How to Heat Your Home

When confronted with the question ‘How should we heat our home?’ most people concentrate on the space heating. Arguably that’s a mistake in itself. In these days of wellinsulated homes, the space heating requirements for a house are pretty similar to the domestic hot water demand, and are a lot easier to deliver. And whilst space heating can be delivered in a steady trickle, often at relatively low temperatures, the hot water is expected to be available on demand and to be replenished in under an hour. Hot water delivery is much more taxing on the heating system than space heating, and therefore this survey of the factors you need to consider begins with a look at domestic hot water.

Domestic Hot Water

We use a lot of water in the home — typically 150 to 200 litres per person per day. And, just as typically, around half of this water is heated so that we can enjoy baths and showers, and use it in the kitchen, washing machine and dishwasher.

So at the outset, we have to budget to heat up something like 75 to 100 litres of water per person per day. That takes a lot of energy: around 5kWh or 6kWh. Multiply that by 350 days in a year (allowing for the odd holiday) and, say, four people living in a house, and you have an annual heating demand of 7,500kWh.

In a new four bedroom house built to current Building Regulations, that’s pretty much the same amount of energy required to provide space heating. If the house is built to higher insulation standards, which will inevitably happen over the coming years, then the space heating requirement will fall by a half or even three quarters from today’s levels; but the demand from domestic hot water will remain little changed. So within four or five years, we can expect the energy demand from domestic hot water to outstrip the space heating demand by a factor of three or four.

The cold water main coming into the house delivers water at a temperature close to the external ground temperature, in a similar manner to the way the collector pipes work in a ground-source heat pump. This averages at around 10°C, although it may vary up or down several degrees from this through the seasons.

Because we mainly require hot water at just over 40°C (comfortable for baths and showers), this means that typically we only need to heat water by 30°C to 35°C. However, in reality, we tend to heat domestic hot water to much higher temperatures, usually between 60°C and 80°C. There are several reasons for this:

• We sometimes want water hotter than this for washing clothes or dishes, although most washing machines and dishwashers heat up the water from cold using peak-rate electricity.

• If we are storing water in cylinders, the hotter the water, the smaller the cylinder needs to be. In terms of energy efficiency, there is no difference between heating 250 litres of water through 35°C or heating 175 litres through 50°C. Each require the same energy (just under 11kWh). All other things being equal, it makes sense to have a smaller hot water cylinder and to blend hot and cold taps when showering or running a bath.

• If you specify an instant hot water system (such as you would get with a combi boiler), you don’t need a water-storage cylinder and you could in theory only heat hot water to the output temperature. However, exactly the same logic applies to combination boilers and they tend to work best by heating smaller volumes of water to higher temperatures and blending the output with the cold tap.

• There is a health issue with storing water below 60°C. It may result in legionella bacteria spreading (though in truth there doesn’t appear to have been a single outbreak of legionnaire’s disease caused by a domestic hot water cylinder).

• Unlike space heating demand, the demand for domestic hot water often comes in short bursts, especially as residents shower or bathe in the morning. A well-designed system can handle this demand and will be able to replenish the stored hot water reasonably quickly. This is known as the recovery rate.

The recovery rate is governed by two factors: the size of the boiler and the size of the heat exchanger or coil in the cylinder.

A low-rated boiler (say 8kW output) will take 90 minutes working flat out to replenish a cylinder with hot water, whereas in a highrated boiler (say 24kW output), the recovery rate will be just 30 minutes. Space heating doesn’t require more than a few kilowatts of power to keep a new home warm, so the size of a boiler is now determined by hot water delivery. And this has led to the development of modulating boilers that can switch between a small burner (say 8kW) suitable for space heating and a large one (typically 24kW) for domestic hot water.

Hot Water Delivery

Electricity: Most cylinders include an electric immersion heater, available as a back-up during a boiler breakdown. Electric heating is frowned upon because of its perceived high cost, but the night-time tariffs, currently charged at around 5p per kWh, now rival the cost of all major fuels except mains gas (currently 3p per kWh). The problem is that immersion heaters are generally slow to heat cylinders and you have to wait till the off-peak night hours to reheat the water, unless you want to pay the punitive 12p per kWh charged by electricity suppliers for peak rate. Thus it only makes sense if you have a big storage tank, capable of handling 24 hours worth of hot water demand.

Heat Pumps: There is one form of electric heating – the heat pump – which sells itself on being low carbon, drawing much of its energy from the ground or air. In many ways, it makes sense to use a heat pump for hot water supply but the decision is not so straightforward. The quoted efficiencies of heat pumps are based on a temperature uplift of 35°C: for each degree higher required, the efficiency drops by 3%. So it pays to design a system where hot water is stored at a relatively low temperature and where the electricity used works on night-time tariffs if possible. If the system is designed well, it should be possible for heat pumps to provide hot water at prices similar to mains gas.
Read more about Heat Pumps.

Biomass Boilers: There is a small but growing interest in biomass boilers, principally fired with wood chip or wood pellets. These are widely used in Austria and Scandinavia and are available with features such as automatic ignition and auto-feed mechanisms — the push-button convenience normally associated with gas or oil. However, they need space to operate in and for fuel storage. At present, the fuel supplies are in their infancy, although prices are pretty favourable.
Read more about Biomass

Roof-mounted Solar Panels: The oldest of the renewable technologies, the solar panel is designed to provide domestic hot water when the sun shines. The size and number of the panels you need should correlate to the number of people in the house (and hence the hot water demand). A well-designed solar panel system should provide most of your hot water during the summer and the shoulder seasons, conceivably halving your domestic hot water bills. However, the payback on solar panels isn’t wonderful — you could anticipate saving £100 to £150 a year for an outlay of perhaps £3,000 to £5,000. However, you do insulate yourself against fuel cost escalation — and who knows what impact that might have in years to come.
Read more about Solar Panels

Thermal Stores: Thermal stores aim to work as hot water batteries, taking energy from a variety of sources and distributing it when required. They can be connected to various heat sources, such as boilers and heat pumps, and manage the input from each to their best advantage both for space heating and domestic hot water duties. You can, in fact, base your entire heating strategy around a thermal store design, though to be effective they do need to be fairly large — typically around 300 litres in size.

