Maximizing comfort and efficiency: Our guide to MVHR systems and cooling

June 2023 was the hottest on record, and July has been a muggy month of storms so far. With several Summer weeks still to go and the schools about to break up for the holidays, it’s safe to say we will probably experience and feel some more uncomfortable weather in the next few months. Each Summer, we are inundated with enquiries about whether MVHR systems can be adapted to add additional cooling capacity and whether MVHR systems provide an effective method for cooling options in new build housing. This has become even more relevant with the recent publication of Building Regulations Part O: Overheating. Here’s our definitive answer to overheating and the impacts of the new regs.

Building Regulation Part O, what is it? And what does it mean for new builds? 

Part O of the Building Regulations has been introduced to limit excess solar gain in new build homes and provide ways in which preventative measures can be implemented to quickly remove this excess heat if your home is at risk of overheating.

In the UK, we have a love of glass, and little thought has traditionally been put into overheating, with our focus being on adding heat, instead of removing it. Climate models currently predict that the global average temperature will rise by 4 degrees Celsius, which could increase the number of extremely hot days from 10% to 37%.

Heat greatly impacts the occupants’ health, behaviour and productivity. It is, therefore, important when designing your new build home that overheating is considered at a very early stage as it may impact the orientation of the building, size of glazing, size of roof overhangs and specification of any other shading structure such as Brise soleil, roof overhang or a veranda.

“Part O is very valuable. It means that we can make more informed decisions about how we design homes and buildings and address design issues that can affect overheating risks right now, which will become more pronounced with climate change,” says Mark Siddall, principal architect and director of research at the Lovely Engineered Architectural Practice (LEAP).

A huge benefit of Part O, which previously has not been addressed by the Building Regulations, is that each room must be addressed individually instead of treating the building as one. This means that previously if one room risked overheating, but no others did, this would be acceptable as, on average, overheating in the home is not a risk. The downside of this is that one room would always be an overheated and uncomfortable space.

Designing out overheating in the first place

The majority of overheating risk in UK properties is due to solar gain through large south facing glazing. The very best approach is to design out overheating in the first place. If your project is designed using PHPP (Passivhaus Planning Package), then your PHPP designer can advise you on the building’s overheating risk and undertake various shading strategies to mitigate this. The Passivhaus Trust and Good Homes Alliance are developing tools to help prevent overheating in new build and retrofit projects.

Reducing overheating

If you are beyond the design stage and are experiencing overheating in a building, a lot can still be done to improve the shading strategy without the need for an active cooling system. Read our blog outlining the various shading strategies open to you.

MVHR as cooling

MVHR (Mechanical Ventilation with Heat Recovery) provides fresh filtered air into a building as well as transferring heat or coolth between incoming and outgoing air. On colder days, the heat exchanger within the MVHR unit transfers heat from the air leaving the building into the incoming air. On hot days the heat exchanger works in reverse with coolth transferred from cooler inside air to help cool the incoming outside air. So MVHR can provide some minimal cooling capacity.

The technology exists to introduce additional cooling to MVHR systems. However, it is not always that effective a solution.

Low air volumes with MVHR

The reason cooling isn’t very effective through the MVHR system is not down to the technology; it is down to the nature of air and heat/coolth. Air does not have a great heat/coolth carrying capacity, so if a space is to be cooled by air, an awful lot of air needs to be supplied to that space. However, MVHR systems are designed to work using a low volume of air. The volume of air that an MVHR system supplies may be much lower than the volume of air potentially overheating in that room. An MVHR system typically provides boost air flow rates of 0.5 air changes per hour to provide fresh good air quality. To provide effective cooling to a space, you typically need around 2 air changes per hour, almost four times the standard MVHR air flow rates.

To use MVHR for extensive cooling would therefore require the supply of much more air requiring industrial-sized MVHR units, larger ducts, larger silencers, insulated ducting etc, something not that feasible in domestic situations and certainly not cost-effective!  One of the purposes of an MVHR system is to reduce energy consumption, so turning it into an inefficient cooling system is counterintuitive.

No zoning possible with MVHR

It is also worth noting that MVHR ventilation with integrated cooling systems cannot be zoned as to which areas will be cooled. It is effectively a large centralised single ventilation zone. This very rarely matches the cooling requirements of the building, as rooms with south facing glazing will need significantly more cooling than those which are north facing.

Where MVHR is suitable for cooling

However, in some cases, MVHR with additional cooling can be a very effective and efficient cooling solution:

  • Situations where there is a very low cooling load to match low air flow rates and a very even cooling load between rooms. If you have a well-designed building, such as a Passivhaus, with very low risk of overheating but you want to pre-cool the air during occasional extreme temperatures, then cooling through the MVHR system might be an option.
  • Small properties with only a few rooms where single-zone cooling can meet the varied cooling demands.

What cooling options are available for MVHR

Where cooling through the MVHR system is required and efficient, there are two main options: water-fed cooling and electrical cooling.

  • Water – similar to the way a radiator works, a cooling coil on the MVHR ducting can receive a cold water supply from for example, a reversible air source heat pump. The supply air on the MVHR system will run over this coil and cool the air. A good example of this is the Zehnder ComfoPost Range
  • Electrical – similar to the way in which a refrigerator, an evaporator will cool down the supply air and warm up the exhaust air. A good example of this is the Zehnder ComfoClime to accompany the Zehnder ComfoAir Q range of MVHR units.

Active cooling systems

If you have a building where overheating is a problem and cannot be designed with a fabric first approach, such as looking at building orientation, window sizes and orientation, and external solar shading, an active cooling system is likely to be required.

This could take the form of a traditional air conditioning system or the use of fan coils in the ceiling of rooms where there is overheating. It is possible to use such systems alongside MVHR systems without affecting the performance of the MVHR system. Internal heat gains from people, lighting, appliances and cooking vary significantly in each room, so recirculating fan coils in single rooms are often the most effective use of active cooling.

 

19th July 2023

One response to “Maximizing comfort and efficiency: Our guide to MVHR systems and cooling”

  1. blank Tim Shepherd says:

    I live in a passivhaus and it took me two years to realise it was easy to overcome the overheating we were experiencing in summer. Simply open the windows, particularly any skylights and as long as a lower floor window is open the wind tower effect will remove the warmer air. This only works if the outside air temperature is cooler than inside so it’s something to do in the evening for a few hours, not during the day. It’s not that often the outer air temperature in the evening stays higher when this strategy does not work.

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