Overheating in homes

This post was written by BSRIA's Saryu Vatal

This post was written by Saryu Vatal, Senior Consultant of BSRIA’s Sustainable Construction Group

BSRIA’s Residential Network organised an event on the 22nd of July focussing on the issue of overheating in homes with an excellent line up of speakers. Nicola O’Connor started the day summarising an extensive research project by the Zero Carbon Hub that brought together input from government, industry and academic experts to understand the challenges around tackling the risk of overheating in homes (http://www.zerocarbonhub.org/current-projects/tackling-overheating-buildings). Chris Yates from Johnson and Starley made an appraisal of the assumptions and requirements within the Building Regulations and associated guidance as well as the implications for mechanical ventilation system manufacturers. Neil Witney from DECC explained the challenges around defining and regulating of overheating within homes, current policies and mechanisms that may be introduced in the future in response to the growing body of evidence highlighting the issue. Paul Ciniglio from First Wessex shared the organisation’s findings from several research projects and experience from their own developments, which resonated with issues highlighted by members of the audience. Bill Gething of Sustainability + Architecture and professor at the University of West England brought into perspective how changes in the way homes have been designed and built over the recent years has led to a shift in the performance of homes. James Ford, partner at Hoare Lea discussed some key considerations for designers to address the issue at early stages, to help minimise risk and dependence on active cooling solutions.

Extent of overheating

Evidence indicates that up to 20% of homes in England may already be overheating. Areas where additional risks have been highlighted include:

  • Common areas in apartment blocks, especially where community heating is installed – these areas are not assessed using SAP as they are outside the dwelling envelope. In reality, being unoccupied spaces these are often not modelled for their thermal performance (and energy use) at all. Community heating is being incorporated in an increasing number of projects and the supply network remains live even in the summer to meet the domestic hot water demand. Ensuring that the specification and installation of insulation for the distribution pipework is adequate is becoming increasingly important as buildings are made more airtight. Often stairwells and circulation areas have a high proportion of glazing and, with recent improvements in the general standard of construction and materials, tend to retain a large proportion of the heat gains. It is now important to incorporate a ventilation strategy for these spaces so that the accumulated heat can escape.
  • Urban areas – the average temperatures in city centres can be more than 4°C higher than rural areas. Flats are more common to city centres and these are often close to sources of noise and air pollution and have limited, if any, potential for cross ventilation. All these factors can combine to limit the effectiveness of natural ventilation in addressing the build-up of heat and not just in the summer. Building designs that incorporate large proportions of glazing in their facades, such as penthouses, if not carefully designed, can require air change rates that are unrealistic to achieve, using natural or mechanical ventilation systems.

Need for a definition

A number of sources and definitions are being referred to currently when evaluating for the risk of overheating in homes. These include CIBSE’s Environmental Design Guide A (2006) which sets standards for comfort, although it is not mandatory to use this to demonstrate compliance with the Building Regulations. Dynamic modelling through tools such as TAS and IES offer the opportunity of making a more comprehensive evaluation than SAP, but this option is skill, time and cost intensive. Building Regulations do not relate to limiting overheating for thermal comfort, just limiting the use of fuel and power for air-conditioning. The minimum evaluation for demonstrating compliance with Criterion 3 of Approved Document Part L of the Building Regulations needs to be carried out using SAP. While SAP is not intended to be a design tool, it is accepted that it is the default tool the industry uses widely.

Research projects have highlighted that dwellings can demonstrate a risk of overheating when evaluated against the CIBSE standard but not when modelled in SAP. Surveys from the Zero Carbon Hub study showed that nearly 60% of the housing providers surveyed had checks in place to assess the risk of overheating. However, only 30% of these housing providers explicitly included the requirement for considering the risk of overheating as part of their employees’ requirements to architects and designers. This suggests a missed opportunity for the issue to be addressed early on in the process, when cost and energy efficient measures may be effectively incorporated.

There are several challenges around the definition of conditions under which overheating can be said to occur as several factors contribute to this, including but not limited to air and radiant temperatures, humidity, air velocity, level of activity the adaptability of the individual. There are several checks that can be built into the design process which can help identify the risk at an early stage and allow for a method for mitigating these to be set up and followed through.

Contributing factors
The energy efficiency of homes in the UK has improved significantly in terms of reduction of space heating loads. This has come about in new homes through Approved Document Part L 1A of the Building Regulations and in existing homes through schemes such as the Green Deal. Homes are now less leaky and better insulated to keep warmth in but attention and emphasis is needed on measures to facilitate the expelling excess heat adequately when temperatures rise.

