Ventilation system specs – HRV or ERV with integrated bathroom exhaust
I am looking for guidance, insight, or feedback from those with experience on my solution for ventilation/air exchanges.
Our home is under construction, ICF primarily, and I am owner-builder. Heating and cooling will be accomplished with hydronic radiant ceiling panels from Messana – no central/forced air.
Building penetrations are limited and construction has proceeded with plans to incorporate bathroom exhaust and humidity control into the ventilation system, the heart of which would be an HRV or ERV. Having read up again including articles on this website, I will likely spec an ERV. We are in zone 5, NE Indiana and the summers are rather humid.
The best option that I seem to have found for bathroom exhaust includes the ZRT – zone register terminal from Aldes. It basically is a motorized damper with a small opening when the damper is closed, allowing for continuous exhaust through the recovery ventilator, but significantly less resistance when needed to boost exhaust in the bathroom.
I would like to find a way to activate/open the ZRT and boost ventilation from the ERV using a combination of motion and humidity sensors in the bathrooms.
I had considered pairing this with an ERV from Aldes, but I also started to look at Fantech. One benefit is that the installer for my open loop geothermal heat pump and radiant system is familiar with Fantech and can easily get materials.
In addition to the plan for bathroom exhaust, does anyone have an opinion on Aldes or Fantech recovery ventilators? I know the boxes aren’t much different, but perhaps there are manufacturer differences in the cores and certainly in the controls.
FYI – the kitchen hood exhaust is separate and I have allowed for independent entrance of make-up air, including a motorized damper controlled by negative pressure in the kitchen flue.
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What is your automation platform to integrate the humidity and occupancy sensors to trigger the dampers? Aldes is not your only option. There are plenty of low cost motorized dampers sold at the big box stores. For example, HD sells Suncourt dampers that are activated by 24 VAC. And they have a set screw so that even in the “closed” position they will allow some airflow. This is inportant because you want the ERV to run at say, 50% all the time, and then when called for it kicks to 100% and the zone calling for the extra ventilation will also send a 24 VAC signal to the damper to open up fully. I plan to do this with HomeAssistant and Z-Wave humidity/occupancy sensors (Aeotec MultiSense) myself but there are plenty of other options too.
Also, I would love to hear some quotes about your Messana install. Considering this for our build as well. I don’t plan on using any of their controllers, just the panels themselves. About how much per sq ft or per sheet did those cost you?
Edit: Regarding your kitchen hood strategy, Energy Star requires no more than 3 Pascal pressure difference between rooms and other kitchen air quality studies have said no more than 5 Pascals (Matt Risinger has a video on this). You really can’t achieve these levels without a powered make up air fan that is sync’d to the same flow rate as the hood. A open duct without a fan won’t be enough. And this is especially important if you have any natural gas appliances inside the home. Watch some of the videos Risinger made about this.
I haven't dove into the ZRT (or other motorized damper) control designs for the bath exhaust - only enough to recognize that there should be reasonable options, such as what you are planning.
My Messana quote was from when we started work on the house, a couple of years ago. I am getting back in touch with them to finalize the order, so I can let you know. I do plan to use their controls, I think with the humidity, dew point, etc, it's just too complicated and I like the concept of their operant temperature sensors.
As far as the kitchen exhaust hood, I may need to consider that. Luckily, it will not be too much trouble to test it or allow to add it if there is an actual problem. All the make-up air duct is in the crawl space and mechanical room, and the register for the make-up air is located in the floor, directly underneath the range. I am using 8" duct for the make-up air, but I can see what you mean about there needing to be a significant pressure difference in order to pull air through the duct without a fan on the make-up side.
I might check out Risinger's video, but I really got turned off by his stuff over a year ago when all the content he started putting out became advertisements - IMO he's sold out.
Why do you think you need motorized dampers in the bathrooms? Keep it simple, you just need the system to be designed to exhaust the bathrooms at 20 cfm continuously with the flow rates adjusted at the exhaust grilles to meet the designed CFM for your standard exhaust rates. Then when you hit the boost button, the ERV will bump up the flow but you aren't worrying about integrating damper control.
This is how we do it in PHIUS (passive house) certified projects and it works great. Here's a excellent presentation from Ryan Abendroth on balanced ventilation in homes: https://www.phius.org/sites/default/files/2022-06/Ryan%20Abendroth%20-%20Ryan%20Abendroth%20Mechanical%20Summit.pdf
If you prefer video format, Andrew Peel (who is an actual mechanical engineer) has a 101 guide to ventilation: https://passivehouseaccelerator.com/articles/the-air-ins-and-outs-of-passive-house-ventilation
I can’t imagine a duct run sized for 20 CFM would perform as expected when the ERV is in boost mode and moving 100 CFM. Even if the ductwork is upsized for the larger of the two rates. Flow rate doesn’t scale proportionally to that extent. Because friction in the ductwork is related to velocity squared. At higher rates you have more uneven distribution, with most of the air trying to escape through the first few supply vents and less making it to the end of the run. But if you open up a damper at the end of the line (assuming that is the room calling for ventilation) then you relieve the friction and encourage more airflow to that room.
