Condensation on Wood Stove / Outside Air Piping

[electrathon]

Stove not in use, a different story. I would have a non-conductive pipe with a simple shut off valve in it. Close it when not in use. The chimney also has the same issue, you are pumping heated air out it too. The more things are closed the less thermal loss you will have when the stove is not in use. With the outside air disconnected you are pumping warm air out and up much of the time. If your draft starts very easy it is likely because it is already moving air.

 

[cjmoran]

Stove not in use, a different story. I would have a non-conductive pipe with a simple shut off valve in it. Close it when not in use. The chimney also has the same issue, you are pumping heated air out it too. The more things are closed the less thermal loss you will have when the stove is not in use. With the outside air disconnected you are pumping warm air out and up much of the time. If your draft starts very easy it is likely because it is already moving air.

I agree. A non-conductive pipe with an internal damper or some sort or shutoff valve would be ideal to keep cold air from flowing through when not in use and thermally isolate the stove from the outside air kit, as long as it meets code. I don't know if I can hook a non-metal tube to the back of the stove. There's no heat there, but I don't know if a hot ash inside could somehow make it's way into the tube out the back of the stove.

I don't know if they make anything like that (pre-made flexible tube, non-conductive, with a damper). I'm sure something like that could be constructed with some know-how.

 

[cd36]

My problem with the outside air kit is the condensation in and on the fireplace itself. The more the fireplace runs the more it condenses. The outside walls of the zero clearance fireplace doesn't get hot enough to melt it off. An insulated duct doesn't help the problem.

It sounds like you're having a bit of the same problem, and I'm not sure how to solve it yet. Even if I insulate the outside of the fireplace the inside will still condense. Really the outside air needs to be preheated a bit before going in the fireplace. Or the ducts inside the fireplace need to be insulated themself.

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[cjmoran]

My problem with the outside air kit is the condensation in and on the fireplace itself. The more the fireplace runs the more it condenses. The outside walls of the zero clearance fireplace doesn't get hot enough to melt it off. An insulated duct doesn't help the problem.

It sounds like you're having a bit of the same problem, and I'm not sure how to solve it yet. Even if I insulate the outside of the fireplace the inside will still condense. Really the outside air needs to be preheated a bit before going in the fireplace. Or the ducts inside the fireplace need to be insulated themself.

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So your getting condensation in the inside of the firebox right where the outside air comes in? I must not be understanding correctly as if it was inside the firebox where the fire is I would think it would get evaporated. Do you have a picture?

 

[cd36]

No its in the outside air duct that goes to my damper. The ice that builds up when it's off does mostly evaporate once a fire is started but the outside wall of the zero clearance fireplace stays icy even with a roaring fire. Here's a picture of the outside.

 

[kennyp2339]

To the OP? Would reducing the diameter of the size pipe be better also, that to me looks like either 4 or 5" pipe, you prob could get away with 3" were it enters the living space and connects to the stove.
I can imagine oak's in very cold (minus zero f) environments will do this, in my area I don't have this problem and I prefer an oak because it reduces drafts from my already leaky house.

 

[webby3650]

OK, form over function. It looks better without the ductwork to the stove, I get that. The pipe is cold though because the outside air is cold. It enters the house on the way to the stove. The pipe absorbs some heat out of the room as the air is traveling through it. Here is the rub though. The cold radiating off the pipe is peanuts compared to the cold air being drawn into your house through air leaks all over without an OAK. Only real difference is that the pipe is cold near to the hot stove, the air leaks in all over the house, like in distant bedrooms that are already cooler. I doubt anyone would be able to pin down this issue, but the common issue brought up is that people have trouble getting heat to the far side of the house. No outside air contributes big to this problem. If you have a single room, overheat all rooms, etc then it really is mute, but if your far rooms are cool it really needs to be looked at hard.

I'm very familiar with how it works..
For the most part, the people we deal with don't even want the chimney visible from the neighbors yard. A big insulated pipe laying behind the stove is definitely not happening. For a serous wood burner it's a good option, for an occasional fire though, I wouldn't suggest messing with it.

