How to Choose a Central Heating System

The concept of space heating was born in the 1950s. Before that, home heating was an ad hoc affair, a coal fire in a grate or an electric two-bar fire moved around the house.

The idea that you could enjoy constant background heat throughout the house, even in the depths of winter, was quite revolutionary in its time. However, it was also extremely wasteful and energy intensive. In the 1950s and 60s, such issues were of no concern as we enjoyed cheap and plentiful fuel supplies.

The oil crisis of the 1970s was, in retrospect, a major turning point in our attitude to energy consumption and home heating. Initially the worries were about keeping fuel supplies running but since then concerns have moved on to the issue of the burning of fossil fuels and the triggering of global warming. Our love affair with space heating shows no sign of withering but we have developed methods of heating and building which require far less energy than we once used. Over the years, the Building Regulations have demanded more and more energy efficiency and this is a process which looks set to continue: another upgrade is booked for 2005.

Whereas a house built in the 1960s would have required something like 200 kWh/m² for its space heating needs, a new home built to todays standards requires between 40 and 50kWh/m², about 20% of the pre-insulated house levels. So, although space heating is now almost universal in new homes, the amount of energy required has decreased five-fold. Partly this is due to more efficient heating systems and better controls but the bulk of the reduction is a result of higher insulation built into the home.

The Holy Grail here is to construct homes that require virtually no energy to keep warm. It is possible. The incidental heat given off by people living in a house and by the lights and appliances they run can be sufficient to keep a house warm through all but the coldest snaps. The key is, as always, to add more and more insulation to the walls, roofs and floors but as your heat loss gets lower and lower, this tactic starts to become prohibitively expensive. There also remains the requirement for domestic hot water, which stays the same whatever the insulation levels in the house. Because of this, its not advisable to build houses without any heating source, though you may be able to abandon notions of a conventional heating system. One demand that you cant realistically design out is the need for copious amounts of hot water. Domestic hot water is now the factor which determines the size of boiler required for new homes: the space heating requirements for most houses tend to be lower than the smallest boilers on the market.

Electric Heat Pumps

A well established technology that transfers large amounts of low-grade heat held within the ground into focussed heat for use within the home. Unlike solar heating technology, heat pumps do require some input power to operate but they aim to output between three and four times more energy than they use which makes them potentially the cheapest and most energy efficient way of heating a house. The downside is cost: good systems are very expensive to install, generally around £8,000, up to three times as much as an oil-fired boiler. There is currently a £1,500 grant available.

Hot Water Requirement

The requirements for space heating and the requirements for hot water cannot easily be separated, although the strategies to deliver each vary quite a lot. The two basic options are reflected in the difference between system boilers and combination boilers: one is to have a large store of hot water which acts as a reservoir for the hot taps, the other is to heat water on demand. Unless space is at an absolute premium, it is usually better and more energy efficient to use a system boiler with a cylinder to store the hot water. This enables the boiler to work in longer, more efficient bursts. The cylinder can either be gravity fed (vented) or mains pressure fed (unvented).

Warm Air Heating

The preferred heating system in North America, warm or forced air heating systems work by installing ducting around the house and blowing heated air around the house. It is quick response heating, suitable for people with in and out lifestyles, but it can also be noisy, dirty and too dry. Warm air heating was widely specified in the UK in the 1960s and 70s but has since fallen out of fashion. You are unlikely to come across it from any British source but there are a small number of specialists from North America and the European Continent that offer warm air heating solutions. One plus is that winter heating can be combined with summer cooling but few people in the UK demand in-built air conditioning.

Underfloor Heating

Most central heating systems are based around wall-hung radiators. However, increasing numbers of people are switching to underfloor heating instead. The standard method is to fit plastic pipes under the floor and then to run warm water through them and thus use the entire floor as a radiator. The plus factors are that you get a comfortable, even heat throughout the house with no cold spots and you dont have your walls cluttered with radiators. The downside is that underfloor heating (UFH) usually costs more and can be slow to respond. Many first-time self-builders decide to install UFH on the ground floor with radiators upstairs.

Solar Power

Solar panels can provide around half of a households domestic hot water requirement in the UK, worth around 50-100/year. The cheapest systems start at around 800 fully installed and there is currently a government grant system. Take advice from the Energy Saving Trust (www.est.org.uk). Solar panels should not be confused with photovoltaic cells which generate electricity and not hot water.

Which Type of Boiler?

