Shared flue and Carbon Monoxide concerns

[dt45]

Hi,
I don't know if this is the right place to ask this question, but I don't know where else to ask.

I'm currently living with my family in central Europe. The house has two floors, which are heated only by two wood stoves.
On the ground floor, there's some older wood stove in the kitchen, which is also used to heat the adjacent bedroom and bathroom, simply by leaving the door open.
On the 1st floor, there's a bigger wood stove connected to the hot water boiler and the central heating for the floor.

Now, what makes me uneasy is that both these stoves are connected to the same chimney (with just one flue). From what I've read, the biggest concern with such setups is that carbon monoxide can leak from one applience through another.
How likely is that to happen and under which circumstances?

 

[Starbrightsteve]

One of my first thoughts was how big is the flue? Does it have the capacity to handle both fires? I would think that as long as the outdoor temperature is higher than the indoor temps the flue gas would go up the chimney if the flue is big enough. How do you make hot water during the times of the year that you don’t need heat? Lastly, has this house had any history of errr, bad things happening?

 

[dt45]

I don't know the exact dimensions of the flue, but it seems big enough to handle both flames, especially since the bottom stove is pretty small and weak.
AFAIK there's no history of bad things happening, but until very recently, no one was living on the ground floor. The reason this even came to my attention was that I was looking to replace that old, inefficient bottom stove with something more efficient and almost every manufacturer explicitly states that the appliance shouldn't (or it's not recommended to) be used on the shared flue.

How do you make hot water during the times of the year that you don’t need heat?

In the summer, all radiators are closed off and that big stove is only used for cooking (for a few hours - which usually makes enough hot water).

 

[jetsam]

Is the flue big enough that you can fit 2 liners in it? I wouldn't want to run 2 wood burner on one flue.

Let's say the upstairs one is down to coals and you light a blazing fire in the downstairs one. That is potentially bad...

Worse yet, Let's say your flue gets partially obstructed with creosote, or bird nests, or a fallen tile. That upstairs stove is now a serious hazard when the downstairs one is burning.

 

[faultymechanics]

Now you have my attention. My current home has a wood insert on the main floor and our heating system, soon to be a pellet boiler, will be sharing the same chimney.

I'd assume the concerns would be the same, though I will say our HVAC person seemed to have no issues with both systems using the same chimney.

It also may have two liners inside, I'm not sure.

 

[dt45]

Is the flue big enough that you can fit 2 liners in it? I wouldn't want to run 2 wood burner on one flue.

The chimney is made from Schidel blocks and there's a ceramic liner in the middle. Something like this:
(broken link removed)
(broken link removed)

I don't think it's possible to fit two liners inside.
Even if it were possible, it's probably out of the question. My parents built this and just the fact that I bought CO alarm and suggested that there my be possible safety issues, made them very angry at me.
They have very toxic mentality with no regard to any safety issues. The only reason I'm currently living here is because I'm trying to save some money on the rent.

Let's say the upstairs one is down to coals and you light a blazing fire in the downstairs one. That is potentially bad...

Can you explain this a bit more? I'm sorry if it's a stupid question, I (obviously) have very little knowledge about all this and I'm trying to understand the underlying dynamics so I can have some understanding of the risks involved.
Also, if anyone else have some info (or useful links) about this topic, I'd be very grateful.

 

[jetsam]

The chimney is made from Schidel blocks and there's a ceramic liner in the middle. Something like this:
(broken link removed)
(broken link removed)

I don't think it's possible to fit two liners inside.
Even if it were possible, it's probably out of the question. My parents built this and just the fact that I bought CO alarm and suggested that there my be possible safety issues, made them very angry at me.
They have very toxic mentality with no regard to any safety issues. The only reason I'm currently living here is because I'm trying to save some money on the rent.


Can you explain this a bit more? I'm sorry if it's a stupid question, I (obviously) have very little knowledge about all this and I'm trying to understand the underlying dynamics so I can have some understanding of the risks involved.
Also, if anyone else have some info (or useful links) about this topic, I'd be very grateful.

