Air Temperature Control on AirMaster Heat Recovery Units
Each AirMaster Demand Controlled Ventilation air handling unit with heat recovery has 2 dampers (main and bypass), both of which are placed on the air supply side leading to the contraflow heat exchanger.
The main damper opens fully as long as the AirMaster air handling unit is in operation, and is closed at all other times. By having the air inlet pathway sealed off during out of service periods, the risk of draughts arising from head-on wind is eliminated.
The bypass damper has a modulating actuator and is used for temperature control. It is actuated when inlet temperature rises above set point. The bypass air pathway has inherently lower resistance than the main path through the exchanger, which enables a proportion of air to make its way directly through to the room without gaining heat from the exchanger. Provided that external air temperature is low enough, this contributes to the cooling effect.
Bypass actuation is the first step taken by the AirMaster air handling unit control system to reduce inlet and room temperature. The process is completely automatic and gradual enough to be imperceptible to the room occupant. There is no manual setting required of the bypass damper.
Bypass operation of the heat recovery ventilation unit is sometimes called ‘free cooling’. AirMaster control systems bring on cooling functions in sequence, with priority given to the lowest running cost method. Thus, after bypass possibilities are maximised, supply air is then ramped up. Only after the bypass is fully open and supply air is at full throughput would the cooling module be brought into service (if there is one).
Temperature control (for cooling)
If inlet temperature rises above its desired setting, during day time operation, AirMaster heat recovery ventilation units act to regain set point in 3 stages:
(1) The bypass damper opens automatically. This diverts a portion of the incoming air around the counterflow heat exchanger, decreasing the temperature of the inlet supply air. No manual intervention or unit stoppage is required to adjust the bypass damper position.
(2) If the bypass damper reaches 100% open without regaining set point temperature (because of external temperature being high), fan speed is then increased to 100% of design throughput. This happens regardless of demand control requirements, which may call for less than 100% flow. The increased air flow enhances the convective cooling effect felt by occupants.
(3) The cooling module is activated (if there is one). If the automatic bypass function has not been sufficient to bring inlet temperature down, effective inlet temperature reduction can be achieved by a cooling module. Cooling modules work on similar principles to those used in a domestic fridge and are designed to deal with fresh air thermal load. They are designed to match the shape of the AirMaster heat recovery ventilation units that they are paired with. They fit between the unit back surface and mounting wall, with side panels which conceal the join to the heat recovery ventilation unit.
Sequential operation of bypass damper, then fan, then cooling module, ensures that the most energy efficiency is given priority. Put another way, the sequence is designed to hold off action by the cooling module as long as possible, as energy is required to power the compressor.
In addition, there is a further cooling mechanism termed night cooling.
(4) Night cooling. This takes advantage of the low temperatures available at night time during periods of summer heat waves. AirMaster ventilation units are programmed to start automatically, during the night following any day when external temperature has reached 26°C. Night air cools down the thermal mass of the room, so that air coming into contact with the same surfaces early the following day would be itself cooled. With night cooling, fan motors are run at the cheapest electricity tariff, helping to reduce the cost of day-time operation by cooling modules
Temperature control by air management provides sufficient protection against frost at direct ventilation unit installations in the UK. No specific provision (such as an electric coil upstream of the air inlet filter) needs to be made.
If inlet temperature should fall below set point, AirMaster heat recovery ventilation air handling units respond automatically by reducing inlet air flow, with the extract fan remaining at the same speed. As there is a reduced flow of incoming cold air now being heated by an unchanged flow of extract air, inlet temperature moves upwards until set point is restored.
Additional temperature control can be provided by comfort heating surfaces placed next to the air supply grille. ‘Comfort heating’ electric elements are designed for winter operation, to take any chill off incoming air. Element ratings start at 500 W (for the AM 150) and go up to 1,670 W (for the AMP 1200). A summary of ratings is given both in the AirMaster Technical Data and the SAV AirMaster Brochure.
Comfort heating can also be made available by finned water tube heating surface. Heating control is maintained by a motorised valve, programmed via the control panel. To protect against frost damage even when the water surface is not operational, a self-controlled valve admits sufficient flow to keep water temperature above freezing. Ratings for water tube surfaces are given in the same sources as referred to in the paragraph above.
Although AirMaster heat recovery ventilation units can raise air temperature in several ways, it is important to remember that they are not designed to be the sole heating system in any room. They work successfully in combination with traditional heating methods as well as HVAC systems.