Viessmann Boiler No Hot Water: Causes and What to Check

Which Viessmann do you have?

Almost every Viessmann gas boiler in a London home is a Vitodens. The number on the front, such as 050-W, 100-W, 111-W or 200-W, tells you the range, and the data badge says whether it is a combi or a system boiler. That matters more than anything else on this page.

On a combi, the hot water is made inside the boiler as you open a tap, so no hot water is a boiler repair. The 111-W is a combi with a small hot water store built into the casing, so it behaves partly like a combi and partly like a boiler with a cylinder. On a system boiler, the boiler only heats the cylinder in the airing cupboard when it is asked, and the fault is often in the controls or the cylinder rather than the boiler.

Fault codes and the reset button

A Viessmann that has stopped itself shows a fault code on the display, usually with a warning symbol. Write the code down before pressing anything. It is the first thing an engineer asks for, and a reset clears it.

F4 is the code people most often search for. It generally means the boiler tried to light and did not see a flame, so check that the gas is on, that a prepayment meter has credit, and in freezing weather that the plastic condensate pipe outside has not frozen. Low water pressure in the heating system is the other common lockout, so check the pressure on the gauge or display. Around 1 to 1.5 bar with the boiler cold is typical for a sealed system.

Press the reset button once and give the boiler a couple of minutes. Our guide on how to reset a boiler explains why two attempts is the sensible limit. A code that comes straight back is the boiler protecting itself, and repeated resets can make a small fault worse.

Checks to make before booking anyone

These cost nothing and sort out a fair share of Viessmann no hot water calls.

• Pressure: low pressure stops the boiler firing for hot water as well as heating.
• Gas: light a hob, if you have one, to prove the supply.
• Hot water setting: make sure the hot water temperature has not been turned right down or the hot water mode switched off on the display.
• Timer: on a system boiler, check the hot water programme is on. A power cut can reset the clock.
• Tap flow: open the hot tap fully. A trickle may not be enough for a combi to fire.
• More than one tap: hot at one outlet but not another points at the tap or its valve, not the boiler.

Vitodens combi: heating works but no hot water

With no code and hot radiators, a Viessmann combi has almost always failed in one of three places. The diverter valve swings the boiler's heat from the radiators to the hot water when you open a tap. If it sticks on heating, you get hot radiators and cold taps. If it sticks partway, the radiators warm up in summer and the hot water never quite gets hot. Our guide to diverter valve replacement cost explains what that repair involves.

The flow sensor tells the boiler a tap is open. When it fails, the boiler stays silent as you run the hot tap but fires normally for the heating. A blocked filter on the cold inlet can do the same by cutting the flow below what the sensor needs.

The plate heat exchanger passes the heat into your tap water, and London water scales it up over time. Hot water that starts warm and fades, or a boiler that lights and cuts out while the tap runs, points here. Our guide to a blocked plate heat exchanger explains how it is confirmed and cleared. The same three parts fail on most makes, and our page on no hot water from a combi boiler covers them in more depth.

Vitodens 111-W: the built-in store

The 111-W keeps a small store of hot water inside the boiler, which is why it gives a better flow than a standard combi. That store has its own temperature sensor and its own loading circuit. If the sensor fails, or the loading side stops working, you can get no hot water, lukewarm water, or hot water that runs out far sooner than it used to while the heating carries on as normal.

The store can also scale up in hard water areas, which slowly cuts the amount of hot water it holds. If your 111-W used to fill a bath and now does not, that is worth mentioning when you book, because it points the engineer at the store rather than the burner.

No hot water from a Viessmann system boiler

With a cylinder, the boiler may be fine and simply never asked to heat the water. Some Viessmann system boiler set-ups use a sensor in the cylinder wired back to the boiler, and the boiler switches its own internal valve over to heat the cylinder. Others use a cylinder thermostat and a separate motorised valve, as on most British systems. Either way, the call for hot water passes through several links, and any one of them failing gives you heating and no hot water.

If the cylinder has an immersion heater, switching it on is a useful test. If that gives you hot water, the cylinder is fine and the fault is on the boiler or controls side. Our guide to no hot water but the heating works goes through the checks on both vented and unvented cylinders.

Repair or replace a Viessmann?

Viessmann boilers are well built, and most hot water faults on them are ordinary repairs to a valve, sensor or heat exchanger. The practical difference from the most common brands is that fewer engineers carry Viessmann parts on the van, so a part may need ordering. That is a reason to book an engineer who knows the range, not a reason to replace a boiler that is otherwise sound.

Replacement starts to make sense when the boiler is old, the same fault keeps returning because scale or sludge has not been dealt with, or a major part is no longer available. A boiler diagnosis visit gives you that answer with evidence rather than a sales pitch.

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. If you smell gas, turn it off at the meter, open the 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 a fault code returns after a reset, the pressure keeps dropping after you top it up, the boiler or the pipes beneath it are dripping, or the taps still run cold after the checks above. Tell us the model on the front and any code on the display so we can bring the right parts. We have 20 engineers across London, and our rate is on the pricing page, agreed before anyone attends.