Vaillant Boiler: Heating Works, No Hot Water

How to read what your Vaillant is doing

Every ecoTEC can show a live status code alongside any fault code. On older ecoTEC plus and pro models with a small two-digit screen, press the i button once and the display changes to a code starting with S. Press i again to return to normal. On newer ecoTEC models with a text display, the same information sits in the menu under the live monitor. Your user manual names the exact steps for your model.

Status codes are not faults. They are the boiler describing each step it is taking. The three that matter for hot water are these.

• S.10: hot water demand, meaning the impeller sensor on the cold inlet has seen water moving.
• S.14: hot water mode with the burner lit.
• S.31: summer mode, or no heating request from the controls. This is normal when the heating is off.

Reading the display is safe. Anything that needs the front panel off, including the diverter valve and flow sensor, is work for a Gas Safe registered engineer.

The tap test, Vaillant style

Turn the heating down at the room thermostat so the boiler is idle, then bring up the status display. Open the kitchen hot tap fully and watch for about a minute. Use a plain tap rather than a mixer shower, which can hold the flow too low to register.

If the code never changes to S.10, the boiler has not detected the tap. The impeller or Hall-effect flow sensor has seized, often with scale, or the small filter at the cold water inlet is blocked. If the cold tap is also weak, the supply is the problem, not the boiler.

If you see S.10 but it never moves on to S.14, the boiler knows about the tap but is not lighting. Look for a fault code such as F.28, the Vaillant ignition lockout. Since the heating works this is less common, so note any code and leave the rest to the engineer.

If S.14 shows and the burner is running but the tap stays cold or lukewarm, the heat is going somewhere other than your tap. If the radiators warm up during the test, the diverter valve is stuck on the heating side. If they stay cold, the plate heat exchanger is most likely scaled.

Settings that look like a fault

Vaillant combis have a hot water temperature dial or setting. Check it has not been turned down to its lowest stop, which can leave the water barely warm. Many ecoTEC combis also have a warm start (preheat) function, which keeps the plate heat exchanger topped up with heat so water arrives hot sooner. Turning it off will not stop hot water, it just means a slightly longer wait, so do not chase it as the cause.

Smart and wireless controls such as vSMART, sensoCOMFORT or a third-party thermostat can have their own hot water schedule or holiday mode. If the boiler shows S.31 and responds to a tap test normally, check those controls before booking anyone.

If the display shows F.22, the heating pressure has dropped too low for the boiler to run at all; see Vaillant boiler pressure for topping up. After a fault has cleared, our guide to a Vaillant boiler reset explains when resetting is and is not appropriate.

What gets replaced, and typical costs

On most ecoTEC combis the diverter valve is a motor head clipped onto a valve cartridge in the hydraulic block at the bottom of the boiler. Sometimes only the motor has failed, which is quicker and cheaper; sometimes the cartridge has seized too. The flow sensor sits in the same block on the cold inlet. The plate heat exchanger is screwed to the same block, so all three parts are reached from the front once the boiler is isolated and drained down.

Typical market prices, parts and fitting together, run roughly £150 to £300 for a flow sensor, £200 to £450 for a diverter motor or cartridge, and £300 to £650 for a plate heat exchanger. Descaling the old plate is possible when it is not cracked, and costs less.

If your Vaillant is still under guarantee, check the conditions before anyone else works on it, since guarantees usually need an annual service record. Otherwise our Vaillant boiler repair visit starts with the £60 call-out at any hour, and the repair price is agreed with you on site before any work starts.

Is it urgent?

With working heating and no leak, a Vaillant with no hot water is inconvenient rather than dangerous. It becomes pressing with young children, anyone unwell, or tenants waiting on a landlord. If water is dripping from the underside of the boiler at the same time, the diverter cartridge seal or the plate heat exchanger gasket may be leaking; turn the boiler off and treat it as a boiler leak.

While you wait, write down every code the display showed during your tap test, including the status codes. An engineer who knows the boiler never reached S.10, or sat on S.14 with the radiators warming, can bring the likely part rather than diagnosing first and returning later. That one note can save a second visit.

For the general version of this problem on any make, see heating on but no hot water, or our no hot water repair page for how a visit works.