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 room 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. This contributes to the cooling effect.
Bypass actuation is the first step taken by the AirMaster air handling unit control system to reduce room temperature. It 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 (AirMaster’s cooling function)
If room temperature rises above desired setting, during day time operation AirMaster heat recovery ventilation units will act to reduce this temperature 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) Supply air flow is increased to 100% of design throughput, regardless of extract air flow remaining at less than this.
This also reduces supply air temperature.
(3) The cooling module is activated (if there is one). The automatic bypass function and night time cooling ensure that the inlet and room temperature are kept down. If this is insufficient, effective temperature reduction can be achieved using a cooling module. Cooling modules 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 the mounting wall, and have side panels which conceal the join to the heat recovery ventilation unit. Cooling modules have a compressor and work on the refrigeration principle.
Sequential operation of (1) bypass damper, (2) fan, (3) cooling module, ensures that the most energy efficient methods are 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. If room temperature during summer day time hours exceeds the maximum set level, AirMaster ventilation units can automatically cool down the room by night time operation. Night air passing over internal room surfaces has a cooling effect, 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 a shortfall in room temperature is detected, AirMaster heat recovery ventilation air handling units respond automatically by both (a) reducing inlet air flow and (b) leaving the extract air flow unchanged. This produces a rise in temperature of the inlet air.
A side-effect to this is a slight air imbalance with under-pressurisation of the room relative to the outside. Whilst operating in this mode, air ingress can be expected through the building structure. If this is considered undesirable, 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 100) and go up to 1,670 W (for the AM 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 a 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 in the paragraph above.
Although AirMaster AHUs with 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.
To find out more about SAV’s heat recovery ventilation systems, please contact us:
by phone: 01483 771910
by e-mail: [email protected]
or complete our Contact Form.
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