The Evolution of Hybrid Ventilation and design requirements

In 2013 Monodraught developed the hybrid thermal mixing based on the Priority School Building Programme requirements, with the Facility Output Specifications forming the basis for the design of the PSBP Schools and ultimately the definitive guide to school design.

The hybrid ventilation systems included our own control system and an internal data logging system using a series of industrial SD cards, to record the system’s performance and room conditions. The hybrid units were developed to include the primary heating and at this point we could gather data on the room energy needed to maintain temperature.

Within this data it was identified that there was an opportunity for reducing the energy use within rooms with a low resistance heat recovery aspect included within the hybrid ventilation solution. In 2020 HVR Zero system was launched, which included our first generation of internet of things (IoT) monitoring and integrated air flow paths to provide natural ventilation, thermal mixing and mechanical heat recovery ventilation (MVHR) using a specific fan power that provided year-round energy savings.

TM54: 2022 plays an important role in evaluating the overall operational energy consumption within buildings (and Hybrid ventilation system design), as heating, ventilation and air conditioning is estimated to make up over 50% of the total buildings energy use.

Our new HVR APX unit takes this development a stage further with lower specific fan powers, optimized heat recovery efficiency, integrated heating and cooling coil and an array of air quality sensor technologies included as standard. These upgraded sensors monitor internal temperature, humidity, volatile organic compounds (VOC), pollution levels (PM2.5), actual air flow rates, and CO2 concentration. These sensor technologies meet the requirements for the WELL building standard which has been adopted for many office buildings. This is then coupled with our IoT platform that allows continual remote data monitoring.

The latest CIBSE TM23: 2022 guidance, which requires ventilation systems to be closed but not sealed during the air testing of buildings, will mean all natural, hybrid and mechanical ventilation systems that are fitted with closing devices will not only need to efficiently modulate the levels of airflow, but provide high levels of airtightness when fully closed.

With a key focus on future climate conditions within building design, with modelling against 2050 and 2080 weather files, the system has been specifically designed for compatibility with renewable heating and cooling solutions such as air source or ground source heat pumps. The APX can future proof buildings with the standard provision for cooling.

The Adaptive Thermal comfort criteria stipulated in BB101 (for schools) and CIBSE Guide A requires three criteria, taken together, to be used to assess the risk of overheating of the building in the UK, using the geographically closest CIBSE Design Summer Year (DSY) weather file. A room or building that fails any two of the three criteria is classed as overheating:

· Criteria 1- Hours of Exceedance: During the occupied hours of the non-heating season (1st May to 30th September), the predicted operative temperature should not exceed the maximum adaptive temperature by 1°K or more, for less than 40 hours (BB101) or 3% of the occupied hours (CIBSE Guide A);

· Criteria 2- Daily Weighted Exceedance: For the severity of overheating, which can be as important as its frequency, the weighted exceedance of the temperature rise and its duration should be less than or equal to 6 during any occupied day of the non-heating season;

· Criteria 3- Upper Limit Temperature: The absolute maximum daily operative temperature for a room, beyond which the level of overheating is unacceptable, should not be greater than or equal to 4°K, than the maximum adaptive temperature (Top – Tmax), at any time.

With regards to internal air quality, CIBSE Guide A criteria specifies that:

· As stated within Building Regulations Approved Document F, office applications and other general non-dwelling spaces require a capability of providing minimum ventilation rate of 10 l/s per person. Other specialist types of area may require additional ventilation.

· If a ventilation rate of 10 l/s per person is achieved, an average indoor carbon dioxide concentration of between 900ppmm – 1150ppm should be achieved.

Whilst BB101 criteria specifies that for natural ventilation or hybrid systems operating in natural mode:

· In all teaching and learning spaces when measured at seated head height, during the continuous period between the start and finish of teaching on any day, the average CO2 concentration of carbon dioxide should not exceed 1500 ppm.

· The maximum CO2 concentration should not exceed 2000 parts per million (ppm) for more than 20 minutes each day.

For mechanical ventilation or hybrid systems operating in mechanical mode, BB101 criteria specifies that:

· The maximum CO2 concentration should not exceed 1500 parts per million (ppm) for more than 20 minutes each day;

In addition to the mechanical and natural ventilation:

· At any occupied time, including teaching periods, the occupants should have the ability to lower the concentration of CO2 to 1000 ppm;

· The extreme maximum CO2 concentration of carbon dioxide should not exceed 5000 ppm during the teaching day.