If your combi boiler heats the radiators but gives no hot water at the taps, the most likely cause is a sticking or failed diverter valve, which switches the boiler between heating and hot water. Limescale in the hot water heat exchanger, which is common in hard-water areas like East Kent, can also cause lukewarm or fluctuating hot water. Both need a Gas Safe engineer to diagnose and repair.
What the diverter valve does
A combi boiler heats your radiators and your hot water, but not both at once. When you open a hot tap, the diverter valve moves to send the boiler’s heat to the hot water instead of the radiators. If it sticks, you may get hot radiators and cold taps, or radiators that warm up every time you run a tap.
Why hard water matters in East Kent
Most of East Kent has very hard water. As it is heated, limescale builds up inside the boiler’s hot water heat exchanger. Over time, that restricts the flow and stops the heat passing through, so the water comes out lukewarm, swings between hot and cold, or the boiler gets noisy when a tap is running.
Other causes
- Low water flow from the mains. A combi needs a minimum flow to fire for hot water. If your cold taps are weak too, see low water pressure at the taps.
- A faulty flow sensor, which tells the boiler a tap is open.
- A thermostat or temperature sensor fault.
Check the hot water temperature setting on the boiler hasn’t been turned down, then call an engineer if that isn’t it.
Covering Faversham, Canterbury, Whitstable, Herne Bay, Thanet, Sandwich, Deal and Dover: see areas we cover.
Get it sorted
Call or WhatsApp Addisons Plumbing & Heating with a quick description or a photo, and we’ll tell you what it looks like and arrange a visit. Free, no-obligation quotes across East Kent. If you ever smell gas, call the National Gas Emergency number on 0800 111 999.
Quick questions
Can I fix a diverter valve myself?
No. It is inside the boiler, and only a Gas Safe registered engineer should open the casing.
Why does my hot water go hot then cold?
Common causes are a combi boiler struggling with low mains flow, limescale in the heat exchanger, or a sensor fault. An engineer can test each one.
Is it worth repairing or replacing my combi?
If the boiler is under about 10 years old and otherwise reliable, a repair is usually worth it. If it is older and breaking down often, ask us for a replacement quote to compare.
