Introduction
Spend enough time researching heat pumps, and you’ll eventually hit a detail that catches a lot of homeowners off guard: your existing hot water cylinder probably won’t be compatible. Heat pumps run at lower flow temperatures than gas boilers, and most cylinders simply aren’t built with enough coil surface area to reheat water efficiently at those temperatures; the result is slower reheat times, lukewarm water, and an immersion heater doing far more work (and costing far more) than it should.
This guide will help you know how to choose a cylinder that actually matches your heat pump and how it is correctly sized for your property and usage without confusing you with technical jargon.
What you need to know
A hot water cylinder designed for a gas boiler and one designed for a heat pump aren’t interchangeable, even though they might look the same from the outside. Gas boilers heat water at high temperatures very quickly, so a relatively small coil can transfer enough heat in a short burst. Heat pumps work the opposite way — they run at lower flow temperatures, gently and continuously, so the coil needs a much larger surface area to transfer the same amount of heat over a similar time frame. Fit a heat pump to an undersized boiler-spec cylinder, and the result is slow reheat times, water that never quite gets properly hot, and an immersion heater picking up the slack — quietly adding to your electricity bill.
A Quick Guide
In a hurry? Here’s the short answer
- Want the most recognised, premium option? → Megaflo (Heatrae Sadia)
- Want close-to-premium performance for less? → Gledhill StainlessLite Plus
- Want the longest warranty? → OSO (verify current terms)
- Already have a Vaillant heat pump? → Vaillant uniSTOR (matched coil sizing and controls)
- Want smart, usage-based heating to cut standing losses? → Mixergy
Brand Comparison
Before comparing brands, it’s worth knowing what to actually look at: coil surface area determines reheat speed with a heat pump far more than tank capacity does, so two cylinders with the same litre size can perform very differently. Warranty length is also worth checking closely — the headline figure often only applies when the cylinder is installed and registered by an accredited installer.
| Approx. coil size | Warranty | Typical price (unit only) | Best known for | |
|---|---|---|---|---|
| Megaflo (Heatrae Sadia) | Manufacturer-specific — check current datasheet | Typically 2–5 yr standard, extendable | Premium bracket | Best-known name, reliability reputation |
| Gledhill StainlessLite Plus | Manufacturer-specific — check current datasheet | Multi-year, competitive with premium brands | Noticeably below Megaflo for similar performance | Value-for-performance “sweet spot” |
| Telford | Manufacturer-specific — check current datasheet | Strong warranty cover cited across sources | Mid-range | Warranty reputation |
| OSO | Manufacturer-specific — check current datasheet | Long-stated warranty (verify current terms) | Mid-to-premium | Longest warranty positioning, all-stainless construction |
| Vaillant uniSTOR | ~5.2m² (verify against current model) | Aligned with Vaillant heat pump warranty terms | Mid-to-premium | Purpose-matched to Vaillant heat pumps |
| Mixergy | N/A — smart top-up heating rather than full-tank coil sizing | Manufacturer-specific — check current terms | Premium | Usage-based smart heating, reduced standing losses |
Coil surface area and warranty terms are the two figures most worth double-checking against the manufacturer’s current datasheet — both are used heavily in marketing and vary between model years.
Sizing guide
Cylinder sizing is usually approached by household size and bathroom count, though the same figures apply whether you’re on a boiler or a heat pump; it’s the coil, not the tank capacity, that needs to change for a heat pump system.
- 1–2 people, 1 bathroom: 120–150 litres
- 2–4 people, 1–2 bathrooms: 150–210 litres
- 3–5 people, 2–3 bathrooms: 210–250 litres
- 5+ people, 3+ bathrooms: 250–300 litres
A useful rule from industry sizing guides: size to the property, not just the current occupants — a single person in a large four-bedroom house should still generally have a cylinder sized for that property’s potential, not their personal daily usage, in case a larger household moves in later.
The genuinely common mistake with heat pump systems specifically isn’t getting the litre size wrong; it’s assuming litre capacity is the only thing that matters and overlooking the coil size that determines how quickly that capacity actually refills.
Installation and compliance notes
Unvented cylinders over 15 litres are notifiable work under Building Regulations Part G3 (England and Wales — Scotland and Northern Ireland have equivalent standards). Only a G3-qualified engineer can legally install, commission, or service one.
Most professional installers are registered with a Competent Person Scheme (such as APHC, WaterSafe, or BESCA), which lets them self-certify the work without a separate application to your local authority. If your installer isn’t registered with a scheme, the installation has to be notified to Building Control directly, either before work starts or via a Full Plans submission — not notifying at all is a criminal offence under the Building Act 1984.
Annual servicing by a G3-qualified engineer is required by virtually every manufacturer to keep the warranty valid — a missing service record is one of the most common reasons a warranty claim gets refused.
Brand-by-brand notes
- Megaflo (Heatrae Sadia) — the best-known name in UK unvented cylinders, premium pricing, and strong reputation for reliability.
- Gledhill StainlessLite Plus — positioned by multiple sources as offering most of Megaflo’s performance at a noticeably lower price.
- Telford — well regarded for warranty cover, sits in the mid-range bracket.
- OSO — stainless steel construction, one of the longest-warranty options in the market.
- Vaillant uniSTOR — designed specifically to pair with Vaillant heat pumps, oversized coil for fast reheat, backed by Vaillant’s large UK installer network.
- Mixergy — smart cylinder that heats only the water needed rather than the full tank, aimed at reducing standing losses.
FAQs
Frequently asked questions
Can I keep my existing cylinder when I install a heat pump?
Usually not, if it was sized for a gas boiler. Boiler-spec cylinders typically have a much smaller coil surface area than heat pump systems need, so keeping one tends to mean slow reheat times and a lukewarm tank. Your installer should check the existing cylinder’s coil size against your heat pump’s output before deciding whether it can be reused.
What size cylinder do I need for a heat pump?
Capacity depends on household size and bathroom count, but coil surface area matters just as much as litres — an undersized coil will bottleneck reheat speed regardless of tank capacity. Your installer should size both together based on your heat pump’s output and your household’s hot water demand, rather than just matching the litre size of your old cylinder.
Does the BUS grant cover the cylinder as well as the heat pump?
The Boiler Upgrade Scheme grant is specifically for the heat pump installation itself, not the cylinder. However, cylinder replacement is often bundled into the overall installation cost by installers, so it’s worth asking for a clear cost breakdown that shows what the grant is offsetting versus what you’re paying for separately.
How often does an unvented cylinder need servicing?
Annual servicing by a G3-certified engineer is generally recommended, and often required to keep the manufacturer’s warranty valid. Servicing checks the safety valves and pressure/temperature relief components that unvented systems rely on, so it’s not something to skip even if the cylinder seems to be working fine.
Final thoughts
The cylinder is easy to treat as an afterthought once you’ve settled on a heat pump brand, but it has just as much bearing on whether your hot water actually feels sufficient day to day. If you’re getting quotes, ask specifically what cylinder is included, what its coil surface area is, and whether the quoted warranty requires installer accreditation to apply. A slightly more expensive cylinder with a properly sized coil will almost always outperform a cheaper one that leaves your heat pump working harder than it needs to.
This guide is for the majority of properties in the UK and the more common hot water needs of the average households. There are much larger properties that will require higher-output units and higher-premium products, which are not covered here.