Baseline PTAC Fan Energy for a Proposed Merv13 + Return Duct System - Goodness Me I had No Idea

3 posts / 0 new
Last post

My final advice on this topic (regarding DOAS for unitary systems and energy modeling for LEED ) would be the following, which I apologize in advance for the long email, but this is actually a complex can of worms that was inadvertently opened, but one that I think needed to be opened and discussed in more detail from a wider view for the eQuest BEM community at large. When I originally saw the post on this topic, it reminded me about this topic and hence thought it would be a good time to discuss.

We obviously have much bigger problems in life right now than what I'm about to cover, especially with the sequestration (i.e. budget knife) set to officially take effect, which could have a significant negative impact on the #1 purchaser of energy modeling/LEED services, the federal government, and also a significant negative effect on the next largest purchasers, the states (i.e. many of mine and your clients fall into these categories). Assuming we will survive all of this, I offer something to help that focuses on at least one tiny problem (relative to the bigger problems in life) we have run into with regards to BEM for LEED projects using Appendix G.

Since we aren't allowed under the current rules to adjust the Baseline fan power allowance for PTACs to account for Dedicated Outdoor Air Systems (DOAS), then I would instead do the following:

Model the Baseline with the 0.30 watts/CFM PTAC fan power allowance only, without any adjustments, and fans run continuously during occupied hours, etc.

Ignore the DOAS in the primary Proposed model. Instead, assume the outside air goes directly to whatever unitary equipment you have, and fans run continuously during occupied hours.

Submit a separate Exceptional Calculation (with EAp2) which shows that a DOAS with energy recovery provides net savings when compared to a DOAS without energy recovery. Properly done, this analysis will account for the additional fan power of a DOAS with energy recovery, which is a result of the additional energy wheel pressure drop, as well as the pressure drop of additional filters to protect the wheel. In other words, a DOAS with energy recovery will use more fan energy than a DOAS without energy recovery. However, net positive savings should result from the cooling and heating energy savings exceeding the negative fan energy savings. If not, then the design team, unfortunately, made a bad decision to equip the DOAS with energy recovery. The DOAS will be equipped with a cooling coil in order to deliver cooled and dehumidified outside air to the returns of the unitary HVAC equipment, where it will then be distributed to the spaces, or considered to be delivered directly to the spaces, depending on the design of the actual DOAS. The DOAS will also be equipped with a heating coil to deliver ~ room neutral air during cooler weather conditions. In other words, you have to accept that preconditioning of the outside air via the DOAS is required, otherwise, energy recovery has no meaning and therefore provides no savings.

If you accept the premise that DOAS are required to properly meet the ASHRAE 62 outside air requirements while properly controlling building humidity levels for projects in humid and mixed humid climates, then I believe the above approach is an imminently reasonable approach. The goal with this approach is not to determine whether a DOAS is required or not, but instead assumes the DOAS is required and the decision is to focus on whether it should have energy recovery or not. Since adding energy recovery certainly increases the cost of the DOAS, you want to be able to tell the design team if the investment is worth it. That's the overall goal. To say that a DOAS wouldn't be required in the 1st place could be an invalid argument in my opinion. I do think whether a DOAS is required is climate dependent. You may not need a DOAS in San Francisco, but trust me, you need one in Little Rock, AR. Saying we can simply open the windows when it's incredibly hot and humid outside just doesn't work. Likewise, saying we can introduce hot & humid outside air into the spaces while cycling the primary HVAC system compressors on room temp only doesn't work either. There are some DX equipment that have the ability to control both to room temp and humidity without reheat, but this form of humidity control is passive and limited, not all that great. I have at my house, a very advanced DX unit (with dual compressors) made by a certain major manufacturer, with both temp and humidity control. I am surprised at the ability/inability of the unit to maintain humidity below the set-point of 55% during humid ambient conditions, and I don't even directly introduce outside air into my house (i.e. any outside air is infiltration only). Other DX equipment is equipped with reheat to control humidity. This can certainly work in the real world, but eQuest (as Maria recently pointed out) doesn't allow the reheat option on PTACs but does on PSZ-ACs. Therefore, another valid option could be (as Maria had suggested) model the PTACs as PSZ-ACs with max humidity control and reheat. I think that is a very valid option and in fact I used that very approach for the Baseline PSZ-AC systems on a previous project located on the Florida Gulf Coast where the PSZ-ACs brought the outside air in directly. This approach provides a proper energy penalty for forcing the Baseline systems to properly take care of space humidity (i.e. set max humidity to 60% on main air-side system tab). One can certainly argue that this approach is valid per Table 3.1 Section 1.b of Appendix G (i.e. temperature and humidity control set-points shall be the same for Proposed and Baseline). I would make the argument that using a DOAS could be considered the "best practices" most energy efficient way to provide for humidity control of the outside air versus the DX/reheat approach. My procedure ultimately trusts the judgment of the mechanical design engineers on the project. If they determined the building needs a DOAS for proper humidity control, then the building needs a DOAS. My procedure simply comes in after that decision has been made and quantifies the relative savings of equipping the DOAS with energy recovery versus not. My procedure assumes that DOAS with energy recovery saves energy, a position that I would think all DOAS equipment manufacturers would agree with. I think it's inappropriate to essentially penalize a project via the current Appendix G fan power allowance values (for system types 1 & 2) when the project has a DOAS, and instead ask the design team and owner to put in more efficient and expensive lighting, HVAC, etc. in order to compensate for the DOAS penalty. In the trenches, that doesn't go over real well with design teams and owners who are working under budget constraints on these projects.

