Skip to main content

Hot water cylinders & unvented hot water cylinders | A Buyer's Guide



How do vented and unvented cylinders work? Which is the best hot water cylinder?

There is a vast array of hot water cylinders on the market (often referred to as a hot water tank). In this guide we talk you through the main vented and unvented hot water cylinder types and when they are appropriate for your system. We review 5 vented hot water cylinders and and 8 invented hot water cylinders from leading manufacturers to find you great value and performance from your cylinder.

Overview of hot water cylinders

Direct and Indirect cylinders

only Indirect are used with central heating boilers.

Open vented copper cylinders

work in conjunction with a cold water storage tank and any boiler.

Unvented hot water cylinders

mains fed, steel hot water tanks, work with any boiler.

‘Tank in tank’ cylinders

high performance hot water cylinders, work with any boiler.

Hot Water Tanks by Type

Direct and Indirect

A ‘direct’ cylinder is heated via an electric immersion and does not work with a gas or oil boiler.

An ‘indirect’ cylinder has coil running through the tank. ‘Primary’ heating water (not fresh water) passes through the coil and is heated by the boiler to around 70 degrees. The heated coil imparts its heat onto the fresh (potable) water in the tank that is used for washing or showering.

Open Vented Copper Cylinders

Open vented copper cylinders work in conjunction with a cold water storage tank. The open vent pipe comes off the top of the cylinder, rises above the cold water storage tank and hooks over into it. The open vent pipe releases excess pressure and heat from the hot water system in the event of an overheat fault situation.

They are not dependent on the incoming mains pressure to deliver hot water at a good flow rate and are best retained if mains pressure is poor. They can be further improved with a pump.

NB it is possible to have an open vented hot water system even if the heating system is ‘sealed’.


  • They provide good hot water flow rates, even if the incoming mains pressure is poor
  • The flow rates can be improved with a pump
  • Copper cylinders are very cost effective to replace


  • Old copper tanks with loose insulation covers are very inefficient
  • They take up space
  • They must be situated at a high point in the system

Indirect Vented

Brand Telford Gledhill Telford Gledhill Heatre Sadie
Material Copper Copper Stainless Steel Stainless Steel Stainless Steel
Standard sizes 90 litres - 220 litres 120 litres - 210 litres 96 litres - 206 litres 90 - 300 litres 100 - 210 litres
Price range £250 - £376 £234 - £526 £225 - £300 £410 - £600 £725 - £1,225
Warranty 10 years 10 years 10 years 25 years 25 years
Insulation Foam insulation 35mm insulation Foam insulation 50mm Envirofoam 50mm Polyurethane
ERP rating (A-F) C - all sizes C - all sizes C: 80 - 117 litres
D: 140 - 290 litres
C - all sizes B: 100 - 120 litre
C: 150 - 210 litres


Unvented hot water cylinders

These are mains fed hot water tanks with an inner coil but no open vent pipe. Instead an expansion vessel allows heated water to expand.

Unvented hot water cylinders are steel rather than copper in order to withstand the increased pressure place upon them. Unvented cylinders come with a whole host of safety devices which must be fitted and maintained by a qualified engineer.


  • When mains pressure is good, they can deliver excellent hot water flow rates
  • They also allow for greater flexibility in siting the unit, i.e. they do not have to be sited at a high point in the system


  • They are more expensive to install
  • They must be installed and maintained by a qualified engineered
  • There are more components subject to long term maintenance costs

Unvented Steel

Brand Telford Heatre Sadia Gledhill Range (Kingspan) Worcester Vaillant Heatre Sadia
  Tempest Stainless Megaflo Eco Stainless Lite Tribune xE Greenstore Unistor Standard Premier Plus
Standard sizes 90 litres - 300 litres 70 litres - 300 litres 90 litres - 300 litres 120 - 300 litres 90 - 300 litres 120 - 300 litres 100 - 300 litres
Price range £480 - £620 £700 - £1275 £515 - £710 £680 - £1125 £670 - £950 £625 - £990 £571 - £805 (including fittings)
Warranty 30 years 25 years 25 years 25 years 25 years 25 years 30 years
Insulation High density foam 50mm foam 50mm Envirofoam 50mm Polyurethane 50mm polystsyrene 50mm expanded polystyrene 50mm insulation
ERP rating (A-F) C - all sizes B: 70 - 210 litre
C: 250 - 300 litres

A: 90 litres
B: 120 - 210 litres
C: 300 litres

B: 120 - 210 litres
C: 250-300 litres
B: 90-210 litres
C: 250 - 300 litres
A: 120 - 150 litres
B: 180 - 300 litres
B: 100 - 120 litres
C: 150 - 300 litres


Jo & Caroline - Heating Heroes

Hero Support

With our years of experience and research, we get you the best boiler package for an efficient, long-lasting boiler


‘Tank in tank’ cylinders

These tanks operate in a very similar way to indirect cylinders, but rather than passing hot system water via a coil, an inner tank of fresh water sits within an outer tank of primary water. The wall of the inner tank transfers heat from primary to fresh water. The most commonly installed in the same way as an unvented hot water tank.

The main advantage of these tanks is the speed with which it can heat water. In households with a high hot water demand, a tank in tank set up will keep up with multiple showers better than an indirect cylinder or unvented hot water tank.


Faster hot water ‘recovery’ times, the unit replaces hot water as quickly as it is used
As hot water is close to being instantaneously produced, the tanks can by much smaller


  • Expensive to purchase
  • They must be installed and maintained by a qualified engineered
  • There are more components subject to long term maintenance costs

Thermal stores

Many homes built in the 1990s and early 2000s were fitted with ‘Gledhill’ thermal stores. These were over-engineered cylinders that provided both heating and hot water, in conjunction with a boiler. Users experienced a catalogue of problems and they are generally being removed.

Modern thermal stores are used for combing multiple heat sources as well as hot water production. They are commonly used when combining a renewable energy or solid fuel stove with a conventional boiler.

Hot water production is usually via an external heat exchanger, such as those used by combi boilers.

Tips for picking the right cylinder

  • If you have a vented cylinder, you are happy with its performance and you do not need the extra space, it is cost effective to replace it with a better insulated vented copper cylinder
  • If you need the space consider moving your vented cylinder into the loft
  • If you have poor incoming water pressure, definitely stick with your vented cylinder
  • If you are adding bathrooms to your property and need to move the cylinder, consider a unvented cylinder or ‘tank in tank’, providing your water pressure is good


Jo Alsop

Heating Hero

The Heating Hub

Let us take care of everything for you and deliver the best heating system for your budget