Heat Pump Cylinder Size Calculator UK 2026

Gas combi boilers can deliver 24-30 kW of instantaneous hot water - no cylinder needed. Heat pumps deliver 4-10 kW, far below shower/bath demand (a hot shower at 40C calls for ~15 kW continuous). The solution: an indirect cylinder stores hot water, reheating slowly between uses.

This changes the sizing rules vs gas system boilers (vented or 150L unvented standard):

• Storage volume must cover daily peak demand: morning rush, evening dishwashing, all-bath family weekends.
• Recovery time matters: a 200L cylinder feeding 4-person morning showers needs ~2-3 hours to fully reheat. If using smart tariff cheap rate (04:00-07:00), that's fine. If reheating during peak hours (16:00-19:00), the cost spike is significant.
• Unvented (mains-pressure) standard: heat pump cylinders are nearly always unvented for mains-pressure shower flow. Requires G3 building reg compliance + annual maintenance.

150L unvented cylinder:

• 1-2 occupants, shower-only household.
• No bath-heavy users.
• Reheat time 8 kW @ 50C: ~1.5 hrs cold-to-full.
• Typical install: studio flat, 1-bed apartment.
• Cost: GBP 1,000-1,400 cylinder unit only.

200L unvented cylinder (most common UK 3-bed):

• 2-3 occupants, mixed shower/bath.
• Single bathroom + ensuite.
• Reheat time: ~2 hrs cold-to-full.
• Typical install: 3-bed terrace, semi-detached.
• Cost: GBP 1,200-1,700 cylinder.

250L unvented cylinder:

• 3-4 occupants, family with baths.
• Single bathroom or master + ensuite.
• Reheat time: ~2.5 hrs cold-to-full.
• Typical install: 3-4 bed family home.
• Cost: GBP 1,400-1,900 cylinder.

300L unvented cylinder:

• 4-5 occupants, multiple bathrooms.
• Strong morning peak demand (2 simultaneous showers).
• Reheat time: ~3 hrs cold-to-full.
• Typical install: 4-bed detached.
• Cost: GBP 1,700-2,400 cylinder.

400L+ unvented cylinder:

• 5+ occupants, multiple bathrooms.
• Smart-tariff peak-shifting strategy (reheat all overnight, never during peak).
• Reheat time: ~4 hrs cold-to-full.
• Typical install: 5+ bed detached, multi-generational home.
• Cost: GBP 2,200-3,500 cylinder.

For households with high-flow fixtures (rainfall showers, multiple ensuite simultaneous use), check cylinder size against fixture L/min:

• Standard shower: 6-8 L/min × 8 min = 48-64L per shower.
• Rainfall shower / high-flow: 10-12 L/min × 8 min = 80-96L per shower.
• Bath: 100-150L per fill.
• Kitchen sink + washing up: 15-25L per wash.
• Dishwasher fill: 15-20L hot water draw (most modern machines cold-fill, but check yours).

Daily peak example - 4-person household:

• 4 × morning shower (8 min each, standard 7 L/min) = 224L.
• 1 × evening bath = 120L.
• 2 × kitchen wash = 30L.
• Total: 374L per day. Peak: 224L within 60-90 min.

250-300L cylinder + 8 kW heat pump comfortably handles this with overnight reheat. 200L cylinder would need a peak-rate reheat after morning showers - costly.

Typical cylinder dimensions:

• 150L cylinder: ~1.2-1.4m tall × 0.5m diameter. Fits airing cupboard, small utility room.
• 200L cylinder: ~1.5m tall × 0.55m diameter. Standard airing cupboard size.
• 250L cylinder: ~1.6-1.7m tall × 0.6m diameter. Need slightly deeper airing cupboard or utility.
• 300L cylinder: ~1.8m tall × 0.6m diameter. Often needs dedicated utility space.
• 400L+ cylinder: ~2m tall × 0.65m diameter. Garage / dedicated plant room.

G3 unvented installation requirements:

• Tundish + expansion vessel (15-30L typical).
• Pressure relief valve discharge - via tundish to outside drain.
• Service area: 600mm above + sides accessible.
• Annual maintenance: pressure check + anode rod inspection (5-yearly replacement).

1. Direct gas combi swap = 150L cylinder: Many operators replace gas combi with smallest heat pump cylinder available - then run out of hot water on day 2 + write off heat pumps as 'cold showers'. Right answer: 200L minimum for any household with 2+ occupants.
2. Ignoring future-proofing: 2-person couple installs 150L. 5 years later: 4-person family with same cylinder = constant peak-rate top-ups. Right answer: install 200-250L if planning >3 year stay + possible family growth.
3. Oversizing 'just in case': 500L cylinder for 2-person household = 50% wasted standby heat losses (insulated cylinder still loses 1-2 kWh/day). Right answer: size for current + 12-18 month projected usage.
4. Ignoring tariff strategy: 200L cylinder + flat-rate tariff = fine. 200L cylinder + time-of-use tariff = need afternoon-evening peak-rate reheat. Right answer: size up by 50% when on smart tariff.
5. Skipping pressure check: Mains water pressure < 1.5 bar = unvented cylinder won't deliver decent shower flow. Test before specifying. Right answer: pressure-test mains supply before install; backup with shower pump if pressure < 1.5 bar.