[BLDG-SIM] A few words about ERVs in eQuest

Karen Walkerman kwalkerman at gmail.com
Fri Nov 9 06:55:28 PST 2007


Hello!

There have been a few posts over the last few months about ERVs.  I
know I posted some questions about them recently...

I am currently doing a rough model of a 43,000 square foot building.
This building houses all sorts of scientific labs and is provided with
183,000 cfm.  The systems are mostly archaic constant volume 90-100%
outdoor air dual duct air handlers.  The building is located in
Vermont.

I let the wizard generate the building shell (4 floors above ground, 1
below, perimiter/zone zoning, 2 watts per square foot for lighting and
equipment) and specified 4.26 cfm/ square foot ventilation air for
each zone.  I created a parametric run for the ERV:

RECOVER-EXHAUST = YES
ERV-RECOVER-TYPE = SENSIBLE-HX
ERV-LATENT-EFF = 0.6000
ERV-RUN-CTRL = WHEN-MIN-OA
ERV-OA-STATIC = 1.5
ERV-EXH-STATIC = 0.75

This should allow the economizer to take over when the cooling needs
can be met by outdoor air.

The result was about $70,000 savings per year.  This number seemed
extremely low as I know of an installed 8,000 cfm 100% oudoor air
system a few miles away that has saved the owners $20,000 per year.

I took a look at Brandon's recommendation that the system be specified
as PSZ or SZRH, but a dual duct system operates significantly
differently than either of these, and specifying SZRH, I still got a
very small cost savings.

Then, I changed the zone ventilation requirements to 0 cfm, and
specified the supply air for each system to be 4.26 cfm/sq ft, and OA
Fraction to be 1.  I left all Parametric run components the same.
Now, I'm coming up with $450,000 per year savings, which is much more
consistent with what I know about the 8,000 cfm system installed
nearby.

Why does the ERV seem to work when the cfm is specified at the system
level, but not when ventilation air is specified at the zone level?

Also, I am still seeing an increase in "space cooling" for the ERV
run.  I understand that the additional resistance of the ERV will
increase the output of the fan energy, and thus the heat of the
outdoor air, but it seems that the ERV should be recovering some cold
energy as well to at least break even on cooling.  Any thoughts?

~Karen~

-- 
Karen Walkerman
Second Law Consulting

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