top of page
  • Writer's pictureLee Morris

Do you know a Combi Boiler from an Unvented Cylinder?

Do you know a combi boiler from an unvented cylinder?

Some of the terms you might read when researching a new heating / hot water system, what they are, and why you might need to know.

Combi Boiler

What is it?

A combination or 'combi' boiler is both a high efficiency water heater and a central heating boiler in a single compact unit. Combi boilers heat water directly from the mains when you turn on a tap, so you won't need a hot water storage cylinder or a cold water storage tank in the roof space.

Good for – a house with one bathroom, a flat. Instant hot water that won’t run out.

Bad for – a house with multiple bathrooms and occupants. Running 2 showers at once for example could cause problems.

Unvented hot water cylinder

What is it?

An Unvented Hot Water Cylinder is a pressurised system which is fed directly from the cold mains water feed, and therefore can deliver hot water at mains pressure, which is great for improving the performance of your shower and bath.

You might have heard of the popular Megaflow by Heatrae Sadia.

Good for – a house with multiple residents, and/or bathrooms.

Bad for – A small flat with single occupant, it would be a bit unnecessary.

Vented hot water cylinder

What is it?

Vented cylinders (also called open-vented cylinders) need to be supplied by a large tank of cold water which is kept in the loft. The natural pull of gravity carries this water down the pipe work to the hot water cylinder which heats the water.

Pro’s – It can be a cheaper system to install than an unvented cylinder.

Con’s – You need cold water tanks in your loft to feed it, which takes up space.

Good for – A property with poor mains water pressure.

Immersion Heater

What is it?

Immersion heaters heat water directly from inside the tank. A strong electric current is passed through the element which heats the water around it. The immersion heater is wired into the mains electrical supply via an isolating switch.

Good for – A property which doesn’t have a gas supply.

Bad for – A property needing constant instant hot water.

Thermostatic Radiator Valve (TRV)

What is it?

A TRV is a self-regulating valve fitted to a central heating system radiator, to control the temperature of a room by changing the flow of hot water through the radiator.

Why would you want it?

To control your heating from room to room. Each radiator can be set to it’s own temperature.

Flow rate

What does it mean?

This is the rate that you receive gas or water into your property. The type of gas meter and the pipework size dictates your flow rate. Boilers will require a minimum rate, and if you have an insufficient rate for your boiler, it will not work properly.

Why do you need to know?

To ensure your appliances are working efficiently and safely, and comply to the manufacturers warranty.

Water Pressure

What does it mean?

The pressure of the water in your property's mains water pipework.

Why do you need to know?

This could dictate the type of heating system, appliances and shower you require.

This covers questions I’m quite often asked. If there’s anything I’ve missed off that you would find helpful, please ask.


Recent Posts

See All


Call for a quote

07985 278542
24 Hour Emergency Call Out​
bottom of page