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Mill Building Office Radical Retrofit: Heating & Ventilation

Our office retrofit scheme is nearing completion and the MVHR team is eagerly awaiting being able to move into its new cosy, efficient and well-ventilated offices. In this blog, we outline the heating and ventilation strategies for the retrofit.

MVHR mechanical ventilation with heat recovery

Unsurprisingly, our MVHR team undertook the MVHR designs for the new offices! Two separate MVHR systems have been designed for the space:  one for the open-plan office space and one for the conference room, kitchen, bathroom and hallway areas.  Two units were needed because of the variable potential occupancy rates.

The MVHR team has worked on new configurations of where supply and extract air valves go, in response to the open-plan space and drawing on the latest understanding of how to minimise the risk of infection from airborne diseases, taking advantage of ‘displacement ventilation’.

Displacement ventilation

Displacement ventilation essentially means supplying air slowly at low level and extracting at high level. The supply air might be supplied at ground floor level and the stale air from the room might be extracted from ceiling height. It is a ventilation strategy that also benefits from the fact that ‘hot air rises’ through convection.

Displacement ventilation is useful for open plan rooms where supply and extract air valves are located in the same room. If both supply and extract valves are located in the ceiling area there would be a danger that fresh the air will just travel along the ceiling and be extracted without circulating through the room – effectively meaning that the ventilation to the room is being ‘short-circuited’ and is not going to where it is needed. Displacement ventilation has also been cited as having a useful role to play in reducing the risk of transmission of airborne illnesses, especially in office settings minimising the horizonal circulation of air by mainly moving air vertically from the ground to the ceiling. Our friend Paul Smith at Greengauge has been very active in researching the positive benefits of displacement ventilation for minimising airborne infection rates, especially in office situations.

This is the first time that our MVHR team has used ‘displacement ventilation’ techniques as it is not usually the way we model airflows for private domestic properties. However, it seems a very useful approach for office/ open plan spaces and we would definitely consider replicating this approach when working on similar projects.

Open-plan office space

A Zehnder ComfoAir Q600 MVHR unit will be used for the open-plan office space.

The supply air ventilation has been designed to be placed at a low ground floor level and the extract valves will be placed at ceiling level as part of the displacement ventilation design.

The air valves for the supply ventilation are a new design to us (Lindab CHA) and are located at floor level. The CHA is described as a ‘semi-circular perforated displacement diffuser for installation against a wall or column’. We need to be careful with the location of these diffusers as we don’t want to create a draught near people’s feet, even though the fresh supply air is at room temperature, having passed through the MVHR heat exchanger.

A note on air valves and commissioning

As the CHA air valve is new to us, we are having to work out a few things such as to how to moderate the airflow through it, which is especially important during the MVHR commissioning process. When commissioning an MVHR system we need to adjust all the air valves in the MVHR system to release differing amounts of air depending on where in the system they are. In general, the air valves early on in the ducting system will need to be most closed to restrict airflow through, while the air valves at the end of the system would tend to be the most open. This adjustment of the air valves ensures that air passes through the ducting sufficiently to reach all air valves and is carefully modelled in airflow calculations.

Conference room, kitchen, bathroom & hallway

A Zehnder ComfoAir Q600 MVHR unit will be used for the second zone of the office, with the unit being located in the Conference room.

We have used a mixture of displacement and our normal ventilation strategies. The conference room has some of the CHA supply air valves at floor level (but not everywhere as we can’t put one in that position because of a radiator). There are more separated rooms in this section of the office and so our more normal approach of locating supply air valves on the walls and extract valves in wet room ceilings (kitchen and bathroom), with air passing through door undercuts is also part of the strategy. Nevertheless, the conference room does have some displacement ventilation approaches, with floor level supply and ceiling level extraction.

CO2 sensors

As the building will have variable occupancy, with sometimes 1-3 staff and sometimes 20+, we are installing CO2 sensors to help ensure the MVHR system is delivering the correct volume of air changes. The MVHR CO2 sensors are connected to the MVHR  units and will set the fan speeds according to occupancy and CO2 levels in the building. The MVHR system will go into ‘Boost’ mode when the CO2 sensors pick up that occupancy is high.


As is our preference on systems which require such a high air flow rate, we have designed using the rigid spiral wound ducting system. As the ducting is fully exposed, this option provides better aesthetics than what a plastic radial system would offer.

Air source heat pump

A heating system for the office is being designed by our sister company Green Building Renewables and is a 8.5 kW Mitsubishi EcoDan air source heat pump (ASHP) which will supply heat to the building via wall mounted radiators.  The ASHP fan unit will be installed outside on a concrete base, with a temperature thermostat and control installed in a central location inside.

Heat pump efficiency

The heat pump’s Seasonal Co-efficient Of Performance (SCOP) is 4.79. This means that on average over a year for every 1 Kilowatt of electricity the system uses it will output 4 Kilowatts of heat. Ignoring primary energy for a second, this compares very favorably with gas boilers which have an efficiency of around 96%. Crucially the national electricity grid is now, on average, delivering less CO2 emissions per kWh delivered than gas, so it’s already more sustainable to install a heat pump. The electricity grid is getting greener every year as new offshore wind farms are brought online.

A solution for hot water

We won’t be needing a hot water cylinder because the office space has minimal hot water demand and the internal gains from the cylinder and pipework would contribute to overheating in summer. Instead, we are installing a 15 litre capacity under sink immersion heater. This will supply enough water to the kitchen and 3 wash hand basins in the WCs. We are installing a shower for colleagues who cycle to the office but this is an electric shower so provides hot water on demand for the occasional use that it will get.

Sizing the radiators

Green Building Renewables has calculated the anticipated heat outputs for each room which enabled us to size the radiators accordingly. We have gone with ‘triple’ radiators which are 50% wider than standard radiators. These allow us to fit the correctly-sized emitter without taking up too much wall space. Heat pumps in retrofit typically need larger radiators because heat pump system run at a lower circulation temperature of 35°C, whereas a gas boiler will run at 60°C.

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