Homes are expected to provide comfortable conditions for occupants all year round and through a range of different occupancy patterns, which may in reality be considerably different to the standard assumptions made in modelling tools like SAP. It is possible that if modelling for thermal comfort is carried out assuming worst case assumptions for occupant density, external conditions and hours of occupancy, many homes would require mechanical cooling. There are, however a number of common sense measures that can be applied to ensure the impact of key contributing factors are minimised. These include controlling solar gains from south and west facing glazing and making provisions for adequate, secure ventilation especially when thermal mass has been incorporated in the structure.
The current extent of overheating in homes must be seen in the context of the anticipated changes in climate. With external temperatures expected to rise with an increased frequency of extreme weather conditions, homes built today must be fit for purpose for warmer summers.

Mechanical cooling?
There has been a rise reported in the installation of mechanical cooling systems in homes in the UK, more noticeably so in the south. While this may be an expected feature in high end homes, the cost of running these systems can be prohibitive, or at least perceived as so, for households where minimising expenditure on energy and fuel is a priority.
There is potential to develop low carbon mechanical cooling systems such as reversible heat pumps. The large scale uptake of these can however have some serious implications for energy supply and the capacity of the grid to accommodate a draw in peak summer months.

Way forward
In addition to affecting comfort, exposure to high temperatures over prolonged periods can have a significant impact on the health and well-being of residents. It is critical therefore to agree on a set of parameters that can help define overheating in homes and this should be carried out with input from bodies such as Public Health England.
Until a definition and modelling strategy is developed, designers and housing providers can refer to several good practice guides and research studies that help embed a common sense approach to design. There is significant potential to mitigate the risk of overheating in homes if early stage design decisions are taken with due consideration for the issue. The limitations of mechanical ventilation systems to help achieve comfort in homes must be acknowledged so that the final burden of an ill-considered design does not rest with the occupants.

References and further reading
Design for Climate Change, Bill Gething and Katie Puckett, RIBA Publishing Feb 2013

To find out more about our Residential Network and to download the presentations from this meeting check out BSRIA’s Network pages.  To find out more about all of BSRIA’s networks contact tracey.tilbry@bsria.co.uk.

Best & Worst Practices Please!

Julia Evans, BSRIA Chief Executive

Julia Evans, BSRIA Chief Executive

BSRIA recently held a workshop on behalf of DECC identifying priorities to promote low carbon heating and cooling in non-domestic buildings as part of the development of its low carbon heat strategy.  Attendees were drawn from both the Young Engineers and Energy and Sustainability BSRIA networks.  Personal thanks to AECOM’s Ant Wilson for chairing the event.

It was a busy day.  It recognised that both new and existing buildings have a pivotal role in reducing greenhouse gas emissions, and by 2050 one of the key requirements will continue to be how we provide heating and cooling.

BSRIA’s Peter Tse and Ian Orme both gave excellent presentations which drew on both good and poor practices identified from more than 50 independently assessed case studies.  These, I felt, answered the questions “what does good practice look like”, as well as “what are the consequences when its not followed”.

The workshop session resulted in many suggestions as to priorities for the future.  There were a couple which caught my eye.

In response to the suggestion that one of the priorities for DECC should be identifying independently assessed best practice and developing exemplars of new technologies, a number of delegates felt that instances of “bad practice” were even more helpful.  It seemed to me that a priority for at least a part of the audience was to know what to avoid doing.  Perhaps this reflects the industry’s receptiveness to messages about risk, and that we often learn most when we make mistakes.  The emphasis on “independent assessment” also resonated.  Many have become sceptical about instances of self-identified “best practice”, and BSRIA’s independent guidance on what works, and what does not, is there to assist the industry do things better.

Another of the workshop themes was on “skills shortages”.  After many years of recession, construction companies have euphemistically “right sized”, and this means that we have lost a lot of great talent from the industry.  Now that there are green shoots of recovery in construction, there is already talk of an exacerbated “skills gap”.  This gap makes it even more challenging for the industry to deliver buildings which meet the needs of their occupiers and where innovation is required to help tackle climate change, and meet the UK’s commitment to “zero carbon” and “very low energy” buildings. This reminded me of another of BSRIA’s strengths – training provision.

BSRIA's 2014/15 Training Brochure

BSRIA’s 2014/15 Training Brochure

Finally there was an astute observation that our recent quest for low carbon buildings has meant that we have worried less about the efficient use of energy, with the net outcome that we can end up with an EPC A rating for carbon design, but a DEC G rating for energy in use.  The move to policies that move us to buildings which are both zero carbon and nearly zero energy use will hopefully remedy this, although I suspect this particular journey may contain further unintended consequences before we reach that goal.

The workshop identified many requirements if we are to create environmentally conscious buildings that meet user needs, and importantly maintain these elements over the life of the building.

BSRIA’s mission remains to “make buildings better”.  As part of my role, I’m listening to our members and the industry what they expect from BSRIA.  I’d like to extend this offer to you, so if you have ideas about BSRIA’s future role, please send them to me: Julia.evans@bsria.co.uk.

