Exhaust fans and make-up air

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PHIUS Certification Manager Lisa White weighs in on a common challenge for passive house designers.

When it comes to exhaust for the kitchen range/cook-top, either a re-circulation hood or direct exhaust hood is allowed in PHIUS certification, and among projects in certification, there has been no dominant approach — we see it both ways about equally. 

When projects use kitchen hoods that exhaust directly to the outside, it’s common practice to provide makeup air relief when the hood is in operation. This is because that additional exhaust airflow is separate from balanced ventilation system, the building is very air-tight, and without pressure relief, the exhaust airflow causes slight depressurization in the building.

PHIUS has been asked to clarify if makeup-air is required for projects when direct exhaust is utilized in the kitchen. The short answer is, it depends. The longer answer is below.

The ventilation balance  requirements for certification are outlined in the PHIUS+ Certification Guidebook v2.1 Section 3.5.3.3, and copied below:

Regardless of type, the ventilation system must meet one of the following requirements for balance:image-2

  1. Total measured supply and exhaust airflows are within 10% of each other. (Use the higher number as the basis of the percentage difference.)
  2. The total net pressurization or depressurization from the un-balanced ventilation system does not exceed 5 Pa. The net pressurization/ depressurization that the ventilation system imbalance causes on the building is determined using the multi-point air-tightness test results graph.

Intermittent exhaust airflow rates for kitchen exhaust hoods are generally much higher than a continuous exhaust airflow rates in the kitchen.  For example, a whole house may have a total of 150cfm continuous, balanced ventilation, and may have a 125cfm kitchen intermittent exhaust hood. With this combination, option 1 above would likely never pass.

PHIUS has established a method for determining compliance with option 2 during design. A stress test must be used to see if this intermittent ventilation system would cause more than 5 Pa of depressurization in the building. For a single unit building, the stress test is simply measuring the effect of turning on the range hood. If that airflow rate causes more than 5 Pa of depressurization in the building, there must be a provision for makeup air. For multi-unit buildings, an appropriate ‘stress test’ has now been defined that is both conservative and realistic.

Read the full Tech Corner Article here: https://www.phius.org/Tools-Resources/TechCorner/Makeup%20Air%20Requirements%20for%20Direct%20Kitchen%20Hood%20Exhaust%20.pdf

And try out the Intermittent Exhaust Allowance Calculator here: https://www.phius.org/PHIUS+2018/Kitchen%20Exhaust%20Calc%20v1.13/Kitchen%20Exhaust%20Calc%20v1.13.htm

 

Countering a policymaker’s concern regarding passive house

We recently heard from a PHIUS constituent who had these comments about an experience with a policy maker who was skeptical about passive building:

In recent discussions about building performance compliance options for our city’s Stretch/Green Code, a committee member raised a concern about a provision for passive house design in commercial projects. He thought that current modeling software isn’t reliable enough for reasonable accuracy and cited it as a “fatal flaw.”

Monitored energy use is tracking closely to WUFI Passive models.

Click on the image for a pdf report: Monitored energy use is tracking closely to WUFI Passive models!

Knowing that there are large and small commercial passive house projects being successfully built, I hope you can speak to his concern. The more specifically this can be addressed – the good and the bad – the better.  We are strong believers in passive house design but need to confidently understand how reliable the available technology is before adding it to our building codes.

Essentially, then, the question many policy makers have is this:

Is Passive House Certification too Risky for Code?

Short answer No.

PHIUS+ Certification is based on the same science, data, and energy programs that building codes are built on. However…

Our community is indeed successfully designing and certifying residential and commercial buildings to the PHIUS+ Passive House Standards.

We have two terrific resource sites for both applications and with great examples. Here are the links to them:

  • https://multifamily.phius.org/
  • https://commercial.phius.org/

WUFI Passive, the design/certification and energy modeling software, has proven to be accurate in predicting energy use. We have the largest pool of certified projects in North America and actual measured data is available for many of those projects.

