ERV extraction rates and toilet stink
Hello GBA community,
So… How many CFM of continuous ERV extraction does it require so that nobody ever knows if anyone else dropped a stinky, no matter how potent or quick the turnover? (think: Guests at a party)
I’m in the final stages of planning and installing a (fairly) elaborate HVAC system on a residential new construction project and I’m still debating ERV extraction rates for our bathrooms/toilet rooms.
I’m planning on 30 CFM in primary bathroom shower + 20 CFM in his toilet closet + 20 CFM in her toilet closet and 20 CFM in guest shower + 20 CFM in guest toilet area and a similar 20 CFM in Gym bathroom + 20 CFM in Gym toilet closet.
I’m considering running a third tube to each of these areas and using a three tube plenum just in case.
My thinking here is that I suspect that the house will actually be WAY over ventilated and I may want to drop the ERV’s back to 160 or 120 CFM each but I will still need to get the stink out. If I slow the units down, the proportional CFM per tube will drop. Yes, I know they have modulating dampers but slow is slow and it may require additional CFM that I can only allow for once without tearing the house apart.
I’ve invested far too much at this point to experience any of what I would consider “quality of life” issues.
Does anyone conclusively know, from first hand experience what it REALLY takes to perfectly evacuate steam from a 20 GPM car-wash equivalent shower and handle toxic #2’s with utter discretion?
Sincere thanks, in advance.
Please forgive my first world problems and know that I came into this with no knowledge of HVAC or Building Science but that I’ve become very interested and involved in it. Also please forgive the TLTR details below.
Here are my particulars: Climate zone 8, Two occupants plus two large breed dogs, ICF construction w/ R50 hybrid spray foam on the roof deck but Lots of glass, 8300 Sq Ft (4000 sq ft main house, 2000 sq ft garage, plus 2300 sq ft of room-in-attic “bonus” rooms – read: game, storage, library), 78k cubic foot volume, 6 ton WaterFurnace Synergy 3 unit (hydronic heated concrete with forced air cooling), Arzel zoning system with 4 zones plus 4 smart zones (divided by use, heat load/exposure and square footage), Santa Fe Ultra 205 dehumidifier, Aprilaire 801 humidifier, two Zehnder Q600 ERV’s that are set up as hybrid system using comfotube and the Zehnder extraction methodology but ducting the supply into the HVAC return.
I have the Zehnders planned to run continuously at 200 CFM and 190 CFM = 0.30 ACH but that’s totally open for discussion.
Ashrae 62.2-2016 = main house 6187 Sq. Ft w/ 4 occupants 216 CFM + Attic 2729 Sq. Ft. w/ 2 occupants 97 CFM = 8916 Sq Ft @ 312 CFM w/ 6 occupants
78k cubic feet = 429 CFM .33 ACH hygenic standard
HVAC engineer recommended 240 CFM, Zehnder recommended 350 CFM and I bumped it up to 390 for some extra toilet exhaust.
PS there is a little square footage discrepancy as the plans evolved but not enough to push the needle much.
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Replies
A 15min air change is a good number to shoot for. So if the throne room is about 50sqft, about 25CFM will get you that.
Always go for the larger unit so you have more boost capacity. They are also more efficient when running at the lower flow rate the building needs. Be careful with such large structure with few occupants. You can easily over-ventilate the place and dry it out in the winter time. I would aim for the lower side towards code min and adjust based on CO2 levels if needed.
No need to run at high flow rate all the time. Add occupancy sensors to each bath and only run the ERV on boost when needed.
Thanks Akos!
I'm totally with you on the humidification concern. This is why I wanted a hybrid system with the ERV sharing HVAC ductwork with both a dehumidifier and a humidifier and letting the ECM air handler do the work of distribution with the full house zoning system enabling the air handler to put exactly as much air exactly where I want it. It took a lot of work to figure out how to only have one set of ducts and one air handler to do this huge space while keeping TSP at 0.68" (basically using it all like a VAV system)
I will go ahead and run a third 10CFM tube to all these places. Its easy enough to simply disconnect it from the manifold/ silencer if I dont want or need it. Its also easy to oversize the silencer manifold by a few ports. This will allow me to lower the total CFM of the unit when running while simultaneously maintaining a higher level of ventilation at the bath extraction points.
Your suggestion of occupancy sensor is also a great idea as opposed to relying only on using a boost switch. Guests... or myself :) can't always be trusted to turn on the fan.
Elegantiron,
The effectiveness of the system is governed not only by the extration rate, but also the location of the vents. To remove moisture it's best placed above the shower/bath enclosure, or just outside it. The optimum location for removing odors is directly behind or to one side of the toilet about 4 ft off the floor.
Malcom_Taylor,
Thank you for weighing in on this. I have long questioned the most efficient placement of the toilet exhaust vents.
Your advice makes sense to me. Instinctively, I want to place them close to where the small comes from. I posted this very question some 8 months ago to reddit and was told to stick with the ceiling... but my ceilings are 10'6" tall. Lots of cubic volume there to evacuate before the odors rise. I'm glad your post will be searchable to others with this question.
One more thing... what type of efficiency do you think one would lose by putting the extract grille a bit lower than 4'... say 15-1/2" on center from the floor so that the toilet itself blocks it from view. check out my attached section view/elevation of the guest bathroom and shower. 3 tube Zehnder plenum shown in purple in the shower ceiling and low toilet wall.
Thanks Again
Elegentiron,
I’m only going on experience with this - and I've never located one that low - but suspect anywhere that draws the air out before it has a chance to mix with the larger volume above the toilet would work well.
The IMC (International Mechanical Code) recommends 50cfm per restroom for intermittent exhaust airflow from restrooms and 25cfm for constant airflow. This follows along with ASHRAE 62.1 & 62.2. It sounds like your exhaust will be continuous which in that case I would use 25cfm per restroom.
https://codes.iccsafe.org/content/IMC2024V1.0/chapter-4-ventilation#IMC2024V1.0_Ch04_Sec403.1
https://www.ashrae.org/technical-resources/bookstore/standards-62-1-62-2
Edit: It should be noted that these are minimum exhaust rates.