During energy audits, our team often finds situations where walls or partitions have been moved or an extension has been added to a building and the electrical and mechanical services have not been considered. This leads to reduced occupancy comfort and energy wastage. The major energy saving opportunities lie in the duct design and the lighting layout. The following case study examines the opportunities brought about by re-examining duct design.

(part 1) HVAC opportunities

The following diagrams show a case study of an existing duct layout where an extension has been added on the west facing windows of the office. Measuring the flow rates of the packaged units servicing the area alerted us to the fact that air velocities were excessive and fresh air rates were greater than 10 litres per second per person.

Figure 1 – Mechanical service duct layout with measured diffuser air velocities

What is the affect of high air velocities and what energy saving options does this present?

High air velocities cause wind chill. Wind chill is a convection process which increases the transfer of heat from surfaces such as skin and clothes.

Figure 2 – Wind chill cartoon from
www.r-p-r.co.uk/wind_chill_table.htm

In figure 1, occupants were complaining of feeling cold even though the temperature in the area was measured at a comfortable 23 degrees Celsius.

Rule of thumb

At 25 degrees C, an air speed of 1m/sec will be felt by the body as 2 degrees cooler.

If air velocities are too high there may exist an opportunity to slow the air handling fan down. This can be done via the installation of a Variable speed drive (VSD) or in belt driven fans, by changing the pulley size. Both of these methods result in fan energy savings.

Note: care must be taken not to reduce the air speed excessively in refrigerant systems as this could lead to malfunction or excessive wear and tear on the unit.

It is also important to consider what happens to the fresh air volume when slowing down air handling systems. If the fresh air intake is set to 10% which provided the occupants with precisely 10 litres per second per occupant and then adjustments are made to reduce the air flow rate by 25%, this would result in new fresh air volumes of 7.5 litres which may be too low. In a fixed fresh air system this may mean opening the fresh air damper (if it is adjustable) and in a modulating system, this will require adjustments at the controller or in the Building Management System (BMS).

An alternative to increasing the fresh air rate may exist in installing CO2 monitoring. The opportunities of which will be the subject of a later blog.

In our case study we have identified that there is an abundance of air volume and higher than required fresh air volumes. The small west positioned packaged unit was installed initially to service a different heat load presented by the west facing glass. Since then the building has been extended and the glass is now internal. This has the effect of reducing the heat load on this part of the office.

By simply reviewing duct design, a complete packaged unit has been removed from service!

By measuring the air volumes at each diffuser we can determine the quantity of excess air and how we can balance the system to improve occupancy comfort. Auditing the diffusers also highlighted some areas that did not require air conditioning such a store room, and a copy room that has been retrofitted with its own split system and extraction system (see diagram above). The following diagram shows the new layout of the HVAC ducts. Note that AC unit 3, and 4, have been extended to allow the removal of AC unit 5. By simply reviewing duct design, a complete packaged unit has been removed from service.

Figure 3 – Mechanical services duct layout after changes

It should be noted that this analysis has been made significantly easier because of the access to up to date mechanical services drawings and accurate floor plans. As any alterations are made to buildings it is important to update the floor layouts, mechanical, and electrical services drawings. If your facility does not have up to date drawings, it may be worth while seeking the services of a drafting company to develop a Computer Aid Design (CAD) set. This will allow the facilities department to track any changes as they occur and allow more efficient analysis of problems for contractors which ultimately will result in faster and more comprehensive analysis of problems.

Changes to temperature settings are a little known but easy way of getting significant carbon savings in offices.

In office buildings the single largest energy user is the heating, cooling and ventilation system. This system will typically account for 40% to 60% of the buildings energy cost and greenhouse gas emissions. A major determinant of how much energy your system uses is the temperature at which it is set to operate. Depending on climate, most offices are set to maintain a year round temperature of either 22OC or 24OC.

Hobsons Bay City Council in Melbourne had temperature settings at the Hobsons Bay Civic Centre adjusted by CarbonetiX engineer Linton Hartfield to allow the temperature to vary between 20 and 25 degrees Celcius. Electricity metering of the air conditioning system showed a 25% energy saving on the packaged heat pump units supplying the building. The air conditioning temperature changes occurred a couple of months after a building extension – which had increased electricity consumption by 10%. According to Environment Officer Rowena Joske, “After the temperature adjustments the electricity bills dropped back to what they were before the extension.” In other words simply adjusting the temperature settings has cut carbon emission by 10%.

Temperature complaints are, however, a major bane to facility managers who can’t seem to keep everyone happy. Do adjustments to temperature settings increase the number of complaints? In the case above more complaints did occur about it getting above 25 degrees in some parts of the building. So the system has been adjusted back in those parts of the building to limit the maximum temperature to no more than 25 degrees.

Comcare, a federal government organisation for public sector employees, has produced guidelines as to what a comfortable office temperature is. According to Comcare the acceptable range of office temperatures is 20 to 26 degrees Celcius, with 20 to 24 degrees recommended in winter and 22 to 26 degrees recommended in summer.

In temperate and cold climates in Australia allowing the temperature in the office to float between 20 to 24 or 25 degrees will significantly cut energy use and carbon emissions as compared to setting the system to maintain exactly 22 degrees. In hotter climates allowing the temperature to go up to 26 degrees instead of making it stay at 24 degrees will similarly save energy and carbon emissions.

For offices in terms of carbon reduction per dollar spent it doesn’t get much better than changing building temperature settings.