To learn more about the BSRIA workshop you can download all the presentations from our website. 

The hidden menace of corrosion in heating and cooling systems

Written by Reginald Brown, Senior Consultant at BSRIA

Written by Reginald Brown, Senior Consultant at BSRIA

Most buildings services engineers will have come across a heating or cooling system that has not received water treatment and still appears to function perfectly and another that has apparently been treated but experienced serious corrosion related failures. Why should one be vulnerable and the other not? The answer is that most common metals are subject to corrosion but the rate of corrosion and risk of failure depends on a variety of factors including the chemical and microbiological environment, temperature, flow rate and not least the thickness of the metal.

In many respects water is the ideal heat transfer medium for building services. It has a reasonably high specific heat, is liquid over a convenient temperature range and is non-flammable, non-toxic and freely available. The downside is that water is an electrolyte that facilitates corrosion in metallic pipework and components. One might think that the obvious solution is to use plastic pipework but this can actually increase the risk of corrosion of the corrodible components that remain.In a steel pipework system, the dissolved oxygen in the system water will rapidly be used up as it reacts with the large area of corrodible surface but the loss of metal thickness should be insignificant. In a plastic pipework system there are few corrodible components so oxygen concentration will remain higher for longer and the corrodible materials will continue to corrode at a high rate. This means that almost all water based heating and cooling systems should have some form of water treatment to control corrosion, and it may be even more important in plastic pipework systems.

The usual construction programme for large building projects involves installation and pressure testing of pipework followed by pre-commission cleaning and commissioning several months later. During the gap between pressure testing and pre-commission cleaning the system may be both stagnant and still contaminated with manufacturing and construction residues. This is an ideal environment for the development of biofilm and corrosion.

In traditional steel pipe systems (using BS 1387:1985 or BS EN 10255:2004 medium or heavy grade pipe) this is not too much of a problem. The relatively thick pipe (at least 3.2 mm for 1 inch nominal bore and larger) can tolerate the initial corrosion due to the oxygen in the fill water and biofilm development during subsequent stagnation conditions. Provided the pre-commissioning cleaning is carried out effectively, ideally with a biocide wash prior to chemical cleaning, there should be minimal impact on the lifetime of the system.

Thin wall steel pipes and steel panel radiators may not be so fortunate. The thickness of 25 mm nominal bore thin wall carbon steel pipe is only 1.5 mm while a typical steel panel radiator is only 1.3 mm thick. If the initial corrosion was spread uniformly across the metal surface it would not be problem but what tends to happen is that small patches of the surface become anodic relative to their surroundings and are preferentially corroded leading to rapid localised pitting. If dissolved oxygen levels persist or are replenished due air ingress, continuing additions of fresh water or permeation through non-metallic materials then the pitting can progress to perforation. Components that should last 25 years can be perforated in a few months. This is one of the most frequent types of corrosion failure reported to BSRIA and can result in expensive remedial works even before the building is occupied.

Water treatment chemicals work by inhibiting the corrosion process, either by coating the surface of the metal (anodic inhibitors) or otherwise blocking the corrosion reactions (cathodic inhibitors). However, inhibitors are not the solution to poor closed system design or operational deficiencies and certainly won’t work to best effect in a dirty system i.e. one with a high level of suspended solids and/or biological contamination. Also, the system operation must allow the inhibitors and other water treatment chemical to be maintained at an effective concentration and circulated throughout the year.

In summary, the factors necessary to avoid pitting corrosion of steel components in closed systems are:

  1. Minimise the delay between first fill and pre-commission cleaning.
  2. Carry out effective pre-commission cleaning of the pipework system.
  3. Establish, monitor and maintain effective water treatment and water quality as soon as possible in the life of the system.
  4. Circulate water throughout the system on a daily basis to avoid stagnation.
  5. Avoid ingress of oxygen from inadequate pressurisation or excessive fresh water additions.
What happens in the first few weeks of the system can prevent pipe corrosion like this over the next 25 years

What happens in the first few weeks of the system can prevent pipe corrosion like this over the next 25 years

What happens in the first few weeks of the life of the system will influence its fate over the next 25 years. You can’t easily see what is going on inside a pipe but get it wrong and you could be looking at major remedial works in a tenth of that time.

A detailed discussion of corrosion and the use of inhibitors and other chemicals is contained in BSRIA BG50 Water Treatment for Closed Heating and Cooling Systems. Pre-commissioning cleaning is described in BSRIA BG29 Pre-commission cleaning of pipework systems. Guidance on the monitoring of water quality in closed systems is contained in these documents and BS 8552 Sampling and monitoring of water from building services closed systems – Code of practice.

BSRIA also runs a Pre-commission cleaning of pipework systems training course and provides independent failure investigations for all types of building plant and systems including pipweork corrosion.

This article was first published in Modern Building Services.

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