How does an MVHR System help stop summer overheating?

Can an MVHR system help to cool your home in summer?

Yes, a Mechanical Ventilation with Heat Recovery system (MVHR) can help to cool your home in summer, and it does it in a way that is quiet and automatic, using only the energy equivalent of two LED lightbulbs.

Compared to an Air-Conditioning (AC) system, an MVHR is nowhere near as powerful at cooling, but an AC system also some has significant issues attached to it (mainly high energy use, cost, noise and health).

This article will provide some key points to decide if you should choose an MVHR or an AC system (or both) for your home to help with cooling in summer.

How can an MVHR be reversed to cool?

In order to explain how an MVHR system helps to cool a home in summer, we have to look at how it heats a home in winter.

All year round we have a need for ventilation in the home – even in summer, when we may still need to keep the windows and doors closed for security, noise and pollen/hayfever issues. The MVHR fans are therefore always running at low level and for the equivalent of a few pence per day.

In winter, the MVHR is extracting warm, humid air from bathrooms, kitchens and utility rooms and passing the heat to the cold incoming air from outside. This airflow exchange is done at a rate that removes CO2, VOCs and excessive humidity and means the incoming air is pre-heated.

In summer however, if it’s warm inside and outside we don’t want to recover the extract heat to push it back into the home. The “smarter” MVHR systems will detect this and will automatically shift to “Summer Bypass Mode“. The warm, humid air from bathrooms, kitchens and utility rooms is exhausted straight outside, bypassing the exchanger, and the incoming air is filtered straight inside.

Diagram to show how summer bypass works in an MVHR by Zehnder

Part of the indoor heat removed is from AV/computer/lighting systems and hot water tanks that would otherwise be contributing to excess heat in the home all through summer.

Never buy an MVHR that does not include proper automated Summer Bypass Mode.

Getting rid of humid indoor air makes us feel cooler

It’s important for indoor cooling that warm, humid air is removed from the home, because excessively high humidity is a factor in what makes us feel hotter. By reducing the Relative Humidity (RH) of the indoor air, we can tolerate slightly higher temperatures, so an MVHR removing humid air will drive down the RH and create a more comfortable indoor environment.

On cool summer nights without wind the MVHR is still ventilating

During the day the incoming air from the MVHR won’t make any difference to the indoor air temperature, but as temperatures cool in the evenings (as they do in the UK) the MVHR will still be bringing in this cooler night-time air, and this helps cool the home overnight – especially when sleeping.

This is a bonus compared to just leaving the windows open, as there’s no guarantee there will be any wind to carry the cooler air into the home anyway (and often summer nights are still and devoid of wind).

How does an MVHR compare to Air-Conditioning?

An MVHR system can’t recreate the power and intensity of the cooling from an AC system. Whereas an AC system can blast in cold air into a room and lower it to 16 DegC within a few minutes, an MVHR moves the air much more slowly. This is deliberate, as an MVHR is designed to ventilate near-silently and over 24 hours. An AC is designed to react instantly, and for shorter periods of time.

If you’ve every slept in a hotel room with an AC system that clunks and drips and hums all night, the difference in acoustics between AC and an MVHR is clear.

Should I get an MVHR or an AC system to deal with cooling?

All modern homes should be considering an MVHR system for their ventilation, as it is the cornerstone to an energy efficient, comfortable and healthy home in the 21st Century.

But the question is at what point a home needs active cooling such as AC, rather than just relying on opening windows, running the MVHR or desk fans? AC is expensive and noisy and space has to be found indoors and outdoors for it, so it has to be considered very carefully before installed.

Air Conditioning indoor unit

In the UK we haven’t traditionally tended to use air-conditioning systems in our homes, but our company is now more often seeing AC installed strategically in our MVHR projects in places where the risk of overheating is highest – typically lofts and ground floor rooms with lots of south-facing glazing.

The decision to install active cooling (eg, AC) will depend on the risk of overheating to the home. Although every home is different, there are certain aspects of a building that will increase the risk of overheating. I’ve explained why homes overheat in more detail in this article.

CIBSE has also produced its own guidance on thermal comfort and how to build for it, but it’s behind a paywall to non-members sadly. It says everything I mention in this blog.

What aspects of a building increase overheating risk

Rooms with lots of unshaded south-facing glazing, and rooms at the top of homes where heat rises (usually lofts) are at high risk of overheating in summer, as are rooms with lots of structural steel frame. Internally, hot water pipes (secondary circulation) and Audio-Visual rooms can also add to overheating, as they generate heat all year round.

An energy model of the property can be created to calculate the exact risk of overheating (given as a percentage of how many days the home will reach 26 DegC + indoors), but if there is no model the above aspects should be considered and mitigated as much as possible. Trees, canopies and external blinds can be useful by preventing solar gain, pipes can be lagged and AV rooms can be placed in garages outside of the thermal envelope.

For some new builds we sometimes suggest living a few summers in the home before deciding on AC, as the overheating period may not turn out to be an issue, or may only last a few days (in the UK) and generally AC can be retrofitted fairly easily in the future with the minimum of disruption.

Is MVHR compatible with AC?

Yes, but only the smarter MVHR units have automatic season detection based on the outdoor temperature. The smarter MVHR will never kick into heat recovery mode in an effort to battle with the AC cooling – the MVHR will always use the outdoor temperature to assess what season the home is in.

Are there any MVHR systems with integrated cooling?

Yes, a few European MVHR units include integrated heat pumps that will reverse in summer to cool the incoming air. This can be a good “middle option” between MVHR only and MVHR + AC, as they are actively cooling the property, but cooling it in the MVHR’s very quiet and efficient way.

There are also in-room portable AC units which seem to do a fairly decent job of cooling down a room, but they will have noise and energy usage issues.

If you’d like to discuss any aspect of your home’s ventilation or cooling strategy, please contact me at Patrick[@] heatspaceandlight.com or leave a comment, and I’d be happy to help. The link to the portable AC above is an affiliate link.

 

 

Patrick

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