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The heating of the air in the rooms is called space heating. A well designed space heating system keeps air temperatures steady within the comfort zone for wearing light indoor clothing (around 16-22°C), whilst allowing slight variations within this range according to different room uses.

Waste heat
Houses generate a surprising amount of background heat from cooking, the waste heat from lighting and appliances (especially computers and televisions), and the body heat of inhabitants (a handy 150 watts for each person in the house). The average house generates 1.5 kW from such waste heat and additional space heating is only needed when the heating load on the house exceeds this level. Experimental houses with a very high level of insulation have been able to meet all their internal space heating from this waste heat alone.

Additional heat
Realistically though, any renovation of an existing building however well insulated will continue to be heavily dependent on additional heating. The options for heating are limited: fossil fuels. The burning of fossil fuels (oil, gas and coal) is the main cause of climate change. They are a leading cause of environmental damage at all levels of production and use. Fossil fuels will run out (oil will start becoming scarce within a generation), but we cannot even afford to wait even this long if we wish to avoid catastrophic climate change. At present, though, fossil fuels are cheap, plentiful and extremely easy to obtain. Of the fossil fuels, gas is the last polluting, producing half as much carbon dioxide as coal and virtually none of the particulate pollutants.

Due to the losses of generation and transmission, electricity is very inefficient when used for heating- a domestic boiler produces twice as much heat as electricity generated from the same amount of gas. 90% of the electricity generated for grid distribution is from burning fossil fuels or nuclear and should be avoided. There is some debate about whether it is OK to use electricity for heating if it is from renewable sources. Many utility companies offer “Green Electricity” which they guarantee comes from renewable sources (a guide to Green Electricity)

In our view electricity is a precious resource which should be prioritised for its most efficient uses: lighting and powering motors. At present we have a very long way to go before we have surplus renewable generating capacity in Britain to provide for heating. So, there is a role for green electricity heating in the longer term, but in the short term we think the more sustainable solution is to reduce consumption and use other fuels as efficiently as posible for heating.

Solar energy is by far the most sustainable source of heat. Unfortunately the period of maximum demand for space heating coincides with the period when there is the least sun. A south facing solar conservatory will generate much useful heat on sunny days, but additional heating will always be needed except in the very best insulated houses.

Bio-fuel is from a sustainable resources which naturally regenerate. Examples of bio-fuels include wood, agricultural waste such as straw, and products manufactured from waste such as alcohol and methane. The most common bio-fuel is wood, and many eco-houses are heated entirely with fuel from a local wood lot. In theory bio-fuels are the only sustainable source of plentiful heat in winter. Few are without problems, though. Burning wood and waste is highly polluting without good filters or an advanced burner.

For the additional heating we decided to combine all three energy sources as best we could.

Solar – a solar panel on the roof heats water. Large skylights and sun porches capture solar heat inside the house.

Bio-fuel – a wood burner in the front room runs on waste wood from the renovation and from trees pollarded in the garden.

Fossil Fuels – we used natural gas for the background central heating.

Given the problems with the last two heating sources, we began by doing as much as we could to reduce the amount of heating needed (called the “heating load”) with insulation and draught reduction. We then used solar heating to pre-warm air entering the house through the porches. We bought a new boiler to maximise the efficiency of the fuel we did burn. Finally we achieved further reduction of consumption by thinking very carefully about how we lived in the house and used the heating.

We decided to replace our 15-year-old boiler with a new condensing boiler. Condensing boilers are remarkably efficient and can convert 85% of the energy in the gas to useable heat compared with our old boiler which had an efficiency of 70% at best. Even though the existing boiler had a few years of life left in it, this was one of the rare times when it was worth buying new technology.

There are numerous government schemes to encourage people to change to high efficiency boilers. Contact your local authority to find out what discounts and subsidies are available. In our case we found that Oxford and most Thames Valley authorities are members of the Green Boiler scheme which passes on to the public bulk discounts on condensing boilers of up to 50%. In addition we obtained a further £100 cash back from the scheme on installation.

Condensing boilers achieve high efficiency by condensing the water vapour in the waste flue gases – this condensing releases heat which preheats the returning water. This mechanism works best if the temperature of the returning water is below 45°C. If the boiler is more powerful than is necessary, or there are too few radiators, then the returning water is too warm for the boiler to work effectively and a condensing boiler loses its advantage over other boilers. For this reason it is very important to size the boiler and radiators accurately.

