ERV dedicated ductwork best practices
Bostonmusicdave
| Posted in General Questions on
Wondering if anyone has details on dedicated Ducting design for erv. I’ve seen the small zehnder tubes but very expensive. My hvac contractor hasn’t done dedicated ducts so looking for guidance in designing this correctly
thanks
dave
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Installing a ducted ERV no different than a forced air system, just much lower flow rates. For a typical 4 bed house, you are looking around 100CFM.
You generally run a main feed trunk from the unit and branch off it to feed each room. You need a fresh air supply to all rooms (bedrooms, living, basement if finished). The main trunk is around 6", feeds to the rooms can be 3" or 4". You want a balancing damper where each feed comes off the trunk, you use this to set the flow (code here requires 25CFM for the master, 10 CFM to every other room).
There is a separate trunk for the stale air, these come from the bathrooms, laundry room and kitchen. If you have more than two bathrooms, it is better to only connect the ERV to the two most used ones and install a dedicated exhaust fan for the rest.
There are some good guidelines here:
https://www.bchousing.org/publications/Heat-Recovery-Ventilation-Guide-Houses.pdf
This article (https://www.jlconline.com/how-to/hvac/state-of-the-art-hvac_o) in JLC, written by Kristof Irwin of Positive Energy LLC in Austin, specs metal ducting, and in Chicago Zehnder has to use metal ducting because of the sheetmetal union. Doable, but you'll have to be crafty with terminating the ducts at the adjustable registers so the system can be commissioned. If you're running with a Zehnder they should be able to recommend installers and they have an installation manual.
There are also articles here discussing the topic:
https://www.greenbuildingadvisor.com/article/ducting-hrvs-ervs
https://www.greenbuildingadvisor.com/question/erv-dedicated-duct-system
Positive Energy specializes in the design of this type of work.
Given the well proven negative health effects of high CO2 levels, I'd design for better than code, with a minimum of 15 CFM/person supplied to closed door rooms.
https://www.krueger-hvac.com/files/white%20papers/article_demand_control_ventilation.pdf
Dedicated exhaust fans in bathrooms will do a better job (because they move more CFM and sometimes because they don't recirculate odors).