Wind down my chimney

Mrwurm · Mar 9, 2002

Hey, any advice for a guy with chimney troubles. I have a double-walled tube that runs up from my furnace and my hot water heater all the up through the roof. I have a steep roof (12,12 pitch??) and the chimney exits the roof about four foot from the soffit. The portion of pipe that sticks out is about 3 ft long. The peak of the roof is at least 10 ft higher than this. On windy days, I get so much wind down this pipe that my hotwater tank burner just gets blown out whenever it lights. I have to relight the pilot whenever we need hot water. I did this test. Removed the vent pipe from the hotwater tank, lit the pilot, turned switch to on. Heater works perfectly with nice uniform flame. I then replace the vent slowly to heat it up so that an updraft will be generated. This is currently the only way I can get hot water on a windy day. I checked the pipes and they are not clogged up. It's quite the opposite. Cold air (Michigan) blows out of the vent pipe with so much force you could blow-dry your hair with it. How can I turn this situation around????

RonL · Mar 9, 2002

Mrwurm

I believe the the vent pipe is supposed to be higher than the peak of the roof. I think there is a formula for this. I'll check my references and get back.

RonL

beenthere · Mar 9, 2002

You might have a couple choices, one being to lengthen the height of the pipe above the roof. From your description, it is a little short. But adding pipe may mean you will have to have guy wires to keep it upright in a strong wind. The other possibility is to add a fan or blower to the stack, providing a forced draft whenever the gas is on. You may at the same time put in a damper which is closed when the draft blower is not on, protecting the flame from being snuffed out.
Best to discuss this with the heating people that designed it, as they may have missed the mark a bit.

Egon · Mar 9, 2002

You will have to extend you chimney so it is three ft. above the top of your roof. I think the three ft. is a code requirement in most places.

Egon

Mrwurm · Mar 9, 2002

The general theme here seems to be to lengthen the chimney pipe. I have heard that a chimney should be higher than the roof peak, but in actual practice it does'nt seem to be the case. I took a look at some of my neighbors homes and some chimneys are one to two feet higher than mine, but none of them are above the roof peak. Most are at least 3 ft lower than the peak. I would need a really tall chimney with guy wires and braces to get up that high. I think I might just add a three foot section to the pipe and see if that does the trick. If I go any higher I will need the fancy supports. The chimney has a fancy cap on it, maybe there are different caps out there that work better at preventing down drafts.

PhilNH5 · Mar 9, 2002

Regarding the 3 foot rule - For a chimney for a wood-burning appliance it is 3 feet higher than anything in a 10 foot radius. Not 3 feet higher than the peak. That's what I recall when I had an additon built on to my old house in MD. 'course could be a local code issue.

Phil

JJT · Mar 9, 2002

Around here we have a 2/10 rule. The chimney needs to be at least 2 feet higher than anything within 10 feet of the chimney. If the roof pitch is truly 12/12 then you need to have 12 feet of chimney sticking thru your roof, (to make it 2 feet taller than the roof 10 feet away.

Mrwurm · Mar 9, 2002

I'll do some measuring tomorrow and see where I stand in relation to the ten foot radius. It's a challenge to get up there but I'll give it a shot. Just looking up there it seems like a six foot diameter sphere with the center in the cap would touch the roof. (seems like the cap is 3 feet from the roof) I think if lengthening the chimney does'nt work I'll get a contractor out here to see about a power vent and or damper. I wonder if these 'high-tech' devices will work with an appliance that has a standing pilot?

patrickg · Mar 9, 2002

If you could perform some experiments to determine if it is a high pressure region around your chimney exit (probably due to wind blowing over the peak of the roof) or an effect of wind redirected by your roof when it comes from certain directions causing the vent cap to "scoop" wind and deflect it down the pipe you might solve the problem without a really tall pipe and guy system although you might not meet the letter of the code. If it is mostly a "high pressure" area surrounding your vent outlet problem then no change to the vent outlet will help. If it is redirected wind interacting with your vent cap then a different vent cap or a diffuser put over the current one could solve the problem. A simple experiment to find out would be to find or construct a clyindrical can (metal 5 gal but bigger is better pail with lid or ...) drill a zillion holes in it so it looks like a sieve and the area of the holes is twice or more the effective open area of the outlet in your vent cap. Make a hole inthe center of the lid to fit your chimney then cut the lid in two through a diameter so you can get it around the chimney pipe and splice the lid together with some light sheet metal (tomato can or ...) using sheet metal screws or... and put the pail back on the lid. This diffuser will ameliorate the effect of the wind direction but allow hot gasses to escape. It will not cure a high pressure problem.

Another design is to cut several vertical slits in the can with small horizontal slits at the top and bottom of the vertical slits. Bend the cut portions to form louvers, all aimed around the circumference in the same direction at about 30 degrees. Supposedly this design converts straight wind that hits it to a rotary motion that creates a slight negative pressure in the center of the vortex and actually helps the exhaust flow. I haven't built this second design so I don't know if it works well or not but I have seen both of these designs made up in stainless steel for flue pipes about 8 inches ID.

In theory you could put an inverted "Y" in your flue pipe and have a blower in one leg and the appliance in the other. When the blower runs it creates a suction on the other leg by venturi action (see also Bernoulli). I've seen these for really powerful boosters for range hoods. I wouldn't have just any body with his name on his work shirt engineer this last approach as it is a powerful technique and can be overdone.

Patrick

Anonymous Poster · Mar 9, 2002

The gas funace in my basement has a booster fan on the exhaust which runs whenever the burner kicks in to maintain a proper draft. If you have a prevailing wind from a certain direction I have seen special chimney caps with a steamlined cowl which faces into the wind and creates a venturi effect as the wind rushes by thus setting up an updraft in the flue. But this may work the opposite if the wind reversed inless it is on some sort of pivot to allow it to "weathervane".