The underlying principle of efficient heating is to size the boiler to the output needed to heat the house for three degrees of frost and then add the water heating and add a sizeable allowance extra. The radiators are then sized according to the heating needs of each room, with a total output that is greater than the output of the boiler (to allow for some radiators being turned off).

We calculated the heating load by adding the heat loss of all the walls, floors and roof using calculations taken from their u and r values (more on calculating heat loss…). We worked out that, allowing for one air change per hour, the Yellow House would lose 179.8watts of heat for every degree difference in temperature between the inside and outside. This meant that under peak load conditions of three degrees of frost, and allowing for the added demand of water heating, the boiler would need a heat output of 3.6 kW. Boilers are still graded in British Thermal Units, 1,000 BTU is approximately 0.27kW, so we needed a 13,000 BTU boiler. Our plumber working by a rule of thumb reckoned we needed a 50,000 BTU boiler for a three bedroom house – three times the actual need. This underlined the importance of doing the proper calculations. If your head spins at the maths, hire a heating engineer to do the calculations.

We could not find a boiler with such a low capacity, and had to make do with lowest capacity boiler available under the Green Boiler Scheme, a 30,000 BTU (8.85kW) Caradon.

Most houses have far too few radiators for the heat output of their boiler. The combined heat output of all the radiators should be somewhat greater than the heat output of the boiler to ensure a low temperature for the return water. Radiator catalogues give a heat output in kilowatts for each model, so it is not hard to work out the output of new radiators or estimate the heat output of existing ones.

To compensate for the overcapacity of our boiler we needed to specify an overcapacity of radiators. We kept most of the existing radiators, upgraded the radiator in the bedroom by 50% to deal with the new double height space, and ordered two new radiators for the bathroom and the kitchen. The final total output of 13kW is a reasonable capacity for a 8.85kW output boiler, as it means the boiler is still working efficiently even when two or three radiators have cut out or are turned off.

The radiators finally fitted were:

Living Room Single fin 3000 Keep None- thermostat room
Kitchen Double Fin 1451 New 22°C
Utility room/toilet None Heated by grey water
Extension Double fin 3053 Keep 18°C
Hall Double Fin 1000 Keep 11°C
Rear Bedroom 1 Single Fin 1417 Keep 16°C
Rear Bedroom 2 (office) None Heated by boiler/tank
Bathroom Double Fin 850 New 24¡C
Front Bedroom Double Fin 2246 New 17°C

The placement of radiators is of great importance. The standard logic of radiator placement is that they should be placed under windows to counteract the downdraft of cold air from the window. If they are placed on the wall opposite a window they will set up a convection current in which hot air rises and sits at the top of the room and pulls cold air across the room from the window.

Well, that is the theory and it makes sense for old houses with leaky single glazed windows. The logic doesn’t apply so well for well sealed double glazed windows, especially the high performance ones in the Yellow House. There is also a stronger counter argument for not placing radiators under windows. Radiators literally pump radiated heat into objects close by- the wall behind the radiator is the hottest place in the room. If a radiator is against an external wall then that heat is going to be wasted (and, of course, windows are always in external walls!). What is more, the hottest air in the room then rises over the coldest object – the window – another source of heat loss. To make matters worse, many people have curtains over the window which drape over the radiator- so the hot air rises up and circulates uselessly behind the curtain. All in all, if you have high performance windows itÕs worth placing radiators against internal walls wherever possible.

The strategies we followed in the Yellow House were:

1. Where possible place radiators against internal walls. We had to make do with existing radiators in most rooms which were all under windows, and the replacement radiator in the front bedroom would only fit comfortably under the window. The new radiators for the kitchen and bathroom are both placed against internal walls with the TRV set to a high setting to encourage them to work as much as possible.

2. When the radiator is on an internal wall, it is worth encouraging it to “pump” heat into the wall behind it. This uses the thermal mass of the house to keep a steady room temperature. For the radiators placed against internal walls, the back of the radiator and the wall behind it are painted with very dark paint to encourage this storage effect.

3. The radiators on external walls need to be discouraged from “pumping” heat into the wall. All DIY stores have sheets of reflective insulation to place behind radiators. It’s expensive to do a whole house so it’s cheaper to make your own and easier to make them to the required size. Ours are sheets of thin cardboard, with old polystyrene ceiling tiles glued on (reused from tiles removed from one of the rear rooms), covered with smoothed aluminium kitchen foil. They are taped to the wall with duct tape. They are about 7mm thick- which is about right as it is important to keep as much air space as possible behind the radiator. If you can’t keep at least 2cm of air space, it’s best not to try.