 

[snavematt]

I just recently had an Explorer 2 installed, I asked about an OAK, and they didn't recommend it, what you are going through is exactly what they told me they thought would happen with my set up. My stove works just great without it. As far as I know an outside air kit is only required when installed in a mobile home (at least here) and as per The manual


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[rwhite]

I'm very familiar with how it works..
For the most part, the people we deal with don't even want the chimney visible from the neighbors yard. A big insulated pipe laying behind the stove is definitely not happening. For a serous wood burner it's a good option, for an occasional fire though, I wouldn't suggest messing with it.

Now I'm seriously wondering if I need one! I went around the back bedrooms and sealed all the sills. That alone raised my temps 3 degrees in those rooms. I have all new sweeps on the doors as well. I burn 24/7 and never gave a thought about incoming air as the stove burns just fine. Does an OAK actually have to go outside? My stove backs up against the garage wall. No flammables or chemicals stored there. Other than momma's car I'm not allowed entry into that space very often

 

[webby3650]

Now I'm seriously wondering if I need one! I went around the back bedrooms and sealed all the sills. That alone raised my temps 3 degrees in those rooms. I have all new sweeps on the doors as well. I burn 24/7 and never gave a thought about incoming air as the stove burns just fine. Does an OAK actually have to go outside? My stove backs up against the garage wall. No flammables or chemicals stored there. Other than momma's car I'm not allowed entry into that space very often

Yes, it needs to go outside, or a ventilated crawlspace. A garage isn't a good idea.

 

[cjmoran]

I just recently had an Explorer 2 installed, I asked about an OAK, and they didn't recommend it, what you are going through is exactly what they told me they thought would happen with my set up. My stove works just great without it. As far as I know an outside air kit is only required when installed in a mobile home (at least here) and as per The manual


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Great. I read the manual yesterday as well and it did seem to just require for mobile home installation. I'll probably leave the OAK permanently disconnected. It doesn't seem to be making any difference having it connected vs. unconnected. It's nice to know that the problem I'm having has been identified before and your installer didn't think it was required. I'm finding out the same thing.

I hope you like your Explorer 2. We really like ours. I'm relieved now that I know why it was rusting. The rust came off easily with a brass bristle brush and it shouldn't happen anymore.

 

[cjmoran]

To the OP? Would reducing the diameter of the size pipe be better also, that to me looks like either 4 or 5" pipe, you prob could get away with 3" were it enters the living space and connects to the stove.
I can imagine oak's in very cold (minus zero f) environments will do this, in my area I don't have this problem and I prefer an oak because it reduces drafts from my already leaky house.

I connected the OAK for awhile and then disconnected it. I thought I felt more of a draft when it was disconnected, but I did it multiple times and found it was just my imagination. It's possible I'm loosing more heat out of my house with the OAK disconnected, but it's hard for me to quantify how much. If it's around 10% than I could live with that as this is not my primary heat source and it's only used periodically.

 

[cjmoran]

No its in the outside air duct that goes to my damper. The ice that builds up when it's off does mostly evaporate once a fire is started but the outside wall of the zero clearance fireplace stays icy even with a roaring fire. Here's a picture of the outside.
View attachment 192233

Hmm. You said this air duct goes to your damper. With the damper closed, does the frost go away? And is this a metal air duct? Could you replace it with something non-conductive so you isolate the outside temperature from the temperature in your chase behind the fireplace? Maybe add some additional insulation as well behind the box?

 

[Karl Hungus]

The Outside Air Myth exposed.
A good an informational read.

your house is 10 years old
did you have a blower door test done?
does your home have a ERV or HRV?
Do you have exhaust fans in bathrooms, kitchen and from a dryer that exhaust to the outside?
Most of these exhaust ducts are no where close to being sealed when not running, they are letting air into your house if needed.
Unless the builder was into very energy efficient homes I doubt your 10 year old home is very air tight.
If it has neither a ERV or a HRV then I doubt its on the very air tight side of things or one would of been installed.

