MVHR and natural ventilation: Comments and case studies

Transcription

1 MVHR and natural ventilation: Comments and case studies Healthy Homes: Ventilating for good indoor air quality 21 November 2012 Professor Rajat Gupta and Rohini Cherian OISD: Low Carbon Building Group, Oxford Brookes University

2 Structure of Presentation Background Case study applications Natural Ventilation NV Mechanical Extract Ventilation - MEV Mechanical Ventilation & Heat Recovery - MVHR Lessons and challenges

3 Background

4 Need for ventilation Appropriate indoor air quality (IAQ) = absence of air contaminants / pollution which may impair the comfort or health of building occupants Conflict between energy efficiency, health & wellbeing of occupants Natural Ventilation - variable, uncontrollable, & occupants - liability rather than asset Move towards mechanical ventilation regarded as inevitable Source: GHA, 2011:Ventilation and good indoor air quality in low energy homes

5 Types of ventilation Passive systems Windows Doors Trickle vents Ventilation slots Hygroscopic materials Active systems Mechanical extract MVHR Air conditioning (Changing climate!) Source: Building Regulations Part F (HM Government,2010)

6 Natural Ventilation Natural ventilation systems rely on pressure differences to move fresh air through buildings. Dependent on design of building form, material and occupant operation. Systems are simpler than mechanical High dependence on occupant behavior and operation. Silent and without direct running costs Things to consider: Efficiency Insulation and air tightness Controls - accessibility and simplicity Privacy visual and auditory Safety - from intruders and in case of fire Potential issues: overheating, draughts, glare Source: University of Tennessee, 2012

7 Continuous Mechanical Extract Mechanical extract ventilation (MEV) system continually extracts air from wet rooms. Typically dual speed, providing low speed continuous trickle ventilation, and highspeed boost flow. Replacement dry air is drawn into the property via background ventilators and by air leakage. Providing a gap at the bottom of the internal doors will allow the free passage of air through the property. Requires regular maintenance to clean filters Best practice standard: Specific fan power of 0.6W/l/s or less Source: EST guidance -GPG268

8 Mechanical Ventilation & Heat Recovery Combines supply & extract ventilation, system is typically dual speed Warm, moist air is extracted from wet rooms is passed through a heat exchanger before being exhausted to outside. Fresh incoming air is preheated via the exchanger and ducted to habitable rooms. Energy saving benefits are only realised for airtight properties Reduces risk of condensation and cold air draughts System must be acoustically treated and provided with fire dampers Best practice standard: Specific fan power of 1W/l/s or less Heat recovery efficiency = 85 %

9 Case studies

10 Case studies in ventilation Whole House retrofit, London Natural Ventilation with intermittent mechanical extract GHA Apple plot, Stawell Continuous mechanical extract CSH Level 5 Homes, Swindon Continuous mechanical ventilation with heat recovery

11 Case study-1

12 Whole House retrofit, London 1992 brick cavity, mid terrace social housing Whole house retrofitted to RfF target of 17kgCO 2 /m 2 /yr Area: 83.7 m² Air tightness: Measured >5m 3 /h/m 2 Temperature : Mean 23 deg C Min 13 deg C Max 30 deg C Relative Humidity (space): Mean 48% Min 25% Max 75% CO 2 level (living space): Mean 630 ppm Min 408 ppm Max 2500 ppm Natural Ventilation with intermittent mechanical extract

13 Natural ventilation for tackling overheating Adequate secure insulated louvers/vents, Movable shutters or shades, operable roof light and internal thermal mass = no mechanical cooling Source: (SOURCE: ZERO CARBON HUB, 2010)

14 Whole House retrofit, London Occupant Satisfaction Very satisfied with ventilation panels in windows and this is used regularly. (Occupant satisfaction correlates with perceived control) Ventilated loft is loved for its aesthetics and light; but the drying system does not seem to be used often convenience issues. Distribution of monitored CO 2 levels within the home reveal that 92% of readings are within 1000ppm.

15 Case study-2

16 EST/GHA study Apple plot, Stawell Four bedroom 139m 2, detached house built in 2009 to CSH Level 5 Timber frame, wood fibre board outer insulation, recycled newspaper inner insulation, OSB internal lining, and timber-clad Air tightness: Designed 3 m 3 /h/m 2 Measured 2.82 m 3 /h/m 2 Relative Humidity (living space): Mean 55 % Min 35 % Max 75 % Temperature (living space) : Mean 20 deg C Min 15 deg C Max 27 deg C CO 2 level (living space): Mean 500 ppm Min 400 ppm Max 1200 ppm Continuous mechanical extract

