BRACKNELL, UK, 12 JANUARY 2001 - The effectiveness of a building control system can be marred or enhanced by the standard of maintenance applied. DAS Legal Expenses Insurance Company Limited considered the fully comprehensive maintenance programme provided by Honeywell Control Systems to be of such a high standard that it contracted the company to upgrade and replace the existing building controls system at its Bristol headquarters.

The result has been greater comfort control for occupants of the 4,000m2 (43,500 sq. ft) building, as well as operational cost savings by improving the control between main plant and terminal units, together with continued maintenance support for all HVAC plant and equipment.

The air-conditioned building employs fan coil units (FCU) to provide heating and cooling to all floors. Variable volume heating pumps supply low temperature hot water (LTHW) from two De-Dietrich gas-fired boilers, variable volume cooling pumps provide chilled water from two Trane air-cooled chillers, with tempered fresh air make-up via a packaged air handling unit.

The complete system is monitored and controlled by a Honeywell Excel 5000 building management system comprising Excel 10 FCU zone controllers to the terminal units, Excel 500 direct digital controllers govern the main plant, linked to an Excel Building Supervisor central computer system.

The Excel 10 uses distributed intelligence to provide local temperature and fan control of the FCUs. Where appropriate the fan coils have been grouped to provide consistent temperature control within the open plan areas. The controller comprises a temperature sensor input and heating and cooling actuator outputs. Where multiple fan coils are grouped together, the Honeywell linear actuators have been connected in parallel using multicore cables. An Echelon LonWorks bus enables data communications between the controllers and the Excel Zone Manager units, which acts as the interface to the Excel 500 systems.

Maximum energy savings are achieved by the multi-layered intelligence of the Honeywell system. The Excel 10 terminal unit controllers are capable of timed control, load optimisation, PIR interface, and demand limit control, supported by features such as night purge, pre-heat, regulated air volume, and system optimisation. The Zone Manager supplies valuable data relating to fan coil valve status to the Excel Building Supervisor, thereby enabling the main plant to precisely control the isolation valves and dampers to all floors. It also receives fire alarm status from the main plant so that in the event of a fire the FCUs can be disabled.

The building's main plant controls have been upgraded to the Honeywell Excel 500 system, which has been interfaced to the existing control panel and switchgear. Both the boilers and the chillers operate a lead/lag regime with the lead boiler setpoint at 80oC and the lag boiler at 75oC and the lead chiller setpoint at 8oC and lag chiller at 6oC. In both cases the lead/lag plant are rotated on a weekly basis by the Excel system.

Since the Excel 5000 came on-line, Karen Dursley, Property Services Manager for DAS, reports that control of the space temperature is now being effectively dictated by the fan coil units. "Previously it was not uncommon for the boilers and chillers to be running simultaneously in order to maintain operational readiness should the FCUs call for heating or cooling. In addition, we are now able to harness any available free cooling or heating from the outside air temperature via the air handling unit," she explained.

Another refinement of the system was to include the Excel Building Supervisor central computer, expanding the previous standalone building controls system into a powerful building services management system. "Being able to easily access the system to view plant status and acknowledge alarms as they happen has greatly improved operational standards throughout the whole building and provides us with a management tool that encourages optimum standards of comfort control and energy management," added Karen Dursley.

Since the Excel 5000 system has been installed, DAS has contracted Honeywell to provide maintenance provision to another of its offices in Bedwas, Wales.

Readers can request further details of Honeywell Building Solutions and Services from: Honeywell Control Systems Ltd., Honeywell House, Arlington Business Park, Bracknell, Berkshire RG12 1EB. Tel: 01344 656000. Fax: 01344 656240. E-mail: uk.infocentre@honeywell.com Web: www.honeywell.com/uk

Editorial contact: Royer Slater
Honeywell Control Systems Ltd
Direct to desk: 01344 656379
royer.slater@honeywell.com