A tale of two houses - 35

You don't have to spend a lot of money to build a home that doesn't need central heating - what's required is the right specification and the correct installation. Patrick Waterfield finds out what it's like to inhabit an energy efficient space, in a family home in Co Down and in a holiday house in Co Donegal

Received wisdom tells of a law of diminishing return where insulation is concerned and thus an optimum level based on purchase price compared to the cost of energy saved. This is true up to a point - the point at which a central heating system is no longer needed! Then the whole equation gets tipped on its head because you save the cost of the heating system, as well as the fuel it would have used over the lifetime of the building.

The No Central Heating standard was designed with this concept in mind, developed by the author of this article and by a local timber frame company specialising in energy efficient dwellings (whose standard product consists of a highly insulated fabric with high levels of airtightness and mechanical ventilation with heat recovery). Arriving at the No Central Heating standard was 'simply' a matter of improving the fabric insulation further and specifying high performance glazing, as well as ultra-low air permeability levels.

The result is a system which, for little overcost on normal construction, can achieve levels of performance of Passivhaus standards. A number of house-building systems are available, notably from other European and Scandinavian countries, which provide excellent levels of energy performance but at a considerable cost premium. Key to the cost effectiveness of the No Central Heating system is the use of industry standard components, such as 140mm timber stud, and local factory and on-site construction. Additional costs for insulation, glazing, mechanical ventilation and high levels of airtightness are offset by the avoidance of a central heating plant (such as boilers) and heat emitters (for example radiators) plus associated pipework, valves and controls.

The walls, roof and floor of the No Central Heating houses are of layers of insulation of different materials, in order to provide excellent thermal performance without the risk of interstitial condensation. All windows, including rooflights and glazed doors, must be inert gas filled, triple glazed units with low emissivity coatings and insulated spacers.

While both the Co Down and Co Donegal houses share a similar fabric make-up and services (no boiler or radiators of course!) one is continually occupied and the other intermittently, which gives us a chance to assess the dynamic as well as steady-state performance of the system.

The mechanical ventilation systems continually extract air from the waste areas (kitchens and bathrooms) and recover up to 85% of the heat content which is transferred to the incoming fresh air. In this way, the dwellings benefit from continuous, filtered fresh air with a greatly reduced energy requirement and improved comfort levels. The system thus also acts as a means of transporting heat around the house, and is sufficient to heat the bedrooms and ancillary rooms to the required comfort levels. In most conventionally-built dwellings this mechanism would not work - the heat loss through the building fabric would be too high and there would also be too much air leakage in and out of the building. But in a super-insulated fabric with very low air permeability it can.

The homes are oriented towards the south, with high areas of glazing on the main facades, thus making use of available solar heat gains directly into the main living spaces. These, combined with heat from cooking, equipment and even people ­collectively termed "incidental gains" - contribute significantly to the heating of the dwelling. Again, in a normal house, these incidental gains would be insignificant compared to the rate of heat loss. However, combined with very low heat loss from the fabric and through ventilation, the incidental gains reduce the "heating season" to around three months, half that of a normal house. Additional heat, when it is needed, is provided by a room heater in the main living area, where higher comfort temperatures are usually required. The mechanical ventilation system, as explained above, helps to distribute this heat throughout the dwelling.

Integral to the concept of these low energy homes is the provision for intermittent heat into the bathroom areas, which also require a slightly higher temperature than elsewhere in the house. This can be provided by a towel radiator, either electric or wet - in the latter case being run on a small pumped circuit off the hot water cylinder. Sensible time control of the towel radiator will limit energy and cost consumption and does not dilute the No Central Heating concept. Furthermore, most of the heat generated in the bathrooms is thus distributed, via the ventilation system, to the rest of the house. Use of masonry walls around the room heater acts as thermal mass, helping to store heat generated and also acting as a heat sink to reduce the risk of overheating in summer - though high insulation levels also reduce excessive solar heat gains into the building.

