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Heating: Beginner's Guide

A beginner's guide to heating your home including vented and unvented systems, choosing a boiler, choosing the right size boiler and a table comparing different fuel costs.

The system that provides your home with heat and hot water is the part of your renovation or self-build project that will present the most options. Picking your way through them and choosing the right elements to suit your home and lifestyle is vital, because this same system is also the part of the home that is most likely to go wrong (and, when it does, cause you huge irritation). It is also, with the exception of the electrics, the only part of your home that produces its own running costs. Added to this, it’s the one area homeowners know least about; and it’s also the single most fast-changing aspect of the housing world.

So it’s no wonder that the majority of renovators and self-builders leave it to others to decide, or even worse pick out a standard solution. It is often these same renovators and self-builders who come to regret not spending more time researching.

The good news is that renovating, extending or, even better, building your own home is the perfect opportunity to seize control of those mysterious pieces of kit and get a basic understanding of how the thing works (and is meant to work) so you can begin to make some decisions.

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The Basic Heating System

At its simplest level, think of a heating system in two parts: the bit that generates the heat, and the bit that distributes that heat around your home. You should also, at this early stage, understand that heat is required in two forms — for space heating (i.e. keeping you warm) and for hot water (i.e. for showers etc). So, the simplest of all systems would have:

  • a boiler (which uses power to heat up water and incorporates a pump to move it around);
  • piping (to move that warm water around your house);
  • emitters (whether it be radiators or underfloor heating);
  • hot water cylinder (to store hot water for use as required, although these are not required with a ‘combi’ boiler — more on which later).

(MORE: Underfloor Heating or Radiators? Choosing Emitters)

Choosing the Boiler

The boiler is the most important part of the heating system and your choice will impact on how the system operates and its efficiency. Your new boiler will by definition be at least 90% efficient (i.e. convert 90% of the energy it uses into heat) and it has to be a condensing boiler (which utilises the heat from the exhaust gasses within the boiler for added efficiency). Most people will opt for a regular ‘system’ boiler but you could also opt for a ‘combi’.

As the name suggests, a combi boiler combines a central heating boiler with a water heater. Hot water is produced directly from the boiler and is given priority over the heating requirements. This is a sealed system and because it heats water instantaneously, there is no need for hot water storage. The main issue tends to be that smaller models struggle to produce the amount of hot water on demand that a larger family needs. Be aware that solar panels won’t work in tandem with a combi boiler system.

(MORE: Boilers Guide)

What Size Do I Need?

Boilers come in different sizes (measured in kW) and you need to specify the right one — a boiler that’s too large will not only be more expensive but will operate less efficiently than an adequately sized model. Bear in mind that plumbers will be more likely to oversize as they don’t want callbacks from problems relating to a small model, and the capital cost is passed on to you anyway. Many of the boiler suppliers offer online guides for choosing the right size. You can have a stab yourself by adding up the required heat output from the radiators or underfloor heating system (taking into account room sizes, insulation levels and window sizes; this can usually be calculated on radiator company websites) then adding 3kW for hot water and a 10% buffer. Typical boiler requirements for a larger detached house would be in the region of 30kW.

The Different Systems — The Vented System

In this type of system, the hot water is heated and stored in a cylinder in the airing cupboard, with the storage tank in the loft supplying cold water directly to the cylinder’s base. The hot water rises to the top of the cylinder (replaced by cold from the storage tank) and is drawn off via a vent pipe to the hot taps. (Because water expands when heated, it needs somewhere to expand to — the vent pipe can accommodate some of this; an expansion tank in the loft takes the excess).

Vented systems can either be direct or indirect — the difference being that indirect systems accommodate a heat exchanger in the cylinder to heat up water in the cylinder itself, while direct systems heat up the water in the boiler for storage in the cylinder (although most direct systems accommodate an electric immersion heater which allows heating of water in the cylinder independently of the boiler).

How to know if you’ve got one: You’ll have a copper cylinder in your airing cupboard and an expansion tank in the loft, as well as a cold water storage tank

The Unvented System (aka Sealed)

In recent years ‘unvented’ systems have become more popular. Unvented cylinders provide hot water throughout the house at mains pressure. The key benefits are good flow rates and fast recharge times (i.e. the time the system takes to refill after being discharged).

Unvented systems require expansion vessels either next to or incorporated within the boiler or cylinder, meaning there is no need for expansion tanks in the loft. As cold water is supplied directly into the cylinder, there is no need for a cold water storage tank. The use of an expansion relief valve ensures the system doesn’t overheat.

How to know if you’ve got one: You’ll have a white cylinder in your airing cupboard and nothing in your loft.

Jargon Buster

Expansion Tank

Because water expands when it heats up, there needs to be room in the heating system to accommodate the additional capacity. In traditional systems this would be in the form of an expansion tank in the loft, but on more modern systems is in the form of an expansion vessel, located either next to or within the boiler or hot water cylinder.

Hot Water Cylinder

The storage vessel which supplies hot water on demand to taps and showers, located in the airing cupboard. Depending on the type of system, it either heats up cold water supplied by the cold water storage tank, stores hot water supplied directly by the boiler, or heats up cold water supplied directly from the mains.


A boiler is a vessel that transfers energy (usually either gas, oil or LPG) into heat to warm up water. The amount of heat it can produce is measured in kW, and typically boilers range in size from 15 to 40kW for domestic applications. It usually incorporates a pump to feed the water through pipes to the radiators.

Heating Fuel Options: Cost Comparisons

Not every home can access mains gas (natural gas). We compare the total cost of different fuel options over two different periods.

Fuel Type 2-Year Cost* Fuel Type 10-Year Cost*
Natural Gas £3,026 Natural Gas £14,130
Electricity £4,220 Solar £16,945
LPG £4,353 Heat Pump £19,052
Oil £6,035 Wood/Biomass £19,198
Solar £7,322 Oil £20,842
Heat Pump £9,335 LPG £21,475
Wood/Biomass £13,224 Electricity £28,747

* Including capital cost and running costs for the period, based on the same 200m2 house built to current Building Regulations (required 15,000kwh per year) and assuming 12% annual change in fossil energy prices, 4% annual rise in wood pellets. Excludes Renewable Heat Incentive Income.

The Least You Should Know

  • Heating systems are either vented (with an expansion tank in the loft) or unvented (with nothing in the loft).
  • Boilers come in different output sizes. You can assess what size you need using online calculators.


One major problem down the track after installation of a Combi Boiler, particularly in small premises, is the failure to install an expansion minivessel on the hot water circuit. The inevitable result is high expansion pressures straining o-rings or washers on Taps, Shower mixers or sink units....leading to significant cost if a new mixer is required to replace one with all the seals blown out.