How Does a Heat Pump Heat Your Hot Water?
A heat pump heats your domestic hot water by circulating warm water through a coil inside a well-insulated cylinder, gradually raising the stored water to a target temperature of 48–55°C for daily use. Unlike a combination boiler that heats water on demand, the heat pump heats in advance and stores it — working best at lower, steady flow temperatures that maintain a higher COP. This is actually more efficient than on-demand heating. A typical 210-litre unvented cylinder is the standard choice for a three-bedroom family home, providing mains-pressure hot water to all taps and showers without a cold water tank in the loft. Unvented cylinder installation requires a G3-qualified engineer under Building Regulations Part G.
Related reading: How much does a heat pump cost in 2026? | Solar panels and heat pumps: the perfect combination
Unvented vs Vented Cylinders: Which Do You Need?
Unvented cylinders
An unvented cylinder is connected directly to the mains water supply. Because it operates at mains pressure, you get strong, consistent hot water flow to every tap and shower — even on upper floors. There is no need for a cold water tank in the loft.
Unvented cylinders are the preferred choice for heat pump installations because:
- Better water pressure — mains pressure throughout the house
- No cold water tank needed — frees up loft space
- More hygienic — no open tank where contaminants could enter
- Better insulation — modern unvented cylinders lose very little heat
- Compatible with all major heat pump brands
The key requirement is that unvented cylinders must be installed by a G3-qualified engineer (see below). All Electromatic M&E Ltd engineers hold the G3 certificate.
Vented cylinders
A vented cylinder is fed by a cold water storage tank, typically in the loft. It operates at gravity pressure, which means water pressure on upper floors can be low. Vented systems are older technology and are less common in modern heat pump installations.
For the vast majority of properties we install across Richmond, Kingston, Twickenham, and Hampton, an unvented cylinder is the right choice.
What Size Cylinder Do You Need?
| Household | Bedrooms | Bathrooms | Recommended Cylinder | Daily Hot Water Use |
|---|---|---|---|---|
| 1–2 people | 1–2 | 1 | 150–180 litres | 80–100 litres |
| 2–3 people | 2–3 | 1–2 | 180–210 litres | 100–150 litres |
| 3–4 people | 3–4 | 2 | 210–250 litres | 150–200 litres |
| 4–5 people | 4–5 | 2–3 | 250–300 litres | 200–250 litres |
| 5+ people | 5+ | 3+ | 300+ litres | 250+ litres |
The most popular sizes we install
180-litre cylinder — Ideal for couples and smaller households. Fits neatly into an airing cupboard and provides enough hot water for a daily shower each plus washing up. Most common for two-bedroom flats and smaller terraced houses.
210-litre cylinder — The sweet spot for a typical three-bedroom family home. Comfortably handles two showers and a bath per day. This is our most frequently installed size overall.
250-litre cylinder — Recommended for larger families or homes with multiple bathrooms.
300-litre cylinder — For larger detached homes with three or more bathrooms, or where hot water demand is consistently high.
Factors that affect your choice
- Number of showers and baths — baths use significantly more water than showers
- Heat pump output — a larger heat pump can reheat a smaller cylinder faster
- Available space — some airing cupboards cannot accommodate a 300-litre cylinder
- Solar PV — if you have solar panels, a slightly larger cylinder can store excess solar energy as hot water, boosting self-consumption
Top Cylinder Brands We Recommend
McDonald Water Storage
A UK-based manufacturer with a strong reputation in the plumbing and heating industry. Duplex stainless steel construction, 50mm+ high-density foam insulation, 25-year inner cylinder warranty, and excellent heat pump compatibility — including twin coil for heat pump plus solar or immersion backup.
Kingspan Ultrasteel
One of the most recognised names in hot water storage. The Ultrasteel range is specifically designed for renewable heating systems. High-performance factory-applied foam insulation, 25-year inner vessel warranty, and designed for low-temperature heating — ideal for heat pump systems.
Telford Cylinders
UK manufacturing with decades of experience. Copper or stainless steel options, factory-applied rigid foam insulation, up to 25-year warranty, and a wide range of twin-coil and heat pump-specific models.
For most installations, any of these three brands will perform excellently. Your installer will advise on the best option based on your specific requirements during the free home survey.
Legionella Protection: The 60°C Pasteurisation Cycle
Legionella bacteria can grow in stored water between 20°C and 45°C. Because heat pumps typically heat water to 48–55°C for efficiency, all properly designed heat pump systems include legionella protection — a non-negotiable requirement under Building Regulations Part G.
How it works
Your heat pump controller is programmed to run a weekly pasteurisation cycle that heats the entire contents of the cylinder to at least 60°C for a minimum duration (typically one hour). This temperature kills legionella bacteria.
Most heat pumps can reach 60°C on their own, though at reduced efficiency. Some systems use the built-in immersion heater to top up the last few degrees, which is perfectly normal and adds only pennies per week to your electricity bill.
