[Equest-users] VAV with Reheat in Summer

Tue Dec 19 07:41:43 PST 2017

Hi Ali (and Kathryn!),

I peeked around the model yesterday and made a couple observations, but forgot to hit send... sorry for the delay.  Not sure if this will totally solve your case but should help you understand what's happening:

  1.  A piece of why you're observing relatively flat heating needs through the year is explained by:
     *   100% of design cooling airflow is being pushed to your zones at all hours the system is ON per zonal minimum flow inputs of 1.0.
     *   Combine the above with your systems being made to push air 24/7/365 via the fan schedule.
     *   Allowing airflow to turn down and/or allowing the system to turn off would reduce reheat caused by sub-cooling of your spaces.
  2.  Was it was intended to input -1 btu/h for baseboard capacities?
  3.  Reinforcing Kathryn's point on the importance of reset strategies in mitigating reheat: it's generally good practice to keep an eye on your hourly supply air temps (which I do see included in your custom hourly report).  Consider for similar reasons tightening up your cooling supply throttling range.  I have observed supplying 51F air when you intend 55F (this really tripped me up on a different model), which can add up to substantial difference in reheat requirements.  I've been advised correctly designed/commissioned air handling system controls these days shouldn't have trouble maintaining supply air to within a degree of setpoint or better.
  4.  Your humidity control setpoints will be the source of at least some of the sub-cooling (and subsequent additional reheat energies).
  5.  You stated "one thermal zone for each VAV at this stage."  Anticipate reheating needs might need to increase if you intend to serve >1 zone per system.

You can always clip off availability for the reheat coils via snap temperature or schedule it off via hydronic loop controls.  You can also more broadly cut off system heating capability with airside inputs or similar loop controls on the primary heating loop.  These are behaviors I regularly encounter for building hydronic plants around my part of the world.  If however you don't address/understand the causes for your reheat first (perhaps with exploring some of the above bullets), you'll risk creating new unmet hours as spaces sub-cool without the reheat they want.

I've only ever visited Vancouver once on vacation during the summer, so can't claim intimate knowledge of the climate, but I recall some pretty chilly evenings (and amazing food!).  Probably stating the obvious here, but wouldn't be surprised to find a system supplying and conditioning ventilation air 24/7 is likely to require space heating through the year.

... hope some of this is helpful!

~Nick

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Nick Caton, P.E., BEMP
  Senior Energy Engineer
  Regional Energy Engineering Manager
  Energy and Sustainability Services
  Schneider Electric

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From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Kathryn Kerns via Equest-users
Sent: Monday, December 18, 2017 4:31 PM
To: Alireza Mohammadzadeh <ali.mo at rpeng.ca>; equest-users at onebuilding.org
Subject: Re: [Equest-users] VAV with Reheat in Summer

My only other advice is to identify which zones are requiring the extra cooling in the summer that the rest of the zones don't need. This is the cause of reheat.  Try setting all the process loads to 0.0 and if that fixes the problem, start adding the process loads back in a bit at a time.

I would also look at the hourly reports and see if you can identify the time of day the reheat occurs in the summer months. This might give you a clue as to the problem.

Kathryn Kerns
Systems Specialist
BCE Engineers, Inc.
| Ph: 253.922.0446 | Fx: 253.922.0896 |

From: Alireza Mohammadzadeh [mailto:ali.mo at rpeng.ca]
Sent: Monday, December 18, 2017 2:13 PM
To: Kathryn Kerns <kathryn.kerns at bceengineers.com<mailto:kathryn.kerns at bceengineers.com>>; equest-users at onebuilding.org<mailto:equest-users at onebuilding.org>
Subject: RE: VAV with Reheat in Summer

Thanks for the reply. Yeah I already set the cooling control to Warmest in the model. If I change it back  to "reset" (default in equest for VAV) my reheat load will be significantly higher.
Even with warmest in cool control, I have a lot of reheat in summer shoulder which doesn't make sense to me.

Ali

From: Kathryn Kerns [mailto:kathryn.kerns at bceengineers.com]
Sent: Monday, December 18, 2017 1:22 PM
To: Alireza Mohammadzadeh; equest-users at onebuilding.org<mailto:equest-users at onebuilding.org>
Subject: RE: VAV with Reheat in Summer

I was having the same issues and had difficulty explaining how the PVAV system works. After a lot of trial and error, I discovered one thing that affects the VAV reheat  in the warmer months and one thing that affects the amount of mechanical cooling in the cooler months.

  1.  The "Cooling Control" variable under the Cooling Tab affects the amount of reheat that occurs in the summer months. There is a great deal of difference in the amount of reheat if you use the ASHRAE baseline 5 delta F reset based on outside air temperature and using the "Warmest" option. Try it, and you will see.
  2.  The "Economizer Low Limit" function located under the Outdoor Air tab, if available, usually shows up as 45 F. Lowering this down to 35 F will reduce the amount of mechanical cooling used in the cooler season.

Hope this helps.

Kathryn Kerns
Systems Specialist
BCE Engineers, Inc.
| Ph: 253.922.0446 | Fx: 253.922.0896 |

From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Alireza Mohammadzadeh via Equest-users
Sent: Monday, December 18, 2017 12:41 PM
To: equest-users at onebuilding.org<mailto:equest-users at onebuilding.org>
Subject: [Equest-users] VAV with Reheat in Summer

Hi eQUEST users,
I'm modelling a building that uses VAV with reheat for perimeter and interior zones. The min flow ratio at zone level can't be less than 1, and I set the design flow to match our design.
I get a lot of reheat in summer shoulder, and when I look at the hourly report it seems like my central coil is cooling down the intake and zone coil reheats it. I only have one thermal zone for each VAV at this stage, and I'm sure that summer reheat is not because of humidity control.
Any suggestion how I can get rid of the large should season reheat?

Thanks in advanced

P.S., I attached INP and PD2 files.

Ali Mo, EIT

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