensor (thermistor)
Lukewarm when the hob or another gas appliance is on	Not enough gas reaching the boiler
Hot water weak and slow as well as lukewarm	Blocked filter on the cold inlet, or low mains pressure

Diverter valve. In a combi, the diverter valve sends the boiler's heat either to the radiators or to the hot water. If it sticks part-way, some of the heat goes round the radiators while you run a tap, so the hot water is weaker and the radiators warm up in summer. It's a common repair on older combis.

Plate heat exchanger. This is where cold mains water picks up heat from the boiler. London's hard water leaves scale inside it, which cuts the heat transfer and makes the boiler work harder and sometimes kettle or cut out. It can sometimes be descaled, but on many boilers it's replaced. A scale reducer on the cold supply helps stop it coming back.

Temperature sensor. If the sensor on the hot water side reads wrong, the boiler thinks the water is hotter than it is and turns down the flame too early.

Gas supply. If the water goes cooler when the hob is on, or the flame looks weak, the gas pipe may be too small for the boiler, or the meter or regulator may be restricting it. That's a job for a Gas Safe engineer to test. If you smell gas at any point, turn off the gas at the meter and call the National Gas Emergency Service on 0800 111 999.

For a wider list of combi faults, see our guide to combi boiler problems.

If you have a hot water cylinder

With a system or regular boiler and a cylinder, lukewarm water usually means the cylinder hasn't been heated enough. Check that the programmer is set to heat hot water and that the cylinder thermostat, usually a dial strapped to the side of the cylinder, hasn't been turned down. If the heating works but the cylinder never gets hot, the motorised valve that sends heat to the cylinder may be stuck. Our guides to no hot water but the heating works and hot water running out quickly go through cylinder faults in more detail.

When to call an engineer

Call an engineer if every hot tap is lukewarm at a modest flow with the boiler set correctly, if the radiators warm up when you run hot water, if the boiler cuts out or shows a fault code while you run a tap, or if the water is getting gradually cooler over weeks, which usually means scale. Don't open the boiler casing yourself: the diverter valve, heat exchanger, sensors and gas valve are all inside the sealed case and must be worked on by a Gas Safe registered engineer.

We charge £95 an hour for labour, plus a £60 call-out that's waived when the job runs over 2 hours. Parts are extra and we confirm the price before any work starts. Our boiler repair service covers combi, system and regular boilers across London, and the pricing page explains how jobs are charged.

Frequently Asked Questions