Heat pump hot water: when to heat your cylinder (and when not to)

How a heat pump heats your hot water

A gas boiler connected to a cylinder can drive 20–30 kW of heat into the water, recovering a 180-litre tank in around 30 minutes. A heat pump works at lower power — typically 6–12 kW of useful heat output in domestic hot water (DHW) mode — and takes longer. A 10 kW heat pump input heats a 250-litre cylinder from cold in around 75 minutes. A smaller 6 kW unit on a 200-litre cylinder will take 90 minutes or more in cold weather when efficiency drops.

This slower recovery has two practical implications:

1. The cheap window needs to be long enough. A two-hour off-peak window may not recover a fully cold cylinder. Plan for at least 90 minutes of heat pump run-time, with margin.
2. You need a cylinder sized for the gap. If your cylinder only holds three hours of stored hot water and there's a six-hour gap between cheap windows, you may run out. Most heat pump installations use 180–250 litre cylinders for this reason.

Why timing matters more than on a boiler

On a flat-rate tariff, when you heat your cylinder is irrelevant — the cost is the same at 3 am as at 6 pm. On a time-of-use tariff, the difference can be dramatic.

Using April 2026 rates as an example: on Cosy Octopus, heating your cylinder during a cheap window costs around 14.5p/kWh. Doing the same at the 4pm–7pm peak costs 51.7p/kWh — 3.5 times as much. A household that heats 1,200 kWh of hot water per year (typical for a three-person home) saves around £120–£130/year simply by ensuring that 1,200 kWh happens at the cheap rate rather than the peak.

That's before any space heating scheduling. Hot water alone is worth the effort of setting a schedule.

When to schedule your hot water

Matching your tariff's cheap windows

The simplest setup for most households is a single overnight schedule: heat the cylinder fully during the cheapest overnight window and rely on that stored heat through the day. For a 200-litre cylinder and typical occupancy, this is usually enough.

If you have a midday cheap window (Cosy, British Gas), a short top-up schedule at 13:30–14:30 adds resilience on heavy-use days without adding significant cost.

When to schedule to avoid

On Cosy Octopus, never run a hot water cycle between 16:00 and 19:00. The peak rate of ~51.7p/kWh means 90 minutes of cylinder heating at this time costs roughly three times what the same cycle costs in the 04:00–07:00 window.

On EDF FreePhase, the amber rate (standard tariff level) applies for most of the day — not a penalty, but not a saving. Concentrate heating in the green overnight window.

The immersion heater question

Most heat pump cylinders include an immersion heater element as a backup. It's useful in two situations: if the heat pump is offline for maintenance, or for a Legionella pasteurisation cycle (which requires heating the cylinder to 60°C, a temperature many heat pumps can't sustain efficiently).

The immersion element draws 2–3 kW of direct electricity and has a COP of exactly 1.0 — it turns one unit of electricity into one unit of heat. Your heat pump turns one unit of electricity into 2.5–3.5 units of heat at current outdoor temperatures. Running the immersion on a whim is the single fastest way to undo your tariff savings.

Rules of thumb:

• Keep the immersion boost switch off by default.
• Schedule Legionella cycles (usually weekly or monthly) to run within a cheap window if your system supports it.
• Use the immersion as an emergency override only — not as a routine supplement.

A practical schedule for common setups

Cosy Octopus household, 200-litre cylinder:

• 04:30: hot water schedule starts (heat pump, DHW mode)
• 06:45: schedule ends (gives 2h 15m — enough for full recovery with margin)
• 13:15: optional 45-minute top-up if afternoon demand is high
• 22:15: optional 45-minute evening buffer before overnight gap
• 16:00–19:00: confirmed nothing is running

EDF FreePhase household, 200-litre cylinder:

• 01:00: hot water schedule starts
• 03:30: schedule ends
• No further schedule needed for average occupancy

Adjust start times to your actual household hot water demand. A family of four with two morning showers may need more stored volume than a couple working from home.

Checking whether it's working

If you have a smart meter and a Bright account, you can see half-hourly import data in Heat Pump Tariffs. Look at your consumption profile:

• Is there a clear spike during your cheap window? Good — that's your cylinder heating.
• Is there consumption during your tariff's peak window? Investigate what's drawing power.
• Is your overnight usage unusually high for the cylinder size? The immersion may be running unnecessarily.

A well-scheduled cylinder shows a predictable overnight or early-morning peak in your consumption chart, with low or zero heating-related consumption during the day.

The short version

• Heat pump cylinders are slower to recover than boiler-fed ones — plan for 90 minutes of run-time, not 30.
• Hot water is the highest-value load to shift to cheap windows.
• Set one main schedule in your cheap overnight window and leave it running.
• Add a short midday top-up if your tariff has a midday cheap period.
• Keep the immersion boost off except for Legionella cycles.
• Check your half-hourly data to confirm the schedule is landing in the right place.

See which tariff delivers the best blended rate for your hot water usage on Heat Pump Tariffs — using your real data, not estimates.