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HVAC system efficiency comparisons

asked 2015-01-16 15:40:05 -0500

anonymous user


updated 2015-11-11 13:02:42 -0500

This is long but may foster some good discussions. Read at your own peril :)

I had a colleague ask me today if we had any information about exactly how much more efficient a few HVAC systems are compared to each other. His project is in pre-SD phase and the design team and owner are trying to come to a decision on which HVAC system type they should use in design. Usually we address this by giving qualitative comparisons ("good, better, best") but this owner was looking for some hard numbers. They want to know that

Client management of expectations is our usual approach to head off situations like these. The answer this owner is looking for is extremely subjective and depends on countless factors (usage, climate zone, building geometry, materials, controls, etc.) It's not really possible to distill a specific and correct answer to such a complex question. Also with it being so early in the design process, nearly all the variables are still up in the air!

So from a "best practices" perspective I want to pose the following questions to the community.

  • How do you all handle situations like these at your practice?
  • Would you typically conduct an energy model in this situation even at this early stage?
  • What if modeling wasn't in the fee?
  • Are there online or NREL tools/resources that I'm not aware of that would be useful here?
  • What studies, research or reference guides to you turn to in these situations?

Thanks all! I hope we can get some good discussions going.


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answered 2015-01-21 08:15:18 -0500

I would personally use the default space types and HVAC system templates in OpenStudio to do this type of analysis. You could also use something like Sefaira Systems, which is a new tool developed explicitly for early design HVAC system exploration. The key is explaining that your results have significant assumptions. If the client is interested in reducing that uncertainty, I think that is the perfect time to discuss energy modeling with them.

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@aparker, @JO, just a note, Sefaira Systems just does design day and sizing. It doesn't do 8760. At least not right now (of course, the product is exactly one day old).

__AmirRoth__'s avatar __AmirRoth__  ( 2015-01-21 08:30:34 -0500 )edit

The AEDG HVAC scripts are also very useful when creating a quick model for early HVAC design decisions. With these and other HVAC scripts you can run the model with several different HVAC system types relatively quickly.

TaylorRoberts's avatar TaylorRoberts  ( 2015-01-21 16:55:29 -0500 )edit

answered 2015-01-16 15:56:03 -0500

IMO this is exactly where energy modeling shines, to answer questions like this, and I would absolutely use an energy model in this situation.

Using a piece of software like eQuest and the Design Development wizard, you can quickly investigate a number of different system types. You won't be able to examine more exotic systems like VRF or other complex designs that don't fit into eQuest's neat little packages, but at least you can check out the majority of the standard system types out there, as well as investigating various ECM's that may wish to implement. A skilled eQuest user could bang out wizard models in a couple of hours, or maybe half a day. A master of EnergyPlus could likely do the same, but my own (somewhat limited) experience suggests that E+ requires a little more finesse and detailed inputs, which you wouldn't have at this stage of the process.

You could also examine a free piece of software like RETScreen - - which will let you try out various pieces of equipment.

In this case, you aren't generating the model for a client, but to help your design process and give more informed answers to the client. Without a model, you'd have to fall back on rules of thumb, and as you stated in your question, there are so many complexities involved in a design that rules of thumb will probably just get you into trouble later on.

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Asked: 2015-01-16 15:40:05 -0500

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Last updated: Jan 21 '15