Combustion air Installation.

[snaple4]

Hello everyone! This is going to be a work in progress over the next few days. Some background: I work in the HVAC field and do almost all the design work for the company I work for (mainly residential and light commercial). I purchased an underground house a bit over a year ago. Had some very unsafe things going on. Fast forward to today I have ripped out old stand alone stove, rock wall that was built around it, the old pipe and the 4” single wall vent for the 80% furnace that ran next to it. I now have a summers heat 2k, class A pipe inside clay, double wall inside. Furnace is now 90+% vented out the side of the house (you can see the pvc in a pic). House itself has almost NO leakage verified by blower door. So that leads me to install a fresh air for the wood stove and house.

This first image shows the 6” to 3” reducer, the rigid flexible metal 6” pipe, and the aluminum waste gate. Thought the gate would be a nice safety feature for peace of mind. The wall pipe goes through is concrete board with metal studs (otherside is open to HVAC closet.)



this second picture shows the 7” pipe (transition inside the HVAC closet) as it exits the HVAC closet and runs along the wall towards the exterior wall.



This 3rd picture you can see the exterior wall. I have to turn left Over the door and down out of the blue area. A central attic fan was installed here. I am now going to use that space to get both fresh air pipes and my kitchen exhaust pipe to the outside. Reason I have to exit here is because the rest of the house is either a load bearing beam or solid concrete. Yes, my ceiling is also concrete with 2’ of dirt on top.


I am using either mastic tape or mud to seal all joints (even on the elbows). I decided to run 7” to reduce the chance of excessive pressure loss in the pipe. I’m using r6 insulation so I shouldn’t end up with condensation inside the pipe. Let me know what you think. I have to wait till tomorrow to finish so I built a fire as is. So far it doesn’t seem to make a difference with the pipe hooked up.

 

[jetsam]

Do not couple the intake air to the stove unless the stove provides an airgap between the duct and the stove.

The reason you always read that the outside air must be lower than the firebox at all points is that if the draft reverses, your air intake becomes your flue, and it is not well suited to that task. I am sure you as an HVAC engineer can imagine the ramifications of using flexible metal dryer duct for flue gasses that can go north of 900 degrees.

The only remedy that comes to mind at the moment is to put a warm air trap outside, and run the intake behind (but not touching) the stove. Shouldn't be a major hassle for you and it will be safe.

I would not burn that stove connected directly to the intake with the reducer as pictured, personally. Would it probably be okay? Yeah, probably.

 

[snaple4]

Thank you jetsam for the feedback. Let’s have some friendly back and forth. I know that directly connecting combustion air originating above the stove is considered bad by many. This house was built without any exhaust fans except for the dryer. I am in the process of installing a vented range hood and two bathroom exhaust fans (good in-line type to expel moisture). I am going to wire my whole house fresh air to energize when these devices activate but there is no guarantee that the exhaust will not over power the forced fresh air for the house(especially if the fresh air fails to engage). This could possibly cause a reversal of the wood stove pipe.

You are correct that the metal flex pipe will not hold 900 degree temps. However, the only time I could imagine a reversal of draft is when the fire is dying out and winds are excessive. Even then with a damper in-line I don’t see how it would be possible. I might be wrong on this but haven’t seen the evidence to prove otherwise. As for a warm-air trap. I find they don’t work. Air isn’t like water and will flow regardless. Most warm air-traps just aid in the mixing of the air so you don’t feel the cold air.

 

[bholler]

Thank you jetsam for the feedback. Let’s have some friendly back and forth. I know that directly connecting combustion air originating above the stove is considered bad by many. This house was built without any exhaust fans except for the dryer. I am in the process of installing a vented range hood and two bathroom exhaust fans (good in-line type to expel moisture). I am going to wire my whole house fresh air to energize when these devices activate but there is no guarantee that the exhaust will not over power the forced fresh air for the house(especially if the fresh air fails to engage). This could possibly cause a reversal of the wood stove pipe.

You are correct that the metal flex pipe will not hold 900 degree temps. However, the only time I could imagine a reversal of draft is when the fire is dying out and winds are excessive. Even then with a damper in-line I don’t see how it would be possible. I might be wrong on this but haven’t seen the evidence to prove otherwise. As for a warm-air trap. I find they don’t work. Air isn’t like water and will flow regardless. Most warm air-traps just aid in the mixing of the air so you don’t feel the cold air.

He is right you shouldn't do it. It is against code because it has happened enough times to cause them to add it to code. No one ever did it thinking it would cause a fire

 

[snaple4]

He is right you shouldn't do it. It is against code because it has happened enough times to cause them to add it to code. No one ever did it thinking it would cause a fire

Do you happen to have the section? I don’t recall that being in our MC or Fire and gas books. Maybe I missed it or it is local?

 

[bholler]

Do you happen to have the section? I don’t recall that being in our MC or Fire and gas books. Maybe I missed it or it is local?