There are conditions that a OAK is beneficial and or needed. but until your house is very air tight you won't need one.


http://www.woodheat.org/the-outdoor-air-myth-exposed.html

 

[electrathon]

The Outside Air Myth exposed.
A good an informational read.

your house is 10 years old
did you have a blower door test done?
does your home have a ERV or HRV?
Do you have exhaust fans in bathrooms, kitchen and from a dryer that exhaust to the outside?
Most of these exhaust ducts are no where close to being sealed when not running, they are letting air into your house if needed.
Unless the builder was into very energy efficient homes I doubt your 10 year old home is very air tight.
If it has neither a ERV or a HRV then I doubt its on the very air tight side of things or one would of been installed.

There are conditions that a OAK is beneficial and or needed. but until your house is very air tight you won't need one.


http://www.woodheat.org/the-outdoor-air-myth-exposed.html

This is taken from the article:
It is sometimes assumed that taking air from outside through a duct saves energy because the stove doesn't use up indoor air and cause outdoor air to be drawn in through leaks to replace it. But the assumed heating deficit is so small as to be insignificant. The average air consumption of a modern wood heater is in the range of 10 - 25 cfm, which is very small compared to the natural leakage rate of houses. Building scientists say that the air in a house must be exchanged at least every three hours, or one-third of an airchange per hour, to control moisture from cooking and washing and to manage odors. One third of an air change in a 1500 square foot house is 4000 cubic feet, or 66 cfm. Note that this is the absolute minimum air change for healthy living and that most houses older than 20 years have natural leakage rates far higher than this in winter. So the air consumption of a wood stove is a tiny part of a much larger exchange of air between the house and outdoors.

The main disadvantage of taking air from inside the house is that the pressure environment can be adversely affected by powered exhausts. However, depressurization caused by powered exhaust flows is predictable and manageable, unlike the more random and unpredictable effects of wind on outdoor air supplies. The worst-case indoor air pressure environment can be measured using the house pressure test procedure, and can be controlled either by limiting exhaust flows or by installing a powered make-up air system.

In general, therefore, wood stoves and fireplaces that are vented by natural chimney draft should draw the air for combustion from the room in which they are located. Where necessary the indoor air pressure should be controlled to minimize depressurization.

The guy actually is making the point that you need outside air. He says 25 CFM is only a tiny increase from the existing 66CFM. That is more than a 30% increase! One third of the air infiltration is related to the stove. That is not a tiny amount, it is a massive percentage of air exchange. He then goes on the finalize the article by saying you may need to add make up air in his last sentence. I believe he is referring to a heat exchanger to bring in warmed outside air while venting the house. They are great and if you are not supplying outside air to your stove the need for one goes up exponentially, but if you have outside air the need for one goes down.

 

[lsucet]

The Outside Air Myth exposed.
A good an informational read.

your house is 10 years old
did you have a blower door test done?
does your home have a ERV or HRV?
Do you have exhaust fans in bathrooms, kitchen and from a dryer that exhaust to the outside?
Most of these exhaust ducts are no where close to being sealed when not running, they are letting air into your house if needed.
Unless the builder was into very energy efficient homes I doubt your 10 year old home is very air tight.
If it has neither a ERV or a HRV then I doubt its on the very air tight side of things or one would of been installed.

There are conditions that a OAK is beneficial and or needed. but until your house is very air tight you won't need one.


http://www.woodheat.org/the-outdoor-air-myth-exposed.html

I don't know what to say about that link. Everything depend on what we want to believe or not. I don't think his/her statement is accurate neither.
For me, outside air is always good, but of course in winter time we don't want that cold air penetrating inside our houses for many reason that we all know and it is not necessary to mention at this time.

combustion needs fresh air. period. Cars don't get the air from inside the cab. if is a good thing to do, manufactures will be doing it and saving on air filters, additionals components plus customers will have less maintenance expenses. Regardless if is an engine or a stove and the amount of air draft, temperature of the air, etc, outside air is always better for combustion.
Now saying so, i understand the debate about it. I think the manufacturers of these stoves, fireplaces and codes, etc are in fault here.
Most stoves with outside air kits except pellet stoves let a good volume of that cold air escape to the inside of the house cause the intake is not air tight to the primary air at the stove. It is just a partially supply air to the stove. most of tube stove gets its secondary air from the inside of the house and i think when primary air is closed, larger volume of that cold air is entering the house. At that point we all can be right and we all can be wrong.

i do not know about other cat stoves but one thing that i like from my Blaze King compare to the other stoves that i have is that the intake is seal to that OAK and it has just single air intake system. I stop feeling that cold air infiltration that i used to feel with the OAK connected to the other tube stoves including when not in use. we all need to do whatever needs to be done but facts are facts. I know is hard for some cause of stove location to have an OAK installed and we all do what we have to do but if you can i will recommend it. But remember most stoves the intake are not air tight sealed to the OAK regardless how it looks and air infiltration to the house will be there all the time. it is up to you.

