What are wet heating systems?

Wet, or water, heating systems heat a home by heating water centrally in a boiler.  The heated water is then pumped around the home in pipes.  It passes through radiators which have large surface areas.  These surfaces radiate the heat out into the rooms.

Homesmart only currently provides smart solutions for wet heating systems.

So if you have a different heating system, for example:

  • electric radiators
  • electric underfloor heating
  • hot air
  • solar heating
  • heat pumps

then we are sorry that we cannot help you.

However, there are a wide range of boiler types that work with wet heating systems, as set out below

Open vent

Open vent, or unpressurised, heater systems are generally older.  The water in the central heating is topped up from a cold water header tank.   Topping up is done to replace any water lost to evaporation or leakage.

The cold water tank needs to be higher than all the pipes and radiators in the system so is normally located in the loft.


Modern heating systems are normally pressurised.  This means that they have a pressurisation unit which takes care of expansion and contraction of the water being pumped round the system.

Being closed systems, pressurised systems stay cleaner and are less likely to have air blocks.

They also take up less space since they do not need tanks of cold water to top up the system.

Different types of boiler

There are many different types of boiler.  They vary in the fuel they use as well as how they work.  Let's unpick some of the differences.

Firstly, the fuel which is easy to understand.  Boilers that work with wet heating systems could be fuelled by gas, electricity or oil.  All of these are compatible with smart heating systems.

Secondly, what the boiler does:


A combi boiler has a special system to heat water very quickly.  So when you turn on a hot water tap, it is able to provide hot water for washing or cleaning on demand.

This means that you do not have to have a cylinder to store hot water ready for use.

The type of boiler is named a "combi" because it combines the production of hot water for your taps with the (separate) heating of the hot water that is pumped round to your radiators to warm your rooms.

Combi boilers are becoming very popular because they

  • produce hot water efficiently when you want it
  • free up space used by hot water cylinders and cold water tanks in other systems
  • provide hot water under pressure which might avoid the need for power showers
  • reduce pipework


Traditional boilers are also known as regular, standard, conventional or heat only boilers.  They require a hot water cylinder and a cold water header tank in the loft.

They work well with older heating systems although they can also be used in conjunction with solar heating.



System boilers provide most of the heating used to produce hot water for washing and for radiators.  However they do require a hot water cylinder to store hot water.

The hot water cylinder normally contains enough water for several people to wash, have a bath or have a shower.  However, it takes some time for the water to heat up again once used.

System boilers do not require a cold water tank in the loft.  They can also work alongside solar heating for a flexible system making use of renewable energy.


A back boiler is a device which is fitted behind a stove or fireplace.  It uses the some of the heat provided by the burning fuel to heat water in a heat exchanger which can then be circulated round the house to heat up radiators or a hot water tank.

The back boiler outputs hot water from the top while cold(er) water is fed in at the bottom.  The water is circulated by gravity with hot water rising (e.g. to heat an upstairs hot water cylinder) and losing its (most of) its heat before returning to the back boiler.

Underfloor heating

From which?  www.which.co.uk/reviews/underfloor-heating-systems/article/underfloor-heating-explained/water-underfloor-heating

With a water-based underfloor heating system, a series of pipes connected to your boiler circulate warm water throughout the floor to heat the space. Alternatively, you can connect the water underfloor pipes to a solar water heating system, air source or ground source heat pump.

When we asked water underfloor heating owners* where they had their underfloor heating installed in their home:

  • 75% had it in their kitchen
  • 67% had it in their living room
  • 56% had it in their hallway
  • 42% had it in their bathroom.

The highest percentage of people (25%) had it installed in two rooms, while for electric underfloor heating, most people (66%) had it installed in just one. They most commonly had it fitted in their bathroom, while the kitchen second most common.

If you have underfloor controller you will also need a room thermostat.
You can control multiple zone with one underfloor controller but reality is that you won’t be able to because they are physically apart - i.e. bathroom and kitchen are most common. So in that case you will need 2 unless they are close to each other.
Hand adjusting the temperature of a central heating radiator by turning a manual TRV

How room temperatures are measured in a multi-zone smart heating system

Smart TRVs (Thermostatic Radiator Valves) fitted to radiators do a great job of sensing the general temperature of a room in most cases.  They will send signals to turn on or off the boiler as necessary as well as turning on or off the flow of hot water to the radiators in the room.

However, if all the radiators in a room are boxed in or if there is only underfloor heating, then you will need to have a single zone thermostat in the room (e.g. stuck on a wall) to determine the temperature and send the signals to control the heating.

If the room is very large, you may need to define it as two zones and have two radiators at different ends of the room each monitoring the temperature and controlling heat at each end.