Indirect vs Direct Unvented Cylinder: Which is Best?

The direct-versus-indirect question is the single most common one we field on a cylinder-replacement enquiry — and the most common one we see answered incorrectly on the quotes our customers forward to us for a second opinion. The choice locks in fifteen to twenty years of running costs. Picking the wrong one on a property with a working gas supply can quietly add £15,000 to a household's energy bill over the life of the cylinder, and there is no easy way to switch later because the cylinder body itself is manufactured differently.

This guide is the version we wish more London households read before signing off a quote. It explains exactly what each type is, what it costs to run on 2026 London tariffs, which heat source each pairs with, and the regulatory framework that applies to both under Approved Document G3 of the Building Regulations. By the end you should be able to read any cylinder quote put in front of you and know within thirty seconds whether the proposed type is right for your property.

What a Direct Unvented Cylinder Actually Is

A direct unvented cylinder is a sealed pressurised hot water storage vessel heated entirely by internal electric immersion heaters. There is no coil and no external heat source. The cold mains feeds the bottom of the cylinder via a combination valve (incorporating a pressure-reducing valve, a check valve, a strainer and an expansion relief valve), the water heats inside the cylinder, and stored hot water at mains pressure leaves the top.

Almost every modern direct unvented cylinder is fitted with two immersion heaters — an upper and a lower element. The arrangement matters because it allows a household to heat only the top third of the cylinder during peak hours (using the upper element) for a quick recovery, and heat the full cylinder overnight on Economy 7 (using the lower element). Cylinders are typically rated at 3 kW per element. A 210-litre direct unvented cylinder will recover from cold to 60 degrees in around three hours on a single 3 kW element.

The body is identical to an indirect — the same stainless steel construction, the same WRAS-approved expansion vessel, the same temperature and pressure relief valve and the same discharge route. The only structural difference is that there is no internal coil and no primary flow and return tappings on the side of the cylinder. The Megaflo Eco Plus Direct, the Telford Tempest Direct and the OSO Delta Direct are the most common models we fit in London.

When direct is the right answer

Direct is correct in four specific situations. A new-build all-electric flat with no gas supply and no realistic route for one. A property on an Economy 7 or Economy 10 tariff where the household is genuinely available to time hot water use against the off-peak window. A small flat conversion where running boiler primary pipework to a cylinder cupboard is impractical and the household has accepted electric water heating as a constraint. A holiday let or annex where occupancy is intermittent and the simplicity of an all-electric cylinder outweighs the running-cost penalty.

What an Indirect Unvented Cylinder Actually Is

An indirect unvented cylinder is the same sealed pressurised vessel but with an internal coiled heat exchanger — usually copper or stainless steel — and a pair of primary tappings on the side of the cylinder. A system or regular boiler (the two types of boiler that work with stored hot water; a combi does not) circulates hot water from its primary circuit through the coil at typically 75 to 80 degrees flow temperature. The coil transfers heat to the stored water without the two circuits mixing.

A single immersion heater is fitted as a back-up — usually in the upper-middle of the cylinder so it can heat at least half the contents if the boiler fails. The immersion is wired through a switched fused spur and is intended for emergency use, not routine use. On most installs the immersion heater sees an hour or two of use a year, total.

Boiler-rated indirect cylinders have a coil of roughly 1.0 to 1.8 m² surface area, sized for a boiler primary flow of around 75 degrees. Heat-pump-rated indirect cylinders have a much larger coil — typically 3.0 to 4.5 m² — because a heat pump only delivers primary water at 50 to 55 degrees, so more heat-exchanger area is needed to move the same energy. A boiler-rated cylinder paired with a heat pump will struggle to reach 50 degrees on the stored side; this is the single most common heat-pump-cylinder mismatch we see across London.

The twin-coil variant

A twin-coil indirect cylinder has two separate internal coils — one for the boiler primary and one for a solar thermal panel (or, increasingly, a heat pump alongside a back-up boiler). The lower coil is fed by the solar circuit and pre-heats the water; the upper coil is fed by the boiler and tops up to setpoint. The Range Tribune HE Twin Coil, the Telford Tempest Solar and the Joule Cyclone Solar are common examples. A twin-coil cylinder adds around £350 to the fitted cost compared with a single-coil indirect of the same size.

Running Costs Compared — 2026 London Tariffs

The numbers below are the heart of the decision. On the 2026 London market, gas costs around 6.5p per kWh on a standard variable tariff and electricity costs around 27p per kWh standard rate or 12 to 16p per kWh on Economy 7 off-peak. The Energy Saving Trust figure for hot water consumption in a typical four-person household is 4,000 kWh per year.