It's probably something that is unlikely in your exact setup, or you'd have seen problems already. But the theory is the same as the picture in my post. Strong drafting appliance downstairs (big fire), weak drafting appliance upstairs (just coals). Both are generating CO. If the air intake exceeds the flue's ability to take it (either in normal operation, or someone opens the door on the downstairs stove), you could end up with the downstairs stove venting through the upstairs stove, worst case. Less bad but still serious is that the upstairs stove can't vent properly due to flue pressure, and it leaks some of its own CO back into the room.

I don't think problems are actually likely if the stoves have been operating for years with no issues already, but stuff happens. Different operators run the stoves differently, the stoves themselves get changed out, unusual atmospheric conditions occur with the weather, a chunk of liner falls into the flue, etc etc. Any one of those has the potential to turn OK into not OK.

I would definitely have a CO alarm by both stoves, especially the upstairs one. (And you can tell your parents that this is just common sense for any wood stove.)

 

[dt45]

Thank you for your answer.
Just the upstairs stove have been operating for years, no one was living downstairs until very recently.
Also, the house has been made much more airtight recently with new insulation and PVC windows and doors.

The CO alarm is currently downstairs, I bought it just recently. The normal reading is 0, but there are peak readings of ~30ppm. I guess that sometimes happens overnight, when there are only embers left in the stove and no draft (or even negative draft)?
Are those peak readings a reason for concern, considering that bedroom and kitchen downstairs are pretty much the same room (separated only by the thin wall and doors which are always open) and this is the place where I'm currently almost 24/7?

 

[jetsam]

30ppm won't kill you, or even make you very sick, but it's an indication that some combustion byproducts are traveling the wrong way under normal circumstances.
https://www.cpsc.gov/Safety-Educati...-Center/Carbon-Monoxide-Questions-and-Answers

I would honestly worry more about CO near the upstairs stove, but issues are possible at both.

CO issues are possible with any stove when you tighten up house. The stove depressurizes the house a little. The fire burns down to embers, draft weakens, and you can get draft reversal where air is pulled down the flue. This replacement air brings all the CO out of the stove with it. This is more likely the more air is being pushed out of the house by other stoves, exhaust fans, dryers, etc. It is also more likely the tighter the house is (less sources of replacement air). Adding an outside air supply to the stove generally fixes this.

 

[lildex93]

CO meters on each level are a must. 30ppm wont kill you. 35ppm is the standard in the workplace people can be exposed to for 8 hours a day for 5 days a week. Keep in mind these people are also going home for those remaining hours to breathe off those levels. Use this to your own habits. I would say limit your time in any levels as much as possible. Is it possible to reload each stove at the same time?

 

[bholler]

CO is only one of the concerns. The flue needs to be large enough to handle both appliances burning at once. When both are it is relatively safe. But if only one is the flue will be dramatically oversized meaning that one appliance won't draft properly. The other appliance will probably also leak air into the flue further reducing draft.

 

[jetsam]

CO is only one of the concerns. The flue needs to be large enough to handle both appliances burning at once. When both are it is relatively safe. But if only one is the flue will be dramatically oversized meaning that one appliance won't draft properly. The other appliance will probably also leak air into the flue further reducing draft.

Oh yeah, imagine the creosote from burning green wood on an uninsulated clay flue with all that cold dilution air coming into it...

I bet if the year-round stove was downstairs and the occasional use stove was upstairs, they would have had some exciting chimney fires by now.

 

[dt45]

Surprisingly, once the fire is going, the draft is pretty good. There are occasional backpuffings, but that's rare, probably only when atmospheric conditions are bad.

Yes, I'll definitely buy the CO alarm for the upstairs (but knowing my parents, if it goes off, they'll just remove the batteries to stop annoying them).

I was less worried about the upstairs because the house is pretty big (it has a newer extension to the old part, separated by pretty thick wall) and the bedrooms are pretty far from the stove - on the opposite side, in the old part.
Downstairs, bedroom is practically in the same room as stove; the chimney is literally in the bedroom, which also makes me wonder: There are cleaning door on the chimney in that bedroom, if they are not fully airtight, is it possible for some CO escape through them?