17 EST/GHA study Apple plot, Stawell Extract location: Kitchen, ensuite and bathroom via ceiling mounted plastic grilles Extract fan: Location: above the built-in wardrobe in master bedroom. Control: Location: isolator switch placed at high level in the wardrobe Activation: an electronic control system responds to humidity levels detected in the wet rooms Householders unaware of the control switch! Continuous mechanical extract

18 Ongoing monitoring data: fans and pumps Electricity use, May March 2012 (3118kWh) Rainwater pump 1% Solar pump 0% MV fan 8% Immersion 16% Light/appliances 63% CH pump 12%

19 EST/GHA study Apple plot, Stawell Occupant Feedback: Overall Air is cold in winter, but is not stuffy. Occupant who suffers from asthma has not had any problems in this house. Specific to MEV Weak, noisy, non-adjustable Does not remove cooking smells sufficiently we were surprised that the kitchen has no extractor fan Residents thought it was mechanical ventilation with heat recovery (MVHR) unit. Continuous mechanical extract

20 Case study-3

21 CSH Level 5 Homes, Swindon 13 CSH level 5 houses for social rental in Swindon, UK Timber frame and hempcrete construction Photovoltaics, Solar thermal, Exhaust Air Heat Pump (EAHP) Air tightness: Designed 2 m 3 /h/m 2 Average Measured 4.7 m 3 /h/m 2 Relative Humidity: Mean 50% Min 40% Max 65% Temperature: Mean 22 deg C Min 26 deg C Max 20 deg C CO 2 level: Mean 600 ppm Min 1300 ppm Max 380 ppm Continuous mechanical ventilation with heat recovery

22 CSH Level 5 Homes, Swindon Mechanical ventilation, hot water heating and under floor heating is provided by an exhaust air heat pump in combination with solar thermal panels Delivered air flow rates satisfy the design guidance Continuous mechanical ventilation with heat recovery

23 MVHR Commissioning review Issues identified in initial post construction evaluation Additional, unplanned bends and offsets in ductwork Dirty filters: system running through final construction Location of filter Connection between vent and duct Continuous mechanical ventilation with heat recovery

24 MVHR Swindon Homes Occupant feedback Overall impression of the building design positive Tenants did not fully understand the mechanical ventilation and felt that it was insufficient, therefore opening windows to provide desirable indoor air-quality House number X experienced higher CO 2 concentrations over longer periods than house number Y. House number X also utilised their doors and windows to provide additional ventilation whereas house number Y did not. Is the MVHR system under-ventilating the house in practice? Continuous mechanical ventilation with heat recovery

25 CSH Level 5 Homes, Swindon Access to control and filters Fan speed switch User controls at top highlighted in yellow. Both supply air and exhaust air filters were examined during this inspection visit and were found to be heavily contaminated with dust particles. Continuous mechanical ventilation with heat recovery

26 Comparing the three case studies Whole house retrofit, Natural ventilation with intermittent local extract EST/GHA study, Continuous mechanical extract ventilation Swindon Homes, Mechanical Ventilation with Heat Recovery Positive Occupants regularly leave ventilation panel open Acceptable indoor CO 2 levels Very good air quality reduction in health problems Occupants happy with temperatures (but in use BUS not carried out yet) Good CO 2 levels Not so positive! Air temperature, humidity and CO 2 levels vary widely throughout the day depending upon external conditions & occupant opening (of doors, windows, skylight) behaviour Extract not strong enough for kitchen odours Internal temperatures lower than expected Miscommunication/ mismanagement of expectations Airtightness targets not met Occupant awareness and understanding of ventilation system is low!

27 Challenges and lessons

28 Closing the performance gap Design Stage Ventilation strategy should be decided with air tightness strategy, from start with all stakeholders Natural ventilation strategies may be compromised by design changes (Good passive design is vital!) Mechanical ventilation systems should meet varying conditions(?) Sufficient space for good ducting must be accommodated at design Avoid integration of the cooker hood into the extract system to reduce risks of clogging Location of access for maintenance - many MVHR filters have been found to need cleaning once every 3 months

29 Closing the performance gap Construction Actual as-built air permeability is crucial to the ventilation performance and must take chimneys etc. into consideration Models represent the physics but not occupant behaviour or construction defects. Type of construction should be considered Sequencing of construction works is critical Installation and commissioning Effective training for installers, quality assurance checking, responsible individuals/ organisation should be identified

30 Closing the performance gap Occupancy and Operation Noise from the ventilation system, may lead the occupants to tamper Operation and control information must be clear and simple with rapid and detectable response Double check occupant awareness and optimum operation with second handover visit (who guards the guardians?) Poor correlation between window opening & external temp caused by lack of occupant awareness can severely affect performance Varying occupancy rates should be considered Occupant ratings for air quality can differ significantly Maintenance Clear maintenance plan with contact information for individuals responsible for each task / problem What is the lifecycle cost of this regular maintenance?

31 Thank you for listening