System boiler: A standard or condensing boiler that directly powers the central heating system and indirectly powers the hot water supply via a coil within a separate hot water cylinder. The heating system can be vented (gravity fed) or unvented (mains fed), as can the hot water cylinder.

Both the central heating and domestic hot water supply are directly powered by the boiler with no stored hot water. The boiler is connected to the rising mains and produces a flow of hot water on demand. Flow rates can be limited. Both standard and condensing combis are available.

Condensing boilers: A boiler with an extra large heat exchanger to improve energy efficiency by reclaiming energy that would otherwise be lost via the flue in the form of a superheated steam.

Modulating boilers: Energy efficiency can be improved by modulating the burner constantly to give the required output instead of the all or nothing approach of switching the burner on or off.

Is anything else possible? Electric boilers are available but are relatively limited on power and expensive to run. Solid fuel boilers are still available, and are now remarkably clean to run, but coal (more usually anthracite beans) is still dirty, dusty, bulky stuff that needs dry storage.

Can I combine my heating and hot water system with anything else? Aga Raeburn and others offer a boiler option on their cookers so they can be used for more than cooking. However, this means the cooker must be running at all times not a problem usually, but if your kitchen gets hot in the summer it is going to get a lot hotter with the range toasting away in the corner. Something to think about for those few really sultry summer days.

Vented or unvented? If you have sufficient mains pressure in your area (min. 3 bar), an unvented plumbing system is best. It does away with the cold water feed tanks for the central heating and cold taps and gives hot water at mains pressure. It also means that the hot and cold supply pressure is balanced, thus eliminating temperature fluctuations when taps are opened. If you have an existing plumbing and heating system and are only replacing the boiler, stick with the unvented system.

Heating Controls

The simplest form of heating control is the on/off time clock which allows heating and hot water to be set for two or more periods a day. The time clock is usually combined with a thermostatic switch which will turn the central heating off when the room temperature reaches a pre-defined level. Thermostatic control is also available on individual radiators (TRVs) or for individual rooms. Increasingly people are splitting their homes into separate heating zones, each switched independently. Control and comfort can be increased further by adding weather compensation: this allows the boiler to adjust its output in reaction to changes in outside temperature before they have affected comfort levels inside the house. This is particularly important for underfloor heating.

Check that your heating system is in full compliance with Part J of the Building Regulations at planningportal.gov.uk.

Case Studies

Heating Solutions 1: Self-build for Retirement

Tony and Denise are both in their 70s and are planning to build a three bed perhaps timber frame home. Both are retired although Denise has a part time job, so they regularly go on mini-breaks but often spend all day around the house.

The Plumbers Solution: Suggesting a combination boiler is not always met with agreement. The loss of the airing cupboard is just not acceptable to some people. Removing the domestic hot water cylinder and installing a small radiator with a thermostatic radiator valve fitted would warm the airing cupboard but not when the central heating is switched off in the summer months. Alternatively, a thermostatically controlled, low output, oil filled electric radiator would work all year round but some people insist on retaining the hot water cylinder. Not a problem, using the Worcester Bosch 28kW condensing system boiler.

An easy decision is to fit a grade three cylinder with factory applied insulation, high efficiency coil and fitted with an immersion heater with an independent safety cut out. This will prevent the cylinder water from overheating should the thermostat fail. This new safety requirement should also be complemented by fitting a thermostatic mixing valve to the bath taps.

Fitting a Myson Kick Space under the kitchen units will act as a convector heater in the winter and provide a cooling fanned stream of circulating air in the summer months.

The Energy Efficiency Experts Solution: Many older people prefer higher temperatures all the time and this increases fuel bills. I've included some extra features to offset the running costs associated with a really warm house. The house has steel column radiators; e.g. Acova better aesthetics than panels and a higher heat output per m2. Theyre oversized so they can operate at peak supply and return water temperatures of 60°C/40°C (instead of the normal 80°C/70°C). Weather compensation raises the temperature of the supply water to the radiators as the outside air temperature falls. Individual room temperatures are trimmed by TRVs on each radiator. In the heating season I would suggest the house is heated all the time, although the occupants should adjust the controls to keep bedrooms and circulation areas slightly cooler than living rooms. Larger, lower-temperature radiators and weather compensation will give almost as good a comfort standard as an UFH system. Note that UFH isnt a great choice in this low thermal capacity house, because of its slow response. In 2002, Gordon Taylor, a retired engineer, installed a heating system with similar features in a 1960s four bedroom detached house. His boiler runs at a measured seasonal efficiency of 96%. The gas bill is about £350/year in a house which was constructed before building regulations!

