Case Study: The terrifying hot backdraft
A perfect storm of bad venting system design
The cold backdraft, which is mentioned in several places in our chimney section, is the result of an outside chimney being unable to overcome stack effect in the house when no fire burns. The cold backdraft produces a creosote smell near the hearth, and can mean a room full of smoke when a new fire is lit. It is annoying, but not hazardous.
The hot backdraft happens when a low fire is burning and the flue gases in an outside chimney are cooled to the point where draft collapses and smoke begins to seep from the stove. Once smoke begins to leak from a stove in that situation, a full hot backdraft can follow quickly. This is a case study documenting the experience of a visitor to woodheat.org who emailed us for some advice.
After reading some of the articles on the site, Karen wrote to woodheat.org and this was how she described her experience:
My fiancé and I recently bought a house that had a 90's era Vermont Castings Resolute installed in the basement. The chimney is insulated double-walled pipe and runs up the outside of the house. Clearly, because of the poor chimney setup we were having draft issues and filling the house with smoke every time we started a fire.
One night, we went to bed and awoke about two hours later to the house filled with smoke and the carbon monoxide detector going crazy (thank goodness we have one). Clearly, some backdrafting happened. Earlier that evening we’d had a runaway fire, so the only thing I did differently than any other night I let the fire go out was to shut the air intake, but I didn't think that mattered since the stove was far from airtight anyway.
Regardless, I decided after that awful night we were getting a new wood stove. We've now set ourselves up with a Lopi 1750 and it's wonderful. We got a nice little device called a Draw Collar, too, that preheats the chimney air so there's no smoking on start-up. Trouble is, I'm afraid to let the fire go out. We haven't had to yet because I work from home, but the time is coming.
We have gone to bed and let the Resolute go out plenty of times and never had an issue. Why did it happen that one night? And how can I make sure it doesn't happen again (short of re-installing my chimney, because we can't afford that)? I'm terrified!
The Lopi 1750 is a good quality mid-priced non-catalytic stove. In an attempt to solve their problems in lighting a fire without smoke, they installed a Draw Collar, which is a flue pipe-mounted electrical heating device intended to get heat into the flue before lighting.
John Gulland responded to her questions:
Sorry to hear about your lousy chimney and the effect it is having on your peace of mind. I don't want to increase your fears, but I think you are right to be concerned.
People often think that dealing with a cold backdraft is as simple as getting the chimney flow to go up rather than down so they can light a fire. Some do that by opening a door or window on the same level of the house to neutralize the negative pressure due to stack effect so a fire can be lit without spillage. Others use hair dryers or butane torches to inject heat into the flue to create some draft. The designers of the Draw Collar figured they could make some money by helping people to overcome the annoying cold backdraft. But none of these tactics solve the root problem, they just temporarily mask it. In extreme cases like yours it is possible that during an overnight burn when the stove is turned down, draft can collapse and stack effect can pull smoke out of the stove. Once this kind of spillage starts, it can quickly become a full hot backdraft. It is most likely to happen in cold weather and when a cold wind cools the outside chimney to the point where it produces so little draft that it can't overcome the negative pressure due to stack effect.
I have worked with clients who have experienced terrifying hot backdrafts in the middle of the night, so I know the risk exists. Cold weather compounds all the problems with outside chimneys. Ultimately, the only solution that would give you the peace of mind you deserve, and the knowledge that a hot backdraft will not happen again, is to put in a chimney that runs up through the house. (You knew that was coming, right?) You might also consider moving your new stove to the main floor where it will be more effective in heating your main living area.
A day or so later we heard from Karen again:
John - thanks so much for your quick response. Of course, last night it got down to -27°F (we do live in northern Wisconsin), and we had the same issue. The Lopi at least did not fill the house with smoke, but the CO was so bad we had the fire department out. We've decided to bring the chimney inside.
We have a chalet-style house, and the wood stove is currently in the basement against one of the end walls, with the chimney exiting just above. We want to just move the stove to the main floor, same general location though - and bring the chimney inside. We'd put the stove in our living room which has vaulted ceilings. I'm thinking we just run the pipe straight up and put it through the roof on one side of the peak, OR have it exiting the gable wall JUST underneath the peak. Any advice? Also, we have double wall insulated pipe for our outside chimney - can we just use the same pipe and bring it inside? It's in good condition.
I'm so pleased that you'll make the change. I know you won't regret it. I think it would be a mistake to have the chimney exit the gable wall. Here's why: 1/ That would introduce two 90 degree changes of flue gas direction, which causes turbulence and flow resistance into the system, making it more likely you would experience smoke roll-out when you open the loading door to add fuel. 2/ A venting system like that would need cleaning more often and would be difficult to service. With a straight system, a good stove like yours, good fuel and good burning technique, chimney deposits are likely to be minimal and non-combustible. You will need to check it annually, but I expect you won't find much build up. To check, all you need to do is disconnect the flue pipe from the stove flue collar, push up the telescopic length about a foot and look up with a mirror and flashlight.
