Increased cooling and heating energy when using cooling-coil fouling module?

asked 2015-09-29 11:35:01 -0500

jfirrantello gravatar image

updated 2015-11-08 12:45:19 -0500

edit: modified to make clear that I'm only modifying cooling coil UA values.

I'm using the DOE Commercial Reference Buildings to perform simulations involving the cooling coil-fouling module introduced in v8.3. For the purposes of this question, I'm going to talk about the Large Office building in Phoenix AZ.

I have modified the rated UA via the coil fouling operational fault modeling to approximately 86% of its "clean" value for cooling coils. This produces some expected effects, e.g. pump energy goes up due to a reduced waterside dT across the coil for an equivalent load. Another "expected" effect is that the amount of cooling energy use goes up. I put "expected" in quotes because I am not aware of what effect fouling has on the chiller models used in EnergyPlus. As a wholly unexpected effect, the gas heating energy goes up. This occurs over the summer months, not the winter months. My question is "Why does this occur?"

A few thoughts thus far:

1) When I convert the cooling energy to cooling load (via COP) and the heating energy to heating load (via boiler efficiency), the amounts just about even out. Is the heating load a result of compensating for over-cooling? Why would over-cooling occur in a situation when UA is reduced?

2) Are the increases in cooling and heating energy unrelated to one another? If that is the case, how does changing the cooling coil UA increase the heating?

3) For the Large Office Commercial Reference Building, the increase in heating seems to be occurring in the warmer (1A through 4A) and drier (2B through 6B) climates, though it is more pronounced in the climates that are both warm and dry (2B and 3B).

Presently, I have been unable to formulate a system operating condition where these results make sense to me. Am more than a bit puzzled, and any help would be appreciated.

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