Using an ERV as the OAK

[oaky]

I am looking into installing a fireplace insert or wood stove into an existing chimney.
In my research, one of the issues that comes up is back drafting. I do have both a laundry exhaust and a kitchen exhaust 20-30 feet away from the fireplace and am concerned that if both were running that could create a backdraft.

I already have an ERV. I could try to run it at high positive pressure. However, it would be hard for it to efficiently provide pressure to that area of the house because it is a whole house ERV and the house has other floors that it is also supplying. We also frequently have strong winds.

This leads one to consider an OAK (outdoor air kit). I have read this article about the downsides of an OAK. This lead me to brainstorm for solutions to the downsides.

I had the idea of providing a direct line of air from the ERV to the firebox (this would split off from the existing ERV fresh air flow). It is possible with an ERV to prevent most backflow to the ERV when the ERV is not running (I run my ERV 24/7, at a low setting, this scenario would be an ERV repair) with a backdraft damper. It is also possible to get an accurate CFM into the firebox by flow restricting this line from the ERV to the appropriate CFM with a special flow restriction damper.

This approach would help with all 3 of the disadvantages I see from an OAK (energy loss, flow reversal, de-pressurization and back drafting). On the backdrafting issue, I am not entirely certain what the result would be. The ERV is providing increased pressure that can push air into the room as well as out the chimney. I think the result would be significantly better because more air would go through the chimney and even if more air did come into the room it would be more diluted.

There's probably an issue with pushing more air into the room when putting in wood. This could be solved by having a powered damper switch near the firebox that turns off the ERV flow to the firebox. This same powered damper would also be used to prevent any possibility of backflow when the ERV is being repaired or otherwise off and to disable flow when there is no fire.

What do you think of this concept?

This approach can be modified to not output the ERV line directly into the firebox, but instead to output it into the home very close to the firebox. This is somewhat similar to cracking a window but is more energy efficient. This would remove the possibility of backflow directly into the line and to the ERV. It would also remove the concern of the ERV pressure causing more air to come into the room both during wood loading and during negative pressure (backdraft). The pressure truly available to the firebox would be reduced, but that reduction would be combating any negative room pressure. Additionally, if the fireplace ERV line is running when there is no fire, that just puts more fresh air in the house at that location instead of wasting that air through the chimney. That makes it possible to not use a special flow restriction adapter as well although an off switch to the flow would still be useful for efficiency when there is no fire, particularly at night when we sleep on the level above. The downside of course is that the possibility of backdrafting is increased, particularly if the ERV is not operating, or is operating on a low setting. Normally when it is windy, I run the ERV on low. However, if a fire is going I might want to keep it running on a higher setting to combat wind pressure increasing the odds of backdraft.

 

[begreen]

Describe the setup in detail. Will this install be on an outside wall? What floor of the house will this be on, is there a basement or crawlspace below? How tall will the chimney be?

This article and discussion may be helpful.

Sweep's Library - Why burn outside air?

Should I burn outside combustion air in my stove or fireplace?

hearth.com

 

[oaky]

It would be installed to an existing open air fireplace on the main level. I will likely go with an insert, but a wood stove vented through the fireplace would be the other option. The open air fireplace has a chimney that goes straight up out through the roof- up through the floor above and the conditioned attic above that.
There is a basement below with an unfinished ceiling through which I would run the ERV line.

 

[begreen]

It's possible the insert would work fine without an oak or ERV modification, especially if the ERV is self-balancing. If there is an ash dump in the fireplace with basement cleanout access, then the could be a path for ducting in outside air.

 

[oaky]

The ERV is a broan AI self balancing. However, it is installed to a central blower by taking air from the return (farther away) and then putting air back into the return. So its self-balancing is working on the pressure in the return which may not be that responsive to pressure swings in the house.

There is an ash dump, but it's closer to the basement floor rather than the basement ceiling which is where the ERV intake is or a new air intake would be.

 

[begreen]

I would not mess with the ERV unless necessary. A hole could be punched in the ash dump higher up and then ducted to outside for a fresh air supply? Some inserts allow changing the intake to accommodate such a setup.