Where we have measured data,we have found that on average we achieve modeled vs. measured results of +-7%, which is the best I have heard of in the energy modeling industry.

That said, there is much confusion out there regarding two passive house certifications. PHIUS Senior Scientist detailed the substantive differences between PHIUS+ and the European approach. In short, ours is a climate-specific passive building standard developed under a DOE grant for North America that has proven to produce very accurate predictions (here is a link to the NREL publication: https://www.nrel.gov/docs/fy15osti/64278.pdf).

The German Passivhaus Institute uses a different standard, not derived from North American climates but instead from only one central European climate (one set of standards for all climates in the world). They also use a different modeling tool, a spreadsheet called PHPP. They have to my knowledge very few projects certified to date in the commercial and large MF residential sector in North America and have not published any modeled vs. measured data.

We initially used their tool for our projects and found a significant difference in modeled vs. measured performance. Overheating in summer was also a problem. In our experience, the performance was off by 25-30% pretty consistently from what was predicted by PHPP. That’s why we switched to a different, more accurate methodology and modeling tool.

So, to the policy maker who raised the concern, I would agree that the PHPP and German standard do have a problem in North America. I do not expect that policy makers take my word for it, either. We encourage code and other officials to vet the standards and design tools carefully before including them. And to all of the PHIUS community who are fighting the good passive building fight, we will be happy to provide you data that proves the performance of the PHIUS+ standard and the WUFI-Passive modeling tool.

Kat

 

Passive Building Standards–What about Embodied Energy/Carbon?

GWPHIUS Senior Scientist Graham Wright is back this week, this time following up on a comment made to his One Cereal Aisle, Many Cereals post from last week. Reader Sarah Larsen (thanks Sarah!) asked about embodied carbon issues. The answer, we thought, deserved its own post, and we hope, discussion.

First, here’s Sarah’s comment:

Graham, thank you for such a thorough and thoughtful post. As a licensed architect, and CPHC I am already inclined toward the PHIUS methods, but continue to be curious about the nerdy differences, how, and why they came to be. I am also incredibly happy with the relationships being developed between PHIUS and code/regulatory agencies; it is critical that energy-efficient building practices spread beyond the geeky few.

The graph shows per capita energy consumption (kg oil equivalent) vs. per capita GDP, PPP (current international $). The size of the bubbles denotes total poulation per country. All values refer to the year 2011. Courtesy European Environment Agency.

The graph shows per capita energy consumption (kg oil equivalent) vs. per capita GDP, PPP (current international $). The size of the bubbles denotes total population per country. Click to enlarge. Data from the Eureopean Environment Agency and the World Bank.

With that said where is the EMBODIED CARBON calculation in our standards?? I can’t stress enough how important it is that we take this into consideration. There are PHIUS certified buildings being developed touting their energy and sustainable credentials who are using products with CO2e paybacks that will almost certainly never be met. We have just 10 years to get this right – a 90 year ROI is not going to cut it! I am very much afraid that if we develop code requirements with blinders on to anything but energy *consumption* we will have the reverse impact that we are working toward – climate catastrophe – by packing our buildings full of foam rather than thoughtfully balancing carbon footprint throughout a life-cycle assessment (LCA).

LCA has some growing up to do, but tools such as Tally are already making better, more broad-thinking choices available to anyone who cares. It is critical that leaders such as PHIUS be talking about embodied carbon and the up-front resource demands of our built environment. I think we could adapt Michael Pollan’s advice on eating as the best way to consume other resources: Build efficiently; not too much; use mostly plants.-– yours in climate-conscious building – Sarah Larsen

Now, back to Graham:

We are aware of the issue but feel we are not yet to the point where we can outright standardize, that is, make hard and fast rules. The Norwegian Institute for Zero Emission Buildings came up with some different definitions depending on how many building life cycle phases are included.