I don't have it off hand no but it says you can't do it in just about every stove manual I have read which over rides code anyway.

 

[jetsam]

And even though draft reversal may be rare:
1) It is rare but potentially catastrophic
2) Your uniquely low air infiltration, and the unusual height of the intake, puts you at relatively high risk.
3) Your uniquely low air infiltration makes the potential consequences of a melted flue in the living room area quite unpleasant.

Imagine what happens if an exhaust fan goes on when it shouldn't and you're asleep. Imagine a plastic bag blowing over the house's regular replacement air intake, or some wag putting a brick on top of it for laughs.


I'm not saying you're definitely going to have draft reversal, but I sure as heck wouldn't chance it.

I don't know how much replacement air volume you have available in the first place, either, but you should be well equipped to figure this one out. Say it's cold out, you have a hot stove, and you open the stove door. Obviously even if there was no fire , you would have a high volume of air going up the flue. With a fire, the speed increases a lot. Given the length and ribbed interior of the intake, can the outside air intake plus the regular house air intake(s) even match that volume? Just opening the door could create an unstable situation with oscillating draft (and please do the math on this question, because it is a concern whether or not you couple the oak to the stove).

 

[Ludlow]

I saw photos of an OAK reversal once. Dont remember where at this point. Even when they are installed at level they can backpuff in the right circumstances. If attached and installed above the firebox, they can turn into a drafting chimney. Just dont do it.

 

[snaple4]

Update:

Talking with a few manufacturers and some fellow HVAC techs I have come to the conclusion that my design will work. I won’t list the reasoning for this because I found a way to run the pipe along the floor(though I will get yelled at by my wife when done). The decision to change was made b/c Woodstock and BK specifically say not to have pipe above stove and I plan on using one of the two stoves to upgrade in the future. The current pipe can still be used for my fresh air for the house so I’m not really out much other than a bit of time and maybe an elbow or two. I’ll update with pictures as I rap things up in the next few days.

 

[bholler]

Update:

Talking with a few manufacturers and some fellow HVAC techs I have come to the conclusion that my design will work. I won’t list the reasoning for this because I found a way to run the pipe along the floor(though I will get yelled at by my wife when done). The decision to change was made b/c Woodstock and BK specifically say not to have pipe above stove and I plan on using one of the two stoves to upgrade in the future. The current pipe can still be used for my fresh air for the house so I’m not really out much other than a bit of time and maybe an elbow or two. I’ll update with pictures as I rap things up in the next few days.

Good decision

 

[jetsam]

I have to doubt that the lowest point of the air intake is below the firebox if the whole house is underground? But lower is better.

Just leave a gap between the stove and the air intake, and the intake becoming a flue won't be an issue.

You should still make sure that the house has sufficient replacement air... if for whatever reason it can't keep up, draft reversal is still a real CO hazard even without the OAK. I don't know how an underground house gets its air, but I wouldn't want to be in a situation where a single fan or circuit board failing (or a single pipe getting a bird nest in it over the summer) could cause something like that.

Probably the highest volume airflow situation is when the stove is full of coals and the door is open, and you are running other stuff that pushes air out (exhaust fans, dryer, etc). The OAK is definitely too small to keep the house pressurized by itself under those circumstances.

 

[snaple4]

I have to doubt that the lowest point of the air intake is below the firebox if the whole house is underground? But lower is better.

Just leave a gap between the stove and the air intake, and the intake becoming a flue won't be an issue.

You should still make sure that the house has sufficient replacement air... if for whatever reason it can't keep up, draft reversal is still a real CO hazard even without the OAK. I don't know how an underground house gets its air, but I wouldn't want to be in a situation where a single fan or circuit board failing (or a single pipe getting a bird nest in it over the summer) could cause something like that.

Probably the highest volume airflow situation is when the stove is full of coals and the door is open, and you are running other stuff that pushes air out (exhaust fans, dryer, etc). The OAK is definitely too small to keep the house pressurized by itself under those circumstances.

The OAK will be attached directly to the stove per manufacturer directions of my current stove and for any future stove. This is because I am worried about backdrafting once I hook up the bath exhausts and range exhaust. I do understand why you say not to attach but I don’t have any worries. The pipe doesn’t come into contact with combustible material within the distance required and I will have a backdraft damper.

Even though the house is underground you still treat the house as a single story structure. I found a point under a window I can exit the envelope for the OAK. Originally we thought that section was concrete as well. Since it is framed I can run the entire length of pipe on the ground. I just have to worry about my wife complaining about the box that will be built around it.

Currently the fresh air for the house is mainly from some poorly sealed areas on the front of the house and the cracked windows. Once I am done with my piping I will have forced fresh air from my whole house dehumidifier; it is capable of dehumidifying and/or forced fresh air. The 90% furnace gets its combustion air from a pvc pipe that runs to the furnace.