Heating Solutions 2: Young Couple Building a Three Bed Detached House

Darren and Kate are in their late 20s, have full time jobs and have no children. They are hoping to build their first home together, a three bedroom detached property, and need advice on a central heating system. Our experts advise.

The Advanced Heating Experts Solution: This type of two bedroom house with the third bedroom suitable only as a cupboard would traditionally be heated by radiators from a combination boiler. It is unlikely for such a home to have underfloor heating as the builder would rule it out because of the capital cost. Yet such a home would benefit greatly from space saving UFH which would provide draught-free comfort. The couple also have the chance to make this a high-spec home. A gas condensing boiler and lots of insulation in the attic and perhaps dry lining insulation on the internal walls, would be the first things I'd specify. The windows could be replaced with double glazing, especially on north facing walls.

The Energy Efficiency Experts Solution: This house has a high thermal capacity but its sporadic occupancy pattern does not justify underfloor heating nor I fear do this couple have a large enough budget. The house should have a condensing combination gas boiler which runs a conventional radiator system. The combi boiler heats the incoming cold water instantaneously. This should be acceptable in a small house which would probably have only one bathroom. There is a time clock which the users can adjust to provide heat late in the evening, early in the morning and all day at weekends. TRVs are necessary only in rooms particularly liable to overheating.

All hot water pipes should be insulated and radiators should be sited under windows where possible to combat down draughts; this would become unnecessary if a house exceeds building regulations by an adequate margin. The recommended hot water system assumes adequate mains pressure. All systems are assumed to have a reverse return layout so that they can be more easily balanced. This is a vital aspect of commissioning the system whether or not it also has TRVs. All systems must have a boiler interlock a mechanism to stop the boiler firing on its own thermostat when no zone of the house is calling for heat or hot water. This feature is critical to energy efficiency.

The Plumbers Solution: As the couple live a busy working life, getting home at different times every evening, the central heating and hot water system must be responsive to immediate and unpredictable demands. Using a 25kW rated Worcester Bosch condensing combination boiler will perfectly match these demands. With a heat output capacity modulating between 7.4kW to the 25kW maximum their home will quickly achieve comfort levels. Underfloor heating will not have the quick response time of panel radiators. Choosing radiators that are pleasing to look at will mean each room has a pleasant focal point. Each radiator will have a TRV fitted which controls the ambient temperature of each room and meets installation best practice standards. A TRV must not be fitted to the radiator located in the room with the room thermostat.

A combination boiler with a 25kW rating can produce a 10.2litres/minute flow rate of hot water at 35°C DT so will serve the kitchen sink, white goods and the bathroom wash hand basin. The bath fill time is not quick but more than sufficient for lazy weekend soaks, so an instantaneous electric shower, such as the Mira Sport 9.8kW will provide adequate winter and excellent summer flow rates for mid-week daily showering as and when necessary.

Heating Solutions 3: Couple with Teenagers Building Four Bed House:

David and Emma are in their 40s and hold down full time jobs, with two teenage children both at school. They are building a large four bed perhaps timber frame detached home. Our three experts advise on heating solutions.

The Plumbers Solution: Massive demands are placed on bathing and showering facilities first thing in the morning, again in the evening, and with teenagers about its anybodys guess as to what happens at weekends and during school holidays. In this large home, room heating comfort levels are easily met with a Worcester Bosch 40kW condensing combination gas boiler. As with all the combis fitted in these scenarios it is wall mounted and will modulate between 11.4kW and 40kW.

Like the other combination boilers, it also modulates in hot water mode as it delivers up to 16 litres/minute at 35C DT. There is a problem here as with an en suite bathroom and family bathroom, kitchen and utility room the hot water demand in the morning is exceptionally high. Using an unvented hot water cylinder to serve the bathroom hot water flow demands leaves the combi boiler to provide the hot water for the kitchen, utility room and downstairs cloakroom.

The Energy Efficiency Experts Solution: The condensing boiler operates a system of natural convectors. With a low water content and a fast response, these suit a low thermal capacity house. Each room has individual temperature control giving the teenagers freedom to use their rooms as study-bedrooms. I suggest the use of a programmable room thermostat to permit night setback and keep the house comfortable for sleeping. Otherwise a timber frame house can cool too much on cold winter nights.

The Advanced Heating Experts Solution: It is essential that this house is economical to run and maintain as it is the house the family is likely to stay in for the longest period in their lives.

