ERV recommendation for replacement
Hi,
I have a new all electric two story house with slab on grade built to the passive house standard but not certified. The pre-drywall blower door test came back with 0.47 ACH@50 with the wood stove, stove outdoor air kit and other mechanical equipment installed.
I currently have the VanEE G2400E ERV installed but after months of troubleshooting a negative pressure when the unit is in recirculation/defrosting mode, we found out it was the damper assembly. The HVAC contractor rebalanced the system again for good measure and replaced the damper assembly under warranty but the unit still exhaust a guesstimate 20 CFM outdoor on those two modes. We spoke with VanEE and they indicated that their damper assembly is not airtight and it was “normal” for the unit to leak around 20CFM therefore refuse to replace the unit for a difference model or buy back the system.
With the negative pressure caused by the ERV unit in recirculation mode, it would pull air from the chimney, making starting fire very difficult due to the downdraft. I would need to either turn off the ERV system or put the unit in continuous mode where there was no issue (balanced system).
Since I live in Ottawa, Canada, the ERV will go into defrost mode below -10*C (14*F) which just recirculated the air over the core, that would cause the negative pressure and we’re concern that it could pull gas from the chimney when starting the fire or it’s dying down. That is why we’re are looking to replace it.
Sorry for the long background story but I know some would ask detail.
QUESTION:
What are some good ERV mark and model for very air tight homes other than the expensive Zehnder? The TVCC (Total Ventilation Capacity Condition) based on the HVAC design is 130 CFM.
Thank you,
Arnold
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Replies
Have you considered opening a window for a few minutes while starting your fire? If your wood stove has an outdoor air intake to supply combustion air, you shouldn't need to worry about it once you close the door.
I have no issues starting a fire if the ERV is either turned off or on continuous mode and the wood stove has a outdoor air supply directly tired.
It would be likely a lot more cost effective to just add a motorized damper (that seals better) to the exhaust duct and a relay to mirror the ERV's recirc damper. That said, 20CFM is a pretty low number to pull any kind of negative pressure unless you have other exhaust only fans that could also be in play.
A downdraft during start, particularly with a longer (cold) chimney in winter can be a challenge (with respect to some back puffing) , even for a balanced house. I just put my setup into a 110 CFM positive pressure mode when loading the air tight fireplace. The fireplace also has an outside air kit.
The 20 CFM is a guesstimation and it could be more but I don't have the proper tool to determine that. I've been told from an engineer that 20 CFM is enough to cause a negative pressure with an airtightness of 0.47 ACH pre-drywall with no exhaust for bathroom fan, drier, water heater or hoodfan.
I have no issues starting a fire in winter if the ERV is either turned off or on continuous mode. When it's very cold outside, I may need to use my heat gun for a couple of minutes to push the cold plug out.
Arnold
I appreciate the response but I've spent the last three months troubleshooting this issue with a few individual and professional in the construction industry and they've concluded the issue is the ERV damper doesn't seal properly on recirculation/defrost mode which is causing the negative pressure. This wouldn't be an issue for older homes or code minimum.
I'm now just looking for ERV recommendation for very airtight home like a Passive House and I am currently in discussion with the manufacture to find a solution or buy the system back.
Thank you,
Arnold
Instead of replacing the ERV, one option would be to install a pre-heater to keep fresh air feed above -10C. I've run the math on this before and the cost of this is not all that much. You can even put it on a timer so it only runs at the times you are likely to run the stove.
You can avoid defrost mode by getting a RenewAire. The RenewAire power efficiency is not good at top CFM but if you get a large model like the EV Premium L and run it at a lower CFM most of the time the power efficiency is really good and you have a lot of boost available. You have to buy an external motorized damper for the intake, but it has built-in controls. That seemed quite odd to me, but now it's looking like a bonus!
The ability to control a RenewAire like most ERVs is rudimentary.
If you want to try to salvage your existing ERV you could add a preheater to the incoming air.
If you don't want to do that and want to risk breaking your ERV you could try to set it to run on a duty cycle, possibly by turning off power to it.
One more though on salvaging- if the pressure from the leak is constant than you could add an exhaust damper that requires more than that amount of pressure to open up. Make sure the minimum settings, etc on the ERV will push that damper open during normal operation.