 

[OhioBurner©]

I'm struggle to understand if insulating the OAK duct will really fix the problem. If the cold air still makes it's way through the pipe, through the stove, and back out the chimney, I would think it would still be cold even if it's insulated. The cold will still conduct to the back of the stove, and I'll still get condensation. It's temperature will just be more isolated from whatever is on the other side of the insulation. Just because it's wrapped in insulation doesn't mean it will not get very cold. Is that right? Maybe there's something I'm missing and I don't understand the thermal dynamics. Note that the chase that the outside air kit is in is behind a metal panel that sits behind the stove. I don't know at want temperature the chase will regulate at. It's not in my home. It's in the chase are under where the chimney liner goes up, under my old pre-fab firebox that the stove now sits in front of.

No, I don't believe insulated the OAK will have much effect on your condensation issue. It could even make the stove slightly colder and exacerbate the issue, but likely not much change. I have an OAK hooked up to one of my stoves that I hardly ran until half way through December. The stove was always significantly colder than the room. Best thing would be to plug the intake so no outside air enters the OAK when you're not running the stove but then you have to remember to unplug it. I prefer not to have to fiddle with things so I just left mine alone and dealt with the fact it was acting as a cold sink (kind of ironic since it's suppose to cut down on heat loss, but all the big proponents of OAK seem to focus just on situations where it's advantageous). Over the summer I had a lot of internal rusting inside the stove too. I think once the burn season is done this year I'll go ahead and plug my intake with a rag or something and maybe disconnect it on the inside too so the stove can breath a little. Probably should plug up the chimney too but I probably won't go that far, I don't condition air in the summer so not much concern about drafts and such then.

 

[electrathon]

No, I don't believe insulated the OAK will have much effect on your condensation issue. It could even make the stove slightly colder and exacerbate the issue, but likely not much change. I have an OAK hooked up to one of my stoves that I hardly ran until half way through December. The stove was always significantly colder than the room. Best thing would be to plug the intake so no outside air enters the OAK when you're not running the stove but then you have to remember to unplug it. I prefer not to have to fiddle with things so I just left mine alone and dealt with the fact it was acting as a cold sink (kind of ironic since it's suppose to cut down on heat loss, but all the big proponents of OAK seem to focus just on situations where it's advantageous). Over the summer I had a lot of internal rusting inside the stove too. I think once the burn season is done this year I'll go ahead and plug my intake with a rag or something and maybe disconnect it on the inside too so the stove can breath a little. Probably should plug up the chimney too but I probably won't go that far, I don't condition air in the summer so not much concern about drafts and such then.

Totally agree with you. If the stove is not in use it is important to close both the air inlet and the chimney. People feel the cold air at the inlet and are not happy, but the chimney is a bigger heat loss than the inlet, you just do not feel it most of the time. If you have a stove that the draft is super easy to start it is usually because the draft is already sucking heat out.

 

[rwhite]

The Outside Air Myth exposed.
A good an informational read.

your house is 10 years old
did you have a blower door test done?
does your home have a ERV or HRV?
Do you have exhaust fans in bathrooms, kitchen and from a dryer that exhaust to the outside?
Most of these exhaust ducts are no where close to being sealed when not running, they are letting air into your house if needed.
Unless the builder was into very energy efficient homes I doubt your 10 year old home is very air tight.
If it has neither a ERV or a HRV then I doubt its on the very air tight side of things or one would of been installed.