 

[dt45]

Is it possible to reload each stove at the same time?

It would be impractical, but now during the winter, both stoves have the fire going on during the day (which probably means that CO can easily escape through the chimney). I'm more worried about what happens during the night when fire dies down to embers, or what happens later when one stove is in use and other one is not.

 

[dt45]

Regarding my previous question about chimney cleaning door - I've searched through older threads and found recommendations for sealing them with silicone, but those were related to the problems with draft. Could that also help in my case (those peak CO readings)?

 

[bholler]

Regarding my previous question about chimney cleaning door - I've searched through older threads and found recommendations for sealing them with silicone, but those were related to the problems with draft. Could that also help in my case (those peak CO readings)?

Possibly. Co spillage and draft are usually related

 

[St. Coemgen]

I also live in Central Europe and I also have have a Schiedel chimney I had installed. And while it was installed many years ago, I do recall the instructions saying one appliance per chimney flue. For all the reasons already stated. Side note: a Victorian house I used to live in had a wood stove downstairs and a fireplace upstairs on a single flue -- and sortly after I moved out the upper story burned down due to a wayward cinder from the lower floor stove. Schidel makes duel flues for multi-appliance installs, and a "Swift" air system one for getting air to the stove in tight houses (if you tightened up your house recently, you may need to find some way to get external air to your stoves).

Regarding sealing the clean out door, you should not need to do this if you installed the Schidel door and access pipe. They come with a fire proof gasket on the fire-stop panel inside the door (the spring on the panel keeps it tight). This gasket is simply fiberglass insulation and you can get a replacement from Schiedel.

However, if someone did this chimney install as a DIY project, all bets may be off. And anything may have been done wrong, out of code, etc. Even my contractor tried to install the flue pieces in upside down till I corrected them.

 

[dt45]

Yeah, it was pretty much a DIY project, so there's probably a lot of things wrong with the setup.

Downstairs stove itself also could be a problem. It's pretty old, inefficient (maybe there are even some cracks somewhere), firebox is very small, so it needs to be constantly reloaded. I want to replace it with something modern&efficient, but I don't know if that won't make things even worse (with CO leaks and possibility of chimney fire).

 

[St. Coemgen]

Downstairs stove itself also could be a problem. It's pretty old, inefficient (maybe there are even some cracks somewhere), firebox is very small, so it needs to be constantly reloaded. I want to replace it with something modern&efficient, but I don't know if that won't make things even worse (with CO leaks and possibility of chimney fire).

Ideally, a properly installed Schiedel chimney is designed to reduce creosote buildup as they are insulated, which keep the pipe warm and minimizes buildup. I sweep once a year and I get maybe 1 to 1.5 liter of soot at the bottom for 6 months of daily burns (on two chimneys). But I do have modern stoves. But of course you can simply sweep your chimney more often if you have concerns.

The CO issues is of course a different issue. As already stated, an outside air source to prevent negative pressure, and a good CO alarm is a must. And as long as you test them frequently to make sure they are working, that should help a lot. And if you have two good reasoanble air tight stoves, that would reduce CO leaking from the stove itself risks (reduce, but not eliminate -- ergo why the CO detector is required).

Regarding replacing the old stove: Quite frankly I would. For one thing, fewer refueling and better efficiency (you burn less wood). But it depends on how much capital you have, and how much a volume you need to heat, and if you are on a budget or not. If on a budget there are a lot of decent "modern" wood stoves in the 6 KW range here that cost less than 300 Euro (if you have a "Tesco" nearby, they often have some -- at least to give you an idea what is available, but those stoves have a rather shorter life span than more expensive ones***). If you have a larger budget, then going to a store that actually specializes in wood stoves is a better option if you want a stove as more a long term investment.

*** My 5 KW Wamsler Visegrad I have had for 10 years, and cost then about 300 Euro, and I am surprised and delighted it still works fine (the narrow tall firebox is a bit difficult to start (there was a learning curve -- top down burn works best), but it can burn really long wood, so can give a long burn if you have long wood). So life span varies. But I did buy that from a wood stove store, and Wamsler is a higher quality German based "mid range" stove manufacturer. My other stove... cost quite a bit more and was a custom import.

