BRE: Ventilation and overheating in multi residential buildings Are they linked?

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1 BRE: Ventilation and overheating in multi residential buildings Are they linked? M Swainson BRE HVAC Engineering swainsonm@bre.co.uk Thursday 20 th October 2016 Part of the BRE Trust

2 Ventilation and overheating the link What is overheating? Why is overheating occurring now? Multi residential buildings why a particular problem? Ventilation in dwellings and heat removal How do we evaluate the risk of overheating? Is ventilation the solution?

3 What is overheating? CIBSE provide industry guidance: Peak temperatures and overheating criteria for the design of buildings, recreated from CIBSE Guide A, Environmental Design (2006) Limiting values for the operative temperature (max) to avoid overheating in free-running mode as a function of the exponentially weighted running mean of the external temperature, from CIBSE Guide A, Environmental Design (2015) ZCHub noted: Sleep quality has been found to reduce during the summer, an effect that is commonly attributed to high night time temperatures (Okamoto-Mizuno & Tsuzuki 2010). This is very likely to be the case during prolonged heat episodes characterised by consecutive hot days and warmer than average nights that inhibit the recovery from daytime heat (Fischer & Schar 2010)

4 What is overheating? HHSRS (Housing Health and Safety Rating System)(Housing Act 2004) As temperatures rise, thermal stress increases, initially triggering the body s defence mechanisms such as sweating. High temperatures can increase cardiovascular strain and trauma, and where temperatures exceed 25ºC, mortality increases and there is an increase in strokes. Dehydration is a problem primarily for the elderly and the very young. Hazard assessment The assessment should take account of the provision for ventilation, particularly night-time ventilation, and the provision and condition of any mechanical ventilation or air conditioning system. Also relevant will be the thermal capacity of the structure and the amount and position of thermal insulation, the extent and orientation of glazing, and the condition of and controls for the heating system.

5 Is overheating a problem? Most work to date has been based on heat waves short term periods of very high outside air temperatures, day and night. Links to mortality demonstrated. Relationship between internal and outside air temperatures poorly understood and therefore the impact of changes in building design and construction unevaluated. Temperature mortality associations in each region in England and Wales, Source: Armstrong et al. (2011), (RR: Relative Risk)

6 Overheating - understanding why? The heat balance is simple: Heat gains = heat losses Sources of heat gain Solar gains conducted through fabric. Re-radiated and convected internally Ventilation and infiltration of outside air Internal gains from services; DHW system, lights, etc. Internal gains from occupants, occupant activities and white/electrical goods Direct solar gains through glazing

7 Overheating understanding why? Fabric insulation well insulated and getting better Airtightness good and getting better Glazing and shading balance of daylight and gains Internal gains / usage patterns defined by building user and overall little changed Thermal mass move towards lower mass with timber frame, plasterboard lining on walls and partitions, etc.

8 Overheating why are flats more at risk? Surrounded by spaces at a similar temperature above, below and on three sides. (single sided flat) Infiltration a function of wind direction and window opening throughout building many flats on lee side of a building get no fresh air though open windows. Communal heating distributed 24/7 to meet DHW requirements. Ventilation designed to meet Building Regulations minimum flows only. Purge ventilation practically impossible to achieve for heat rejection.

9 Overheating not a result of a heat wave The separation of the inside and outside environments means the weather can, in many cases play a relatively minor role in the internal environment.

10 Overheating understanding why? Heat gains in a block of flats communal heating Rooms at 20ºC Void at 40~50ºC Rooms at 20ºC Water at 80ºC 45.0 C SP C

11 Overheating understanding why?

12 Overheating The other side of the equation The heat balance: Heat gains = heat losses + heat rejected But how do we reject heat from dwellings and most UK buildings? Ventilation natural or mechanically driven Ventilation of buildings is for the provision of fresh air and removal of pollutants; moisture, odours, etc. AD-F 2010 states:

13 Overheating The other side of the equation CIBSE Journal August 2014 Is this really good design? Is this provision of effective purge ventilation? (Yes, but not for heat rejection)

14 How do we evaluate overheating - modelling SAP. Not really a tool for looking at overheating (AD-L) Dynamic thermal modelling. The perfect tool, if only we could define all the inputs..

15 How do we evaluate overheating - modelling How is the opening of this modelled? SAP 2012 version 9.92, dated October 2013

16 How do we evaluate overheating - modelling Urban heat island can be captured in a DSM, but the micro climate has significantly more impact and is largely ignored in modelling.

17 How do we evaluate overheating - modelling Employer's Requirements specify all pipes to be insulated with insulated pipe supports throughout, but What is assumed in the modelling?

18 How do we evaluate overheating - modelling Monitored temperatures show significantly greater stability, modelling did not capture true dynamic thermal changes.

19 How do we evaluate overheating - modelling SAP. Not really a tool for looking at overheating (AD-L) Dynamic thermal modelling. The perfect tool, if only we could define all the inputs.. Currently working on a broader model that takes some account of the: Local environment deep urban or green space. Micro environment shading, albedo, etc. Ability to realistically achieve purge ventilation rates. Confounders to opening windows at night pollution, security, noise, etc.

20 Overheating is it linked to ventilation Primary sources of heat gains are relatively simple to define. Local / micro modifiers are complex and little studied. Modelling is based on idealised buildings, in reality pipes are not insulated as specified, windows not opened as assumed for plausible reasons, etc. Ventilation is the key means of rejecting heat, therefore it needs to move from We met the Building Regs. to a primary part of the Building Services: Fully designed to meet AD-F and heat rejection flow rates. Commissioned. Maintainable. (Follow the lead of non-domestic buildings possibly consider central systems to minimisesome modifiers)

21 BRE: Ventilation and overheating in multi residential buildings Are they linked? Mich Swainson BRE HVAC Engineering Thursday 20 th October 2016 Part of the BRE Trust