Finally, we fitted Thermostatic Regulator Valves (TRVs) to the radiators. TRVs turn off the radiator when a desired room temperature is reached. They are a useful little energy saving device as they adjust heat output to the daily variations in room temperature (such as from the sun or from the cooker) and they compensate for any radiator that is oversized for an individual room. They are also cheap (£15). A TRV should never be fitted to radiators in the room holding the central heating thermostat or the thermostats will compete with each other.

Setting the optimal temperatures for the house has major implications for energy use. The government information on energy saving always urges people to turn down their thermostat by 1 degree, but this makes it sound like people are being told to make a sacrifice. A better way of putting it would be to say “live in a house which is warm enough to be comfortable but cool enough to be healthy and keep yourself clear headed”.

The government (and the default setting on most thermostats) assume a basic temperature in living rooms of 21°, but we find this too high. We have experimented with different temperatures and find that we are happy with 18° as the standard living temperature. This, incidentally, is also the temperature currently recommended by doctors for houses with babies. Once in a while we might turn it up a degree for an evening, and there are valves on the radiators (see below) so that individual rooms can be kept warmer or cooler. The bathroom, for example, in the centre of the house can be kept as hot as we want (a toasty 24°).The bedrooms are kept at around 17°C.

When we set the individual TRV settings, our logic was:

• Keep bedrooms cool (17°)
• Keep living rooms warm (18°)
• Keep circulation spaces (hall and stairs) cool (11°). It is worth having some heating in them or there is a cold blast every time a door is opened and this can play havoc with the main thermostat.
• Turn the radiators in unused rooms to frost (5°)
• Keep central rooms at the core of the house (kitchen and bathroom really warm so the heat can work out into the rest of the house (22/24°C) Also these rooms had radiators against internal wall which are worth giving a higher setting (more on radiator placement… above)

It is definitely worth investing in good quality temperature controls. The more flexibility you have over the heating controls the more options you have for tailoring it to your exact needs. The differences in efficiency and comfort can be enormous.

Old designs of temperature controls were very inflexible. Central heating and water heating were usually run off the same timer. This is an extremely inefficient arrangement; if the central heating is on all day the water is also on all day and the entire tank is kept hot constantly.

The qualities to look for in heating controls are:
• Flexibility- the ability to set different central heating temperature levels for different times of day for each day of the week.

• Simple ways to override the programme. One is the ability to simply turn the heating up or down a degree without having to change the programme. Another is an off switch- so you can turn the heating off if you’re going to be out. Most modern heating systems have a whole range of options for lowering the heat for a while- but it’s a pain to have to programme the thing. Nothing beats an off switch.

• Instructions and controls that can be used by anyone. No small thing- some systems are impenetrable.

• For houses with a water tank, a separate timer for the hot water. The most efficient way to heat water is to heat up the tank in a short but concentrated burst during a time when hot water is not being used and then to use the hot water up gradually. A separate timer allows you to set a specific water heating time.

We chose the Honeywell CM67 which is a good standard system. We can recommend it. The water is separately controlled by a separate timer, a Danfoss 103ES. The Honeywell, like many modern central heating systems, allows you to set different temperatures for different times rather than having crude on and off times. We found that the house seems far hotter than it really is when the radiators are on (because they warm us by radiation as well as just warming the air) so it is best to raise the temperature in steps. At night the temperature is allowed to drop, but if it drops below 15 (which it only does on extremely cold nights) the heating may start up briefly.

With trial and error we have found the best settings for our needs are:

3.00am 15°
6.40am 17°
8.00am 18°
10.00am 17.5°
5pm 18°
10.30pm 15.5°

These are the setting for our needs- we have a baby and often work from home so we need constant warmth. Houses that are not occupied during the day would have a very different timing, allowing the temperature to fall a long way during the day. Well insulated houses with good thermal mass will tend to hold their heat a long time. For much of the winter the heating only runs in the morning and the house coasts on its stored energy until the evening.

The water is timed for two bursts in the morning and late afternoon. They are timed for 6.40 to 8.00 in the morning and 5 to 6 in the evening. The golden rule is that hot water cannot be used during these periods. They are deliberately timed to coincide with times when there will be a demand for central heating too- the boiler is most efficient when it is under maximum load.

