DOAS system

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Hi All,
I would like to know what is DOAS system for what purpose it is
used .I have only heard about it but not used in any of the projects. It wil
be so nice if anyone can tell for what applications it is used.

Sambhav Tiwari.

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On 27/03/2010 17:36, sambhav tiwari wrote:

Dedicated Outdoor Air System. It is an airhandler that delivers treated
outdoor air to the zone. It can be used with variety of systems. I used
it a lot in conjunction with terminal units (fan coils). If properly
designed, DOAS will limit humidity raise at part load conditions.

Dusko Nikolic's picture
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Hi ,Sambav,

DOAS dedicated Outside air System can be used in "active" chill beam system.


Sanjay Cholekar

Sanjay Cholekar

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Dear Sambhav,

There is good information available from ASHRAE which you should read about

A brief understanding is that certain HVAC systems, such as chilled beams,
VRF and water-source heat pumps, are used to remove zone sensible cooling or
heating loads. They are very effective at this function but do not provide
outdoor air ventilation for the zones. As a result, a separate HVAC system
is provided for the express purpose of providing ventilation air. This is
called a Dedicated Outdoor Air System (DOAS).

James V. Dirkes II, P.E., LEED AP

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This discussion has perhaps underlined the differences in terminology
between the US (where a significant part of development takes place)
and the rest of the world.

For example, in the UK 90%+ of systems using an AHU are "DOAS" but I'd
never heard of the term before dealing with E+.

Another thing we have come accross is the applcation of Fancoil
systems appears to vary significantly between the US and EU. In Europe
most fan coil systems are augmented by "DOAS". They can also be quite
advanced pieces of kit with electically commutated (EC) fan motors for

This is merely my observation from E+ where the Fourpipe fancoil
object is quite dumb (i.e. Single speed fan that doesn't modulate on/
off with the load).

I say quite dumb... It's obviously more clever than me!

Chris Yates's picture
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I know you've already gotten several responses to your question; however, for further insights, check out the numerous publications on Dedicated Outdoor Air Systems by Dr. Stanley Mumma at Penn State. Start at:


Larry Degelman

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I resisted jumping in on this discussion earlier, but Chris' comment
motivated me to say that my experiences in China are very similar, and
that perhaps it's the US that's out of step with a lot of the world.

The standard HVAC system in Chinese buildings, i.e., at least 90% of
the time, are fan-coil units with a "DOAS" to provide conditioned fresh
air. The "DOAS" are essential because of the high outdoor humidity
during the summe. Of course, they're not known as "DOAS"; I only learned
that terminology when I explained the system to a US consultant.

It seems that when the US HVAC industry went to all-air systems in the
50's (?), fan-coil systems became stigmatized as archaic and their
designs stagnated. That's not the case in other parts of the world.

Joe Huang

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Joined: 2011-09-30
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Adding a personal note on the fan-coil units and DOAS - in the office I was
previously in I setup a occupant survey when we were in the process to
attain LEED Silver certification (we go it!). The office was in a 20 some
floor office tower with fan coils on the perimeter for the envelope load,
the interior was arranged as an open office space and served by 8 VAV
boxes. This office had almost no complaints from any occupants in the open
offices. The majority of thermal comfort / acoustic complaints were from
individuals seated in the individual office suites at the core of the
building (where they had no control of their environment).

The office also had blinds to block solar radiation that occupants could
choose to use - and everyone did. Also we ended up purchasing small desk
fans and a few Herman Miller C2 personal comfort devices for occupants in
the offices to provide them a level of individual control. This could be
called points-chasing but everyone appreciated the devices and was happy.
The Herman Miller C2 devices can be used for heating and cooling - or if
you place your computer with the business end facing towards you (like I
do) you have a free 400W heater.


Shariq Ali EIT, LEED AP BD+C

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Similar to the experience of Joe about China, fan coil based system are
quite popular in India too, but, except for the green building community,
not many people are using DOAS. Rather, I would say, not many people are
aware about importance of the same.

best greetings,
Jyotirmay Mathur

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