What size hot-water cylinder do I need for a heat pump?

Rule of thumb: 45 litres per person in the household. So 90L for a single occupant, 180L for a 4-person family, 250L for 5+. This is more generous than the old gas-boiler rule (35L/person) because a heat pump takes longer to reheat — typically 2–4 hours from cold to 50°C — so you want a tank big enough to ride out a busy hour without falling back on the immersion. For households with high hot-water demand (multiple showers in quick succession, baths, etc.), bump to 60L per person. For households with low demand or where space is genuinely tight, you can go down to 35L/person if you also accept that you'll occasionally need the immersion as a top-up.

Why can't a heat pump replace a combi boiler like-for-like?

Combis heat water instantaneously on demand at high power (24–35 kW for the hot-water side). A typical heat pump is 5–14 kW — nowhere near enough to heat water as fast as you use it. So heat pumps NEED a storage cylinder where water sits at temperature waiting for you, while a combi has no storage. Practically: if you have a combi, the install includes installing a cylinder somewhere (airing cupboard, utility, garage, loft), which adds £800–£2,500 to the install and needs physical space ~600×600×1800mm. The combi itself usually gets removed; sometimes it's kept as backup but most installs disconnect it entirely.

Should I heat the cylinder once a day or continuously?

Once or twice a day on a SCHEDULED cycle is the right answer for almost all UK heat-pump installs. Heat pumps are most efficient at lower flow temperatures, so a slow heat-up to 50°C overnight or early morning gives better COP than topping up on demand. Two cycles (e.g. early morning + mid-afternoon) suit households with evening showers; one cycle (overnight) suits households whose hot-water use is concentrated in mornings. Continuous keeping-warm is a bad idea — it forces the pump to run frequent short cycles at higher flow temps for marginal hot-water gain, hurting COP. Your installer will set this up during commissioning; you can tweak the schedule via the unit's app or controller afterwards.

What's the weekly Legionella cycle and why do I need it?

Legionella pneumophila bacteria can grow in hot-water cylinders held below 50°C. HSE Approved Code of Practice L8 requires hot-water systems in domestic dwellings to reach 60°C at least once a week to kill any bacteria that have grown in the stagnant layers. Heat pumps run more efficiently at 50°C than 60°C, so the standard pattern is: heat pump heats to 50°C daily, then ONCE A WEEK the system uses the backup immersion heater (3 kW resistance element) to bump the cylinder to 60°C for an hour. The weekly cycle adds maybe £1–£2/year to your electricity bill — negligible. Most installs schedule this overnight (e.g. 2am Sunday). It's automatic; you don't think about it.

What's the difference between vented and unvented cylinders?

Vented cylinders use gravity-fed water from a cold-water tank in the loft, so hot-water pressure is limited to the gravity head (usually 0.3–0.5 bar — okay for taps, weak for showers without a pump). Unvented cylinders are connected to mains pressure (3–6 bar typical), so taps and showers run at full mains pressure with no need for a pump. Almost all modern heat-pump installs use unvented cylinders because mixer showers + mains-pressure feel are now standard expectations. Unvented cylinders need a G3 qualified installer for the safety-relief plumbing and must have annual safety-valve checks. Cost difference: unvented adds ~£300–£600 over a vented cylinder of the same size.

Can I use a smart tariff to heat the cylinder on cheap electricity?

Yes — and you should. The pattern is: shift the cylinder reheat cycle to your tariff's cheap window. Octopus Cosy and Octopus Go have defined cheap-rate hours (typically 4am–7am and/or 1pm–4pm). Octopus Agile and Tracker have variable cheap periods that change daily. Most heat-pump controllers can schedule the cylinder cycle to a fixed time (works for Cosy/Go) or accept commands from a home-automation system (works for Agile via Home Assistant or similar). Savings: a 4-person household heating 180L from 15°C to 50°C twice a day uses ~6 kWh/day. Shifting that to a 12p cheap rate vs 28p flat rate saves ~£0.96/day = ~£350/year just on the cylinder cycle. Combined with running the central heating on the same shifted schedule, smart-tariff savings on a heat-pump install often pay for the install premium over a like-for-like gas replacement within 5–7 years.

Hot water planning for a heat pump install: UK 2026 guide

Why hot water is its own design problem

On a gas-boiler retrofit, hot water is an afterthought — the existing cylinder or combi keeps working, you only change the heat source. On a heat-pump retrofit, hot water is half the design conversation. Three reasons:

Heat pumps are slow. 2–4 hours to reheat a cylinder from cold to 50°C, vs ~20 minutes for a gas boiler. So you need bigger storage.
Heat pumps don’t love 60°C. COP drops sharply above 50°C, so the operating sweet spot is ~48–52°C. Lower than gas-fired hot-water systems typically run.
Combi homes need a cylinder added. ~50% of UK homes have combis; for those, the heat pump install also installs a cylinder + relocates the hot-water plumbing.

These three together mean hot-water planning isn’t just “pick a tank size” — it involves layout, scheduling, tariff alignment, and a Legionella protocol. Worth getting right at the design stage.

