An integral part of any energy efficient building is mechanical ventilation system. It not only ensures the correct exchange of stale air in the building, but also provides heat recovery from the exhaust air back to the supply air. This reduces the operating costs of the building and improves thermal comfort in rooms. The advantages of heat recovery ventilation are often mentioned in relation to the winter period, but what happens in the summer season? Can mechanical ventilation also cool a house effectively? Let me answer these and other questions below.
The main function of mechanical ventilation is to provide fresh air into rooms and remove dirty exhaust air outside. Air exchange takes place in the heart of the heat recovery unit. It allows up to 90 percent of the heat from the exhausted air to be recovered. By reducing heat losses, the maintenance costs of the building are reduced in the total energy balance.
However, it is important to remember that there are four seasons and keeping warm is not always desirable. We encounter this situation during summer nights, when warmth is still present inside houses and the temperature outside begins to drop. At such a time, everyone in the house would certainly like to open a window and ventilate the room – but this type of action conflicts with the idea of the mechanical ventilation system, which should provide comfort without opening windows.
To avoid lack of ventilation in summer, heat recovery ventilators may use solutions that can prevent the heat exchanger from heating up the air in summer. There are also cooling devices dedicated to heat recovery units that can reduce the temperature of summer air by up to several degrees.
In order to disable heat recovery in summer and thus prevent unnecessary heating of the air in summer, ventilation bypasses are used. Bypass operation is based on the air flow through the heat recovery unit with the omission of the heat exchanger. It is used only in summer to cool a building – when the outside temperature is lower than the inside temperature.
In this situation, the bypass opens, causing the supply air to pass outside the heat exchanger, resulting in the supply of cooler air from the outside. In the winter period the bypass may protect the heat exchanger from freezing (frost). In heat recovery units, depending on the manufacturer, there are two types of bypasses:
• Automatic – opening or closing of the damper occurs without user intervention, however the user can set operating parameters of the device.
• Manual – adjustment is made by removing the heat recovery unit’s service damper and replacing the heat exchanger manually with the so-called summer cassette.
The greatest efficiency and convenience is provided by the use of an automatic bypass. In such a solution, the built-in controller of the air handling unit is responsible for engaging the bypass. It can be used to set the desired room temperature above which heat recovery will be switched off. The bypass is mainly used in cross-flow and counter-flow heat exchangers. Bypassing heat recovery in rotary heat exchangers is accomplished by stopping the rotor.
A bypass – a damper or another element in the heat recovery unit used to bypass the heat exchanger allowing cool air to be used in summer
The purpose of mechanical ventilation is to improve the living conditions in houses, it allows you to remove dirty and stale air outside and provide clean, filtered air inside. In summer, nothing prevents you from opening the windows in your home, in which case the airing will result in a natural exchange of air in the building. The question is, should we switch off the heat recovery unit when airing our house? In principle, there is such a solution via a window reed switch, which will switch off the heat recovery unit when a window with a reed switch is opened.
We should remember that ventilation is also intended to remove stale air, but it turns out frequently that there are no windows in the rooms from which we want to remove the stale air. These are usually bathrooms, dressing rooms, pantries or technical rooms. Therefore, in some heat recovery init we can find the “open windows” operation mode. This mode switches off the supply fan in the air handling unit, leaving the exhaust fan running. In such a ventilated house, the air handling unit consumes less energy, negative pressure is created in the house and open windows allow the inflow of fresh air from the outside.
A bypass built in the heat recovery unit may reduce the intensity of the building heating in summer, but it will not reduce the temperature of rooms during hot days. Therefore, in addition to a bypass, mechanical ventilation can be equipped with appropriate devices for cooling rooms in summer. Depending on the variant applied, we can use a ground heat exchanger, a duct mounted cold water cooler or a freon cooler for cooling the rooms.
A ground heat exchanger is a very good complement to the mechanical ventilation system. In its simplest form, it is a pipeline buried in the ground – through which ventilation air enters. The mechanism of operation of a ground heat exchanger is based on the ability of the ground to stabilize the temperature at a certain depth throughout the year; the average temperature of the ground at a depth of about 1.5 meters is approximately 4°C.