With regards to implementing this procedure, I'm not just speaking academically here; I've been through the LEED review fire on this one and this is exactly how I handled this for a real LEED project that involved a dormitory on a university campus. My exceptional calc was accepted by the LEED reviewers, and the project ultimately achieved LEED Gold certification. It made sense to me that the exceptional calc was accepted because exceptional calcs are supposed to be for things that provide savings, but are not covered currently by Appendix G, and one can make the argument that Appendix G does not cover DOAS for Baseline systems. This exceptional calc will take the DOAS from being an ugly minus (i.e. negative savings) on your project to being a plus, a source of savings.

The exceptional calc was a special eQuest model built exactly for modeling DOAS with energy recovery.

My hope is that the next version of eQuest will be able to directly model energy recovery for DOAS, and make life easier on all of us eQuest modelers. Incidentally, I have not heard anything about a new version of eQuest, so if anybody has any information to share on that, would love to hear about it. I really hope that something new is coming at some point in the future, and soon. :)

I apologize that I'm not a regular contributor on this forum, because frankly, I find it very difficult to make time along with all other demands that we face. However, I spent a lot of time on this topic at one point and so it was a topic of great interest to me and one where I thought I could make a contribution to the eQuest BEM community at large.

If you guys have interest in the exceptional calc procedure and eQuest model that I developed specifically for modeling savings associated with DOAS with energy recovery, let me know, and I will be happy to share it for your use (at your risk) on your projects. I'm sure it's not perfect, and I'm sure some holes can be shot in it, but I'm willing to put it out there for the benefit of the eQuest BEM community.

Sorry for the very long email.

Have a great weekend! :)



James A. Hess, PE, CEM, BEMP

James Hess's picture
Joined: 2011-09-30
Reputation: 601


What was the nature of the exceptional calculation? Did you justify using less power in the proposed model than the actual design? (Less fan power?) If not, why was an exceptional calculation required? What do you change in your eQUEST Proposed model when you "Ignore the DOAS"?


[Senior Energy Engineer 28Jun2012]

Bill Bishop's picture
Joined: 2012-02-25
Reputation: 7


10-4 on your email .

I did account for the DOAS fan power, just in the Exceptional Calc, versus the main eQuest model.

I did so in order to specifically show that the net energy savings provided by the DOAS energy recovery exceeded the DOAS fan power penalty associated with energy recovery.

I did this as an Exceptional Calc because eQuest cannot directly model energy recovery for DOAS. It turns out that eQuest can model energy recovery for DOAS, but you have to pull out a big back of tricks in order to pull this off. Consequently, it was logistically much easier to keep the eQuest DOAS energy recovery model separate from the main model.

This will make more sense if you take a detailed look at the exact procedure, for which I have attached. I needed to take a few minutes to remove any mention of the project or owners name, because I did not specifically get the owner's permission to publish this prior to sending this out. Hopefully, they won't mind.

Here are a few points on the materials:

* Main procedure is described in this document: Modeling DOAS with ER in eQuest.pdf

* The exceptional calc eQuest ERU files are also included: DOAS with ER.pd2 and DOAS with ER.inp

* Within the eQuest model, I have made heavy use of Global parameters

* Go to the Global parameters and toggle the "ER On-Off Flag" to turn energy recovery On or Off.

* You run eQuest with this flag On to simulate the DOAS with energy recovery, and Off to simulate no energy recovery.

* For this example project, I've included the eQuest ES-D reports with and without energy recovery.

* The ESD reports provide the information to populate the following document: DOAS with ER Exceptional Calc Results.pdf

* That document has the results of the Exceptional Calc which are then uploaded to the Exceptional Calc section of EAp2.

* This project was using DES for heating and cooling, and we used the Option 1 approach. Consequently, we had stipulated rates for CHW and HW, per the DESv2.0 guidelines.

* The parameters of the DOAS units are included in the file, ERU Rated Performance Data Summary.pdf, though I believe I've modified these via separate worksheet to account for the fact that the DOAS units actually used much less fan power (~ 24% less) when balanced in the field to design airflow conditions (i.e. original fan sizing was conservative by the design engineer).

* Within eQuest, I am using different supply and exhaust fan power values depending on whether we are running the energy recovery scenario or not. Check the custom user expressions and code to see how I did this and linked back to the global parameters.

* Basically, you enter the design outside airflows, exhaust airflows (unbalanced situation, some local exhaust vs. system exhaust), and the fan power values. Confirm the other assumptions are reasonable, hit the run button, and will have results in under 7 seconds (impressive if I might add).

* This particular project had two DOAS units serving the entire dorm (~ 120,000 GSF total), about 8600 CFM each, with ~ 7900 system exhaust each.

I'll have to answer additional questions later because I need to run, but this will give you guys plenty to chew on for now.

Thanks! :)



James A. Hess, PE, CEM, BEMP

James Hess's picture
Joined: 2011-09-30
Reputation: 601