ERV ducting question
I’m building a house in SC and plan on it being tight enough to need mechanical fresh air. I was planning on using an ERV and would rather not have the Panasonic grille so I was going to put a unit in the cellar. I’m struggling with ducting. In reading Martin’s article where he discusses this very topic in FHB issue 248, I wanted to ask, why could you not tap into your return ducts for exhaust and then tap into the supply trunk line for fresh air delivery. Would this not operate very similar to the fully ducted system described as the most efficient? I would tap in to specific ducts to maximize return and supply delivery locations….Would I need to interconnect the AHU with the ERV so the ERV is cut off if the AHU is operating and install backdraft dampers in the lines?
I feel like I’m missing something obvious…
Thanks Chip
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
When the air handler is running, the supply ducts are at a higher pressure and the return ducts are at a lower pressure. In your proposed setup, the ERV would need to work against this pressure difference. Interconnecting them so that the ERV only runs when the air handler is off might overcome this. However, if you right-size your heating and cooling equipment, will there be enough time with the air handler off for the ERV to accomplish its job?
Some ERV manufacturers support sending the incoming air into return ducts. Some of them require an interlock which forces the furnace fan to run when the ERV runs while other manufacturers do not. Some ERV manufacturers even support using different points along the return duct for both incoming and outgoing with an interlock. There seems to be consensus that these options are inferior to dedicated ventilation ducts but disagreement about how much worse.
How long will the system run is a good question. I'm going variable speed on the forced air system. I'm also doing hydronic radiant which is quite rare down here but something we always wanted. So I guess I this more as a cooling season issue and being variable speed, the system is probably going to run longer than a simple one stage system. I guess part of the answer would lie in how tight the house actually is and how much fresh air is required would it not? That won't be known for a while....
Thank you for your thoughts Reid.
Chip,
Reid is correct.
Trying to integrate an ERV or HRV with ductwork that distributes space heat is extremely difficult, and always involves compromises, higher energy bills, and reduced performance. Your approach -- coming up with a method that violates the recommendations of the ERV or HRV manufacturer -- is likely to get you into trouble fast and result in unsatisfactory performance.
Here's my advice:
1. The best approach is always to provide dedicated ventilation system ductwork rather than to try to share ducts with a heating or cooling system.
2. If your budget is too constrained to do it right, consult the installation instructions of the equipment manufacturer and follow the manufacturer's ducting advice.
3. For more information, see Ducting HRVs and ERVs.
Thanks Martin, I have a friend who works with the National Comfort Institute and he agreed on dedicated ducting. In designing the Mechanicals, I think I have found a place where I can put the ERV and it will be out of they way somewhat and also have fairly short duct runs to get fresh air into the Master Bdrm and Great Room along with a convenient spot to pull the exhaust from. I'll study the PEX layout and look for clear locations to cut in register boots.
Thanks for the help.