ASHP Hot Water in Summer: Legionella Cycles, DHW Priority, and Efficiency Tips

TL;DR:

• Heat pumps heat domestic hot water differently to boilers: they typically heat to 45-50°C rather than 60°C+, which affects DHW priority mode settings and legionella risk
• Weekly legionella protection cycles (heating the cylinder to 60°C+) are mandatory but usually use an immersion heater — plan these for off-peak tariff hours
• In summer, your ASHP can be more efficient heating DHW than in winter, but DHW priority mode can interrupt space heating unnecessarily — adjust your settings seasonally

If you have an air source heat pump, domestic hot water (DHW) is one of the settings most people never touch after installation. And for most of the year, that’s fine. But understanding what’s happening — and making a few seasonal adjustments — can meaningfully improve efficiency and reduce your bills.

How ASHPs Approach Hot Water Differently

A gas boiler fires hot combustion gases through a heat exchanger and can raise water to 70°C or more almost instantly. Your heat pump can’t do that. It transfers heat from outdoor air to refrigerant to water, and it does this most efficiently at lower temperature lifts — typically heating water to 45-50°C in DHW mode.

That’s hot enough for bathing, washing up, and showering. Standard guidance says 45°C at the tap is sufficient for domestic use. But it’s below the 60°C threshold recommended by the Health and Safety Executive for legionella control — which is where the legionella protection cycle comes in.

Legionella Protection Cycles: What They Are and When to Schedule Them

Legionella bacteria can multiply in water stored between 20-45°C. To prevent this, heat pumps are programmed to run a weekly “pasteurisation” cycle that heats the cylinder to 60°C or above and holds it there long enough to kill bacteria (typically 1 hour at 60°C, or shorter at higher temperatures).

The problem: your heat pump struggles to hit 60°C efficiently. Most ASHPs use their built-in immersion heater to top up the cylinder temperature during this cycle, effectively using electric resistance heating — which is far less efficient than the heat pump’s normal operating mode (COP around 1.0 vs 2.5-4.0 for the heat pump itself).

This matters for your bills. If you’re on a smart tariff like Octopus Agile or Go, schedule legionella cycles during cheap overnight periods (01:00-05:00 in the UK) rather than during daytime peak rates. On most Vaillant, Daikin, and Mitsubishi heat pump controllers, you can set the exact time the legionella cycle runs.

How to check: Go to your heat pump controller’s DHW settings menu. Look for “DHW pasteurisation,” “anti-legionella,” or “legionella protection.” Set it to run at a fixed time during your cheapest electricity rate window.

DHW Priority Mode Explained

When your heat pump is running and the hot water cylinder needs a top-up, “DHW priority mode” tells the system to temporarily stop space heating and direct all heat pump capacity to the cylinder.

This makes sense in theory: you get hot water faster. In practice, during milder weather or summer, it can interrupt space heating runs unnecessarily — particularly for zones with underfloor heating, which needs steady heat input to maintain floor temperature.

Summer setting: In summer, you may not need space heating at all. In this case, DHW priority mode won’t interrupt anything useful. But if you have a heat pump that also provides cooling via a reversible cycle, DHW priority can interrupt cooling — worth checking your controller settings.

Shoulder season setting (spring/autumn): DHW priority on for 30-60 minutes maximum is usually a good default. If it’s set higher, a long DHW heating cycle can leave rooms cooler than you’d like.

Most installers leave the default at 30-60 minutes. If you’re finding your house goes cold during DHW heating events, reduce the maximum DHW priority time.

Summer Efficiency for Hot Water

Here’s the part many heat pump owners don’t realise: ASHPs are often more efficient at heating hot water in summer than in winter.

The reason is the temperature lift required. If the outdoor air is 20°C and you’re heating water to 48°C, that’s a lift of 28°C. In winter at 0°C, the same target temperature requires a lift of 48°C. Smaller lifts mean better COP — your heat pump uses less electricity to move the same amount of heat.

During a UK summer, your ASHP heating domestic hot water might achieve COP 3.5-4.5. In winter, the same task might achieve COP 2.0-2.5 with the immersion topping up the deficit.

Practical implication: In summer, you can often reduce how frequently you need to use the immersion heater for regular DHW top-ups. Make sure your DHW setpoint (the temperature you’re maintaining in the cylinder) is set by the heat pump directly rather than relying on immersion top-ups. Most heat pump controllers have a “DHW setpoint” temperature — 45-48°C is typical. If yours is set to 55°C+, the heat pump will struggle to reach it without immersion backup, wasting electricity.

Cylinder Temperature and Hot Water Running Out

One complaint sometimes heard from ASHP owners: “we run out of hot water.” Usually, this isn’t a heat pump problem — it’s a cylinder sizing problem, or the DHW setpoint is set too low.

A 200-litre cylinder at 48°C contains roughly the same usable hot water as a 200-litre cylinder at 55°C, because at 48°C you’re using more of it at closer to actual temperature rather than mixing with cold water. But if your family showers sequentially in the morning, you may simply need the cylinder to hold more heat.

Options:

• Increase DHW setpoint to 50-52°C (the heat pump can usually reach this without immersion help at most outdoor temperatures)
• Schedule a morning top-up in your DHW programme — most controllers let you set multiple DHW heating windows per day
• If you have solar PV, route surplus solar generation to your immersion heater in summer using a power diverter (Eddi, iBoost). This gives you essentially free hot water on good solar days and keeps the cylinder topped up

Settings Worth Reviewing Seasonally

These aren’t universal — every installation is different depending on your household’s hot water demand, cylinder size, and tariff. But they’re a starting point for reviewing your controller settings when the seasons change. Most manufacturers publish app guides; Vaillant’s myVAILLANT app, Mitsubishi’s MELCloud, and Daikin’s Daikin Residential Controller all let you adjust these settings remotely once commissioned.