ASHRAE Journal - March 2015 - 26
TECHNICAL FEATURE
Energy-Efficient
Makeup Air Units
BY HUGH CROWTHER, P.ENG., MEMBER ASHRAE
Unless you live in paradise, delivering makeup air to most buildings is expensive.
Table 1 shows the amount of work it takes to heat and cool air (based on Chicago
conditions) for a standard rooftop unit (a unit that recirculates air with typical air
conditioning loads) and a makeup air (MUA) unit. Note the standard unit conditions
represent 400 cfm/ton (53.68 L/[s·kW]) with 80°F dry bulb/67°F wet bulb (26.7°C dry
bulb/19.4°C wet bulb) return air conditions. It can be seen that a MUA unit requires
more than twice the cooling and five times the heating work as a standard unit.
For many HVAC solutions a dedicated outdoor air
system (DOAS) is required such as variable refrigerant flow systems (VRF), ground source heat pumps
(GSHP), and chilled beams (Figure 1). Many process
applications (labs, industrial processes, garages, etc.)
also require makeup air (MUA) systems. All these
applications require some form of make up air unit
that can move and filter outdoor air as well as heat
and cool (depending on location and application).
Since these units consume significant energy in most
applications, a discussion on how to improve their
efficiency is warranted.
A basic MUA unit has to meet certain minimum performance requirements:
* 80% efficient (indirect fired) gas heat1 (assuming a
gas heat unit);
* 10 EER (Energy Efficiency Ratio)2 if DX cooling is
required;
* Fan performance is generally marginalized but is
accounted in the unit EER requirement (assuming there
is cooling); and
* Basic wall construction called out in the product
specification. A basic unit is typically single wall, 0.5 to 1
in. (13 to 25 mm) fiberglass insulation.
To move beyond a basic unit, four areas of improvement will be considered: gas heat, DX cooling, fan performance, and casing performance (Figure 2). The energy
usage calculations are based on a 10,000 cfm (4700 L/s)
MUA unit in Chicago. The cost3 of gas is $0.79/therm
and electricity is $0.10 kWh. The carbon dioxide (CO2)
equivalent conversion4 for natural gas is 0.510 CO2e
lb/kWh (0.232 kg/kWh) and for electricity is 1.670 CO2e
Hugh Crowther, P. Eng., is a consultant to Price Industries. He is past chair of ASHRAE Standards Committee.
26
ASHRAE JOURNAL
ashrae.org
MARCH 2015
ASHRAE Journal - March 2015
Table of Contents for the Digital Edition of ASHRAE Journal - March 2015
Contents
ASHRAE Journal - March 2015 - Intro
ASHRAE Journal - March 2015 - Cover1
ASHRAE Journal - March 2015 - Cover2
ASHRAE Journal - March 2015 - 1
ASHRAE Journal - March 2015 - 2
ASHRAE Journal - March 2015 - Contents
ASHRAE Journal - March 2015 - 4
ASHRAE Journal - March 2015 - 5
ASHRAE Journal - March 2015 - 6
ASHRAE Journal - March 2015 - 7
ASHRAE Journal - March 2015 - 8
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ASHRAE Journal - March 2015 - Cover3
ASHRAE Journal - March 2015 - Cover4
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