I'm not a mechanical engineer, but I have worked with quite a few residential ERV/HRVs on various passive house projects. I haven't seen a system yet with motorized dampers at the exhaust grilles on the projects I've worked on or any of the projects I've seen from others. Possibly this is due to the systems having ECM motors so that they can adjust their speed as needed to compensate for a portion of the friction losses to deliver the desired CFM.
However, from my understanding, the point of the boost mode is to get a bulk of the humidity out of the bathroom to prevent mirror fogging and then the continuous lower rate addresses the rest of the humidity in the space. This is different to the intermittent exhaust you normally have in the bathroom where it needs to run at a high CFM to remove the moisture during it's short run time.
If you want a better answer on this, I'd suggest either calling Zehnder: https://www.zehnderamerica.com/contact/ or getting in contact with a qualified MEP engineer who works in the low-energy home space like Kristof Irwin from Positive Energy: https://positiveenergy.pro
I think you are overthinking it as these low continuous flow rates in a balanced system will address the moisture issues and the boost function is designed to just take that edge off from the initial high moisture loads of showers and baths.
Here are two great episodes from Kristof about ventilation and ERVs: https://positiveenergy.pro/building-science-podcast/2019/7/15/the-v-in-hvac-part-1 and https://positiveenergy.pro/building-science-podcast/2019/7/15/the-v-in-hvac-part-1-t75r4
I agree w/ Matthew's logic. Also, why have an energy penalty with increased fresh air exchanges (temperature and humidity) just to exhaust a single bathroom - as opposed to the whole house?
Dustin, the ERV operates in balanced mode all the time, even when in boost mode, so if the exhaust rate is bumped up from 20 to 100 CFM for boost, the supply rate will increase by 80 CFM as well. There's not a huge energy penalty since boost will be used a small portion of the day. An hour in the morning, hour at night, and an hour for cooking. Not really going to be a huge issue. I've played around with ventilation rates in my WUFI models and that amount of increased ventilation really doesn't have a big penalty. It's only when you over-ventilate most of the time where it affects the energy usage.
+1 Keep it simple. No need for any fancy damper controls for bathrooms.
A somewhat balanced trunk and branch duct setup will stay mostly in a balance even at higher flow rates. Or at least close enough not to matter.
If you have more bathrooms than your ERV can support in boost mode, install regular exhaust fan in the least used bathrooms.
@Akos "A somewhat balanced trunk and branch duct setup will stay mostly in a balance even at higher flow rates."
That's exactly why I think the motorized dampers make sense. If you have a temporary high humidity load in only one zone, why would you not want to "re-balance" the flow to maximize exhaust from the bathroom in use?
My biggest concern to forgoing the automated dampers in the bathrooms and relying solely upon boost is that it will not be sufficient to expediently exhaust the humidity. Especially since we are installing Messana radiant ceiling panels and humidity control is key to preventing condensation.
It looks like a typical bathroom exhaust fan is about 80-110 cfm. Perhaps that is way too much, I lack knowledge in that regard, but I can only imagine that I would need to seriously upsize the ERV to get that flow from just one bathroom in a balanced system, running in boost mode, without a motorized damper to reduce friction/increase flow at the desired, targeted location.
I will check out Kristof's podcasts - I listened to one in the past and that's how I became sold on Messana.
Yes I think this is what the disagreement comes down to. A set-and-forget duct system will work okay when in boost mode, but it will allocate that boosted exhaust across all the connected rooms. By having motorized dampers you can “spot” ventilate just the room you need to, and by doing this you can utilize one single ERV for more rooms than you would otherwise with the set-and-forget method.
Pretty much, if you have enough flow to vent your bath on boost then you are fine.
If you don't, you can do it with supplemental vents or with damper controls.
I also find that I rarely use the boost for my bathroom, the ERV running on low is enough to clear a shower.
I do use boost often when cooking, at which point it doesn't matter how that flow is split between bathrooms. I guess it can be argued that it would be better to divert all airflow to the kitchen area pickup in that case.
Thanks for all of your input, and feel free to add more if you think of it.
I'll guess that based upon the discussion that an ERV will be just fine to exhaust humidity from my bathrooms - that I needn't worry about the water vapor exchange in the core.
I would also suppose that running the ERV in boost mode to exhaust a bathroom will be less of a concern because the efficiency of the core in maintaining indoor humidity is less with higher airflow, correct?
As an aside, after listening to Kristof's podcasts with Chris Smith of Zehnder, I want to spec an ERV which can continuously ventilate to meet my desired number of air exchanges. Do I need to worry about defrosting the core in the winter in CZ 5A - will I still be able to meet my requirements for ventilation if defrost cycles are necessary?
Any suggestions for how to size a central dehumidifier? And assuming I should duct it independently from the ERV (and don't have central heat-A/C), where I should locate the vents?
I think I will be fine going with the Panasonic 200 - I did calculations from the side deck benwolk posted above. Thanks
The dehumidifier is no longer of concern. Messana has spec'd a hydronic air handler, basically a chiller. They say it will provide dehumidification and 2 tons of cooling capacity.
I'm stoked to see this installed.