Troubleshooting Draft Problems in Chimneys

Transcription

1 Troubleshooting Draft s in Chimneys Draft is the difference in pressure between the inside of the chimney and inside of the house. This is affected mostly (though not completely; see info on fans for example) by the difference in temperature between the inside of the chimney and the air in the house. Most reasons a chimney won t draft fall into seven categories, and there are a good number in the eighth Other as well. Since all draft problems boil down to the same thing: i.e. not enough vacuum outside to drag the exhaust out of the house, they re all related and you could make a different set of classifications if you wanted I m sure. Here s how I break them down. 1. Obstructions and Excessive Drag 2. Sizing s 3. Temperature s 4. Externally Forced Downdraft 5. Dynamic Wind Loading 6. Internal Vacuum 7. No Replacement Air 8. Other

2 Obstructions and Excessive Drag Obstruction is pretty obvious, but some of the other is less so. As the flue gasses move, there s friction against whatever they touch. This friction can be sufficient to create a problem exhausting smoke from the appliance. Birds or squirrels nest in chimneys sometimes. After a chimney fire the puffed creosote can restrict or completely block a chimney. The bases of old chimneys can often be blocked by fallen mortar or masonry. The bases of gas flues are often blocked by the bits of tile that shale off as the moisture from the gas exhaust destroys the tiles over time. Chimney covers can be too low, i.e. close to the top of the flue. In the winter, especially in gas flues, ice can block chimneys. The vapor not only condenses, but also freezes before it exhausts. Misaligned flue tiles creating a obstruction and/or drawing air from outside the flue (but inside the chimney chase) and/or creating turbulence (which is also an obstruction itself.) Smoke chamber is rough and or leaky, causing turbulence and/or loss of draft by introduction of outside air before the flue. Connector is too long and/or has too many bends and/or doesn t flow uphill. Remove the nests. Beware of the dust; don t breathe animal feces because it can kill you. Thorough cleaning, probably with specialty tools and/or chemicals. Chimney fires usually damage tile chimneys too, so careful inspection is in order. Remove the blockage and reline the chimney. Remove the blockage and reline the chimney. Get a cover with more height. The ice is probably invisible and usually will be several feet down the chimney. It can be thick, but most easily busted up by hitting in the center. The real solution is to have the flue relined with an insulated round liner. Reline the chimney Parge the smoke chamber with Chamber Cure. Make sure the pipe pitches uphill at least 2% (¼ per foot) Remove as many els as possible, and replace 90s with 45s, or even flex connectors where you can. Shorten the run if possible

3 Cleanout doors and other air leaks into the chimney (such as the smoke chamber or misaligned tiles mentioned above) competing to introduce air into the flue. Seal them up Reline the chimney

4 Sizing s All chimneys have a certain capacity. Think of it the extremes: Picture a huge fireplace eight feet wide and five feet high. You could have a monster fire in there. Now picture it with a little chimney only four inches round. Common sense tells you that 4 round chimney won t vent a fireplace that big. In real life, the problems come in where it s more marginal, or there are rough surfaces that cause friction for the smoke. There are sizing charts and ratios to help insure that a chimney is sized properly. Firebox is too large for the chimney (or you can say, chimney is too small for the firebox) 1. Change the firebox opening size (make it smaller) Use a Smoke Guard. They will reduce the opening size either 4 or 6 and that often makes enough difference. Raise the firebox floor. A layer of firebrick may do the trick. Put on glass doors. Doesn t solve the problem when the doors are open but probably helps when closed. 2. Change the chimney size (make it bigger) Reline the chimney with a properly sized flue. 3. Put on a top fan; the fans are somewhat expensive, but this can be the best solution.