 

[dt45]

Thank you for your detailed answers St. Coemgen, they are very helpful.

I think I'll go for a new downstairs stove. I don't care so much for life span, as I don't plan to live here for very long, so I'll go for some cheaper option.
I'll have to ignore the "no shared flues" warning which now has pretty much every manufacturer.

Regarding airtightness - This old downstairs stove is very far from airtight, I guess modern stoves sold in EU these days are much better in that regard, which will probably help with reducing CO risk downstairs.
Upstairs stove is also very far from airtight, but I can't do anything to change that.

Outside air supply seems like an interesting idea, but I don't know if it'd be a good idea to try that on a shared flue (i.e. possibility of draft reversal).

 

[jetsam]

Yes, I'll definitely buy the CO alarm for the upstairs (but knowing my parents, if it goes off, they'll just remove the batteries to stop annoying them).

In the US, a Kidde brand CO alarm with a 10-year sensor and lithium battery is down to about $20 now.

They're convenient, safer (no more dead batteries), and you need a screwdriver to remove the battery, which is not designed to be user-replaceable

Looks like similar products are available in the EU- here's one from amazon.de for €20.

Amazon product ASIN B07885T2S1

 

[St. Coemgen]

Thank you for your detailed answers St. Coemgen, they are very helpful.

Glad I could help, even if in a small way. In the end, you have to work within your givens and limitations and that must be rooted in reality, and be realistic what you can and can not do..

Example of reality: To install an outside air source for one of our two stoves, I had to install that during renovation, which required poking a hole in the 60cm thick stone wall that is our house wall, and running a pipe under the new concrete slab bring poured, to reach the stove location in the center of the floor plan. I can certainly understand.... you are not going to do that.

 

[dt45]

In the US, a Kidde brand CO alarm with a 10-year sensor and lithium battery is down to about $20 now.

They're convenient, safer (no more dead batteries), and you need a screwdriver to remove the battery, which is not designed to be user-replaceable

Looks like similar products are available in the EU- here's one from amazon.de for €20.

Thanks, that's good to know. Kidde brand is also popular in Europe, it's easy to find them here.

 

[dt45]

Glad I could help, even if in a small way. In the end, you have to work within your givens and limitations and that must be rooted in reality, and be realistic what you can and can not do..

Yeah, I wish reality was different (i.e. if they just used the common sense and asked themselves: "OK, connecting multiple stoves on one flue seems convenient, but what could go wrong?", things would be different), but it is what it is. I'll have to see how new stove will behave in this conditions.

I'm still a bit worried about potential long term exposure to smaller doses of CO. I'm thinking about buying real CO meter (which can reliably read out smaller CO concentrations), but they are pretty expensive.

 

[jetsam]

Thank you for your detailed answers St. Coemgen, they are very helpful.

I think I'll go for a new downstairs stove. I don't care so much for life span, as I don't plan to live here for very long, so I'll go for some cheaper option.
I'll have to ignore the "no shared flues" warning which now has pretty much every manufacturer.

Regarding airtightness - This old downstairs stove is very far from airtight, I guess modern stoves sold in EU these days are much better in that regard, which will probably help with reducing CO risk downstairs.
Upstairs stove is also very far from airtight, but I can't do anything to change that.

Outside air supply seems like an interesting idea, but I don't know if it'd be a good idea to try that on a shared flue (i.e. possibility of draft reversal).

Outside air should be safe on the downstairs stove. The air supply needs to be at all points lower than the stove (to prevent the draft reversal you already mentioned), so it wouldn't be possible to install it safely on the upstairs stove.

I think you would be making the whole system safer with outside air on the bottom stove, (@bholler ?) especially if the house is getting depressurized due to the recent weather sealing. The downstairs stove won't contribute to the depressurization, which makes the upstairs stove safer when draft is weak.

Whether or not it is practical to install it is another question! Sometimes it's pretty easy, sometimes you need a big hole in a 60cm thick stone wall....