There are conditions that a OAK is beneficial and or needed. but until your house is very air tight you won't need one.


http://www.woodheat.org/the-outdoor-air-myth-exposed.html

I don't think there's any doubt that most homes can operate without an OAK. It's a matter of the path the air has to travel to get there. Either cold air directly into the stove or pulled through the vents, cracks etc and then to the stove. Either setup can obviously have it's issues you take the lesser of the evils.

 

[Karl Hungus]

This is taken from the article:

Building scientists say that the air in a house must be exchanged at least every three hours, or one-third of an airchange per hour, to control moisture from cooking and washing and to manage odors. One third of an air change in a 1500 square foot house is 4000 cubic feet, or 66 cfm. Note that this is the absolute minimum air change for healthy living and that most houses older than 20 years have natural leakage rates far higher than this in winter. So the air consumption of a wood stove is a tiny part of a much larger exchange of air between the house and outdoors.

The main disadvantage of taking air from inside the house is that the pressure environment can be adversely affected by powered exhausts. However, depressurization caused by powered exhaust flows is predictable and manageable, unlike the more random and unpredictable effects of wind on outdoor air supplies. The worst-case indoor air pressure environment can be measured using the house pressure test procedure, and can be controlled either by limiting exhaust flows or by installing a powered make-up air system.

In general, therefore, wood stoves and fireplaces that are vented by natural chimney draft should draw the air for combustion from the room in which they are located. Where necessary the indoor air pressure should be controlled to minimize depressurization.

The guy actually is making the point that you need outside air. He says 25 CFM is only a tiny increase from the existing 66CFM. That is more than a 30% increase! One third of the air infiltration is related to the stove. That is not a tiny amount, it is a massive percentage of air exchange. He then goes on the finalize the article by saying you may need to add make up air in his last sentence. I believe he is referring to a heat exchanger to bring in warmed outside air while venting the house. They are great and if you are not supplying outside air to your stove the need for one goes up exponentially, but if you have outside air the need for one goes down.

What your missing and assuming is that he says a MIN. for health is 66cfm. THAT'S A MIN. for health reasons.
Hes not saying an average house has 66 cfm leakage. Only a SUPER TIGHT very small house will ever see a leakage rate that small.

Did you miss his next sentence?
"most houses older than 20 years have natural leakage rates far higher than this in winter"

I had my late '70's tri level tested in 2010. It had a leakage rate of 1775cfm! 1775 cfm equates to 4.58ACH50 in my house!

Note from my test.

"Given the conditioned floor area, house volume and external surface area of the home, this is considered average in regards to air infiltration."

So my house with 1775 cfm leakage is average for its size and age.

here is some other notes from my test.

"newly built energy star certified homes in WI average around 1.50 to 2.50 ACH50."

If my home was at the average of a newly built energy star certified home built 7 years ago assuming 2 ACH50 it would equate to 775 cfm!

If I had 66cfm leakage that would equate to .17 ACH50.

The tightest most efficient house standards, Passive House, requires .6ACH50. More than 3 times the 66 cfm.

Forget 66 cfm, its not the real world for air leakage, 660 cfm is a closer average of the houses out there.


Another gem.

"old farm houses can be as leaky as 10ACH50 or more."

I had no significant air leakage at my wood stove.

In my average leaky 40 year old house the 25 cfm for the wood stove is 3% of the air leakage. Insignificant.

I had the test done to improve the energy efficiency of my home. I followed alot of the recommendations.
If your serious about the energy efficiency of your home start with a good energy assessment and follow the recommendations.

You'll know what your leakage is and how insignificant 25 cfm for a wood stove is.

 

[cd36]

I'm not sure how much leakage my house has, I've never had it tested. It's a new build though in Manitoba which has pretty strict guidelines. All walls/ceiling are vapor barriered with acoustical or spray foamed as needed. Rim joist in crawlspace is completely spray foamed.

When I turn on the stove fan I can notice a difference when opening doors in the depressurization caused by the fan. I have an hrv as it is required by code to have one due to the tightness of the houses. I haven't tried running my wood stove without the OAK but am considering it based on my condensation issues, just not sure how it'll work in my house.