Co Donegal

Construction type

One and a half storey timber frame

One and a half storey timber frame

U-values

0.l3W/m2K for walls, 0.11 W/m2K  for roof,  0.11W/m2K for floors and 1.0W/m2K for window units 0.l3W/m2K for walls, 0.11 W/m2K for roof,  0.11W/m2K for floors and 1.0W/m2K for window units

Airtightness < 2 m3/(hm2) at 50Pa

House size 2,000 sqft1,700 sqft

Energy rating BA2

Space heating heat recovery ventilation pre-heatsheat recovery ventilation pre-heats air; 5.5kW wood-burning stove;air; 4.5kW wood-burning stove with bespoke towel radiators fed by hotback boiler; towel radiators for water system, both approx.bathrooms fed by hot water system - 100W in size200W and 400W in size

Hot water heating Approx. 6m2 of flat plate solar36 evacuated tubes supplemented by panels supplemented by an air the stove with back boiler source heat pump (COP of 3)

 The County Down house

The County Down House, which is owned and occupied by Darren and Ashleen Annett and their two young children, sits on a site between Darren's parents' house and that of the parents of Darren's long-term friend and architect John Lavery. The house comprises a main open plan living/dining/kitchen area, plus additional first floor living room, three bedrooms, two bathrooms, a study and a utility/shower room.

A plasterer by trade, Darren was more aware than most people of the opportunities for influencing all aspects of the design of a dwelling, including the energy side. He was also open to considering innovative products and practices - within cost constraints-of course. As the design phase took shape Ashleen was expecting their first child and the cost factor became even more important in their minds. The prospect of removing, at a stroke, the single greatest running cost of most houses, appealed greatly to them both.

The house faces more or less due south, fortunately dictated by the tight site, and incorporates a high level of glazing on the south facade and correspondingly low levels on the north side. The only fixed source of heating in the home is a wood-burning stove, located in the main living room. During the last severe winter, the timber frame provider became concerned for Darren and Ashleen and rang them to check that they were warm enough. He needn't have worried! Even with outside temperatures around -10°C, internal temperatures were maintained around 20°C. Ashleen reports that they needed to light the wood­burning stove most days for three months during the winter. This required a certain level of intervention, which perhaps would not suit everyone. However, the same heat input could be provided readily by, say, a wood-pellet burner with automatic ignition start, controlled on a timer.

Unlike the Co Donegal house the wood burning stove provides heating only, the solar hot water panels (located on the main roof pitch) being supplemented by an air-source heat pump. The heat pump comes as an integrated package with the hot water cylinder and can be controlled on time and/or temperature to fill in the gaps left by the solar panels. This approach has the advantage over the Donegal house of separately controlled heating and hot water systems. As with the Donegal house, small towel radiators in the bathrooms are run off a loop from the hot water cylinder.

Naturally, the heat pump runs on electricity and represents a certain energy and cost component (the latter of which can be reduced if operated on a night rate tariff). Darren reports that their electricity bills are around £40 a quarter more than normal, which also includes the ventilation system fans. However, some small gains are made through savings in lighting, which is almost all LED. Overall, the cost benefit is clear, the additional electricity cost being far outweighed by that of running a central heating system in a standard (or even well insulated) new build property.

The saw-tooth profile indicates day to night readings. Even during the coldest part of the winter from mid-to-end December 2010 (outside temperatures about -5°C) the internal temperatures are maintained at 18°C to 20°C, occasionally rising much higher when the occupants demanded it. The high variation in temperatures is due to the heating system not being controlled.

The County Donegal House

The house featured here was the prototype of the No Central Heating House and was built for myself and family. The house is intermittently occupied, being used currently as a second home. It faces southwest, to avail of views and solar gains while also addressing privacy with respect to neighbouring dwellings. High levels of glazing are deployed on the southeast and southwest facades and correspondingly low glazing ratios on the other sides.

The house comprises three bedrooms, plus a study/bedroom and two bathrooms, in addition to open plan living/dining and kitchen areas and a large porch. Heating for the whole dwelling is provided by a small wood-burning stove located in the main living area, which is open plan to the dining area and kitchen. The stove is fitted with a back boiler which produces hot water, fed via a simple gravity system to the hot water cylinder. This complements, seasonally, an evacuated tube solar water heating system located on the south-west roof pitch. Indeed, the two systems complement each other so well that the electric immersion back­up has never been used.

