NAT Air Handing Unit Controller Natural Ventilation Control
AHU / DIN / NAT / .../ NH / ...
Main Features Supports Natural Ventilation Applications Controls Supply & Extract Fans, Heating and Cooling Batteries Constant Supply, Constant Return or Variable Supply Air Temperature control Night Cooling Additional Submodule are Available to Add Extended Features for Natural Ventilation Schemes

Summary Features

General

The natural ventilation version of the AHU controller is generally similar to the standard (FRE, RET and SUP) versions of the AHU. However, the NAT version combines all three control options in the one product, the choice is set by configuration setting SPTY. Note however that in the NAT version the Supply setpoint calculation is changed from that of a standard AHU (more details below).

The natural ventilation version of the AHU is available as a non-housekeeping module only.

Most of the features within the natural ventilation version are the same as those in the standard AHU range, but there are also some additional features which relate to its functionality in natural ventilation applications. All standard AHU features are described in the documentation for the standard AHU versions. However any features relevant to natural ventilation applications and additional features unique to the NAT version, are as follows.

Also see the Application Example at the end of the feature details.

Supply Setpoint Calculation (NAT version only)

The supply setpoint calculation is changed from that of a standard AHU, zero demand produces a setpoint equal to the space setpoint. The setpoint is modified from this value based on demand and MINS (cooling) and MAXS (heating). If driver rescale is set on the AHU the supply setpoint will modulate between 0 and the minimum rescale value. For example if the cooling is set to be rescaled from 50% to 100%, the valve will only operate for the latter half of the cooling demand, then the supply setpoint will vary between space setpoint and MINS for cooling demands from 0 to 50%. This is done so that the Damper Controller can be used to attempt to match the supply temperature requirements before the mechanical plant is utilised.

The Supply setpoint and the supply temperature are passed to the Damper/Fan controller which attemps to control both air flow and air temperature as best it can.

Control Mode (NAT version only)

The AHU has a new configuration variable CMDE control mode, this is used to force the Supply Loops off (bypass), in this mode the Output equals demands from Space Loops which in turn in SPTY2 are the received demands from the Zone. CMDE can be set to 0 both loops Bypass; 1 heating controlled, cooling by passed; 2 Heating bypassed cooling controlled; 3 both loops in controlled mode.

This has been done to attempt to separate the room temperature control from the tempered air flow and temperature. The AHU acts essentially as an actuator to the Zone, with redirection to the heating driver in cooling. Note the valve will not open if the medium temperature is not suitable.

Night Purge (NAT version only)

The conditions required to run the night purge have been extended to include checking that the Outside temperature exceeds a configurable set value (POUT found at C185 default 18.0 degC) at a configurable time (PHRS found at C183 default 16.0 = 1600hrs real time). This requirement is defined in the Max Fordam specification for Granta Park. Setting PHRS to zero disables this feature.

Night purge normally runs only the AHU fans, in some applications cooling demands from the AHU controller are used to operate windows or similar, in this case a window actuator module might be used registered as a sub module of the AHU controller. To get this module to open the windows while the controller is in Night Purge mode set configuration variable C189 NPSD Night Purge Send Demand.

Heating Hold off (NAT version only)

If night purge has run then the heating can be Held Off until after a specified time (HHRS found at C184 default 10.0 = 1000hrs real time) the next day. Setting HHRS to zero disables this feature.

Two-Pipe Operation (NAT version only)

The controller can be set for Two Pipe operation (switch TWOP) where it receives the flow temperature of the supply medium from its Heat Source and determines whether heating or cooling is allowed dependant on whether this temperature is greater or less then the current space setpoint (setpoint supervised from the Zone).

Application Example - Granta Park

For a complete overview of this real application go to the "Case Example" option under Natural Ventilation in the product menu on the left of the screen.

AHU Control

The controller is set for Two Pipe operation (switch TWOP), it receives the medium temperature from its Heat Source and determines whether heating or cooling is allowed dependant on whether this temperature is greater or less then the current space setpoint (setpoint supervised from the Zone).

The heating outputs from the AHU control the underfloor valve, the cooling outputs are not used. The driver cooling signals are re-routed to the heating driver after the driver rescale function so the driver re-scaling can be used to adjust the control response of the valve.

The Cooling demand is sent to the AHU sub modules as normal and also to the Cool Source which controls the Atrium dampers with Wind sensitive Actuators.

The Supply sensor is positioned to measure the air temperature at floor supply grills. This sensor is used to provide a limit to the range of the air temperature which is allowed to be passed to the room. It is also used to shut down the AHU and its sub modules if the supply air temperature falls below 16 deg C (MINS), or 10 degC if night cooling is in operation.

The AHU is set to be SPTY 2, the demands from the zone controller set the required supply setpoint which the Damper Controller then tries to maintain, the control will not be very precise because the air flow rate will vary with wind speed and direction and the heat or cool pickup from the underside of the slab is ill defined.

The supply setpoint calculation is changed from that of a standard AHU, zero demand produces a setpoint equal to the space setpoint. The setpoint is modified from this value based on demand and MINS (cooling) and MAXS (heating). If driver rescale is set on the AHU the supply setpoint will modulate between 0 and the minimum rescale value. For example if the cooling is set to be rescaled from 50% to 100%, the valve will only operate for the latter half of the cooling demand, then the supply setpoint will vary between space setpoint and MINS for cooling demands from 0 to 50%.

The AHU has a new configuration variable CMDE control mode, this is used to force the Supply Loops off (bypass), in this mode the Output equals demands from Space Loops which in turn in SPTY2 are the received demands from the Zone. CMDE can be set to 0 both loops Bypass; 1 heating controlled, cooling by passed; 2 Heating bypassed cooling controlled; 3 both loops in controlled mode.

This has been done to attempt to separate the room temperature control from the tempered air flow and temperature. The AHU acts essentially as an actuator to the Zone, with redirection to the heating driver in cooling. Note the valve will not open if the medium temperature is not suitable.

The Supply setpoint and the supply temperature are passed to the Damper/Fan controller which attemps to control both air flow and air temperature as best it can.

If required a slab sensor is fitted to the spare sensor input channel, this is used for monitoring.

The night purge routine has been extended to include an extra test as defined by Max Fordam spec. Night purge will only happen if all the normal tests are passed and the Outside temperature is above 18degC (adjustable) at 16:00 (adjustable). Also if the night purge has run then the heating is disabled until 10:00 (adjustable) the next morning.

Options and Product Codes

Air Handling Unit Controller for Natural Ventilation Application

AHU / DIN / NAT / [battery types] / NH / [variant]

Battery types

Variant

SC Controls Ltd

PO Box313
Wadhurst
East Sussex
TN5 6JL

phone 08707 606040
fax 08707 606041
e-mail seachange@sccontrols.co.uk
http:// www.seachange.co.uk