5 Temperature s What we re talking about here is not having enough temperature in the flue gasses to maintain buoyancy. Hot air rises, right? What if the air isn t hot enough? It s nice to get as much heat into the living space as possible, but chimneys just have to have a certain amount to do their jobs too. You have to allow some heat loss up the chimney. Connector pipe is too long or has too many els: quite common. Excess air entering the flue and cooling the gasses too much. Chimney is cold: quite common, especially with outside masonry chimneys. Smoke heat lost in the smoke chamber See solutions for obstructions Use double wall stove pipe to keep gasses warmer. Check cleanout doors Rebalance the barometric draft regulator Reline the chimney and insulate the liner. In some cases you may be able to insulate without relining. Parge with Chamber Cure to Reduce turbulence Decrease residence time of gasses Insulate from the outside

6 Externally Forced Downdraft This is less common that most people think it is, but comes quickly to mind as checking for obstructions. So said, sometimes it really is the problem. Downdraft can occur when a hill, tree or structure is in the way of the wind. The wind can t go though (at least not easily) so it goes over the top of the obstruction, and then curves down again before leveling out. Once in a while that means it actually blows right down a chimney. Chimney too low behind an addition or other building. Tree in the way Hill in the way Raise the chimney to 2 feet above a line with in 10 feet of the structure. This can be done by Adding sections to a factory built chimney More courses of brick on masonry chimneys. Assuming minimum height requirements are already met, put on a VacuStack. Put on a top fan Remove all or part of the tree Wait until the leaves fall off before having a fire. Put on a VacuStack This is most unlikely, but Move to a condo on the beach? Raising the chimney may help. Put on a VacuStack. Put on a top fan.

7 Dynamic Wind Loading A fairly common problem, this is when the wind hits one side of the house, and has to go around it. This causes a high pressure on the windward side and a low pressure on the lee side. That low pressure means a relative vacuum acting on the openings, leaks etc on that side of the house- and pulling air from the house (and causing a downdraft though the chimney as the house tries to replace the air being sucked out of it!) Can only happen when the wind blows. Make sure all the windows and doors are closed on the leeward side of the house Seal leaks on the leeward side of the house. Crack a window on the windward side of the house Install a make up air kit; I like them in the basement, but think the instructions nowadays say put it upstairs. Put on a VacuStack. Put on a top fan

8 Internal Vacuums Sometimes there are reasons why a house pulls a vacuum somewhere beside the chimney that s stronger than the chimney s draft. Competing fans: Bathroom, kitchen, attic, even radon fans all compete with the chimney for replacement air, and the chimney may well be the place they get that air. Hot attic: if the air in the attic is hot, it creates its own chimney effect on the house, and it can often overcome a chimney. Competing chimney: This is like the competing fans. The solutions are similar too. Overpower the other fans with a chimney top fan. Sometimes this is solution is about all you can (or will) do. Particularly for restaurants this may be the only solution. Turn off the competing fan(s) Crack a window to help re-pressurize the house. If it s a chronic problem, cool the attic. Perhaps an attic fan, or better gable ventilation. Seal up the cracks, doors etc. that lead into the attic. Most practical solution is to just not use the fireplace on days where the attic can get that hot. On those days it s probably not very cold out anyway. Put on a chimney top fan. Put on a chimney top fan. Don t use the competing fireplace(s) Crack a window to help re-pressurize the house.

9 No Make-up Air Since the 1970 s houses have gotten pretty tight in the interest of not wasting heating energy. The problem is that if you use an exhaust fan, a furnace, a fireplace etc.- anything that needs to move air out of the house, that air has to be replaced- regardless of the temperature of the outside air (I don t care if it s forty below zero, you have to replace that air.) A tight house can make this very difficult. House is so tight it won t let in replacement air. Crack a window or door Install a make-up air kit Change to a fireplace or appliance using a direct vent (handles its own air exchanges)

10 Other- Sometimes More Complicated and Usually Less Common The reasons for these problems are varied. No Barometric Damper: This is not a problem of too little chimney draft, rather too much (over-drafting) Chimney too large Chimney smokes intermittently in an apartment or condo Over-drafting: too much Venturi effect due to high winds On oil and gas flues, the pressure in the fire chamber has to stay fairly constant for the flame to stay on. For this reason they put in barometric draft regulators. They belong in the stovepipe going to the wall, and need to be updated when they get rusty and don t swing freely anymore. This is most often a problem with gas flues, though the dynamics apply to anything. Basically if the flue is too large, you can t maintiain enough temperature inside to keep the gases going up, so they cool off and fall right back down (and into the house.) The solution is to reline the flue with a properly sized liner. Might be the elevator pulling or pushing a block of air in the elevator shaft. You can t do anything about the elevator, but you can crack a window. Try closing the fireplace damper to a small opening Install a top sealing damper