Keeping Cool

Overheating Protection

In recent years we were approached more often to provide cooling solutions to homes. Modern homes often feature large glazing, and in conjunction with light-weight building structures – without thermal mass – these can pose a high risk of overheating. This can happen even in Scotland, as the overheating risk is less dependent on external temperatures than on the amount of solar gains.

An overheating risk analysis should be part of every design of dwellings right from the start. It is not good practice to ignore the issue and leave it up to the M&E team. Mechanical cooling is fairly power hungry and expensive to run and should be avoided if possible.

Following cascade of mitigating factors should be considered:

1) Structural shading: e.g. overhangs, brisolei, shutters

2) Other forms of shading: e.g. greenery, trees, blinds

3) Optimizing the g-value of glazing: This way solar gains through windows can be reduced.

4) MVHR system (well insulated and high performing): If solar gains are mitigated, the MVHR will help keeping the dwelling cooler by deflecting the heat of the incoming air to the outside and recovering cold to the inside.

5) Purge ventilation by cross ventilation: remember that with MVHR systems, windows can still be opened.

6) Mechanical purge ventilation: 4-10 air exchanges will help getting rid of solar gains.

7) Thermal mass: e.g. exposed stone or concrete. In warmer climates this will only work in conjunction with shading.

8) Thermal absorption: e.g. phase changing materials.

9) Mechanical ventilation with cooling: e.g. MVHR with mechanical cooling or evaporation/ adiabatic cooling. Please note that this will only provide sufficient cooling in conjunction with shading.

10) Air conditioning as last resort.

Overheating can also be caused or contributed by un-insulated heating or hot water pipework within the thermal envelope. Special attention should be paid to constant operating pipes, e.g. instant hot water loops. We had one case of hot water pipes from an AGA stove that emitted so much heat that the dwelling was overheating in winter.

Also heat radiation into service zones where MVHR ducting is installed can add to the overheating risk.

Summer Bypass Function of MVHR Systems

Most of our MVHR units have an in-built summer bypass function:

The bypass opens when the extract air temperature exceeds a certain limit.

The more the fan speed, the more effective is the bypass ventilation. However the cooling effect is limited, but also low-cost, as it is passive. In contrast air conditioning can be much more powerful, but also uses a lot of electricity (it needs about 3-4 times more energy to actively cool a space than to heat it).

Solar gains can be very powerful and can bring a few kW of heat into your building, especially if there is large panels of glazing. The bypass function is not designed to counteract all solar gains, but to counteract these, as much as possible, in a passive way.

Unlike air conditioning, dwellings with MVHR do not need to have their windows kept closed, in order for the system to work. So feel free to open windows and doors, if you like. However if you don’t want to open them, the MVHR system is designed to provide good air quality without opening of windows and infiltration vents.

The bypass opens and closes usually fully automated, according to the extract air temperature readings. Ideally it is a 100% bypass, without air leakage.

With the newer PAUL units, you can see if the bypass is open or closed in the information menu/ current device status.

You can change the bypass comfort temperature for the PAUL units. The temperature is measured in the MVHR unit (the combined extract air, which reflects the average temperature in the dwelling). However the threshold temperature cannot be adjusted to less than 20 degrees C, in order to prevent unintentional conflicts with the heating. The bypass does not open, if the outside air temperatures are below 13 degrees, in order to prevent condensation on the supply ducting within the house. Your technician can lower this temperature to 12 degrees.