Why does my Vaillant heat pump stop heating when hot water is on?

Because the system runs in hot water priority mode. When a reheat starts, space heating pauses until the cylinder reaches its target temperature.

Why is my hot water temperature higher than the target I set?

That’s usually the anti-legionella cycle. It temporarily heats water above 60°C once a week for safety.

Why doesn’t my Vaillant heat pump reheat hot water when scheduled?

Hysteresis probably hasn’t been met. Even within a schedule, the reheat won’t start until the temperature difference (target minus probe reading) is greater than your hysteresis value.

What should I set Cylinder Charging Hysteresis to?

A good starting point is 7K. Increase it if your probe is low in the cylinder or you want fewer, longer reheats. Avoid setting it too low or you’ll get inefficient top-ups.

What’s the best hot water temperature for a Vaillant Arotherm heat pump?

Around 50°C works for most homes. It’s a balance between usable hot water and system efficiency. Depending on your probe location, the actual water at the top of the cylinder will likely be hotter.

Does Cylinder Charging Offset do anything?

No. On a heat pump, that setting has no effect, it is a leftover from gas boiler control software.

Can I automate hot water reheats with a smart tariff?

Yes. You can set schedules for off-peak times or use something like Havenwise to automate it around your tariff windows.

Is it better to use a Mixergy smart cylinder?

Mixergy gives more control and visibility. You can see exactly how much hot water you have and its temperature without guessing from probe placement.

Vaillant Arotherm Plus Legionella Cycle

Introduction

The Vaillant Arotherm Plus legionella cycle is a built-in safety feature that periodically heats your hot water above 60°C to reduce the risk of legionella bacteria.

If you ever notice your cylinder temperature going higher than your normal setpoint, this weekly anti-legionella run is usually the reason.

It can be scheduled to run on a specific day and time, or disabled if it is not needed, with sensible timing helping to avoid hot water clashes and higher electricity costs.

This article is part of my wider series of Vaillant Arotherm guides, aiming to demystify the settings that matter most for efficiency and comfort.

Vaillant Arotherm Heat Pump Guide & User Manual

And this article is an excerpt from the Vaillant Arotherm Basic Hot Water Settings article.

Vaillant Arotherm Basic Hot Water Settings

Vaillant Arotherm Plus Legionella Cycle

If you notice that your hot water temperature sometimes reads higher than your chosen target, it could be because of the Vaillant Aotherm plus legionella cycle.

The legionella cycle frequency can be scheduled once a week to take your hot water beyond 60°C using the heat pump.

You can choose the day in this menu item, or turn it off if not required:

Settings → Installer Level → 00 → Installation Configuration → Domestic Hot Water → Anti-legio. day

This Heat Geek article on legionella is worth a read: “Legionella and Water Temperature, What You Need to Know“.

If you decide you do need to run anti-legionella protection, you will also need to choose a time of the chosen day to complete this:

Settings → Installer Level → 00 → Installation Configuration → Domestic Hot Water → Anti-legio. time

Pick a day and time when there is little or no hot water demand, such as overnight. This avoids running the cycle when you might need hot water and also lets you take advantage of cheaper off-peak electricity if you have a time-of-use tariff.

Note: If the Absence/Holiday function is activated on the heat pump, the anti-legionella function is not carried out. But as soon as the Absence feature is switched off, the anti-legionella function will run.

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