In short
On a combi, heating without hot water usually points to the diverter valve, the hot water flow sensor or a scaled-up plate heat exchanger. On a cylinder system, check the hot water timer and cylinder thermostat first; a stuck motorised valve is a common cause. Do the simple checks, then call a Gas Safe engineer.
First, which system do I have?
- Combi: no hot water cylinder; the boiler heats water as you run a tap.
- System or regular boiler: a hot water cylinder in a cupboard, and with a regular boiler, tanks in the loft.
Combi boiler: heating but no hot water
Safe checks
These checks are safe for a homeowner. Anything on gas parts, electrodes, the diverter valve or inside the casing is for a Gas Safe engineer.
- Check the hot water temperature setting on the boiler has not been turned down or set to frost.
- Run a hot tap fully open. A combi needs a minimum flow of water before it fires for hot water.
- Check cold mains water at the kitchen tap. If the mains is off or very weak, the combi cannot heat hot water.
- Look for a fault code on the boiler display, write it down and look it up in our fault code search.
- In freezing weather, check the condensate pipe outside. A frozen pipe locks the boiler out for heating and hot water. See how to thaw a frozen condensate pipe.
- If the simple checks fail, stop and book a Gas Safe engineer. Do not open the boiler casing or work on gas parts.
Likely causes
- Diverter valve: switches the boiler between heating and hot water. If it sticks on heating, the taps stay cold.
- Flow sensor: tells the boiler a hot tap is open. If it fails or is blocked, the boiler does not respond.
- Plate heat exchanger: heats the tap water. Limescale can block it, giving lukewarm or fluctuating hot water.
Cylinder system: heating but no hot water
Safe checks
- Check the programmer has hot water switched on, and the times and clock are right.
- Check the cylinder thermostat, usually strapped to the side of the cylinder, is not turned down.
- If you have an immersion heater, it can be a useful backup while you wait for a repair.
Likely causes
- Motorised valve: opens to send hot water from the boiler to the cylinder. A failed valve motor is a common fault.
- Cylinder thermostat or programmer: not calling for hot water.
- Wiring or the boiler: less often, a fault in the wiring centre or the boiler itself.
What if I have hot water but no heating?
That is the reverse problem: check the programmer, the room thermostat and its batteries, and that radiator valves are open. On a combi, a stuck diverter valve can also cause it. Our heating controls questions go through it.
What CombiCert would check
We confirm whether you have a combi or a cylinder system, check timers and thermostats, and then test the parts that split heating from hot water — diverter valve and sensors on a combi, or the motorised valve and cylinder controls on a stored system. Repairs from £90 + VAT.
When to call a Gas Safe engineer
Diverter valves, flow sensors and plate heat exchangers are inside the sealed boiler case, so only a Gas Safe registered engineer may replace them. Valves and controls on a cylinder system should be left to a heating engineer. Book a boiler repair and tell us the make, model and any code.
Need a hand? CombiCert in Telford
CombiCert Ltd is a Gas Safe registered (621146) gas and heating company based at The Old School House, Park Lane, Telford TF3 4TG, working across Telford and Shropshire for homes and businesses. We are open Monday to Saturday, 08:00 to 19:45 (closed Sunday).
Boiler repairs and services from £90 + VAT. Commercial work by quote.
See Boiler repairs in Telford, or call or WhatsApp 07393 220694, or email hello@combicert.co.uk.
We cover Telford and nearby, including Wellington, Madeley, Dawley, Oakengates, Shifnal, Newport, Ironbridge and Shrewsbury.
Sources
Frequently asked questions
Why does my combi give heating but no hot water?
The most common causes are a stuck diverter valve, a failed or blocked hot water flow sensor, or limescale in the plate heat exchanger. Check the hot water setting, run a tap fully open and note any fault code. The parts are inside the boiler, so a Gas Safe registered engineer needs to replace them.
Why has my cylinder stopped heating water?
Check the programmer has hot water switched on and the cylinder thermostat is not turned down. If both are right, the motorised valve that sends hot water to the cylinder may have failed, which is a common fault. An immersion heater, if you have one, can heat water while you wait for the repair.
Why is my hot water lukewarm on a combi?
Lukewarm or fluctuating hot water on a combi often comes from limescale in the plate heat exchanger, a low flow at the tap or a faulty sensor. Check whether it happens at every tap. If it does, book a Gas Safe engineer. A scale reducer on the mains feed can help in hard water areas.
Can a frozen condensate pipe stop my hot water?
Yes. If the condensate pipe freezes, a condensing boiler locks out and stops both heating and hot water. In freezing weather, check the plastic pipe outside, thaw it with warm, not boiling, water and reset the boiler once. If it keeps freezing, ask an engineer to look at the pipe route and size.
This answer is general information, not a substitute for an on-site assessment by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.