Adjusted HRV Calculations
Hi all. I have been looking at options for HRV’s for a new, super-insulated home under design. While the Zehnder CA350 would be great, I am pretty sure it may be out of the price range (the exchange rate between US and CAN $ is killing us right now). In my hunt for alternate units using the HVI directory (http://www.hvi.org/proddirectory/CPD_Reports/section_3/index.cfm) for ventilation equipment, I’m finding it challenging because from what I’ve learned, the HVI uses kind of a bad testing method for equipment, which can often inflate the efficiencies of the units. It does this by measuring the difference between the outdoor air temperature on the intake side, and the fresh/supply air temperature coming out of the actual unit. This means that air leakage between the airstreams, poor thermal separation between the exhaust and supply ducting in the unit, and latent motor heat can “heat up” the incoming air, making it seem like more excellent heat exchange is happening, when it is actually just “dirty heat” coming from poor design and construction.
In light of this, I came across the following article by PHIUS: (http://www.phius.org/PHIUSPlus2015docs/2015-3-13%20ERV.HRV%20Protocol.pdf). In section 9.3.3.1, it offers a means to compensate for some of these issues in calculating wintertime efficiencies. The problem is, I’m not a physicist, so the formula they are referring to is greek to me. Further down in the article, they state:
“1. For units with HVI certification, use an adjusted SRE for winter performance by adding back the fan power to the SRE equation (add supply fan energy to the numerator, deduct exhaust fan energy from the denominator). This method provides for a mathematical adjustment for exhaust air transfer and case heat transfer, while avoiding double counting fan energy. See spreadsheet for calculations. For units with multiple airflow rating points and efficiencies, PHIUS staff will develop appropriate guidance for selecting the correct efficiency rating to use in modelling.”
I’m wondering if anybody can turn this formula into more plain english for me, explaining what each of the variables in the formula refers to, and perhaps giving an example of how this formula could be applied to a unit so that I (and we all) have the ability to perform this “efficiency” check ourselves, to compare to units tested by the PHI (like the Zehnder products), which compares interior air temperature to the exhaust temperature coming out of the unit, which penalizes the unit’s efficiency if it has all kinds of air leakage and motor heat losses, etc.
Or, if this kind of calculation is not possible from the kind of data one can get from the HVI directory or typical spec sheets for different units, please let me know that as well, so that I don’t try to waste time on an impossible task.
So far, from talking to different people, the Zehnder products just seem to be in a class all their own, with more modestly priced North American units all just kind of being mediocre examples in terms of efficiency, noise, etc. Having installed one Zehnder myself in a new construction we did for a client, and having seen many North American models go in by HVAC contractors in all of our other houses, I am inclined to agree, but without actually being able to have a more comparable testing metric (as PHIUS seems to outline how to do), it is hard to actually know the difference. The Zehnder was sure WAY quieter than every other model I’ve ever been around!
Thanks for any help that can be offered!
PS- So far it looks like the vanEE G2400HE unit is a pretty good North American alternative, with an ECM motor, but any experienced reviews would be great. We’ve installed other vanEE units in houses we’ve done, but I have never been incredibly impressed with them, but recognize they are decent for their price point.
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Replies
Burke,
I'm adding a comment to your thread, in hopes that other GBA readers will respond.
I'm familiar with the Passivhaus contention that North American HRV manufacturers use a method for calculating efficiency that isn't as precise as the method used by the Passivhaus Institut in Germany. My reaction: these efficiency differences aren't worth worrying about unless you are building a Passivhaus.
The annual energy use differences are relatively small. Don't sweat it.
-- Martin Holladay
Hi Martin. Thanks for keeping this in your crosshairs. Since it will be for a potential Net Zero build, I was hoping this calculation would help determine if the annual energy offset between a standard versus ultra-efficient model justified the cost, or was money better spent on more PV capacity.
Burke,
You may want to check out this article, which provides a calculation method that might interest you:
Are HRVs Cost-Effective?
-- Martin Holladay
PHIUS has already done the work for you, Burke. See the spreadsheet here, that takes HVI and data and modifies it for PHIUS energy modeling use: http://www.phius.org/Tools-Resources/Protocols-Calculators/HVI-Winter-Ratings-modified-for-PHIUS-modeling_June2016.xls
See column AE on the "Summary" tab for the heat transfer values that PHIUS uses. Note that these values are specific to particular airflows.
See these presentation slides as well for more information on the background: http://www.phius.org/NAPHC2014/Semmelhack-HRV-presentation.pdf
Thank you both for your feedback. Great article, and great to see your Powerpoint. That Excel document from PHIUS is perfect! Exactly what I was looking for John- thank you! It is curious, however, that apparently every single "Adjusted SRE" is actually improved over the HVI spec'd SRE. I expected them to be slightly less after adjustment, but not quite the across-the-board 12% reduction that PHPP demands of non certified units. Why would they actually go up after accounting for the motor? Is it because the PHIUS adjusted efficiency is using a lower airflow to calculate this, whereas HVI would use the average of all airflows (with the high speed settings having much poorer heat transfer rates)? Curious.
Burke - the energy modeling software (mostly WUFI Passive for PHIUS projects) has an input for heat transfer %, AND a separate input for fan power. HVI's SRE % already deducts supply fan power from the numerator and adds exhaust fan power to the denominator of the equation. If we used SRE in our modeling, we would be essentially counting fan power twice. The modified SRE adds back the estimated supply fan power (50% of total fan power) to the numerator and deducts the estimated exhaust fan power from the denominator....so the modified SRE is ALWAYS greater than the SRE.
The modified SRE is still NOT the best tool for comparing ERVs and HRVs. Units with excessive fan power waste heat that gets added to the supply airstream wind up with "artificially high" modified SRE in the PHIUS calcs. It's the correct value for our energy model input, but it doesn't necessarily mean it's a good unit. The best ERV's and HRV's have very high heat transfer AND very low fan power. Many practitioners fail to realize how important low fan power is to the overall performance. That's why we added a rough "Winter COP" calculation in column AP....sensible energy recovered divided by energy input.
The easiest way to compare performance of ERV/HRV is to look at summer performance instead of winter, as watts shows up as a penalty here.
And remember vendor can cheat at summer performance by rating it at a low cfm.
I see- thanks for the further clarification. So that means that those units utilizing ECM motors with low energy consumption not only straight up cost less to operate, but they also have more realistic efficiency numbers, since they can't be generating as much waste heat to artificially inflate their performance. Makes sense.