Can an MVHR system be used for cooling?

This summer we have been inundated with enquiries about whether MVHR systems can be adapted to add additional cooling capacity. We give our definitive answer below.

Designing out overheating in the first place

blankThe 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 against this. The Passivhaus Trust and Good Homes Alliance are developing tools to help prevent overheating in newbuild and retrofit projects.

Reducing overheating

blankIf you are beyond the design stage and are experiencing overheating in a building there is still a lot that can be done to improve the shading strategy without the need for an active cooling system. Read Enhabit’s blog outlining the various shading strategies open to you.

MVHR as cooling

 

Zehnder ComfoAir Q MVHR unit in situ

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 cool 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. A 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 which is almost four times the standard MVHR air flow rates.

To use MVHR for extensive cooling would therefore require the supply of  much, much more air requiring industrial sized MVHR unit, 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 and those with north facing less.

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.

Active cooling systems

If you have a building where overheating is a problem, and it cannot be designed out with a fabric first approach, such as looking at building orientation, window sizes and orientation, external solar shading, then 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 all vary significantly as well in each room so recirculating fan coils cooling single rooms are often the most effective use of active cooling.

 

Tom Heywood, MVHR Design Manager, Green Building Store & Kaspar Bradshaw, Project Engineer, Enhabit

18th August 2021

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