 

[stoveliker]

I would under no circumstance pipe an ERV directly into the air inlet of a stove.
You are risking loosing control over the stove
And it's still a simple pressure that could, in principle be overcome by reverse draft, leading to exhaust pushing out of the air inlet into the unsuitable ducting (upon which it's nicely spread throughout the home). It's a matter of absolute pressure values and how are you certain that your setting will prevent back draft under all conditions? And that it won't push the stove into run away?

I don't think code inspection would approve this anyway.

 

[EbS-P]

As long as the ERV condition side outlet is not behind a closed door I don’t see any reason for connecting it in any way to stove.

My thinking if the house a a slight positive pressure the. You are fine. Get an ERV that allows fine tuning. I don’t know anything about self balancing. The details matter in this case.

I would rather have a configurable ERV that allows me to set different in and out flow manually if I was going to be operating a stove.

Last and possibly most important question. Did you have a blower door test? Do you have any rough estimate of your natural ventilation rate?

 

[oaky]

The Broan AI series allows for setting different CFM for intake versus exhaust. So it can be set to positively pressurize the house. But in a multi-story house that might mean 30+CFM of imbalance to achieve 10 CFM in just the floor of the fireplace. Meanwhile there is a bathroom with an exhaust fan, a kitchen exhaust, and a vented dryer all on the fireplace floor.

With the ERV running the air is always circulating through the central blower- this will help counteract some of the pressure issues- if there is negative pressure on the first floor, more air will come out to the first floor registers and more will be taken in on the second floor returns.

I don't have a blower door test but I am planning on getting one and could before I get a wood stove.

 

[oaky]

I agree with the comments that it isn't worth the risk of hooking the ERV up directly. It's more of a brainstorming exercise.

But I do think that putting in a fresh air ERV4 register close to the firebox could be a viable approach that is risk free.

 

[EbS-P]

This really a challenge to air seal existing construction below 2.5 AHC50. And once you get that tight with decent insulation, dehumidification is often needed in the summer. At that point a ventilating dehumidifier should be explored. A single ERV just doesn’t provide enough dehumidification of the Necessary fresh air.

Do some research beforehand it could save lots of headaches or even mold!

My priorities in order of importance . Insulate, Air seal, humidity control, fresh air ventilation to control CO2 and other indoor pollutants, good wood burning appliances chimney/flue routing and lastly OAK if needed.

This all begins with a blower door test or at least FLIR imaging. But if you are adding/ have a high cfm kitchen exhaust hood you probably need a power fresh air vent/ ERV switched to come on with the hood.

Don’t be scared about venting with unheated or cooled air as long as it is dry. That is just the cost. More here…https://www.hearth.com/talk/threads...d-indoor-air-quality-hearth-com-style.198427/

I have spent way to much time learning about all of this and my conclusion is that you need to look really hard to find a an HVAC professional that really is proficient when it comes to high performance system design. They can rule of thumb it for an average house that they have previous experience with but once you add decent air sealing, insulation, big exhaust fans, ERVs, possible whole house dehumidifier and a wood stove you are not average!

Way too long

 

[oaky]

This really a challenge to air seal existing construction below 2.5 AHC50. And once you get that tight with decent insulation, dehumidification is often needed in the summer. At that point a ventilating dehumidifier should be explored. A single ERV just doesn’t provide enough dehumidification of the Necessary fresh air.

Your are in the carolinas with a lot more humidity. I haven't had humidity issues before or after the ERV. My issue is having enough humidity in the winter- I am finding the ERV is helping a lot with that.

I have spent way to much time learning about all of this and my conclusion is that you need to look really hard to find a an HVAC professional that really is proficient when it comes to high performance system design. They can rule of thumb it for an average house that they have previous experience with but once you add decent air sealing, insulation, big exhaust fans, ERVs, possible whole house dehumidifier and a wood stove you are not average!

I don't think this HVAC pro exists. It's very rare anyone asks them to install an ERV and wood stoves are rare here and I doubt HVAC companies are consulted when people install gas inserts or make other chimney changes. There is one other home in my neighborhood that has both a wood fireplace insert and an ERV. I didn't choose to go with that HVAC installer because a different one worked with me to come up with a better ERV install.