No Hot Water but the Heating Works: What It Usually Means

Radiators warm, taps cold. It is a frustrating fault, but a useful one, because it tells you the boiler can light, the gas is on and water is moving round the heating. The problem is almost always in whatever sends heat to your hot water, and which part that is depends on whether you have a combi or a cylinder.

Why would the heating work but not the hot water?

Because heating and hot water share the same burner but take different routes. If the radiators get hot, the burner, gas supply and pump are working. The fault is in the part that switches heat over to the hot water side, or in the controls that ask for hot water in the first place.

Is it the diverter valve on a combi?

Often, yes. A combi's diverter valve decides where the hot water from the heat exchanger goes: round the radiators or through the plate heat exchanger to your taps. If it sticks in the heating position, the radiators stay warm but the taps never get hot. A sign of a sticking valve is radiators warming up whenever you run the hot tap.

Diverter valves are a very common combi repair. You can see how the job is approached in our report on a diverter valve replacement on a combi in Kidsgrove. Depending on the boiler, the fix may be a replacement actuator head, the valve cartridge, or the full valve.

What else stops a combi making hot water?

Apart from the diverter valve, the usual causes are a flow sensor that does not detect the tap being opened, a plate heat exchanger clogged with limescale, a hot water temperature set too low, and mains water flow too weak to trigger the boiler. Each produces slightly different symptoms.

• Flow sensor. A combi only fires for hot water when it senses water moving. If the sensor sticks or fails, the boiler never knows you have opened a tap.
• Plate heat exchanger. In hard water areas like Stoke-on-Trent, scale builds up on the hot water side. Typical signs are lukewarm water, temperature swinging up and down, or a noisy boiler on hot water.
• Low mains flow. A combi needs a minimum flow to start. A partly closed stopcock or a blocked inlet filter can drop you below it.
• Settings. The hot water dial or a preheat or eco setting may have been changed.

We cover the effect of local water in more detail in our hard water tips for Stoke-on-Trent homes.

What about system and regular boilers with a cylinder?

With a hot water cylinder, the first things to check are the programmer and the cylinder thermostat. If the hot water channel is set to off, or the cylinder thermostat is turned down, the boiler will heat the radiators and ignore the cylinder. If controls are fine, the motorised zone valve for hot water is the next suspect.

On a two-zone setup, a separate valve opens for the cylinder. If its motor fails, the valve stays shut and the cylinder stays cold. On older systems with a single three-port valve, a sticking valve can cause heating and hot water to behave oddly together. Many cylinders have an immersion heater, which can keep you in hot water while you wait for a repair.

What can you check yourself first?

Check settings, controls and the water supply before booking a repair. These checks take a few minutes and sometimes solve the problem outright.

1. Run a hot tap fully open, not just a trickle, and see if the boiler fires.
2. Check the hot water temperature setting on the boiler or controller.
3. On a cylinder system, check the hot water timer channel and the cylinder thermostat dial.
4. Make sure the stopcock is fully open and cold mains flow looks normal.
5. Look for a fault code and check it in the manual.

A frozen condensate pipe is very unlikely at this time of year, and it would stop the heating as well, so it can usually be ruled out in late spring and summer.

Is it worth repairing, or is it time for a new boiler?

On most boilers, a diverter valve, flow sensor or zone valve repair is well worth doing. These are common parts and are usually available for mainstream makes. The picture changes on an older boiler that already has other faults, where the engineer should give you an honest comparison.

Getting your hot water back in Stoke-on-Trent

Our boiler repair visits are £90 per hour for diagnosis and repair, and the engineer will confirm what has failed and what it will cost before replacing anything. Many common diverter and sensor parts can be sourced quickly for mainstream boilers.

We cover Stoke-on-Trent and the surrounding towns in Staffordshire, south Cheshire and Derbyshire, including boiler service in Nantwich and boiler service in Trentham. Call 07896 677083 to book.