Boilers

Getting the Right Size

It was once common practice to oversize boilers by around 30%, but nowadays this is considered wasteful, making it important to specify the right size for your home. This is decided by means of a heat loss calculation and will be affected by the house’s size, the materials used and the level of insulation and airtightness, as well as your hot water requirements, i.e. how many bathrooms are there? A heating engineer can work this out for you, but you can also use a free online calculator — try the one at sedbuk.com.

Once you know the heating requirements of each room in kilowatts, you can size your heating emitters – i.e. radiators or underfloor heating – then you will know the boiler you buy will be big enough to heat the whole system efficiently.

Condensing Boilers

Before the advent of the condensing boiler, the heat contained within the combustion products that were discharged from the boiler – which could be anything up to around 180°C – were simply wasted.

Condensing boilers were designed with the purpose of extracting as much of the heat energy in the fuel as possible, and turning it into useable heat to warm the house. This means they burn less fuel for the same amount of heating. Since 2005, condensing boilers have been made compulsory by the Building Regulations for new domestic central heating boilers.

Where to Position it

Boilers are ideally positioned on the ground floor in houses (a requirement for oil-fired boilers), within the kitchen or a utility area. Careful consideration must be given to the position of the flue as condensing boilers produce a plume of steam which can be a nuisance. See the Building Regulations Part J for detailed advice and diagrams.

Integrating Renewables

A well-sized solar water heating system could provide up to 60% of your annual hot water requirements, but careful thought needs to be given to how the central heating system will work as a whole. Firstly, you must buy a compatible boiler — not many combi boilers work with solar, but most regular and system boilers will be fine. Secondly, you will require a twin-coil cylinder, which will be heated via heat exchangers from both the boiler and the solar system. If hot enough, the volume of stored water can be fed directly to the tap (i.e. 60°C plus). If the water is not up to temperature, the boiler compensates, using less fuel overall.

Ground-source heat pumps are often designed to be the sole source of heating and hot water in the home, but many models can be configured to take a supplementary heat source such as an oil or gas-fired boiler.

Buying the Best Boiler

It's important to buy the most efficient boiler you can, close to 90% (check sedbuk.com), in an appropriate size. Aim for at least a five-year warranty. The boilers that work to the highest standard are the fan-assisted room-sealed type — i.e. it takes air from outside the building and combustion products are forced out using a fan.

Which type? 

COMBI

How they work: Combis work as sealed systems, providing hot water for both the taps and central heating system, heating the water directly from the mains as and when it is needed — meaning there is no need for a hot water storage cylinder, or a cistern in the roof space.

Advantages: Combis are quicker, easier and cheaper to fit than system boilers, as well as space saving due to the lack of a cylinder or cistern. Water is delivered at mains pressure, so you can enjoy a more powerful (although not ‘power’) shower.

Disadvantages: It’s a priority system, so it only satisfactorily deals with one heating need at a time. While fine for small families with one bathroom, larger families will experience poor flow rates when multiple outlets are used at once. Performance is also dependant on the diameter of the pipe entering the property: if it’s less than 22mm, then a combi is a bad choice.

SYSTEM

How they work: System boilers are fitted to sealed heating systems, but unlike combis work on the principle of storing hot water in a cylinder, so they can feed several outlets at once at mains pressure. There’s no need for a cistern in the loft and the expansion vessel is built in.

Advantages: Ideal for larger homes with higher demands, and as they have most of their major components built in (i.e. expansion vessel and pump), installation is quicker, cheaper and neater. Flow rates are usually high as water is delivered at mains pressure, and hot water is instantaneous.

Disadvantages: Will run out of hot water if overused. Some installers claim they are more complex and prone to problems than regular boilers, such as pressure loss.

REGULAR/CONVENTIONAL

How they work: Regular boilers are now largely bought as replacements for homes with an open-vented heating system (i.e. supplied by means of a feed and expansion cistern in the roof space, meaning the system is open to air). Like system boilers, they work on the principle of stored water and require a separate hot water cylinder.

Advantages: The water out of the taps will be at a good flow rate (not to be confused with pressure) and hot water can be supplied instantaneously. This is the ideal setup for a ‘power’ shower, which requires a cold water feed from the cistern and a separate electric pump.

Disadvantages: They’re more expensive to install, needing more components and pipework, and also take up more space. They can suffer from low pressure if the cistern is not located high enough, meaning additional shower boosters may be required. Hot water can run out.

Note: Sealed or Open? In a sealed circuit, the system is filled to approx 1 bar pressure and then sealed. Unlike an open-vented system, there is no cistern, so an expansion vessel handles excess water. Sealed systems are more efficient.

Main components of a sealed heating system

A boiler heats water on demand (combi) or tops up the cylinder (system).	The cylinder stores a supply of hot water for instant use around the house. When it gets up to temperature, a thermostat switches off the supply.
An expansion vessel is required in sealed, pressurised systems to accommodate any excess water.	A range of controls are required to operate the system. On a more advanced setup, controls can be weather-compensated.

The Installation

Unless you happen to be a qualified, registered installer, you cannot fit your own boiler — so check with Gas Safe (gas) or Oftec (oil) that your installer is registered. However, you can keep an eye on things to ensure that they are doing a proper job (it’s worth contacting a few installers initially to see if their advice varies). The first thing you need to check is that they have correctly sized the boiler and system. As mentioned on p.144, you can get an idea yourself by using the calculator at sedbuk.com, where you can also evaluate boiler ratings.

You must ask your installer to demonstrate how their advice complies with the recent changes to the Building Regulations Part L. You can view Part L at planningportal.gov.uk, along with Part J for guidance on the variety of flue options, siting and information on the condensate. A common problem is for the condensate pipe to freeze, so internally check it is a minimum of 22mm in diameter and externally at least 32mm. Check all pipework is insulated.

Ensure that the heating system is properly cleansed and flushed before the boiler is installed, and that inhibitor chemicals are added afterwards.

Inside a Boiler

1. Fan Assists the evacuation of combustion gases

2. Gas valve

3. Boiler thermostat Controls the temperature of the hot water flowing from pipes to the radiators

4. Flow connection

5. Drain point For draining the system

6. Return connection

7. Heat exchanger The hot gases transfer their heat to the water

8. Air/gas manifold

9. Syphonic trap Disposes of the condensate by discharging a given quantity of condensate fluid at a time, reducing the chance of freezing

Heating Controls

A modern central heating system uses a range of controls to work at optimum efficiency. New (gas or oil) boilers should include the following:

Full programmer: Allows the water to come on at specific set times.

Room thermostat: The thermostat has boiler interlock, so when a set temperature is reached, an electrical contact is broken inside the thermostat to switch off the electrical supply to the heating circuit.

Cylinder thermostat (where applicable): Fixed to the cylinder, so that when the top reaches around 60°C, it switches off the electrical supply to the heat exchanger coil.

TRVs: Thermostatic radiator valves should be on all radiators, except in rooms containing a thermostat. It automatically closes off the water supply to the radiator when the desired temperature is reached.

Automatic bypass valve (if necessary): The pump runs for a few minutes after the boiler has been turned off; this stops static water in the heat exchanger boiling due to residual heat.

Dwellings should normally be divided into two space heating zones with independent temperature control (one in the living area). The heating circuit and hot water must be controlled separately.