The outside air introduced to the exchanger (e.g. tube heat exchanger) is preheated in winter before it enters the air handling unit. In summer a ground heat exchanger is the cheapest air conditioner – it lowers the temperature of air entering the building by several degrees. At present there are tubular, plate and glycol ground exchangers available on the market which differ in the design of individual components. Ground exchangers require a lot of ground space for implementation, if this is not possible we can opt for a duct mounted or freon cooler.
A common solution for cooling ventilation air is a cooler mounted downstream the air handling unit in the air intake duct. Such a cooler is usually made of copper tubes and aluminum fins, while the casing is manufactured from galvanized steel. These devices are factory-fitted with condensate drainage. In summer, the cooler lowers the supply air temperature. The air is then introduced into the rooms through a network of supply ducts, thus lowering the temperature inside the building.
The cooling source for the water cooler is a ground source heat pump with passive cooling capability or an air source heat pump with reversible operation. Then, in summer, the heat pump delivers a refrigerant to the cooler, at a temperature of e.g. 12°C, cooling the supply air down by a few degrees. It is worth remembering that in such a solution a heat pump can heat domestic hot water or cool the building.
An alternative to the duct cooler are glycol coolers, fed directly from the ground source heat pump. In summer, the lower source, i.e. the ground is not fully used – then we can use the heat pump and add a separate glycol circuit to supply the glycol cooler. The control of the pump activation can be carried out mostly by means of a weather sensor or a duct sensor.
A less commonly used solution are duct mounted freon coolers with the design very similar to conventional water coolers. The difference in their method of operation is that there is a freon exchanger in the cooler, which cools the flowing air. The source of cooling for freon coolers are air heat pumps – there is a system with a refrigerant, usually R410A. The cooler is connected to the external unit from the air conditioner, for example, on the wall of the building. However, such a solution is rarely used in residential buildings due to the freon cooler which is very expensive. This type of equipment is commonly used in service and public buildings.
It is worth mentioning that mechanical ventilation is designed to ensure efficient air exchange. When designing mechanical ventilation we stick to the assumptions that the exchange ratio in the building is maintained in the range of 0.6-1.0 air changes/hour. If we want to cool the building, we must not only lower the temperature of the ventilation air, but also increase the exchange ratio in the building, for example, to 2-4 air changes/hour. Unfortunately, each heat recovery unit has its own performance limitations, so usually the HRV operating with the highest speed reaches a maximum of only 150% of the nominal output. Higher speed means higher level of heat recovery unit noise and higher energy consumption costs.
On the other hand, we cannot cool a building with air at, say, 18°C – for the sake of our health, we should keep the difference between the indoor and outdoor temperatures to a maximum of 7°C. So if it’s 30°C outside, the comfortable supply air temperature is 23°C. With these assumptions, it is reasonable to conclude that cooling with ventilation is a good solution in buildings that are not exposed to a great expansion of solar radiation.
For example, these are single-story buildings without roof windows, with canopies and external blinds. If this is not the case, e.g. in a building with a maximally glazed south elevation – an additional air conditioning system is required to achieve temperature comfort in summer.
To ensure proper operation of the ventilation system and proper cooling of the building in summer, for buildings with a large amount of glazing, it is worth considering designing a separate ventilation and air conditioning of the building. The air conditioning system can be designed as a split or multi split solution. That is, with the use of one main air conditioner (split) e.g. in a representative room or with the use of several smaller units in rooms, e.g. rooms in the attic (multi split). Wall-mounted air conditioners are common, but duct mounted air conditioners, which can deliver cooled air to rooms using a manifold and insulated ducts, are even more popular. In addition, in winter, devices with sufficient power can serve as the main source of heat.
It is a good idea to entrust the dilemmas relating to the correct design of the ventilation system and building cooling methods to a sanitary designer. A sanitary designer will not only prepare a correct balance of ventilation air, place supply and exhaust air diffusers and choose a heat recovery unit – a sanitary designer will also suggest a suitable system for cooling rooms. Cooling by means of a separate air conditioning system, e.g. multi-split, will certainly provide adequate thermal comfort in summer, but it is associated with the highest costs. In this variant, there are also costs associated with the implementation of cooling connections and condensate drainage between units. If you only need to pre-cool the air, it is worth using a duct mounted freon cooler in combination with a reversible heat pump – alternatively a ground heat exchanger if the source of cooling is not a heat pump.
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