Re: [EnergyPlus_Support] autosizing vs hardsizing HVAC system, which one is better choice?

I agree completely with Ned. Â

I always prefer actual data from measurement or site visit and have frequently found that existing equipment is oversized - sometimes by 200-300%.Â The more information you have verified, the more confidence you have about the model results.

Some data is much harder or more costly to obtain, so an estimate is OK.Â Infiltration is an example.Â Boiler efficiency is another example; you should never expect that a 20 year old piece of equipment performs the same as when new!Â Part load performance is another aspect that is probably never the same as when the equipment was new.

For all of the estimated / approximated values, the calibration effort can be used to assess the actual values as well as the model's sensitivity to each variable. Â

For one project, gas consumption for a 10 year old boiler matched the utility bills only when the boiler efficiency was 52%.Â Because we had verified many variables, we were confident and did not need to guess about them.Â That made the model results more believable and let us focus on the boiler performance.Â Further investigation revealed that 60 year old condensate pipes in a below-grade tunnel had required frequent repair due to corrosion.Â This, in turn, suggested that the leaks caused the boiler to use a lot of makeup water and greatly decreased its overall efficiency.Â The client replaced the aging pipes and saw greatly reduced gas usage.

On Tue, Jun 16, 2015 at 12:28 PM, 'Edward G. Lyon' eglyon@xxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

Â

It seems your goal is a model of an existing building that can be calibrated to some data you have.Â Autosizing will not work well for you unless the actual system happens to match the autosize selection.Â Entering existing mechanical system components will tell you how the system may be under/over sized as E+ will still calculate zone loads for comparison.Â With existing equipment as a base model it will be easier to evaluate specific equipment changes.

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That said, autosize models are sometimes easier to use to get things working, after which you can modify components individually and quickly identify model problems rather than have to identify random sources of model errors. Â Â

Â

Ned Lyon, P.E. (MA, WV)
Staff Consultant

SIMPSON GUMPERTZ & HEGER
781.907.9000Â main
781.907.9350 directÂ
617.285.2162 mobileÂ
781.907.9009Â fax
www.sgh.com

Â

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx]
Sent: Tuesday, June 16, 2015 12:14 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] autosizing vs hardsizing HVAC system, which one is better choice?

Â

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I was wondering, what is the best way to model HVAC equipment for an already existing building? Is it better to select autosizing and let EnergyPlus to do the job OR entering all the data from manufacturers' manuals? Hardsizing will also include getting the coefficients for boilers, VRF system, heat recovery elements etc.

If Autosizing and using default equations/coefficients is the answer then will it make calibration a challenging task to perform?

Thanks

James V Dirkes II, PE, BEMP, LEED AP
CEO/President
The Building Performance Team Inc.
1631 Acacia Dr, GR, Mi 49504

Direct: 616.450.8653
jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx

Website l Â LinkedIn

Studies show that four out of every three people have a hard time with math.