Modelled annual running cost to deliver 4,000 kWh of hot water:

• Indirect on a 90% efficient condensing system boiler — 4,000 ÷ 0.90 = 4,444 kWh of gas at 6.5p = £289 per year.
• Indirect on a 320% seasonal-COP air-source heat pump — 4,000 ÷ 3.20 = 1,250 kWh of electricity at 27p = £338 per year (standard rate) or roughly £200 per year if shifted to off-peak with a cylinder timer.
• Direct on standard-rate electricity (27p) — 4,000 kWh at 27p = £1,080 per year. This is the worst case and the one we most often see customers locked into by accepting a direct cylinder quote on a property with a working gas supply.
• Direct on Economy 7 with all heating overnight (14p average) — 4,000 kWh at 14p = £560 per year. Competitive with a heat pump, well above a gas indirect.

The gap between indirect on a gas boiler and direct on standard-rate electricity is £791 per year. Over a 15-year cylinder lifespan that is £11,865 in today's money — and energy prices are not getting cheaper. This is the single largest hidden cost we see in the London cylinder replacement market.

If your existing setup is a working system or regular gas boiler and a vented hot water tank in the airing cupboard, the right replacement is almost always an indirect unvented cylinder of the same configuration. Our London cylinder replacement service covers like-for-like indirect swaps from £1,495 fitted, G3 certified, with the existing boiler reused.

Which to Pick — By Heat Source (Combi, System, Heat Pump, All-Electric)

The decision tree is short. The heat source you already have, or are planning to install, dictates the cylinder type.

• Combi boiler (no stored hot water at present) — Neither. A combi heats water on demand; the entire point of a combi is to avoid stored hot water. If you need stored hot water (a household with multiple bathrooms, or a high-flow shower) the right move is to convert the combi to a system boiler and fit an indirect cylinder. We cover this conversion in our London boiler service hub.
• System or regular gas boiler — Indirect. Always. The boiler is already the cheapest heat source per kWh you have available, and an indirect cylinder is built to be heated by exactly this kind of boiler. The immersion heater is the back-up.
• Air-source or ground-source heat pump — Indirect, and specifically a heat-pump-rated indirect with a 3 to 4.5 m² enlarged coil. Mixergy, Telford Tempest Slimline, Joule Cyclone HP and Range Tribune HP all manufacture heat-pump-rated variants. Never pair a heat pump with a direct cylinder.
• All-electric property, no gas supply — Direct, twin-element, on Economy 7 or Economy 10 if at all possible. The upper element handles peak-time top-ups; the lower element runs overnight for the bulk reheat.
• Solar thermal panel (with a gas boiler back-up) — Indirect twin-coil. Lower coil on the solar circuit, upper coil on the boiler. Around £350 more fitted than a single-coil indirect; pays back over the cylinder life through reduced gas consumption.
• Smart cylinder, mixed renewables — A Mixergy X (top-down heating with cloud control) installed as an indirect with electric back-up gives the most flexibility for changing heat sources over the cylinder lifetime. See our Mixergy installation page for the spec and price.

Regulations: G3, WRAS, BS 7593, Benchmark, BS 6700

Both direct and indirect unvented cylinders are subject to the same regulatory framework because the safety risk is the same — stored hot water at mains pressure, expanding as it heats, with the potential to flash to steam if a thermostat fails. The framework is mandatory; an unvented install outside it is non-compliant, voids manufacturer warranty, and is grounds for buildings insurers to refuse a claim.

• Approved Document G3 of the Building Regulations — Mandates that any unvented hot water vessel over 15 litres is installed by a person with current G3 qualifications (BPEC HWSS or City and Guilds 6189, renewed every five years). The discharge route from the temperature and pressure relief valve must terminate safely in a visible position to the outside, with the pipework sized per the tables in Approved Document G.
• Water Supply (Water Fittings) Regulations 1999 and WRAS — Every wetted component (cylinder body, combination valve, expansion vessel, T&P valve, discharge pipework, isolation valves) must carry WRAS approval under the regulations. The cylinder body itself is type-tested and listed.
• BS 7593:2019 — The standard for treatment of water in central heating systems. On an indirect install the primary circuit must be pre-treated and dosed with an inhibitor (typically Fernox F1, Sentinel X100 or Adey MC1+) per BS 7593:2019. On a direct install BS 7593 does not apply to the stored water side, but the cold water supply still needs to meet the regulations.
• Benchmark — The HHIC commissioning scheme. Every unvented cylinder install must be commissioned and the Benchmark logbook completed and signed by the installer. The logbook is the document the manufacturer asks for to validate a warranty claim — usually 25 years on the cylinder body, 2 years on parts and labour.
• BS 6700 / BS EN 806 — The legacy and current water-services design codes for sizing cold supply and discharge pipework. The D1 discharge pipe from the T&P valve must be copper to BS EN 1057 and sized per the Approved Document G table — typically 22 mm for cylinders up to 300 litres.
• Building Control notification — Mandatory for every unvented install. ERL self-certifies via the WaterSafe Competent Persons Scheme; the household receives a Building Regulations Compliance Certificate by post within 30 days of install. See our G3 engineer page for the certification chain.