The house would have underfloor heating on the ground floor to save wall space and allow modern open plan living. The ceiling could be high in some places and the windows could be full height. Radiators and towel rails could be used on the first floor and in bathrooms. Used a couple of hours in the morning and then in the evening, they would be cheaper to install than UFH and are less obtrusive as their size could be quite small as so much less heat is required in bedrooms. The system should be powered by a gas condensing boiler with mains pressure hot water cylinder.

The cylinder which supplies hot water for bath, showers and sinks, can be heated by the boiler and in summer by the sun. The second coil in the cylinder is connected to hot water solar roof panels. It is important to realise that heating the hot water requires more boiler power than warming the space in the house.

Heating Solutions 4: Empty Nesters, Large Four Bed House:

Tom and Margaret are classic empty nesters, in their 50s. Both children have left home and the couple hope to build a smaller house. Both work full time although Tom is based at home. Three experts advise on their heating specification.

The Plumbers Solution: With the man working from home all day whilst his wife is out, this home will be well served with a Worcester Bosch 35kW condensing combination gas boiler. Modulating between 11.4kW and 35kW the property will be efficiently heated. This is especially necessary as only the working areas need to be heated i.e. lounge, study, kitchen and downstairs toilet. The bedrooms and upstairs bathroom can be zoned using a motorised two port zone valve set to actuate in the morning and evening.

The boiler is possibly over spec but if the couple ever want to convert the roof space it is the correct choice to accommodate extra radiators. A hot water delivery of 14.3 litres/minute at 35°C DT from this combination boiler provides for an excellent flow rate for the en suite shower with thermostatic mixing valve and simultaneously provides hot water at the basin.

The Advanced Heating Experts Solution: Here comfort and energy savings are the priorities. The house may be in the countryside where natural gas is not available. Then an oil-fired boiler is the most economical choice followed by LPG fuel. Here the condensing boiler is a must. The house may well have a conservatory that is in use all year round. It would have durable tiled floors in the conservatory, kitchen and hall. All would need underfloor heating to keep warm and chill-free in winter and often in summer. Such a house could be visited or even occupied by a younger family.

Energy saving is not just about insulation, fuel and boilers. It is also about temperature control for comfort and optimum use. Waste of energy can occur due to overheating rooms or heating unoccupied areas.

The Energy Efficiency Experts Solution: Several areas of this high thermal capacity house need to be warm all of the time to keep Dad and his occasional business visitors comfortable. The house should have an underfloor heating system. The bathrooms have radiators in an effort to eliminate the need for electric towel rails (which have 4x the running cost and 2x the CO2 emissions of gas-fired central heating). With the houses high thermal capacity, and regular occupancy, precise temperature controls are more important than time controls. A weather compensation system regulates the temperature of the water supplied to the underfloor pipes, and the temperature in each room is individually controlled. TRVs are required on each circuit.

Heating Solutions 5: Young Family Large Four Bed House

David and Leanne have a small baby. Leanne works two days a week whilst the baby is at nursery, and stays at home on other days. David works full time. Two experts advise on the best heating solution for them.

The Plumbers Solution: The couple and baby require heating comfort levels most days of the week so the property would suit the installation of a 30kW Worcester Bosch condensing combination gas boiler. Able to modulate its central heating output from 7.4kW to 30kW it also delivers an increased hot water flow rate of 12 litres/minute. This will help Mum with the extra washing and bathing. Using a room thermostat and panel radiators throughout the property will achieve the desired comfort levels, however, as baby grows, fitting a low surface temperature radiator in the nursery and play room is a very important consideration.

If Mum and baby are both in the kitchen, to keep baby's tiny toes and feet warm an under tile electric mat can be fitted.

Fitting thermostatic mixing valves to the combi boiler supplied shower and to the hot and cold bath taps in the under bath void will prevent accidents.

The Energy Efficiency Experts Solution: Because of uncertainties over occupancy patterns and the owners budget, on balance I feel that the condensing boiler should supply a radiator system, not UFH. The hot water system here (and in Solutions 3-5) includes a mains pressure storage tank, giving a high standard of hot water service but the energy efficiency features included should minimise the running costs. I recommend a gas-fired condensing boiler and SEDBUK A or B single panel radiators controlled by a programmable thermostat plus TRVs in rooms prone to overheating I would specify a high performance cylinder with a minimum volume of 50 litres per occupant. I would also suggest aerating taps; e.g, single-lever mixer, low-flow shower heads (7 litres/min. at mains pressure) and that the cylinder and all pipes should be insulated.