Vaillant boiler with no hot water

What it usually means

Vaillant give you more information than most manufacturers, and using it properly is the fastest route through this fault. An ecoTEC display shows two kinds of message. Codes beginning with F are faults and the boiler has stopped. Codes beginning with S are statuses, describing what the appliance is doing at that moment during entirely normal operation. Confusing the two sends people to the phone when the boiler is working as designed.

One status in particular explains a common version of this complaint. There is a deliberate waiting period after the boiler has been running, during which it will not fire again immediately, and Vaillant publish it as a status rather than a fault. If your hot water goes cold partway through, or the boiler refuses to relight straight after the heating has been on, look at the display before assuming a component has failed — an appliance in a documented pause is not a broken one.

The next Vaillant particular is how the boiler knows you have opened a tap. On these appliances hot water demand is sensed by water actually moving through the boiler, and if that sensing element sticks — which it does, with scale or debris, and more often on older units — the boiler never learns that anybody wants hot water. Everything else works, the heating is faultless, and no amount of turning taps produces a response. It is one of the more satisfying faults to find because the symptom is so absolute and the part is small.

Then there is the code that has been misdiagnosed more than any other on this platform. Vaillant's pressure-and-pump fault appears when the boiler cannot see the pressure change it expects when the pump starts, and it stops the appliance doing anything, hot water included. It gets blamed on the pump, on the sensor and on the board in roughly equal measure, and the honest position is that it needs someone who knows the platform rather than someone replacing parts in order. If that is the code on your display, say so when you book.

Vaillant also have their own preheat arrangement, which changes how quickly water arrives without changing whether it arrives at all. With it enabled the boiler holds itself ready and hot water is quick; with it off you wait longer at the tap. A household that has recently had a service, a new control fitted or a power cut may find the setting has moved, and the resulting delay reads exactly like a fault.

After all of that, the ordinary mechanics apply as they do to any combi: a diverter valve stuck towards the heating circuit gives hot water only while the heating runs, a scaled plate heat exchanger gives a gradual decline over years rather than a sudden stop, and low system pressure stops everything. And if your Vaillant feeds a cylinder rather than being a combi, the hot water is made outside the appliance, so the cylinder thermostat and the motorised valve come first.