Boiler reviews: Features explained
Inside a typical condensing boiler
Gas condensing boilers
Nearly all new boilers are now condensing types. Condensing boilers are far more energy-efficient than traditional boilers, as they cunningly reuse heat that would otherwise be wasted.
Some heat generated by traditional boilers disappears up the flue in the form of hot waste gases. But condensing boilers use some of the heat from these flue gases to heat water, making the boiler far more efficient.
Types of boiler
There are two main types of condensing boiler: combination or heat-only.
Combination boiler
This is the bestselling boiler type for gas central heating in Britain. A 'combi' provides heat for your radiators and domestic hot water on demand.
The name comes from the fact it combines all the functions of a traditional boiler, hot water cylinder and cold water tank setup. All the major parts are in one unit, which makes them convenient to install.
There are different types, but they all work by taking cold water straight from the mains supply and heating it as you need it. When you turn on a tap, water is drawn from the mains, heated in the boiler and comes straight out of your tap piping hot.
This means there's no need for a separate hot water storage cylinder or cold water feed cistern. But there's a small time lag between turning on the tap and getting hot water, and the flow rate is limited by how quickly the boiler is able to heat up the water.
The advantage is they require less space than heat-only boilers (because there is no storage cylinder), and avoid heating water that's not used immediately. On the down side, the flow rate limits how much water you can use at one time – no filling the kitchen sink while running a bath, for instance.
You might also miss using the (wasted) heat from your hot water cylinder in your airing cupboard.
A heat-only boiler
Conventional heat-only boiler
Heat-only boilers provide heat for radiators directly, but they need to be connected to a separate hot water storage system including a hot water cylinder, cold water cistern and, in some cases, an additional expansion cistern.
These arrangements take up more space than a combi system, but are able to provide hot water to several sources at once and are more suitable for larger houses.
Unless a cylinder is very well insulated it usually loses heat into its surroundings, thereby wasting energy. And the amount of hot water you can use is limited by the size of your cylinder.
For instance you might run a bath, then need to wait for the boiler to heat up the water in the cylinder again before you can run another.
Boiler size
A boiler is sized to suit your home's hot water demands
Most manufacturers make each of their boilers in a range of different heat outputs, typically ranging from 12 to 40kW.
A higher number means a boiler can heat water more quickly, so it can supply hot water to more radiators or taps. The greater the number of radiators or taps you're likely to use at a time, the higher the boiler kilowattage you're likely to need.
As homes have vastly different numbers of radiators and bathrooms these days, it's difficult to generalise about what size boiler is needed for a flat, cottage or larger house. Your boiler will be sized according to the amount of heat and, in the case of a combi boiler, hot water you require.
Your installer will size the boiler according to how many rooms and radiators you have, as well as the number of hot water taps or devices that use hot water.
When buying a new boiler you'll need to get your hot water demand assessed individually by your plumber or installer.
Controls
Effective heating controls can increase the efficiency of a central heating system. They can also help you minimise energy consumption by ensuring each room is at the right temperature for comfort, while avoiding overheating.
Timer or programmer
An electronic timer or programmer decides when the boiler is able to run. A seven-day timer makes it possible to set a different heating pattern for weekdays and weekends.
Some timers allow different patterns for each day of the week; this can be useful for people working part-time or on shifts.
Intelligent heating controllers
These combine several of the basic controls and can also learn how long it takes for a house to heat up in different weather conditions. They also often allow for different temperatures to be set between day and night.
Weather compensators
These measure the temperature, either internally or externally, and delay switching on the central heating on milder days.
Full zone control
Most homes have a single heating zone – the only controls in the rooms are by TRVs (see below).
When a new central heating system is installed, it's possible to fit a full zone control that has different pipe loops and separate thermostats for two (or more) areas. This can save significant amounts of fuel in larger houses
Cut heating costs by turning your thermostat down
Thermostats
Room thermostat
This is best located in a living room or bathroom, which you'll probably want to be the warmest room in the house. The thermostat records the room's temperature, and if this is at or above the set level (20°C is usually adequate), stops the boiler from operating the central heating.
A programmable room thermostat lets you choose what times you want the heating to be on, and what temperature it should reach while it's on. Some allow you to select different temperatures in your home at different times of the day and days of the week.
Thermostatic radiator control valves (TRVs)
These detect the local air temperature and switch individual radiators on or off, depending on how hot the room is. It's not a good idea to have a TRV on the radiator in the same room as the main thermostat as, if you alter the TRV, the thermostat will adjust itself to be in line with this local radiator.
They usually have a fat valve at one end, marked with a * and numbers from one to five. The * setting is to protect against frost.
Cylinder thermostat
This thermostat is strapped to the outside of a hot water cylinder near the bottom. It works by sensing the temperature of the water inside the cylinder, switching on the water heating when the temperature falls below the thermostat setting.
Oil-fired boilers
More than 1.5 million homes in the UK use oil for their heating, and this number is growing as more homes are built in the countryside, away from mains gas.
The average cost of oil heating for a three-bedroom house was £1,044 in 2007-2008, compared to £568 for mains gas and £1,310 for liquid petroleum gas (LPG), the other fuel of choice in the countryside.
Most UK domestic oil-fired boilers use kerosene.
Types of oil-fired boiler
There is a wide range of external, cabin and internal (kitchen or utility) condensing oil-fired boilers on offer.
Heat-only and combination condensing oil-fired boiler types are both available. Most oil-fired combination boilers have an internal hot water store to supply domestic hot water rather than the instantaneous heating more common in gas boilers.
Most condensing oil-fired boilers are floor standing. Many have extended flue options which help to site the flue terminal so that the plume of water vapor they produce will not be a nuisance or visual annoyance.
They will also need to be plumbed in to allow acidic condensate liquid to drain away.
Efficiency
To conform with building regulations, since 1 April 2007 oil-fired boilers must have a Sedbuk efficiency of 86% or more (unless an exception is allowed). In practice, only condensing oil-fired boilers can meet this efficiency requirement.
Typically a new condensing oil-fired boiler will have an efficiency of 92 to 93% compared with 85% for a new non-condensing boiler and 60 to 70% of older systems.
Hot water and oil-fired combi boilers
Oil boilers generally limit the hot water flow rate to ensure the water is as hot as it should be. The hot water flow rate is lower than a gas combi or hot water cylinder system.
We also found that it’s common for the hot water temperature to decline as more water is used.