Home Design and construction

Severe overheating in UK homes is normally due to the solar gain through glazing and the thermal gain/mass from the type and method of construction.

Prevention is better than a cure and the UK Building Regulations Part O sets out conditions where the home designer is now responsible for assessing the potential overheating risks and to design out overheating as much as possible. This is achieved with interventions that stop the property overheating in the first place by using products like heat reflecting underlay, solar control glazing, shading (including overhangs) and effective insulation methods etc.


A/C or MVHR cooler

If you have an overheating issue in normal weather conditions (outside a high temperature summer season) you will need to look at combating overheating via shading and glass treatments. If all else fails an A/C system may be required as a MVHR Comfort Cooler will not be effective on its own.

If you have an occasional overheating issue during warm sunny days a comfort cooler fitted onto your MVHR system may be suitable solution for your home.

As an A/C system will only provide targeted room cooling or heating, it doesn’t provide ventilation. If you want A/C you will also need a MVHR ventilation system. This is one reasons why MVHR with Comfort Cooling is an attractive option.

Please note, that if you are installing both systems simultaneously a MVHR unit suitable for A/C controlled rooms should be chosen.


Purge Ventilation

Purge ventilation in certain cases is required to comply with standards (Building Regulation Part O) and is the extraction of air at a high level of approx. 4 ACH (air changes per hour) to expel heat with the use of purge fans or mechanical heat recovery systems.

Purge ventilation is also required under Building Regulation Part F to rooms that do not have openable windows or windows that have insufficient openings.

Although it is possible to achieve purge ventilation for cooling carful design would be required as to limit air and noise disturbance


MVHR Summer By-Pass

Most MVHR (Mechanical Ventilation with Heat Recovery) units that BPC Ventilation supply have a Summer  By-Pass feature.

With the by-pass switched off (normal weather conditions) the MVHR provides fresh filtered air into a building as well as transferring heat between the outgoing and incoming air.

When the by-pass is switched on (normally automatically) during hot days, the air by-passes the heat exchanger to provide some minimal cooling similar to the external air temperature.

e.g. if the air is 24 inside and the air is 20 outside the unit will provide filtered air at approx. 20 degrees to the rooms.

Note: this is not Comfort Cooling or Direct Cooling and should not be relied on to provide property cooling as the system will not ‘cool’ lower than the outside temperature.


What is a MVHR Comfort Cooler?

A MVHR Comfort Cooler works by cooling the external incoming air being supplied to or by the MVHR unit to less than the external air temperature.

Comfort cooler design criteria.

In most cases, a MVHR with an additional cooling system can provide an effective and efficient comfort cooling solution but it is not as easy as just installing a Comfort Cooler Unit to your MVHR system, and I will explain why.

Air does not have a great heat/cooling carrying capacity, so if a space is to be cooled by an air system, 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. Typically, a MVHR system operates at air flow rates of approximately 0.5 ACH (air changes per hour) to provide fresh good air quality. To provide effective cooling to a space you will require a minimum of 1 ACH.

To use a Comfort Cooler on a MVHR system you will require a larger MVHR unit, larger ductwork and all the ductwork insulated so the ducted air doesn’t get warmed up by the time it reaches the rooms you’re trying to cool.

This is the reason why fitting a cooling coil (to lower the temperature of the air being supplied) to a MVHR system which is designed to operate at the standard 0.5ACH and with uninsulated ductwork, is not an effective a solution.

BPC Ventilation have inhouse technicians that specialise in MVHR comfort cooling design and simply send in a set of plans for a free proposal to design@bpcventilation.com


Types of MVHR cooling

Evaporative cooling coil

An evaporative cooling coil is an inline coil filled with tap water to cool the air passing through it, but performance is negligible in a MVHR system.

Water Cooling Coil

A condensing water-cooling coil is filled with chilled water (normally generated by an outdoor reversible heat pump) and installed in the ductwork after a MVHR unit or fitted to provide zoned areas like upstairs and downstairs rooms.

DX Coil

The other option is to install an A/C direct expansion coil (DX coil) to the ductwork but even though DX has a higher performance you will require an external unit and connecting pipework using liquid refrigerant gas. This option is normally only used on commercial systems.

MVHR Hybrid comfort cooler

A newer popular all in one solution is now available from some leading manufactures like Zehnder and Nuaire that BPC Ventilation supply use an encased heat pump with a DX coil that fits on top of the MVHR unit to provide comfort cooling. This provides a single unit ventilation system with comfort cooling which is easier to install and operate through one controller.