The Boiler: The traditional option is to heat the hot water via a boiler. Mains gas is much the cheapest option: only where it’s not available should you consider using one of the other fossil fuels, such as oil or LPG. The water is stored in a cylinder. These cylinders are now very well insulated, losing very little heat during a 24-hour cycle.

A popular alternative is to use a combination boiler which heats water on demand. Although more expensive, they do away with the need for cylinders and so save space. They are best suited to smaller households where only one person is likely to be using the bathroom at a time, as they suffer from low flow rates.

Space Heating: Radiators vs Underfloor: For many people, the choice of how to heat their home comes down to either radiators or underfloor heating, but which is best for you?

If you get your domestic hot water strategy right, it’s relatively easy to satisfy your space heating requirements. The first part of the process is to carry out a heat-loss calculation. This involves working out how well your structure hangs onto heat, itself a reflection of how well built the house is. There is a direct relationship between the energy efficiency of the building and the size of the space heating system: the better insulated the house, the less work the space heating has to do.

Even if you only need the space heating on occasionally, you still need to build in some method of delivering it around the house. In Britain, the two main choices are wall-hung radiators or underfloor heating (UFH). Radiators are relatively cheap and quick to heat up, but can look ugly and cause hot and cold spots around the rooms. Self-builders in particular have been keen adopters of warm water UFH systems, which aim to deliver a steady trickle of heat throughout the day. Another advantage of UFH is that it works at lower temperatures: typically the water circulating in the UFH pipework is no more than 45°C whereas radiator systems need water between 60°C and 80°C. This on its own doesn’t mean that UFH is more energy efficient than radiators, but it does mean that it works well with heat pumps and thermal stores.

What's the Alternative?

Warm Air: In North America, warmed air is the common form of space heating. It’s not widely used in Europe, although there are one or two companies offering it to UK self-builders. The main advantage of warm-air systems is that they can also be configured to cool homes in summer, but as it’s now five years since we had a prolonged heat wave in the UK, built-in air conditioning doesn’t appear that high on many people’s feature wish list.

Whole-House Ventilation: Many self-builders are choosing to install whole-house ventilation systems which, whilst not adding any heating capacity, are a means of distributing heat. If you choose to have a focal-point fire, stove, or cast iron kitchen range, you are likely to be producing more heat than you require from a conventional space heating system. A well-designed mechanical ventilation system will transfer this heat around the house and even out the temperature fluctuations between rooms.
Read more about Ventilation

Insulation, Not Space Heating: A small but increasing number of self-builders are building homes without any conventional space heating delivery and choosing to rely on a woodburning stove plus a mechanical ventilation system with heat recovery. You need a very well insulated house to consider this option and, even with this, it’s probably incapable of delivering year-round temperatures above 20°C, which is what many have come to expect from a central heating system. But in an era of seemingly ever increasing fuel prices, this may well be a strategy that we will see more widely adopted. However, do bear in mind that even such a seemingly radical leap doesn’t address the problem of heating domestic hot water. You still need to install a heating system for this.
Read more about insulating your home

Heating Systems: Comparitive Costs Through Time*

How long you plan to live in your house should have an influence over the type of heating system you choose, as this chart explains.

A heating system should be looked on as an investment. There is a capital cost involved in installing it and an ongoing running cost which, after 10 to 15 years, will be cumulatively larger than the installation cost. The longer you plan to stay in the house, the more sense it makes to invest in a system with low running costs. As the chart above shows, those systems that cost the least to install can, over time, work out to be far the most expensive option.

The principal fuel used by the Victorians and Edwardians was coal and it was burned either in open fireplaces or in cast iron ranges. Coal was dirty and smoky to burn, so there was a big emphasis placed on keeping rooms well ventilated. Frequently, fireplaces were built into all the living rooms and bedrooms but only lit in one room. Consequently, house design revolved around lots of small rooms.

The advent of town gas and the Clean Air Act combined to switch the country onto the delights of central heating and hot water on demand. Few houses now don’t have space heating and very few still use coal. We’ve switched en masse to mains gas, where it’s available, and to oil or LPG where it’s not. As a result, we’ve embraced open plan living and big glazed openings.

Is all that about to change once more, now that fossil fuels have rocketed in price? Are we going to re-embrace the Victorian ideal of small, cosy rooms, or can we reinvent our housing to enjoy the comforts of 21st century living without the cost that now entails? If gas and oil are to become prohibitively expensive and are deemed too environmentally damaging, what will replace them? Heat pumps? Biofuel? Solar panels? Or so much insulation that we don’t need heating systems anymore?

Saving Water

Surprisingly, the single most cost-effective way of reducing energy bills for domestic hot water is to specify appliances that are water efficient. It’s not difficult nor expensive to reduce your average water demand by around 30% by using aerated taps in kitchens and on shower heads, and by looking for water-efficient dishwashers and washing machines.

*at 2008 prices.

Further reading:

• A Guide to Heating and Plumbing
• The Death of Central Heating
• Underfloor Heating or Radiators?
• Building a house that heats itself