Key points
- It is automatic — once set up by your installer, you do not need to do anything
- It runs weekly — usually in the early hours when you are not using hot water
- It uses minimal extra energy — the cost is negligible compared to total heating bills
- It is a legal requirement under Building Regulations Part G
- Your installer will configure it during commissioning
Related reading: Heat pump maintenance: what you need to know
Hot Water Schedules: Getting the Timing Right
Typical hot water schedule
Most households run two hot water heating periods per day:
- Morning session: 04:00–07:00 (heating the cylinder in time for morning showers)
- Afternoon session: 14:00–16:00 (topping up for evening use)
The heat pump heats the cylinder to the target temperature (typically 50–55°C) and then stops. A well-insulated modern cylinder loses only 1–2°C per hour, so hot water remains available for many hours.
Optimising for solar PV
If you have solar panels, you can shift your hot water schedule to coincide with peak solar generation:
- Main session: 10:00–15:00 (heating hot water using free solar electricity)
- Top-up session: 05:00–07:00 (only if needed, using grid electricity)
This approach maximises your solar self-consumption. Some smart controllers (Mitsubishi MELCloud, Vaillant sensoAPP) can adjust the schedule automatically based on solar generation.
Immersion Heater Backup: Belt and Braces
Every heat pump hot water cylinder comes fitted with an electric immersion heater. This serves important purposes:
- Emergency backup — if your heat pump develops a fault, the immersion heater can heat your water independently while you wait for a service visit
- Boost function — quickly top up the cylinder temperature when demand is unusually high (for example, guests staying)
- Legionella pasteurisation assist — tops up the temperature to 60°C+ if needed during the weekly cycle
The immersion heater is not designed to be your primary heat source — it runs at a COP of 1.0, making it three to four times more expensive than the heat pump per unit of heat. Use it as a backup and boost, not as a replacement.
G3 Certification: A Legal Requirement
If you are having an unvented hot water cylinder installed, the law requires that the work is carried out by an engineer who holds a G3 Unvented Hot Water qualification — a requirement under Building Regulations Part G: Hygiene in England and Wales.
What is G3 certification?
The G3 qualification demonstrates that an engineer has been trained and assessed in the safe installation, commissioning, and maintenance of unvented cylinders operating at mains pressure.
Why does it matter?
Unvented cylinders operate at significantly higher pressures than vented systems. If installed incorrectly, there is a risk of over-pressurisation. A G3-qualified engineer understands safe installation of pressure relief valves and expansion vessels, correct commissioning of temperature and pressure controls, and Building Regulations compliance.
Building Control notification
Every unvented cylinder installation must be notified to your local Building Control department. A G3-qualified installer will handle this notification as part of the job, or it may be covered automatically through a competent persons scheme.
Before hiring anyone to install a heat pump hot water cylinder, ask to see their G3 certificate. All Electromatic M&E Ltd engineers hold current G3 qualifications, and we handle all Building Control notifications as part of our service.
How Electromatic Can Help
Electromatic M&E Ltd offers free home surveys across London and Surrey (TW, KT, SW postcodes). We handle BUS Grant applications (subject to eligibility criteria set by Ofgem), install ASHP and solar PV from a single contractor, and deliver within 2–4 weeks of survey confirmation. All work is carried out under MCS certification via our accredited umbrella partner. Our G3-qualified engineers design and install complete heat pump systems, including the hot water cylinder, legionella protection, and Building Control notification.
Call us: 07718 059 284 | Email: admin@electromatic.uk
Frequently Asked Questions
Can a heat pump heat water as quickly as a combination boiler?
A heat pump heats water differently — it heats a cylinder in advance rather than on demand. With a correctly sized cylinder and a sensible schedule, you will always have hot water when you need it. Recovery time (the time to reheat a fully depleted cylinder) is typically 1–3 hours depending on the heat pump size and cylinder volume.
Do I need to keep the immersion heater switched on all the time?
No. The immersion heater is a backup and boost function. Under normal operation, your heat pump heats the cylinder and the immersion heater remains off. You only need it if the heat pump is out of action or you need a quick top-up of hot water beyond what is scheduled.
Will my water pressure change if I switch from a combination boiler to a heat pump with an unvented cylinder?
In most cases, your hot water pressure will actually improve. A combination boiler is limited by its flow rate (typically 10–12 litres per minute). An unvented cylinder delivers hot water at mains pressure, usually 15–20+ litres per minute. The result is better pressure at every tap and shower.
How often does a hot water cylinder need servicing?
Hot water cylinders require very little maintenance. An annual visual check of the pressure relief valve, expansion vessel, and immersion heater is usually sufficient — this can be carried out as part of your annual heat pump service. The cylinder itself typically lasts 25+ years.
Can I use my existing cylinder with a new heat pump?
Possibly, but it depends on the cylinder’s age, condition, size, and type. Older cylinders may be too small, poorly insulated, or incompatible with the heat pump’s operating temperatures. Your installer will assess this during the home survey and advise whether your existing cylinder can be retained or needs replacing.
The information in this article is for general guidance only and does not constitute financial, legal, or technical advice. G3 certification requirements are set by Building Regulations Part G (England and Wales). Always verify current requirements with your local Building Control authority. The Boiler Upgrade Scheme (BUS) grant of £7,500 is subject to eligibility criteria set by Ofgem — not all properties qualify. Electromatic M&E Ltd operates under MCS certification via our accredited umbrella partner. All installations comply with Building Regulations Part L and MCS standards. E&OE.
Written by Electromatic M&E Ltd — ASHP & Solar installer, London & Surrey (electromatic.uk)
Last updated: April 2026 | Electromatic M&E Ltd, Company No. 13837345
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