The annual service is also mandatory under the manufacturer warranty terms. Our annual unvented cylinder service is £155 fixed and covers the expansion vessel pre-charge check, T&P valve test, anode inspection where fitted, and the Benchmark service record.

Brands, Sizes and Fitted Prices in London

The major manufacturers all produce both direct and indirect variants in the same body. Picking a brand is mostly about warranty length, lead time and engineer familiarity. ERL stocks Megaflo and Telford as the default and orders Joule, Range, Mixergy and OSO to order with one-day lead times across the M25.

2026 London fitted prices, indirect unvented (gas-boiler-rated), supply and fit including G3 certification, Benchmark logbook, expansion vessel, T&P valve, discharge pipework, removal and disposal of the old cylinder, Building Control notification:

• 120 litre — From £1,495. Suits a one-bed flat with one bathroom.
• 150 litre — From £1,595. Suits a two-bed flat with one bathroom, two to three occupants.
• 180 litre — From £1,745. The London sweet-spot for a three-bed terrace with one to two bathrooms.
• 210 litre — From £1,895. Three to four bed, two bathrooms.
• 250 litre — From £2,195. Four to five bed family home, two bathrooms.
• 300 litre — From £2,595. Five-plus bed, three bathrooms, freestanding bath.

Direct unvented variants are £100 less than the equivalent indirect across the size range. The Megaflo Eco Plus carries a £250 premium over the standard Telford or Range body and is the cylinder we fit most often in central London — primarily because the warranty length (25 years on the cylinder, lifetime on the immersion boss) is the longest on the market. See our Megaflo installation page for the full Megaflo Eco Plus spec.

For Telford Tempest (the next most common model in the ERL fleet) see Telford Tempest installation London. For the slim-profile Joule Cyclone (useful where the cylinder cupboard is tight) see Joule Cyclone installation London.

The Five Mistakes We See on Replacement Quotes

1. Direct cylinder quoted on a property with a working gas boiler — The single most expensive mistake. Saves £100 on install, costs £700 to £800 per year in extra running cost. We see this most often on quotes from electricians who do not also do plumbing — they are comfortable with immersion heaters and not with boiler primaries, so they default to direct.
2. Boiler-rated indirect cylinder paired with an air-source heat pump — The 1.4 m² coil cannot move enough energy at 50 degree primary flow temperature, and the cylinder will not reach setpoint in reasonable time. The fix is a heat-pump-rated cylinder with a 3 m²-plus coil. This is the most common heat-pump-cylinder mismatch we see.
3. Twin-coil specified where single-coil is enough — A twin-coil cylinder fitted for a future solar installation that never happens adds £350 to the install cost with zero benefit. Specify twin-coil only when the second heat source is a firm plan, not an aspiration.
4. Wrong expansion vessel size — Both direct and indirect need an expansion vessel sized to roughly 8 to 12 percent of the cylinder volume. Undersized vessels lead to T&P valve weeping and tundish drips within months. See our expansion vessel replacement page for the fix.
5. No second immersion heater on a direct cylinder — A direct cylinder with a single immersion is technically permitted but practically a downgrade. A twin-element direct cylinder costs £20 more in materials and gives the household upper-only or full-tank heating flexibility. Always specify twin-element on a direct install.

FAQs

The FAQ schema at the foot of this page covers: the structural difference between direct and indirect, why direct cylinders are incompatible with combi boilers, the relative install cost, heat-pump compatibility, when Economy 7 makes a direct cylinder economic, and whether you can convert between the two types.

If you need help picking between direct and indirect for a specific London property — or want a second opinion on a quote you have already received — call 0207 046 1363 or WhatsApp 07456 975436. A 5-minute conversation with the property type and existing heat source is usually enough to give a confident recommendation. For broader context on cylinder upgrades, see our London unvented cylinders hub or our central heating service page.

Save the number — 0207 046 1363 or WhatsApp 07456 975436. G3 certified, Benchmark on every install, 25-year cylinder warranty.

John Alexander N. — Director, Emergency Repairs London