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Which Wiser Kit Do I Need? The Most Common Domestic Boiler Systems Explained

Wiser smart thermostats are compatible with most boiler makes and models, but it’s important to consider which boiler system type you have when selecting your Wiser kit. Here we’ll guide you through the most common boiler system types to help you find the ideal Wiser products to suit your home. Also try our online Product Selector to find the right kit in just a few short steps.

 

COMBI BOILERS – WITH ONE HEATING ZONE

A combination or ‘combi’ boiler system combines a water heater and central heating boiler into one compact unit. These space-saving systems are increasingly popular amongst homeowners upgrading their older central heating systems and, according to the Energy Saving Trust, may be more efficient for smaller households with a low hot water demand – when hot water is heated and stored in a separate tank, heat can be lost when it’s not used.

CHARACTERISTICS OF COMBI BOILERS (one heating zone)

  • Central heating and hot water is supplied by a single boiler unit, with no separate water tank.
  • Fed directly from cold mains water pressure.
  • Hot water is available on demand when hot water taps are used.
  • Requires a one-channel Wiser kit (Wiser Thermostat Kit 1 or Wiser Multi-zone Kit 1)

 

COMBI BOILERS – WITH TWO HEATING ZONES

Now that we’re clear what a combi boiler is, some combi boiler systems have two separate heating zones, controlled by two thermostats.

CHARACTERISTICS OF COMBI BOILERS (two heating zones)

  • Central heating and hot water is supplied by a single boiler unit, with no separate water tank.
  • Fed directly from cold mains water pressure.
  • Hot water is available on demand when hot water taps are used.
  • Requires a three-channel Wiser kit (Wiser Thermostat Kit 3) – note that you won’t require the third channel, because the hot water control in a combi system is not separate. A two-channel kit is not suitable for a combi system with two heating zones as the second channel in the two-channel kits is only suitable for hot water control in a conventional boiler system (with a separate hot water tank).

 

ADVANTAGES OF COMBI BOILERS

  • Ideal for smaller homes, due to not requiring a separate water tank.
  • Installation and servicing is usually cheaper than other system types due to less equipment and plumbing - required.
  • Hot water is available on demand independently of the central heating, without having to wait for a water cylinder to heat up.
  • May be more economical to run for smaller households than other boiler system types according to the Energy Saving Trust.

DISADVANTAGES OF COMBI BOILERS

  • A combi boiler requires good water pressure, so may not be suitable in some residential areas where pressure is low.
  • Combi boilers only allow for one hot tap, bath or shower to be running at a time, by running more than one it will significantly reduce the pressure to each, which can be a problem for larger households with high demand for hot water.

 

CONVENTIONAL BOILERS (with unvented hot water cylinder)  ONE HEATING ZONE

Also known as sealed or system boiler systems, a conventional boiler system with an unvented hot water cylinder uses a hot water cylinder that’s separate to the boiler but does not require a cold water tank usually typical of vented gravity-fed systems.

CHARACTERISTICS OF CONVENTIONAL BOILERS (with unvented cylinder and one heating zone)

  • Consists of a central heating boiler and a separate, pressurised hot water tank fed directly by the cold water mains feed, so the tank heats and stores hot water at mains pressure.
  • Absence of a cold water tank.
  • Requires a two-channel Wiser kit (Wiser Thermostat Kit 2 or Wiser Multi-zone Kit 2), to control the central heating via one channel and hot water via the other.

 

CONVENTIONAL BOILERS (with unvented hot water cylinder) – WITH TWO HEATING ZONES

This is set up in the same way as regular conventional systems, but in this case the system has two heating zones being controlled by the boiler, with separate programming for each usually with two thermostats. This may be the case in larger or extended homes.

CHARACTERISTICS OF CONVENTIONAL BOILERS (with unvented cylinder and two heating zones)

  • Two heating zones on separate thermostat controls.
  • Pressurised hot water tank that’s separate from the boiler.
  • Absence of a cold water tank.
  • Requires a three-channel Wiser kit containing two smart thermostats (Wiser Thermostat Kit 3), to control the central heating via two of the channels and the hot water via the third.

 

ADVANTAGESOF CONVENTIONAL BOILERS

  • Does not require a cold water feed tank, therefore more suitable for smaller homes.
  • Provides hot water to any number of taps at mains pressure.
  • More economical to run than a gravity-fed system.
  • Stores hot water, so beneficial for larger families that use lots of hot water.
  • Allows separate hot water control, unlike gravity-fed systems.

DISADVANTAGES OF CONVENTIONAL BOILERS

  • Hot water flow depends on the cold water mains pressure, so if this is supply is lost, you will soon lose the hot water supply.
  • Hot water is not available on demand as with a combi boiler.

 

CONVENTIONAL BOILERS (with vented hot water cylinder and upgraded, fully pumped central heating)

This type of system is effectively an upgraded gravity-fed system, where the central heating has been upgraded to be fully pumped, while the hot water remains gravity-fed. This type of upgrade allows separate central heating and hot water timer control, unlike standard gravity-fed systems, due to the addition of motorised valves and a cylinder thermostat.