 

[Highbeam]

I'm thinking that the whole house is not underground since it has windows and you can see the car parked outside. Think of it as a daylight basement with dirt above instead of a main floor. It is possible to have a totally legit outside air hookup to this stove in this home. Lots of people put stoves in daylight basements.

One important lame fact about your current stove. It has several air intake holes that are not routed to that outside air connection. In fact, when burning, only the very reduced primary intake hole is being fed by your duct. When you put a BK product in you will be getting 100% of the combustion air from outside as you should.

I don't know why you chose a 6" pipe though in this application. 4" would have been plenty and far more aesthetically pleasing.

 

[snaple4]

One important lame fact about your current stove. It has several air intake holes that are not routed to that outside air connection. In fact, when burning, only the very reduced primary intake hole is being fed by your duct. When you put a BK product in you will be getting 100% of the combustion air from outside as you should.

I don't know why you chose a 6" pipe though in this application. 4" would have been plenty and far more aesthetically pleasing.

To keep static pressure in the pipe low and because I have 6” leftovers from jobs. We don’t use anything smaller than 6 usually. I may in the future go from 6-4 or 3 out of the gate but for now it will work.

This stove sure isn’t the quality of a BK.Just out of curiosity, were are the other intakes? Since the base isn’t completely sealed I figured it would still pull some air from the house.

 

[Highbeam]

To keep static pressure in the pipe low and because I have 6” leftovers from jobs. We don’t use anything smaller than 6 usually. I may in the future go from 6-4 or 3 out of the gate but for now it will work.

This stove sure isn’t the quality of a BK.Just out of curiosity, were are the other intakes? Since the base isn’t completely sealed I figured it would still pull some air from the house.

There are some slight variations but most of the summers heat/englander/timber ridge designs have a hole right above the nipple that you clamped the OAK pipe to. That's your secondary air supply, full throttle all the time. Then, up front, under the front corners but inside the pedestal (if you have a pedestal) will be two holes that feed the doghouse air hole, full throttle all the time. Your primary air control only controls the air wash which is where the OAK pipe connects. With a mirror through the OAK hole you can see the slider.

 

[jetsam]

The OAK will be attached directly to the stove per manufacturer directions of my current stove and for any future stove. This is because I am worried about backdrafting once I hook up the bath exhausts and range exhaust. I do understand why you say not to attach but I don’t have any worries. The pipe doesn’t come into contact with combustible material within the distance required and I will have a backdraft damper.

Even though the house is underground you still treat the house as a single story structure. I found a point under a window I can exit the envelope for the OAK. Originally we thought that section was concrete as well. Since it is framed I can run the entire length of pipe on the ground. I just have to worry about my wife complaining about the box that will be built around it.

Currently the fresh air for the house is mainly from some poorly sealed areas on the front of the house and the cracked windows. Once I am done with my piping I will have forced fresh air from my whole house dehumidifier; it is capable of dehumidifying and/or forced fresh air. The 90% furnace gets its combustion air from a pvc pipe that runs to the furnace.

I am sure you know this since you are an hvac guy, but maybe this will help someone else later... ERV and HRV units are just the thing for the ultra-tight space I was (apparently incorrectly) assuming it to be. (I certainly hadn't imagined windows in an underground house.)

 

[snaple4]

I am sure you know this since you are an hvac guy, but maybe this will help someone else later... ERV and HRV units are just the thing for the ultra-tight space I was (apparently incorrectly) assuming it to be. (I certainly hadn't imagined windows in an underground house.)

ERV and HRV units certainly have there place. If I was building from scratch I would heavily consider one. For my house it made a lot more sense to go with a ducted de-humidifier. Even if I used one of those systems I would still need a dehumidifier.

I finished running and sealing up the duct pipe. Once the mastic mud is dry I will finish insulation. I will have to buy a 6” cover so I have it blocked off for now. I won’t burn tonight but maybe tomorrow if it doesn’t get to warm outside.

Also, ignore my wire mess inside the furnace closet. On my todo list to clean up once I get all the pipes ran.

 

[snaple4]

Started a fire last night. Noticed little difference with burn but wasn't using the dryer. This morning the first thing I noticed as I was reloading the fire, NO smoke spill. Usually as I am adding wood and it catches fire some smoke spills out. I tested a bit after things got going but still a bit smoky and opened the door. Still no smoke coming out. I also noticed last night that I did not feel the draft from the window. Wasn't that cold so tonight and tomorrow will be the test on if it helps with that part.

 

[spudman99]

I assume you have adequate coverage with CO2 detectors, you seem pretty well tuned into this thing. Cheap insurance in case something does go wrong and you are not there or asleep.

 

[snaple4]

I assume you have adequate coverage with CO2 detectors, you seem pretty well tuned into this thing. Cheap insurance in case something does go wrong and you are not there or asleep.

Yes sir. Wired smoke/CO throughout plus a CO/propane detector in the LR plugged in at floor level.