You can probably use parts of the chimney, as long it is a current model you can still buy parts for. You'll need to buy some additional components, like a cathedral ceiling support and roof flashing, plus a bunch of flue pipe but that's about it. You'll need to get the make and model of the chimney and get exactly the right components for it. If you go online or to a dealer, you should be able to find a parts catalogue for the chimney so you can specify exactly what you need.
Summary of the situation
Here is a detailed review of how these hot backdrafts might have happened.
- The house is a chalet style, meaning it has a vaulted ceiling, creating a very tall heated space, almost as tall as the chimney. The height from the basement floor to the top of the main floor vaulted ceiling would be at least 25 feet. Using the stack height and temperature difference chart here, we can determine that at an outside temperature of -30°F, which is a 100 degree difference from the normal 70°F room temperature, the stack effect pressure in the basement would be as much as -16 pascals. That is a lot of negative pressure, considering that good chimney draft is only about 25 pascals.
- While the exhaust at the flue collar exit might be about 300°F when the stove is turned down for an overnight burn, it is the average temperature between the stove and the top of the chimney that determines the draft experienced at the stove. When a stove is turned down the flow rate through the system slows, which gives more time for heat loss to the environment. As the night proceeds and the fire recedes to the coal bed phase, the chimney flow rate and temperature fall further.
- The very long run of chimney up the outside of the house functions as a heat exchanger, releasing heat from the flue gases. A cold wind against the chimney could cause further cooling. And for most brands of metal chimney, the joins between sections are not airtight at all, meaning that very cold air can be drawn into the flue. Masonry chimneys are also somewhat porous.
- The original stove, the VC Resolute, is a sidedraft combustion design, meaning that when the bypass damper is closed, the exhaust must flow down to the coalbed level exit before passing through a heat exchanger at the back of the stove and exiting the flue collar. It also has a top loading capability when the top griddle is lifted. Although the griddle is gasketed, it is held closed only by gravity, so leakage can still occur under low draft conditions. Once leakage begins, less exhaust heat reaches the chimney, further reducing the flow rate and temperature, leading to a full hot backdraft.
- Theoretically, the new stove should have been much more resistant to spillage because it is a welded steel conventional updraft which does not force flue gases to go down. The main leakage sites are probably not in the stove at all but at the flue collar and flue pipe joints. When the hot backdraft happened with the new stove, it did not fill the house with smoke, but set off the CO detector, suggesting that a full hot backdraft had not occurred, or that the leakage sites were small enough to resist significant exhaust flow into the house.
- If we assume a temperature of 300°F at the flue collar, and for the moment assume that there was no heat loss at all from the chimney, the draft would have been about 36 pascales. We don’t know how much heat loss there would have been from the chimney but we can assume it would be a lot considering the -27°F outside temperature that night and low flue gas flow rate. If the flue gases lost half their heat before exiting the chimney top, the result would be chimney draft very close to the negative pressure in the house due to stack effect.
- The hot backdraft is not a static event; in fact nothing about a wood fire is static. There is a kind of feedback loop in the interplay between chimney draft and temperature. As the flue gas temperature falls, so does chimney draft and flow rate. That means less combustion air is fed to the fire, which results in a lower firing rate. The lower firing rate further reduces the temperature and gas flow rate, which in turn reduces draft and firing rate. And so on until chimney draft is no longer able to expel the exhaust to the outdoors.
Hot backdrafts are not very common, but they can happen and when they do, they terrify the house occupants and shake their confidence in the safety of their wood heating system. Being aware of the hot backdraft and the conditions that cause it leads us to some useful conclusions, including:
Wood stoves that are connected to outside chimneys are almost always prone to cold backdrafting at standby when no fire burns. This makes the room smell bad and makes fires hard to start without smoke coming into the room, but the cold backdraft is not hazardous. Systems that can cold backdraft might suffer a terrifying hot backdraft under severe conditions, that could include a tall heated envelope created by a vaulted or cathedral ceiling, a tall chimney offering plenty of surface area for cooling, and very cold outdoor temperatures and high winds.
The potential for cold backdrafts which can be inconvenient and annoying, and hot backdrafts which can be terrifying and dangerous is why we at woodheat.org so forcefully recommend that all chimneys should be installed inside the building envelope. The risk of hot backdrafts is also why we don't feel good about telling people that correcting a cold backdraft is as easy as opening a window, or heating the flue with an external source.
We at woodheat.org have also argued against the recent decision by the US EPA to require all wood stoves to be tested for efficiency and for the efficiency figure to appear on certification labels. Now that EPA certified stoves have very high combustion efficiency (in order to keep emissions low), the only way stove manufacturers can boost a stove's efficiency is by increasing heat transfer efficiency which produces lower flue gas temperatures. The new EPA rule on efficiency ratings will surely set off a competition among manufacturers to post ever higher efficiency ratings. We fear the result will be more hot backdrafting, and the potential for terrifying more people who heat with wood and even worse, exposing them to carbon monoxide poisoning. We are aware that arguing against efficiency ratings is seen as a form of heresy, but we think the proponents of ratings have gone into the decision without any knowledge of the potential risks.