We installed an attractive 1930s solid fuel burner in the living room, with the flue pipe running into the old chimney. The burner is intended to get the living room really cosy as an occasional treat on really cold nights. If it was a major source of space heating we would have invested in a modern efficient model which produced less smoke. Although designed for coal, it runs perfectly well on wood. We are currently burning timbers removed during the renovation (especially old joists) and pollarded wood from trees in the garden. This wood would normally have been burnt in the open, so this is effectively free heat. In the future we will continue to use scrap wood from skips and demolition sites (it is important in this case to avoid anything that has paint on it).

Solar power comes in two forms: water heating systems and photo-voltaic electricity generation. Both systems require an unshaded south facing roof to be effective. Orientations of 15° either side of south make relatively little difference to the amount of solar energy received, but the energy declines for greater divergences from south. Our South West is still within the range for solar viability.

Photovoltaics are still relatively new and expensive technology which many people see as the future for power generation. Sue Roaf is an Oxford based architect who has built her entire roof from PV panels and produced more electricity than she can use herself. There is a detailed discussion of the technology in her book Eco-House.

We considered photo-voltaics for the Yellow House and found them to be too expensive for our budget. Prices are coming down so fast that we will certainly consider them again in a few years. We found that solar water heating is far more viable. The technology is well established, the systems on the market are well designed and very durable, and there is enough competition to shop around for prices.

We found that two main problems face the consumer: it is hard to find a cheap and reputable installer, and it is virtually impossible to find information on the comparative performance of different solar systems.

We consulted the Centre for Alternative Technology (which provides a provides a good general briefing booklet) and asked for quotes from members of the Solar Trade Association. We finally opted for the current market leader in Britain, Thermomax Evacuated Tubes. They are very well made, will last at least 20 years and have performed very well. We went with a local installer recommended by Thermomax.

It is still expensive, though. Our system cost £3,250, and it saves us around two thirds of our annual water heating which used to cost a total of £135 per year (the monthly hot water cost is calculated from the monthly energy consumption in the summer when the only heating was for hot water). An annual saving of £90 will still take 36 years to pay back! Luckily there is every reason to believe that energy prices will rise significantly during this period and change the economics.

But this is not the whole story. As part of the solar system we obtained a large new water tank to replace our dreadful old one (which lives on as a grey water tank in the utility room), which was no small saving. There are also hidden savings from being able to turn off the boiler for half the year- reducing wear and tear on the boiler and pump.

In the end, though, the decision to have a solar heater is not made on cost alone – it is an investment in the vision of a world running on clean cheap solar power. If you have that vision the immense satisfaction in having a huge free solar hot bath is worth the extra cost.

There are two main systems: flat plates in which water is passed through pipes inside a sealed unit, and evacuated tubes in which the sun’s heat is captured by plates in glass vacuum tubes and passed through an armature to the circulating water. Evacuated tubes are considerably more expensive but perform far better under clouded skies.

During the summer season (April-September) the tubes on the roof provide almost all of the hot water. We try to live within the natural limits of the system. Because our roof faces south west the period of peak solar generation in summer is between 11am and 6pm. We try to avoid using water during this period so that the tank can reach its maximum temperature. We also keep an eye on the weather forecasts- during a period of clear sunny weather we are extravagant with hot water, having deep baths and doing as much clothes washing as possible. During periods of cloudy weather we cut back on hot water use as much as we can. With constant piles of nappies to wash it is rarely possible to keep entirely within the natural cycles, and sometimes we need to give the tank a boost.

There are many ways to improve the efficiency of the hot water system which, when added together, lead to considerable savings. Most of these can be tried with an existing water system without any structural changes or significant investment

Insulate pipes and the hot water tank
Insulate all hot water pipes that will be in regular use for small quantities of water- such as pipe runs to the kitchen and bathroom sink- and pipes that will be under heavy sustained use- such as between the boiler and the hot water tank. The more insulation around the hot water tank, the better. With a solar systems, the insulation around the tank makes all the difference for keeping the water hot overnight for showers the next morning. Our tank was already insulated and we have added further insulation.

Turn down the thermostat on the water tank
In most households the thermostat on the water tank is set for an absurdly high temperature- indeed in most households no one even knows where it is! (It’s usually a small box attached to the side of the tank about half way up with a dial with temperature settings). Immediate economies can be achieved by setting the thermostat to the temperature of water that is actually needed rather than heating water up to 75°C and having it sit around all day only to dilute it with cold water when you need it. As a rule of thumb, water is hot enough for a shower at 37°C, a bath at 40°, a washing machine cool wash at 40°, and a dishwasher economy wash at 60°C. Tank thermostats are particularly inaccurate, so the best strategy is probably to turn it down little by little until it gets too cool.

