The question about the system type of HVAC

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HELLO,
Everyone, I am a learner ,now I have a question about the system type of HVAC .When the cooling source is chilled water coils and the heating source is hot water coils ,the system type of HVAC have many choices, such as Standard VAV with Hot Water Reheat and Reheat Fan System with Hot water Reheat. In the Air-side system type of the Wizard interface, the parameter information is no differences between the two system type(Standard VAV with Hot Water Reheat and Reheat Fan System with Hot water Reheat),but the result is so difference.I want to know the two system where is different?
I am sorry that my English is not better .thank you again.

Yongcun's picture
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Joined: 2014-05-23
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I assume you are talking about energy usage with the big difference. It
depends on how you have the defaults set up but I think you will find
the system with the larger energy usage is reheating the air in the
cooling season. It is exercising humidity control by over cooling the
air with the cooling coil to knock more moisture out of the air stream
then heating the air up to not over cool the people in the space.
This method is frowned upon right now as it isn't very energy efficient
if you are running your boilers in the summer time to provide the reheat
and running a second set of pumps to move the hot water. It actually is
pretty much banned in some jurisdictions by energy conservation
regulations. It is the best way to control humidity in the air stream.
There are some great ways to make it more energy efficient by using the
heat off the cooling system so you don't have to use the boilers. So
using free heat you just have dual pumping energy as the draw back but
considering all the other methods deployed such as DOAS and it's
associated costs I personally don't think in the end there is much of an
energy penalty. It would probably be a much cheaper system to construct.
I'm a bit of a contrarian and don't like a lot of the new green rules
that put a straight jacket on my ability to design the best system for
the situation.
Bruce Easterbrook P.Eng.
Abode Engineering

Bruce Easterbrook's picture
Joined: 2011-09-30
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Very simply, a variable volume(VAV) system with HW reheat varies the volume of air thru the system, thus lowering cooling coil usage. When the airfow gets to a minimum point, the reheat coil may kick on to increase air temp.
The standard reheat system is a constant volume system, which will use more cooling and probably more heating as well.
See if these are the results you are seeing.
John R Aulbach, PE

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------Original Message------
From: ???
To: "equest-users at lists.onebuilding.org"
Date: Saturday, May 17, 2014 9:57:23 PM GMT+0800
Subject: [Equest-users] The question about the system type of HVAC

HELLO,
Everyone, I am a learner ,now I have a question about the system type of HVAC .When the cooling source is chilled water coils and the heating source is hot water coils ,the system type of HVAC have many choices, such as Standard VAV with Hot Water Reheat and Reheat Fan System with Hot water Reheat. In the Air-side system type of the Wizard interface, the parameter information is no differences between the two system type(Standard VAV with Hot Water Reheat and Reheat Fan System with Hot water Reheat),but the result is so difference.I want to know the two system where is different?
I am sorry that my English is not better .thank you again.
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John Aulbach's picture
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