Comfortable indoor ventilation
The summer bypass function for MVHR units is a crucial factor in reducing discomfort in the summer months. Due to evolving technology, summer bypass means different things on different MVHR units, so you shouldn’t just accept that a product you are specifying has a bypass function and tick that box.
Here is a look at the evolution of summer bypass technology, what currently exists and what to consider during specification:
1.Humble beginnings: If it’s HOT, turn it off.
The first solution used to combat excess heat recovery in the summer months involved simple manual controls which switched off the unit’s supply motor. A basic and simple method, this type of thermal transfer is now banned unless it’s used alongside trickle vents, thanks to the ErP Ecodesign directive of 2016. Switching a fan off during the summer can generally not be considered as a thermal bypass facility, in part due to the impacts of negative pressurisation, especially when open flue combustion appliances are used.
2.Moving on: Slow it down.
The next step in the summer bypass evolution involved a more active solution, but one that risked compromising Indoor Air Quality (IAQ). Some units facilitated thermal transfer or heat recovery by simply slowing down the supply air fan speed. This method causes negative pressure, leading to uncontrolled replacement air which doesn’t necessarily target key areas such as bedrooms. Negative pressurisation also means the air coming into a home isn’t filtered, potentially bringing in harmful pollutants such as pollen and NOx. With air quality fast moving up the agenda in terms of building design and regulations, any solutions that might compromise IAQ will have a direct impact on health.
3.True Summer Bypass and State of the art solutions: Everything in modulation
Since then, true summer bypass functions have been designed into units, whereby a physical bypass bypasses at least 90% of air volume flow. However, many of the options on the market which do deliver true bypass still lack flow control or constant volume motors. Without flow control, the bypass channel is nearly always smaller, leading to a higher pressure drop within the system, potentially reducing the volume of air by over 50% and impacting on running noise levels. Most also just rely on external temperature as an activation or deactivation point, not considering other factors including the internal temperature, especially in the later evening when it cools down outside (heat recovery comes back on) but it’s still roasting inside!
To be most effective a bypass needs to react to its installed environment, considering all aspects that contribute to the discomfort felt – a modulating bypass that can be open, closed, or somewhere in between, with just enough heat recovery occurring to achieve the right level of comfort. This ensures that during a cooler summer night the modulating bypass can remain active, ensuring the supply temperature remains above the dew point and keeping internal temperatures bearable.
The way a summer bypass works is much more than stopping or starting the heat recovery process, it's connected to comfort, air quality and noise levels… all of great importance in the home.
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