Direct or indirect hot water cylinder
Updated: 17.10.2026
Match a direct or indirect hot water cylinder to the appliance that supplies the heat before you compare litres or price. A direct cylinder uses its own immersion. An indirect cylinder takes heat through a coil from a boiler or heat pump, and usually keeps an immersion as backup. Every product in our [hot water cylinder comparison](/hub/hot-water/hot-water-cylinder) is a direct electric wall store, and none describes a coil. The set fits electricity-only heat. Peter F. checked the listings, with no lab test. A qualifying purchase can earn a commission at no extra cost to you.
Match a direct or indirect hot water cylinder to the appliance that supplies the heat before you compare litres or price. A direct cylinder uses its own immersion.
Match a direct or indirect hot water cylinder to the appliance that supplies the heat before you compare litres or price. A direct cylinder uses its own immersion. An indirect cylinder takes heat through a coil from a boiler or heat pump, and usually keeps an immersion as backup. Every product in our hot water cylinder comparison is a direct electric wall store, and none describes a coil. The set fits electricity-only heat. Peter F. checked the listings, with no lab test. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Heat source | Element | Supply | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  TESY TESY BiLight 30 litre | Direct electric | 1,500 W | Plug not stated | 8.8 | about £139.99 | View offer |
| Alternative |  TESY TESY BiLight 100 litre horizontal | Direct, unvented | 3,000 W | 1.5 mm cable | 7.2 | about £259.99 | View offer |
| Preis-Tipp |  TESY TESY BiLight 50 litre horizontal | Direct electric | 1,500 W | 16 A fused spur | 6.5 | about £159.99 | View offer |
On this page
Immersion or coil
Inside a direct cylinder, an immersion heats the domestic water around it. If that element is the only heat source, its rating and recovery time are the everyday limit. A timer can shift heating to an off-peak tariff, but the store still has to cover the gap between cycles.
An indirect cylinder has a coil. Primary water from a boiler or heat pump passes through it and warms the stored tap water without mixing. The coil has to suit that heat source. Vented or unvented does not tell you whether a coil is fitted. Read unvented or vented hot water cylinders separately.
When electricity is the only heat
A direct cylinder makes sense in an all-electric flat or annexe. The immersion is the heat source, not a backup. If it fails, there is no coil behind it.
In this comparison the TESY 30 litre and 50 litre horizontal models state 1,500 W, the GEESEN 50 litre states 2,000 W, and the TESY 100 litre horizontal states 3,000 W. The 50 litre horizontal also states 1 hour 18 minutes from 15°C to 60°C, under the listing’s conditions. The 80 litre TESY prints 1,500 kW and the 120 litre prints 220 W beside 220 V. Neither line supports a recovery comparison.
Electricity-only does not mean plug-in. The 50 litre horizontal TESY calls for a 16 A fused spur, and the 100 litre horizontal listing requires a 1.5 mm cable. Use the hot water cylinder buying guide for the rest of the box.
When a boiler should heat the store
A home with a system boiler normally needs an indirect cylinder. An Energy and Utilities Alliance explainer contrasts a household immersion of around 3 kW with roughly 12 kW that a boiler may deliver to a cylinder. Those are examples, not a specification for every job. Many indirect cylinders keep an immersion as backup. It is not the everyday heat source, and leaving it on can duplicate the boiler’s work.
None of these six listings states a coil. The 100 litre horizontal TESY describes one cold inlet and one hot outlet, not primary flow and return. A coil on a different TESY ASIN cannot be copied onto the 120 litre product here. If the boiler should heat the water, these six are not the store.
Why a heat pump needs the right coil
A heat pump also uses an indirect cylinder, but the lower flow temperature needs a larger coil. These are a different product class from the direct wall stores here. A small boiler coil can recover too slowly on a heat pump, and adding an immersion to a direct cylinder does not create that coil.
A rule of thumb for heat-pump systems below 10 kW uses about 45 litres per person. That is only a starting point. Work through what size hot water cylinder you need with the designer rather than buying the nearest number from this electric set.
What this comparison can and cannot offer
Once direct electricity is confirmed, the 30 litre TESY is the lowest price with a readable 1,500 W element, the horizontal 50 litre needs a fused spur, and the 100 litre horizontal states 3,000 W but needs extra installation parts. This set cannot offer a boiler coil, a heat-pump coil or a conversion of a direct vessel. If a system boiler or heat pump should heat the water, leave these six and ask for an indirect cylinder designed for that source.
Common questions
What is the difference between direct and indirect cylinders?
A direct cylinder heats its stored domestic water with an electric immersion element. An indirect cylinder receives heat through a separate internal coil connected to a boiler or heat pump. Primary heating water passes through the coil and does not mix with tap water.
Can I use one of these six with a system boiler?
Not as the boiler-heated store described by these listings. All six are direct electric products and none states a primary coil. A system boiler normally needs an indirect cylinder with documented flow and return connections plus a suitable coil.
Does an indirect cylinder still have an immersion heater?
Many do. The immersion usually serves as backup when the boiler is unavailable rather than the normal main heat source. Check the exact cylinder and control scheme, because inclusion, rating and operating strategy vary.
Can a heat pump heat a direct cylinder?
Not through the water circuit if the direct cylinder has no coil. A heat pump needs an indirect cylinder with a heat exchanger designed for its lower flow temperature. The cylinder may also include an immersion for backup.
Why can a normal boiler coil be a poor heat-pump match?
Heat pumps usually operate with a smaller temperature difference than boilers. A larger coil surface helps transfer enough heat under those conditions. An undersized coil can lengthen recovery and reduce system performance, so the designer must match both components.
Are all six cylinders in this comparison direct?
Yes. Each listing describes electric storage and none states an internal boiler or heat-pump coil. Do not copy a coil feature from another TESY ASIN onto these products. Verify the exact model before ordering.
Related pages
Sources
Public UK sources listed in the research digest, and the amazon.co.uk listings for these six cylinders. Prices in the digest were captured on 16 October 2026.
- Approved Document G (2024 amendments) (retrieved 16.10.2026)
- Part G FAQ (retrieved 16.10.2026)
- CNC Building Control: unvented hot water cylinder (retrieved 16.10.2026)
- Hot Water Association, unvented note (retrieved 16.10.2026)
- Hot Water Association, sizing a hot water cylinder (retrieved 16.10.2026)
- HSE: legionella and hot and cold water systems (retrieved 16.10.2026)
- Regulation (EU) No 812/2013 on legislation.gov.uk (retrieved 16.10.2026)
- TESY BiLight series (retrieved 16.10.2026)