It is also to worth setting a low tank temperature if you have a condensing boiler because the boiler stops condensing as soon as the return water (which will be the same temperature as the tank) goes over 45°C.

Occasionally one is warned that a cooler tank could develop Legionnaires disease. Personally I believe this to be a very low risk given the high turnover of water in a domestic system. To the very best of my knowledge, Legionnaires disease has never developed in a solar hot water system which typically has a tank temperature between 40 and 50 degrees C.

In the Yellow House the main demand for 40°C water for baths and showers and the economy washing machine cycle in the washing machine. However, we also need 60°C water for the dish washer and washing nappies. Because we have a condensing boiler we have settled for the compromise of 50°C for the tank temperature.

HEATING & WATER: Place the cold water feed tank inside the insulated envelope of the house
It stands to reason that the warmer the water entering the hot water tank, the less additional heat will be needed to heat it up. In most houses, the cold water comes from a feed tank, typically in the loft. It is insulated to prevent it freezing, but gets extremely cold in winter nonetheless. In a few houses the water enters a pressurised tank directly from the mains. The temperature of our mains water in winter is a chilly 6°C.

In the Yellow House we placed the feed tank at the apex of the roof on the mezzanine. Because it is inside the living space it warms from the spare heat that collects at the top of the house. This is not ‘free’ heat- it is just the same amount of energy as would be needed to heat the water in the boiler- but it is spare heat. Most of the warming of the tank happens at night when the warm air that collects at the top of the house really is waste heat.

We drilled large holes in the board supporting the tank to encourage air to circulate around it. This pre-heating adds at least 10° to the temperature of the cold water before it enters the hot water tank, saving 15% of the energy required to heat it. Having the water tank inside the living space can seem a little intimate. At first we heard constant stomach rumblings from the tank; gushing, sloshing and then hours of dripping. The solution was to fit one of the many valves designed to muffle the water inlet. We used the Fluidmaster “Quiet Float Valve” and have not heard a peep out of the tank since.

There is an enormous potential for creativity in DIY pre-heat systems. Experimental eco-houses have built simple solar collectors to preheat water, and run pipes through grey water tanks (and even through compost heaps) to warm the water entering the hot tank. Even very slight warming is worthwhile for reducing the heating load.

We considered supplying the toilets with grey water collected from rain water. There are commercial systems available, but the require water treatment systems, pumps and storage tanks in the roof and separate plumbing. We felt they were too expensive and use too much space for a small terraced house. The Nottingham Ecohouse has a rainwater system and the are details on their website.

Instead we decided on a simpler system to reuse waste bath and shower water to flush the downstairs toilet in the utility room. We saved the old copper hot water tank from the old heating system and put it on a shelf above the toilet (an old toilet given to us by a friend). The 50mm waste pipe from the bath enters the tank at the top. A pipe leads from the bottom of the tank into the cistern of the toilet. There is a tap below this which allows the tank to be drained if needed. There is an overflow pipe from the top of the tank which is fed directly into the waste pipe. The tank is sealed (which building control required as a condition for the system), but the tank can be opened if necessary for cleaning and maintenance.

This tank has made a major contribution to reducing water use in the house. What is more, the heat from the bath water warms the house. The 40 litres of water of luke warm bath water it holds contains over 2kWh of energy, equivalent to running the main boiler for 15 minutes. In practice the warming effect of the tank is considerable, helping to dry clothing hung on the racks in the utility room.

There is always a danger with grey water systems that the water can become infected and start to smell. We found that most of the time the water smells, as one would imagine, like old bath water and sometimes strangely like tea! However, if the tank is not being regularly flushed out with fresh water it can start to smell. The trick is to use the toilet as much as possible and to fully drain the tank before filling it with bath water. We also drain it before going on holiday. We have only once had a real infection which we solved by giving it a thorough cleaning. The system can probably do with an annual cleaning.

We considered options to reduce the use of water in the upstairs toilet. The easiest option, putting a brick or two in the cistern, permanently reduces the flow. We found a far better option: an ingenious device called the “Ecoflush”. This allows each flush to be set to low, medium or high flow. There are good instructions and installation took a few minutes. We strongly recommend it. It is available from Gesek Ltd for £20, or through the CAT catalogue.

For garden water we installed a large rainwater butt behind the extension fed by a down pipe from the main roof (not the turf roof). This is raised 50cm off the ground to ensure pressure in watering hose.

Finally, to ensure that these water efficiencies paid off, we contacted our water supplier and asked them to install a meter. Our water bills fell by two thirds.