[Equest-users] Benefit to modeling duct losses in cold climates?

[Chris Baker]

Thanks Chris.

I'm only implying that a small amount of duct loss (compliant with ASHRAE 90.1) in that the loss would heat the "DUCT-ZONE" somewhat.
That heat would still be contained within the building which could potentially better insulate it during months of extreme cold.

In other words the heat going into "DUCT-ZONE" due to the duct loss would still be heating the building.
It would just be going into an unconditioned space.

I'm just not sure if the heat contained within an unconditioned zone is factored into an annual simulation.

Duct losses lower the supply air rate somewhat which have to be made up (in the form of more energy usage by fans and heating systems).
If there is any potential cost benefit caused by this you would have to cancel that out in the form of the added heat performance in winter caused by the duct/heat losses.

In other words equest would have to model an advantage to having warmer unconditioned spaces in the cold winter months to make it worthwhile.

I think I'm going to experiment with this once I finish my current model.
I doubt it will matter much but if there is cost savings - why not model it if it is still ASHRAE compliant.

-Chris


From: Jones, Christopher [mailto:cjones at halsall.com]
Sent: Wednesday, March 25, 2015 11:48 AM
To: Chris Baker; equest-users at lists.onebuilding.org
Cc: Joe Huang
Subject: RE: Benefit to modeling duct losses in cold climates?

Chris,
I suggest you go back and read the DOE22Vol2-Dictionary manual.  If you are talking about the DUCT-AIR-LOSS, then yes, the duct zone must be a plenum or unconditioned zone.

If you are talking about  the heat/cool energy lost (DUCT-DT or DUCT-UA) then the duct zone can be a plenum if one exists or any zone on the system, you specify the DUCT-ZONE.  Only 1 zone is specified.

Christopher Jones, P.Eng.
Tel: 416.644.4226 * Toll Free: 1.888.425.7255 x 527

From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Chris Baker
Sent: Tuesday, March 24, 2015 7:33 PM
To: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: [Equest-users] Benefit to modeling duct losses in cold climates?

So I haven't yet modeled my current project with duct losses.  This is a 13,000 sf building in Alaska (Climate zone 8).

Equest only models duct losses in terms of how it affects unconditioned zones (it does not model duct losses to conditioned areas).
But over the course of an Alaskan winter (October thru March) any duct losses would act to somewhat heat the plenum space.

So would there be a benefit in thermal performance over that period?
Has anyone experimented with this?

This would obviously add an additional load requirement to the air-side systems and the pumps/air systems would obviously use slightly more power
(especially for PSZ systems) to make up for any duct losses.

I think the only potential benefit would be thermal performance but I'm not sure if equest takes into account thermal energy in a plenum (unconditioned space)
when it models the thermal performance of a conditioned space beneath it during the course of 1 model year.

If it does, I think the only potential benefits during winter months would be in extremely cold climates like Alaska.
Otherwise you are just adding more heating and power loads on the pumps, fans, etc...  Which over the course of 1 year would have an impact.

HOWEVER...
If you model proposed duct losses the same as baseline might there be a greater overall cost savings in proposed model while keeping the baseline ashrae compliant?
ASHRAE (at least the 2013 version) has equations for determining duct losses in the baseline model.

The proposed building would obviously retain the thermal energy more efficiently than the baseline and it might be an overall benefit in cost savings in terms of heating costs
Regardless of how much additional energy is used to make up for the duct losses on the system.

Both models would act to contain the heat expelled by the duct losses but the proposed model would do a better job, basically, than baseline even though the duct losses are the same.

Chris Baker
CCI CAD Drafter


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