Sizing the cylinder

UK 2026 sizing convention for heat-pump cylinders:

1 person: 90–120L
2 people: 150–180L
3 people: 180–210L
4 people: 210–250L
5+ people: 250–300L+

Adjust upwards if you have:

Multiple bathrooms used in quick succession.
Baths rather than showers (a bath is ~80L of hot water).
Power showers or rain heads (12+ L/min flow).
A whirlpool or large free-standing bath.

Adjust downwards (cautiously) if you have:

Genuine space constraints (loft installs in low eaves, etc.).
Spread-out usage patterns (no clustered showers).
Comfort with occasional immersion top-ups during busy weeks.

Default rule of thumb: 45 litres per person. That works out at 180L for a typical 4-person household and is the most common quote-stage default.

Vented vs unvented — the easy decision

Unvented cylinders are now the default for 90%+ of UK heat-pump installs. Why:

Mains pressure to all taps and showers without needing a pump. 3–6 bar typical.
No loft cold-water tank required. Frees up loft space and removes a maintenance item.
Better mixer-shower performance. High-flow rain heads + body jets need mains pressure; gravity-fed vented systems can’t deliver.

Vented cylinders persist where mains pressure or flow is low (some rural locations on shared supply), or where the existing loft tank is hard to remove. In those edge cases, your installer may suggest an accumulator + pump combo with a vented cylinder.

Cost difference: unvented adds ~£300–£600 over an equivalent vented cylinder, and the installer needs to be G3 qualified for the safety plumbing.

The heating schedule that gives best COP

Counter-intuitively, the best schedule for heat-pump hot water is NOT “keep the cylinder hot all day” — it’s scheduled cycles. Two patterns cover most households:

Single overnight cycle — heat to 50°C between 1am and 5am. Works for households whose hot-water use is concentrated in mornings (showers before work/school). Tank stays warm enough through the day for evening washing-up.

Twin daily cycle — heat to 50°C at 4am–6am AND again at 1pm–3pm. Works for households with evening showers or anyone who wants more reserve capacity. Better fit for busy 4-person households.

Both patterns let the heat pump run when ambient temperatures suit it (early-morning being cold but not the COLDEST), and align with most UK smart-tariff cheap-rate windows. Your installer will program the schedule during commissioning based on your usage patterns + tariff.

The weekly Legionella cycle

HSE Approved Code of Practice L8 requires hot-water cylinders to reach 60°C at least once per week to kill Legionella bacteria. Heat pumps run 50°C as the daily target, so the weekly cycle uses the integrated 3 kW immersion heater to boost the cylinder to 60°C for ~1 hour.

Cost: ~3 kWh × £0.28 (flat rate) = £0.84 per week = £44/year. Half that if scheduled to a cheap-rate window. Negligible.

This happens automatically. Most installers schedule it for 2am–3am on Sunday so it never affects your daily usage. You don’t need to do anything.

The backup immersion — and when it fires

Every heat-pump cylinder ships with a 3 kW backup immersion heater. It fires in three circumstances:

Weekly Legionella cycle — automatic, discussed above.
Holiday boost — if you arrive home to a cold cylinder and want hot water in 30 minutes rather than 3 hours. Manual button or app trigger.
Heat-pump fault — if the unit fails, immersion can run the hot-water side until the installer attends.

The immersion is NOT for daily use — it’s 3–4× more expensive per kWh of hot water than the heat pump is. Confirm during commissioning that the controller prefers the heat pump and only falls back to immersion in defined scenarios.

Smart-tariff alignment

Most heat-pump tariffs (Octopus Cosy, Octopus Go, British Gas HomeEnergy) have defined cheap-rate windows of 5–7 hours per day at a third to a half of the flat rate. Scheduling the cylinder cycle inside the cheap window can save £150–£350/year on a typical 4-person household.

Variable tariffs (Octopus Agile, Tracker) require a smart controller or Home Assistant integration to shift the schedule daily — more setup but bigger savings, often £400+/year if you also shift the central-heating run pattern.

See the upcoming heat pump tariffs comparison for tariff-specific guidance.

Where the cylinder physically goes

A typical 180L cylinder is roughly 600mm diameter × 1500mm tall; a 250L is 600mm × 1800mm. Add 100–150mm clearance on top for the immersion + pipework. Common locations in priority order:

Existing airing cupboard — easiest if it already housed a hot-water cylinder. Often the old cylinder was smaller, so the new one is a tight fit.
Utility room or downstairs cupboard — common in homes that converted to a combi and removed the original cylinder.
Garage (if attached + insulated)— works if there’s a frost-protection plan; cylinders should not freeze.
Loft — possible but pipework runs longer + you lose head-pressure unless unvented. Confirm joist strength can take the full-cylinder load (~250kg).

Your installer should propose the cylinder location during the MCS site visit and confirm it in the written quote. If you’re not happy with their proposed location, raise it before signing — relocating after install is expensive.

The summary

Hot-water planning for a heat pump comes down to: cylinder size (45L/person rule of thumb), unvented vs vented (unvented for 90% of installs), heating schedule (1–2 cycles per day, not continuous), weekly Legionella cycle (automatic), and tariff alignment (move the cycle to cheap-rate hours). Combi-replacement homes also need physical space for the cylinder, which is usually the airing cupboard or utility room. Get these decisions made at the quote stage — they affect cost, comfort, and running expense for the next 15+ years.

Hot water planning for a heat pump install in UK 2026