As we are outside of the USA, ASHRAE 90.1:2010 is a new requirement that came with v4 and our designers have no experience in interpreting and applying the mandatory requirements.

Can you please give your opinion on the following?

1. Lighting Control - 9.4.1 – Manual or Partial Auto to 50%

The requirement states that “Any automatic control device required … shall either be manual on or shall be controlled to automatically turn the lighting on to not more than 50% power, except in the following spaces….”

Assuming that we have an open plan space that is larger than 10 000ft2, we require one manual or automatic control device per 10 000ft2 (

a) The proposal is the split the 10 000ft2 space into small 100ft2 zones (about 4 – 6 desks) and provide a motion sensor in each zone. This way the multiple automatic control devices only switch on 1% of the power of the space each, meeting the “<50% power for automatic on” requirement. Would this meet the intent?

b) The alternative would be to provide each 10 000ft2 area with a manual on switch and have a time scheduled night-time shut off for up to 25 000ft2 zones. This option seems to meet the “letter the law” better, but seem to miss the intent of providing saving compared to 90.1:2007.

2. Space Control - (a) – Bi-level Control / Dimming

"The controlled lighting shall have at least one control step between 30% and 70% (inclusive) of full lighting power in addition to all off."

An exemption states that bi-level control is not required in “ c. Spaces types with lighting power allowance of less than 6 W/m2 (see Table 9.6.1).”

If a space such as an open plan office has an installed lighting power density of less than 6 W/m2 would it not require bi-level/dimming control? Daylit zones would still have dimmers controlled by photo-sensors

3. How are these requirements typically implemented in large open plan offices, bearing cost-effectiveness in mind?