Power induction unit/ reheat delta T

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I am working on a building with air handling units on each floor. Interior zones are served by the AHU while exterior zones are served by the AHU as well as FC unit which provides furthur heating and cooling. I modeled the system as a Power Induction Unit with series PIU to represent the FC units. I used a reheat delta T determined from AHU and FC's spec'd heating capacity and air flow rates and allowed eQUEST to autosize and distribute airflow. Thanks to the advice given from members on this list I was able to reduce my unmet hours to a reasonable range.
I am working on the baseline model and am again having issues with unmet hours. As mentioned the heat capacity of VAV systems are determined by the design flow rate and the reheat delta T. In order to reduce unmet hours I would increase flow rates to particular zones by manually increasing Min design flow rates with my reheat delta T set at 20F which is what is required by ASHRAE. I've been at this for a while and my building is basically a wind tunnel, I have floors with supply cfm upwards of 100,000 CFM and I still have thousands of unmet hours. (my throttling range are at 6)
Am I allowed to change reheat_delta_T to a larger value?
For a power induction unit with series PIU boxes, is there any way for the boxes to provide zone cooling? Looking at hourly reports for "zone coil cooling" I have 0's.
Much of this might be brought about by the fact that my building heating and cooling load has doubled in the baseline model.
Any advice would be much appreciated.
Date: Mon, 27 Aug 2012 09:40:41 -0400

John Shen's picture
Joined: 2012-11-12
Reputation: 0

ASHRAE does not mandate anything for reheat delta T in the baseline. You
can make this whatever it needs to be to meet the load, within reason of
course. Maximum recommended discharge air temperature to the zone is 15
deg F above the room temperature setpoint. Reheat delta T's of 30-40 deg F
are not unreasonable for the baseline building.

Robby Oylear, PE, LEED AP

Robby Oylear's picture
Joined: 2011-09-30
Reputation: 202

It seems to me you have gone overboard on your zone air flows which will
generate unmet hours and also waste energy. This is all a balancing act
between several conflicting items, air flow just being one. I don't
think your delta T is the problem. Terminal units can heat and cool and
it sounds like yours are not cooling. I would also suspect your system
is fighting itself. You are going to have to troubleshoot what is going
on and figure out a solution. Your sim reports will tell you how much
heating and cooling each zone requires. From there you can figure out
how much air you need to to accomplish this. Compare that to how much
air you are putting in. I don't do a lot of these types of systems to
give you a quick answer with the information I have. Most of the time
there is no quick answer anyway. All HVAC systems follow a logical
pattern. You need to find where and why you have deviated. As you can
see eQuest will take you way out into left field if you let it.
Bruce Easterbrook P.Eng.

Bruce Easterbrook's picture
Joined: 2011-09-30
Reputation: 0
John Shen's picture
Joined: 2012-11-12
Reputation: 0

A single system can be a lot of problem. There is just so much
variation between the zones most times and there are not enough
T-stats. I don't know the number of zones and the space size but if you
are down to the 100 hour unmet range I think you are as close as you are
going to get it. One other thing to look at is the timing of the unmet
hours. Many times I have found, more so with higher mass buildings is a
one hour warm up is not enough and you will get a lot of unmet hours
right at the start of the day. Sometimes bringing the AHU off night set
back 2 hours before is enough to get rid of these shoulder hours. There
can be a big difference in the heat and cooling capacity required from
maintaining a set point verses trying to recover the space from a 4
degree set back. Equipment these days is sized closer to the line and
it takes a little more time for the system to get the space under control.
Bruce Easterbrook P.Eng.

Bruce Easterbrook's picture
Joined: 2011-09-30
Reputation: 0