It was always intended that towel radiators ­either wet or electric - would be installed in the bathrooms. During the past cold winter, it was found that, in running the wood burner sufficiently to maintain comfort conditions in the living area, an excessive amount of hot water was produced ­there being no means of separately controlling the two outputs from the stove. Wet towel radiators were thus installed into the bathrooms, run off a pumped circuit from the hot water cylinder. This had a triple effect - not only was a heat leak provided for the hot water cylinder and comfort temperatures improved in the bathrooms, but the distribution by the ventilation system of heat from the bathrooms reduced the load on the wood burner.

Lighting is all LED, mostly "warm-white" 4.5W downlighters, which provide a similar light output and quality to halogen fittings while using a fraction of the energy. Of course LEDs produce much less heat than incandescent lamps. However, it is a much more efficient strategy to let the heaters produce heat and the lights provide light.

The intermittent occupancy of the house means that it takes a little while to warm up on first arrival - but this is of the order of a couple of hours after firing the stove, rather than a couple of days in the case of a standard masonry built dwelling. The house is built in an exposed location facing the Atlantic coast so you might expect us to get lots of noise, but the high level of airtightness, combined with very well draught proofed triple glazed windows, makes for an almost eerily quiet interior, even when the westerly winds are blowing their hardest...

Cost considerations

In order to build a dwelling that's comfortable to live in and does not require central heating, you will need to bear in mind two major cost considerations: the extra insulation and the triple glazing required to achieve the necessary thermal performance. Other costs such as mechanical ventilation with heat recovery (MVHR) and solar water heating might have been incurred in any case on a conventional build, although both are central to the concept of the No Central Heating house - the MVHR because it preheats the home and the solar collectors because there is no boiler to produce hot water.

I would estimate a cost of about £4,000 for the solar panels, £4,000 for MVHR, £4,000 for the additional insulation (above the cost of complying with the building regulations), and £2,000 for the higher specification on the glazing (overcost as compared to building regulations compliant double glazing), or equivalent in euros.

Towel radiator costs are minimal and stoves could have been specified as room heaters anyway, although prices were approximately £1,000 for the Co Donegal house and about £1,800 for the Co Down house.

Against this, about £10,000 is saved on a boiler and heating. In addition, the cost of fuel saved each year is likely to total £1,000, so that means that within about four years the extra cost of making your house thermally efficient will have been amortised. It also means that after this time, if you have chosen the central heating route, you will be paying more than if you hadn't!

An important factor with these two houses is that they were built without recourse to grants and full costs were paid for all components and systems, although the Annetts were able to avail of the (soon to be defunct) Low Carbon Homes scheme two-year rates rebate. This shows that ultra low energy housing is affordable - indeed, the No Central Heating system has also been used recently in six social housing units in East Belfast, with the promise of very low running costs and a considerably reduced risk of fuel poverty. Following the County Down and Donegal houses, two other private houses of this specification have been completed with more underway.

As we draw ever closer to the Government's target date of 2016/17 for "carbon neutral" new dwellings, all new housing will need to adopt these principles. Self-builders in particular can move well ahead of legal minimum requirements and future-proof themselves against rising energy costs by adopting this concept. The experience of these houses shows that it works not only in theory but in practice, with very little impact on lifestyle except for a greatly reduced carbon footprint and low energy bills!

Note: All costs include supply and installation but note that these are very rough estimates. When calculating your payback you should also consider the electricity cost of running the MVHR fans and, if there is one, of running the heat pump, though these should each be under £lOO/year or €lOO/year, where relevant, for the equivalent to the dwellings featured here .

The house was occupied for one day around 20 Nov 2010 and from 29 Dec 2010 to 1 Jan 2011, during which time the wood burner was fired up. The occupied period was not during the very coldest part of the winter and the wood burning stove was not run at its highest output. The impact of the heating system can be seen during occupancy; when the house is vacant internal temperatures follow outside temperatures. Night time losses during unoccupied periods are high because no curtains were drawn. Note that the impact of solar gains on the internal temperature are reflected in the spiked profile indicating day to night readings.

Patrick Waterfield is a Chartered Engineer and a Fellow of the Energy Institute based in Belfast