Vaillant Boiler No Hot Water: Causes and What to Check

Combi, system or regular: which Vaillant do you have?

Vaillant's ecoTEC range comes in all three types, and they look very similar on the wall. The model name on the front or the data badge underneath says which one you have. An ecoTEC plus or ecoTEC pro combi heats mains water as you use it, so there is no hot water cylinder in the property. A system or regular boiler heats a stored cylinder, often a Vaillant uniSTOR in newer installs, and the taps draw from that.

The quickest test is to look for a cylinder in an airing cupboard or loft. If there is one, the boiler is not a combi. That split matters: a combi with no hot water is a boiler repair, while a system boiler with no hot water is very often a controls or cylinder fault with the boiler itself in good order.

Read the display before you press anything

Vaillant boilers report two kinds of code. Codes starting with S are status codes, telling you what the boiler is doing at that moment, such as waiting for a demand or running for hot water. They are not faults. Codes starting with F are faults, and the boiler has stopped until the cause is dealt with or it is reset.

Write down any F code before resetting, because a reset clears it and it is the most useful thing you can give an engineer. F.22 points at low water pressure in the heating circuit. F.28 is a failure to light, and our page on the Vaillant F28 fault code goes through the causes. F.29 is a flame that lit and then went out, covered in our guide to Vaillant F29. Press the reset button once or twice at most. If the code comes back, stop, because the boiler is protecting itself from something a reset will not cure.

Checks worth making before you call

A surprising number of no hot water calls end at one of these.

• Pressure gauge: with the boiler cold, most sealed systems sit around 1 to 1.5 bar. Well below 1 bar and the boiler may refuse to fire. Topping up through the filling loop is in the user manual for your model.
• Gas supply: light a hob if you have one. No gas there means a supply or prepayment meter issue, not the boiler.
• Hot water temperature: check the hot water setting on the boiler or your Vaillant control has not been turned right down or switched off.
• Programmer: on a system or regular boiler, check the hot water channel is on, not just the heating. A power cut can reset the clock and leave the hot water off.
• Condensate pipe: in a hard frost, check the white plastic pipe outside. A frozen condensate stops the boiler, and warm, not boiling, water thaws it.
• Several taps: hot at the bath but cold at the kitchen means a tap or local valve problem, not the boiler.

Vaillant combi: heating works but the taps run cold

This is the most common Vaillant hot water fault, and it lives in one of three places. The diverter valve moves the boiler's heat from the radiators to the hot water when you open a tap. If it sticks in the heating position, the radiators get hot and the taps stay cold. If it sticks partly across, you can get warm radiators in summer and lukewarm water at the tap. Our guide to diverter valve replacement cost explains what that repair involves.

The second is the hot water flow sensor on the cold inlet, which tells the boiler a tap is open. If it stops registering, the boiler never knows you want hot water and stays silent when you open a tap, but fires normally for the heating. A small filter on the cold inlet can also block with debris after mains works in the street, cutting the flow below what the sensor needs.

The third is the plate heat exchanger, where the heating water passes its heat to the tap water. London's hard water scales it up until it can barely pass heat. The signs are water that starts warm and never gets properly hot, or a boiler that fires and cuts out while the tap runs. The same three parts fail on every make of combi, and our guide to no hot water from a combi boiler covers them in more depth. If you are comparing makes, the same faults on a Worcester are in our Worcester boiler no hot water guide.

Hot water takes ages to arrive on an ecoTEC combi

If the hot water does come, but only after a long wait, check whether your combi has a preheat or warm start setting and whether it has been switched off. With preheat on, the boiler keeps its plate heat exchanger warm so hot water arrives quickly. With it off, which is often called eco mode, the boiler saves a little gas but the first hot water takes longer. The user manual for your model shows how to check it. A long wait with preheat on, or a delay that has crept in over months, points back at scale or the flow sensor.

No hot water from a Vaillant system or regular boiler

With a cylinder, the boiler is often perfectly healthy and simply never being asked to heat the water. The request runs from the programmer or Vaillant control to the cylinder thermostat or sensor, then to a motorised valve that opens the route to the cylinder coil and tells the boiler to fire. A break anywhere in that chain gives you heating but no hot water.

The motorised valve is the usual culprit, with a motor or switch that fails with age. The cylinder sensor or thermostat can fail or be knocked down. If the cylinder has an immersion heater, switching it on is a quick test: hot water from the immersion proves the cylinder is fine and the fault is on the boiler side. On a mains pressure uniSTOR or other unvented cylinder, our guide to no hot water but the heating works covers the valve and thermostat checks in more detail, and our Vaillant uniSTOR cylinders page covers the cylinder itself.

What we do on the visit

We start with what the boiler reports about itself, including its fault history, then work through the hot water circuit in order rather than swapping parts on a hunch. On a combi that means proving the flow sensor sees a tap opening, checking the diverter moves across, and reading temperatures either side of the plate heat exchanger. On a system boiler it means testing the call for heat, the motorised valve and the cylinder sensor.

Most of these calls end in a repair. Replacing the boiler becomes the honest advice when it is old, parts are hard to get, or the same part keeps failing because scale or sludge has not been dealt with. A boiler diagnosis visit answers that with evidence.

Taking the case off a gas boiler is gas work and must be done by a Gas Safe registered engineer. That includes the diverter valve, the flow sensor and the plate heat exchanger. Check the card of whoever turns up. If you smell gas, turn it off at the meter, open windows and call the gas emergency line on 0800 111 999.

When to call us

Call us on 020 4652 5111 or send an enquiry if an F code comes back after a reset, if the pressure keeps dropping after you top it up, if water is dripping from the boiler, or if you have made the checks above and the taps still run cold. Give us the model name from the front of the boiler and any code on the display. We have 20 engineers across London, and our rate is on the pricing page, agreed before anyone attends.