How to Split ERV CFM Between Rooms
Hi,
I’m choosing an ERV and I think I’m on the right track in terms of total CFM but I’m not finding much detail on how to split it amongst the various rooms in the home.
Would love some help on the room split and also if I should go bigger on the ERV (it’s ~$150 to go from a 130CFM 65% efficient unit to a 160CFM 75% efficient one) Attached the lower floor plan as the kitchen/dining/living is all one space, so might require more CFM on the exhaust side?
Details are:
New construction 2350 sqft home with 2 occupants and planning on installing a Broan AI Series 130 CFM ERV (B130E65RS)
https://www.broan-nutone.com/en-us/product/freshairsystems/b130e65rs
The unit will be set at 110 CFM because that’s the max Broan recommends when using their tandem exhaust.
https://www.broan-nutone.com/en-us/accessory/vtyik1
I’m planning on splitting the CFM for each room in the following manner:
Supply 110 CFM
– Powder 27 sqft, 9′ ceilings = 27.5 CFM
– Upstairs Bath 68 sqft, 8′ ceilings = 27.5 CFM
(I wouldn’t bother with the upstairs bath but Broan’s manual states that at least one supply has to be upstairs on 2 story homes)
– Den: 150 sqft, 9′ ceilings = 14 CFM
Other Details
– Master Bedroom almost never has the door open, and has it’s own return with no jump duct.
– Bedroom 2 is a guest room with jump duct, door open unless occupied.
– Bedroom 1 has a jump duct and I end up sleeping there half the time after being booted from the master for snoring. Door open.
– Den is my office and has the doors closed 1/4 of the time.
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Replies
You've got the supplies and exhausts reversed from what is typical. Bathrooms normally get exhaust to deal with humidity and odors. Even if you have separate bath exhausts, it's still a good idea. Otherwise you might short circuit the two systems, where a significant amount of the fresh air from the ERV gets sucked out by the standalone bath exhaust fans. The kitchen should be an exhaust to try to eliminate some of the cooking particulates before they spread through the house. The biggest exhaust flow should be from the kitchen.
The upgrade is a no brainer. 65% efficiency is awful, and even 160cfm is on the skimpy side when you consider boost functionality. 110cfm is not good. That means not using the tandem duct, which is also a good thing. I really don't buy the "no cross contamination" claim.
Thanks for the reply Trevor.
My fault for switching up the labels, I'm definitely going to be drawing stale air from the bathrooms, kitchen, and utility and providing fresh air to the bedrooms and living room.
Unless you plan on having two people sleeping in the guest room frequently I would reduce that flow a bit and increase all the others a bit. Maybe to 20 or 24 CFM. Maybe even lower and set the system to a higher flow overall when you have guests.
If you have an ECM fan which the unit you link does, a larger unit set to lower flow has three benefits. Lower electric use, higher heat recovery efficiency, and more headroom for boost operation.
Thanks Charlie.
Do you know of any rough guidelines for how much air to send to the living room/den in my install?
Power consumption of ECM equipment is proportional to system pressure, so you drop the pressure, you can significantly reduce power use.
Because of this, with a large unit like that, I would not go with a tandem hood, go for individual intake and exhaust.
Our code calls for 25CFM supply to the master and 10CFM to all rooms (this includes living room). More is always better for air quality but costs energy to heat/cool even with decent ERV efficiency. Usually a good approach is to design for code but size ducts/registers for boost operation. This way you can start out at code min ventilation and increase it if you find CO2 levels are too high.
For stale air pickups, you want about 60CFM out of the kitchen and around 40CFM out of each bathroom and 10CFM out of laundry room. With a large unit like that, you have plenty of capacity, so I would add a stale air pickup upstairs in the hallway as well.
The fresh air supply out of the ERV would be a bit cold in the winter time, make sure to locate them in such a way in that is not directly blowing at people (far from people, up high, blowing along the ceiling is a good spot).
Thanks for the info, Akos.
Regarding the utility room, its a large source of bad smells with the cat litter box in there - would you up the CFM to account for that?
Related question - litter box is in the top corner of the room. I can locate the stale air vent top center of the room and it will be 8 feet from the litter box, but also 2 feet from an HVAC register. If I put it in the opposite corner of the room it will be 15 feet away from the litter box but 8 feet away from the HVAC register.
Is either a better choice?
Thanks!
I've only had to deal with cat litter box once. There it was adding in the smallest (50cfm) bathroom fan I could easily find above the box. Worked great.
If possible, it is definitely worth while to put the stale air pickup directly above the litter box, the distance from the HVAC register is not a problem. some litter boxes can even be directly plumbed to a vent.
As for the amount of flow there, you can always design for more flow there and have an adjustable damper.