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[DickRussell]

Readers on this thread need to bear in mind that the standard blower door test depressurizes a house to 50 pascals (that's about 0.007 psi) below ambient pressure. That pressure difference, while seemingly small, translates to 1.044 lb/sq.ft and would create a net 25 lb force over a 4x6 picture window, or 125 lb over a wall 8 ft high by 15 ft long. It's easily low enough for the structure to withstand without imploding, but high enough so that the air leakage required to maintain that pressure difference is dominated by the tightness of the structure, with little effect from wind pressure or air density difference caused by very cold outside air temperature. It's a standard measure of the tightness of a house, but it isn't a direct measurement of just what air leakage there will be without that deliberate depressurization.

"Natural" air leakage results from pressure differences caused by wind pressure and air density differences due to temperature. Air at zero F is about 15% heavier than air at 70 F. So a house will see most leakage when it's bitter cold and windy outside, and it won't leak much at all when it's in the 60s and calm. Then leakage will be dominated by exhausting devices, such as dryer, range hood, and woodstove or fireplace. Otherwise, "natural" leakage, under worst outside conditions, can be very crudely approximated by dividing the leakage rate at -50 pascals by a number between 15 and 20. That number varies with the configuration of the house and its orientation within its surroundings.

So a house test at, say, 1800 CFM at -50 pascals might leak 100 CFM without depressurization, worst case. Running the dryer, range hood, or woodstove will produce depressurization, and the house will leak accordingly. If the house volume is, say, 2000 sqft times 7.5 ft ceiling height, or 15,000 cuft, then the ACH50 becomes 7.2, which isn't particularly tight, but perhaps better than a lot of older houses.

Bottom line, the air need of a woodstove likely is less than the "natural" leakage when it's cold enough outside to warrant use of the stove, but not necessarily a "tiny" fraction of that leakage. Most houses, even recent builds, likely leak enough to support operation of a woodstove without OAK. However, a really tight house may not be leaky enough to prevent backdrafting when starting up that stove, with or without an OAK. I know this happens if the dryer or range hood is running when I light the stove, and I have an OAK. Once the fire is burning well and the draft is established, then I can turn on the dryer or range hood, on low to medium. Without the OAK, I strongly suspect I'd backdraft.

 

[Coyoterun]

The ACH50 is a very useful way to compare "leakiness" among different houses in different climates. But it's not a real world number.

I do have some real world numbers, surely there are enough energy nerds on this forum to play around with them.

During a recent cold spell (Jan. 4-Jan 7) my high efficiency propane furnace was burning about 4 gallons of propane per day. The average outside temp was below 10* F. The thermostat was set at 68*F, but the majority of the heated space is only about 65* That space has about 16k cubic feet of air, plus there are another 8K cubic feet in the basement that is heated to about 55*F.

We use about 1.5 KWH of electricity, but the time you figure that the dryer vents to the outside, hot water runs outside directly, etc, maybe only 1/2 of that stays in the house as waste heat?

.75 KwH is about 2600 BTU/hr, 4 gallons propane/day through a 95% efficient furnace is about 14,400 BTU/hr. So a total of 17K btu/hr heating the house.

I found an equation here that seemed pertinent. (broken link removed)

BTU Output = Temperature Rise (∆ T) x CFM x 1.08


Solved for CFM = BTU/ ( ∆T x 1.08)

Let's imagine that 100% of the heating energy is going to condition outside air, that inside conditioned air is leaving as fast as the furnace can heat new air. I know that's not the case, but doesn't that give a theoretical max rate of outside infiltration?

Is 50 a reasonable number for ∆T?

In that case:

CFM = 17,000 / (50 * 1.08) = 315

In my house, that's only about 0.8 ACH actual.

I know that's a wild approximation considering what went into it. But does it have any usefulness at all in this discussion? I would think it means that the actual outside airflow through my 100 year old farmhouse envelope during a breezy 10*F day is less than 300 CFM. What do you think that it means?

 

[cd36]

I skimmed through your post as I'm watching the hockey game right now, but if I understand correctly you assumed that all heat you put into the house was due to infiltration your air changes per hour will be inflated as you are ignoring the large amount of heat lost due to conduction through wall/ceiling/windows/etc.

So your actual infiltration rate will actually be a fair bit lower than what you calculated.

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[lsucet]

sorry but i have my brain fried by that formula.lol