# [Bldg-sim] Heat Recovery - Effectiveness Calculation

Alec Stevens astevens at dmiinc.com
Thu Jul 10 05:40:52 PDT 2008

```Mark,
Given your selection data, I would have calculated effectiveness as
(35.5-6)/(55-6)=60%.

However, I think it is worthwhile to get some selections at other than
design conditions, because it is likely that this type of system is most
effective at the coldest outdoor air conditions.  With some different
selections, you might calculate a weighted average efficiency, if eQuest
only allows a single number to be input.

Also, don’t forget to account for the added pressure drop (and fan power) on
the air-side supply and exhaust streams, and the pump power requirement for
the run-around loop.  If the HR coil pressure drop is substantial, it might
be worthwhile to consider a coil bypass option for the many hours between
about 50ºF and 80ºF when you will not need this heat recovery.

Sincerely,
Alec Stevens, PE, LEED AP
DMI

35 Walnut Street, Wellesley, MA 02481
p: 781-431-1100 x11 f: 781-431-1109
e: astevens at dmiinc.com
-----Original Message-----
From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Mark Prince
Sent: Wednesday, July 09, 2008 6:05 PM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Heat Recovery - Effectiveness Calculation

Good afternoon,

I'm currently modeling a laboratory building HVAC system that has a sensible
heat recovery system.  The heat recovery system is a water/glycol run-around
loop type system.  In eQuest there is an input, under the airside system
tab, that allows you to fill in the effectiveness of your heat recovery
system.

My question is this, how should I calculate this effectiveness?

Currently, I've calculated the effectiveness based on the design documents.
According to the equipment schedules the exhaust air is at 72 F, and the
outside air heat recovery coil has a scheduled performance of 6 F entering
and 35.5 F leaving.  Based on these numbers I've calculated a design
effectiveness of 44.6%.  Please see the below equation:

35.5 - 6     =   0.446
72  -   6

However, a colleague of mine is wondering if this is the correct way to
determine effectiveness for the eQuest program.  He is wondering if the
effectiveness might be calculated based on the design air temperature (DAT)
that is always leaving the air handler (55 F), or some other way.

Does anyone know?

Sincerely,

Mark Prince
Affiliated Engineers
5802 Research Park Blvd.
(608) 441-6668
mprince at aeieng.com

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