Residential Baseline HVAC System Type (Not Again)

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I have reviewed the list archives on this issue and found some conflicting opinions for residential buildings. To give some background, I am working on a model for a large adaptive reuse project that includes the conversion of a warehouse building to residential apartments. The proposed design includes split system heat pumps in the units (Electric Heat) and several RTUs (fossil fuel heat) serving as DOASs w/ energy recovery conditioning the corridor spaces and supplying/exhausting air from the residential apartment spaces. The corridor spaces exceed 25,000 square feet and 5 floors. I have the proposed design squared away, but I was hoping for an endorsement of my approach to the baseline before reconfiguring almost everything under my air-side HVAC tab.

For the baseline systems my strict reading of Table G3.1.1A along with the exceptions under G3.1.1 would indicate that for my baseline design I would do the following:

-Apartment spaces: System 2 PTHP.
-Corridor spaces: System 7 VAV w/ Reheat.

I am hesitant to plunge forward, as System 7 seems an unrealistic baseline for this application. I am also a bit confused as to how I should model the outside air delivery in my baseline. I believe I should deliver outside air via the central systems as I did in the proposed design rather than directly at the PTHP in each unit. Since the building is in Climate Zone 3A, I would not include an economizer, but I might have to include exhaust air heat recovery for cooling if the design supply air capacity for these systems exceeds 5,000 CFM, per G3.1.2.10. Am I interpreting that correctly?

To top it off, there was also a school of thought within the archives that one should model the whole baseline building with System 2 for this situation. It would certainly be less complicated, but create issues around fuel use type and fan power.

Any feedback is much appreciated.

Robert Stephenson

Robert Stephenson2's picture
Joined: 2013-06-23
Reputation: 0


I've recently dealt with a similar situation with a third party reviewer, and the conclusion was that the baseline would be System 2 (PTHP) for the apartments, with a DOAS supplying ventilation. The reviewer provided an unofficial interpretation response from ASHRAE from when this question was asked in the past. Technically then, you indeed may end up with heat recovery on your baseline DOAS, although since common practice (at least in my region and limited experience) is to exhaust from the suites directly, you may be successful in arguing that the baseline should be considered to exhaust at the suite level as to not penalize your design for ducting exhaust back to the DOAS for energy recovery. In that case you could claim exemption h, the largest exhaust source is less than 75% of the supply flow.

Good luck!

Riley Beise, P.Eng., BEMP, LEED Green Associate

Riley Beise2's picture
Joined: 2013-02-14
Reputation: 200

I wanted to follow up on this issue, as I received some clarification from the USGBC reviewer. Also, this project is pursuing LEED for Homes certification, not one of the commercial ratings, so this won't apply to all LEED projects.

Under LEED for Homes, given some of the ambiguity of Table G3.1.1A, one may model the whole building as "Residential", Systems 1 or 2. There is the option to go with a direct read of the table and use System 7 for the common spaces, but I will not be pursuing it.

Since this building has a Proposed Design with DOAS (fossil fuel heat) and split system heat pumps (electric heat) in the apartments, the HVAC system type is considered hybrid, and my baseline systems will be System 1 throughout the building.

I can't say I am not pleased by this direction, as it makes for a much simpler modeling process, but it will likely create some savings issues with the fan power for the DOAS in the Proposed Design, as well as the differences in fuel cost for electric vs. gas.

Thanks for the feedback. This was a fun refresher on the ins and outs of Appendix G for me.

Robert Stephenson2's picture
Joined: 2013-06-23
Reputation: 0