We can move more aggressively to put up some resources and guidelines. There are a couple of books I have found that seem to be helpful for making design decisions:

Most of the thinking I have seen focuses on materials, but I am not convinced that is a broad enough view of it. Suppose one uses low embodied carbon materials but this requires a lot of expensive skilled labor. The money paid for that fans out into the general fossil-powered economy, which has a certain emissions intensity per dollar, so there are emissions associated with all that economic activity, so maybe you don’t come out ahead.

From that point of view, low-cost construction is low-emission construction, and everyone is trying to do that already. I remember back in 2013 or so, the IEA or the EIA put up an interactive data browser that let you see what they were thinking in terms of scenarios to limit warming to 2 degrees C. It was clear that they were not counting on much of a contribution from the building sector. It’s pretty clear that the reason for this is that the turnover of the building stock is too slow, there just isn’t enough time anymore.

As I recall in their 2 C scenario for the U.S., almost half the total savings came from decarbonizing the generation of electricity. The “electrify everything” movement has strengthened since, Architecture 2030 seems to be on board with that, recognizing that anything done in new construction standards doesn’t do anything about the existing buildings, which is most of the problem within that next-ten-years window — so the priority is to stop emissions from existing buildings by electrifying. This is part of the reason we modified the overall energy criterion for PHIUS+ 2018 — it allows for off-site renewables now and that is compatible with the “electrify everything” idea. Some people, like ILFI, go even further and just ban all combustion from building operations.

But back to the embodied emissions. Here is an idea I had about how that might be written into a rule. Let’s call it PHIUS+ Equity & Carbon 😀

It would address both equity and embodied energy/emissions with adjustment to the Source energy limit based on construction budget, on the idea I mentioned above, that embodied energies/emissions are roughly indicated by how much money is being spent, along with the emissions intensity of the national economy.

The adjustment would be based on the idea of limiting the total operating + embodied, so, operating + embodied < 714+221 = 935 kWh/sf for example.

To generalize:

SE * 70 yr + $CB * NatkWh / NatGDP < 10.2 kWh/sf.yr * 70 yr + $170/sf * NatkWh / NatGDP

where

SE = modeled net source energy for the project [kWh/sf.yr]   — about 10.2 kWh/sf.yr
$CB = project construction budget [$/sf]   — about $170 /sf, U.S.
NatkWh = National source energy use [kWh]  — about 29 trillion kWh, U.S.

NatGDP = National gross domestic product [$]   — about 19 trillion USD

This is mathematically equivalent to adding a penalty for high-budget construction and a credit for low-budget construction, which is in the equitable direction, on a world wide basis even. The emissions intensity of the Indonesian economy is actually higher than the U.S., I make it out to be 0.44 kgCO2e / USD, while the U.S. is 0.29 kgCO2e/USD. But they catch a break with the above scheme because their cost of construction is a lot lower, about $60/sf instead of $170 for the U.S., so multiply those two numbers together and the Indonesian building industry is putting out 0.44×60 = 26 kgCO2e/sf while the U.S. building industry puts out 0.29×170 = 49 kg/sf.

I switched to carbon accounting for a minute there, but I still prefer source energy accounting. It’s a pretty good proxy for emissions because 80% of the primary energy is coming from fossil fuel burning, but I like that source energy penalizes nuclear for being nonrenewable, whereas straight carbon accounting gives it almost a free pass.

Solving the above equation for Source Energy criterion:

SE < 10.2 kWh/sf + [($170/sf – $CB) / 70 yr] * NatkWh / NatGDP

This would also tend to do a favor for retrofit projects if they are able to save construction cost by reusing the structure — and that is the high-embodied-emission part, for concrete and steel buildings, from a materials point of view. The $170/sf could be generalized to a national average value as well, $NatCB. For residential projects, just substitute for the 10.2 as 3840 kWh/person * ResOcc / iCFA.

Probably only the operating-energy part of it tapers to zero in future. More emphasis can be put on initial-cost/embodied-energy savings by shortening the time frame. At 20 years instead of 70 they are about equal in the initial example. This would favor retrofits even more.