Last week, we received two incredibly similar inquiries related to the ruling in ASHRAE 90.1 from table G3.1.1a that “All conditioned spaces in the proposed design shall be simulated as being both heated and cooled even if no heating or cooling system is to be installed.”

Here is a sample of the question we received:

I have a space in the proposed that is heated only. It has a peak heating load of 12 Btuh/SF, but the baseline peak is 20 Btuh/SF. In this climate zone, heating is defined as conditioned when the peak load is greater than 15 Btuh/sf. Since the proposed is not conditioned and the baseline is conditioned, do I treat the space as unconditioned, semi-heated, or conditioned? If it is conditioned, how do I proceed?

Response:

That's a funny issue. To be clear, I'm assuming you are using 90.1-2007. In terms of heating for your climate zone, conditioned is defined as >15 Btu/h·ft2. Semi-heated is always defined as >3.4 Btu/h·ft2. By the way, semi-heated = a minuscule amount of heating. I have scarcely seen or heard of semi-heated outside climate zones 1, 2, or 3. Of course, the setpoint could largely determine that.

Since you have mentioned 15 Btuh/sf, that means your model is in climate zone 4 or 5.

In any case, the conditioned vs unconditioned should be based on the proposed value, which in your case, is not conditioned nor is it unconditioned. Therefore it is semi-heated. However, because of the climate zone, it is very possible that any small changes will move you into the “conditioned” definition.

You have two choices:

1.   Model it as semi-heated, noting that any minuscule changes might make the space become conditioned.

2.   Model it as conditioned, which means adding cooling to the proposed. Option 1 might be the technically correct option, but you could stick to Option 2 to play it safe and LEED review should have no problem with this, unless for some strange reason it yields significant savings. There's other annoyances that arise from Option 1 because the baseline envelope actually changes to the semi-heated requirements from tables 5.5-1. This could work in your favor since those envelope conditions are “worse”, but it’s likely that this is a case of splitting hairs.

What to do:

Option 1)
If the space is unlikely to become conditioned, you could stick with Option 1 and model it as semi-heated in the baseline. In general, most changes requested by the LEED reviewer will RAISE the heating load.

Option 2)
You could be conservative and assume it is conditioned. Usually, all you need to do is keep things the way they are and add a single zone cooling unit to the baseline. Then you add the exact same cooling unit to the proposed, using the same efficiency and capacity.

To get really picky, you can also change the cooling setpoints on these fictitiously cooled spaces, provided the setpoints are the same in the proposed and baseline. I've heard that many people have adjusted these until it was 5 Btu/h·ft2 in cooling (making it conditioned, but just barely!). This makes the impact minor.

How would you handle this issue? Do you have a method of your own? Let me know what you think in the comments below.