CHARACTERISTICS OF CONVENTIONAL BOILERS (with vented hot water and fully pumped)

  • A system that was originally fully gravity-fed that has been upgraded/retrofitted with motorised valves, such that the central heating is now fully pumped - a recommended upgrade if full replacement of an older gravity-fed system is not possible.
  • Timer and thermostat control of the central heating is available independently of the hot water, due to a cylinder thermostat fitted to the hot water cylinder.
  • Four 22mm or 28mm copper pipes connected to the boiler (sometimes three pipes if an injector T is fitted, but a fourth 22mm or 28mm copper pipe will still be nearby).
  • A cold water feed or expansion tank is situated higher than the hot water cylinder, usually in the loft or attic.
  • Cold taps are fed by mains pressure (so turning off mains water will only turn these off).
  • Requires a two-channel Wiser kit (Wiser Thermostat Kit 2 or Wiser Multi-zone Kit 2), to control the central heating via one channel and hot water via the other.
  • May also be referred to as BiFlo, TwinZone, Y Plan, S Plan, S Plan Plus.
Try this to identify this system type:

On the 2-channel programmer where you set your schedules, try setting the hot water side to off and the central heating side to on. If only the central heating side comes on and only your radiators get hot, you have independent control and thereby, a fully pumped system.

If, however, but turning on the central heating side, the hot water also turns on, this would indicate a gravity system.

 

ADVANTAGES OF CONVENTIONAL BOILERS (with vented hot water and fully pumped)

  • Can provide good pressure in some residential areas where mains water pressure is low, particularly if the water circuit is also upgraded with pumps.
  • Hot water can be stored within the cylinder, beneficial for larger households where a lot of hot water is used.
  • Older systems upgraded with pumps provide better thermostatic control of the central heating and the ability to control it independently of the hot water.

DISADVANTAGES OF CONVENTIONAL BOILERS (with vented hot water and fully pumped)

  • Typically less efficient and more expensive to run than more modern systems.
  • Less suitable for smaller homes due to the requirement for a water cylinder and cold water tank in addition to the central heating boiler.
  • Sludge and debris can accumulate in cold water tanks which will need clearing periodically.

 

GRAVITY-FED SYSTEMS (conventional systems with vented hot water cylinder and non-pumped central heating)

An older and more traditional system type, gravity-fed (or vented) installations feature a conventional boiler and utilise gravity and convection to heat water and distribute it to taps and radiators, rather than the water being pumped. Cold water from tank(s) in the loft flows down by gravity to a water cylinder where it is heated, it’s then forced to hot water taps and radiators at low pressure.

Wiser is not compatible with traditional gravity-fed systems, because this type of system normally lacks water cylinder thermostats, motorised valves and external temperature controls, resulting in very limited thermostatic control which Wiser needs in order to work.

Wiser thermostat and multi-zone kits 2 and 3 may be compatible with gravity-fed systems that have been retro-fitted or upgraded with pumps and cylinder thermostats - Please speak to our Technical Support Helpline on 0333 6000 622 for further advice on your system’s compatibility.

Gravity-fed systems are no longer fitted new and do not comply to current building regulations, therefore homeowners are increasingly opting to replace older gravity-fed systems to either combi or fully-pumped conventional boiler systems.

CHARACTERISTICS OF GRAVITY-FED SYSTEMS (non-pumped)

  • A distinctive feature of a gravity-fed system is four 22mm or 28mm copper pipes connected to the boiler (sometimes three pipes if an injector T is fitted, but a fourth 22mm or 28mm copper pipe will still be nearby).
  • A hot water cylinder is situated higher than the boiler (normally in an airing cupboard) and lacks a cylinder thermostat.
  • A cold water feed or expansion tank is situated higher than the hot water cylinder, usually in the loft or attic.
  • Cold taps are fed by mains pressure (so turning off mains water will only turn these off).
  • The central heating is not available independently of the hot water.
  • Typically there is only ON/OFF boiler control, without any time controls.
  • In residential areas with low water pressure a gravity or gravity/pumped system is usually the best option. Some systems incorporate a pump to help provide greater pressure at the taps.
Try this to identify this system type:

See if you have a time controller on your system or if it just on and off at the boiler. Controlling your system by turning the boiler on an off could indicate a gravity system.

On the 2-channel programmer where you set your schedules, try setting the hot water side to off and the central heating side to on. If doing this automatically turns on the hot water side as well, this would indicate a gravity system.

 

ADVANTAGES OF GRAVITY-FED SYSTEMS (non-pumped)

  • Can provide good pressure in some residential areas where mains water pressure is low.
  • Hot water can be stored within the cylinder, beneficial for larger households where a lot of hot water is used.

DISADVANTAGES OF GRAVITY-FED SYSTEMS (non-pumped)

  • Typically less efficient and more expensive to run than more modern systems.
  • Central heating is not available independently of the hot water – water will always be heated when the heating is on whether it’s used or not.
  • Less suitable for smaller homes due to the requirement for a water cylinder and cold water tank in addition to the central heating boiler.
  • The usual lack of a water cylinder thermostat and motorised valves means very limited thermostatic control.
  • Not compatible with Wiser due to the inability to control hot water separately.
  • Sludge and debris can accumulate in cold water tanks which will need clearing periodically.
If you have a gravity-fed system, Wiser will only be compatible if it is fully pumped, by that we mean upgraded with motorised valves and a cylinder thermostat.

 

If you're not sure whether your system is fully pumped, would like to know what changes may need to be made to your gravity-fed system to be compatible with Wiser, or would like to purchase Wiser for a fully pumped system, we encourage you to to contact our Support Team who'll help you find the kit you'll need, call 0333 6000 622 today.

To find the right Wiser kit for you and find stockists, don’t forget to try our Product Selector today to find the right Wiser kit for you today in just a few short steps.