IECC 403.5 - Economizers

[breakingthescience]

Hi all,

I wanted to get some other opinions on the interpretation of the energy code economizer requirements. I am in Climate Zone 5, Chicagoland, for what its worth. IECC 403.5 requires economizers on systems over 54,000 btu/h cooling capacity. The second part of that requirement says

"The total supply capacity of all fan cooling units not provided with economizers shall not exceed 20 percent of the total supply capacity of all fan cooling units in the building or 300,000 Btu/h (88 kW), whichever is greater."

This statement essentially avoids a design using a bunch of individual systems, at 4-tons or less, totaling over 25 tons of capacity. But what if I have a building with a total cooling load of only 10 tons? Can I use two 5-ton systems without economizers because I am under the 300,000 Btu/h limit? or do ALL systems over 54,000 Btu/h require economizers regardless?

If I really wanted to avoid economizers, I could use three 3-ton systems and still satisfy the code. So, can I use two 5-ton systems without economizers?

Thanks

 

[nuuvox000]

No, you can't use two 5-ton systems without economizers because they are over 54,000 BTU/h. This is a poorly written section of code imo. But I read it as follows:

The total supply capacity of all fan cooling units NOT provided with economizers shall not exceed 20 percent of:

1) The total supply capacity of all fan cooling units in the building
OR
2) 300,000 BTU/h

Whichever of the two options is greater.

So up to 300,000 BTU/h in the building you can only have up to 60,000 BTU/h of cooling without economizers (And no one system can be greater than 54,000 BTU/h in capacity). After that, it can only be 20 percent of the total.

 

[FL_Gingineer]

Why can't you just use the smaller 5 ton systems throughout the building with 2-position, or modulating, outside air dampers, along with a barometric relief (exhaust) on the return side of each system? Obviously the building layout would dictate if this is possible, but just a thought if you must stick with smaller systems.

 

[GT-EGR]

Agree with responses so far - both requirements apply. The code developer came to some tradeoff conclusion that once a unit gets that big it is worth the cost/complexity of including economizer. And that second requirement I imagine deters people from just putting in more units of individual smaller tonnage (although I’m pretty sure two units serving the same zone is considered one system and you have to add their capacities).

In case you aren’t aware - there are water-side economizer options, it doesn’t have to be a traditional outside air economizer - or you could also go with the premium efficiency route that they allow for instead.

 

[breakingthescience]

All good input. The big thing I was questioning is the exception: "The total supply capacity of all fan cooling units not provided with economizers shall not exceed 20 percent of the total supply capacity of all fan cooling units in the building or 300,000 Btu/h (88 kW), whichever is greater."

So, you're allowed to have up to 25 tons of cooling without economizers, or 20% of the total building load if that turns out to be more than 25 tons.

I guess that would mean individual systems would still need to be under 54,000 btu/h each. So, you could technically have six(6) 4-ton systems without economizers.

If I have a small building that only has a 20 ton cooling load, I could technically use five(5) 4-ton systems and not need an economizer. But I am not allowed to use four(4) 5-ton systems with economizers?

Let's ignore why I SHOULD use economizers, or how to do it, or other exceptions (This is in climate zone 5 and can't use efficiency exceptions)


Thanks

 

[GT-EGR]

Your cooling load is 20 tons - so assuming that is all done by individual fan systems - 20% of that is 4 tons, but 300,000 is greater than that.

So you can install up to 300,000 BTU/h of non-economized fan coil units as long as non of them are individually above 54,000 BTU/h.