What is this pipe?

In a townhouse n'hood near mine, many of the houses have a pipe coming out of the wall, and I wonder what it is.
It's a white plastic 4" right angle pipe connector that comes out about 4 inches** and then turns to point down. Nothing is connected at that point. It's at the same level as the garden faucet so I guess it comes out of the ceiling of the basement.
**It only comes out as far as the right-angle piece puts it. There is no other pipe showing.
Some rows of homes have this, but others don't. The ones built maybe around 1997 have them, and the ones built 2 or 3 years later don't. (There are some others that don't that I thought were also built the same year as the first group, but if the construction year determines if there is a pipe or not, maybe I'm wrong.)
I've met a couple of the people who live there, but the odds one of them knows what the pipe is for seem slim to me.
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Probably the exhaust for the high-efficency furnace. But there is usually a second pipe to suck combustion air in.
aem sends...
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On Sat, 26 May 2007 02:57:43 GMT, aemeijers wrote:

4" is too big for a furnace. Maybe a water heater - mine is like that.
BTW, I've seen new construction with high efficiency heating that do exhaust to the outside, but take combustion air from inside - just a short stub on the furnace intake with a plastic grille on it. I thought that was wierd.
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wrote:

That's interesting. I woudl have thought a water heater fire is much smaller than a furnace fire, and would use a smaller exhaust pipe.

That is wierd, but otoh, don't all the previous prior to highficiency furnaces, suck air from the inside and exhaust it to the outside.
I guess colder air has more air in the same volume, and that's good, but indoor air is pre-heated and that's also good, right? Or maybe those are the exact opposites, and only one can be good.
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mm wrote:

Sounds like either a dryer vent, or a vented gas grill like a Jenn-Aire or such.
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On Sat, 26 May 2007 02:59:34 GMT, Robert Allison

Maybe. Some of these houses have dryer vents, the kind with louvers that close. I have to go look again to see if any houses have both.t.

These houses are smaller and cheaper than mine. I can't imagine such a grill installed in whole rows of them. Especially if the pipe is coming from the basement, which it seems to be.
As to the furnace intake, aem, maybe there was one. I'll have to go back and look. What size would it be? There might have been one or two 1- or 2-inch pipes I didn't account for. I asked about the 4 inch pipe because it was so noticable.
I may go tomorrow afternoon.
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wrote:

The only other choice is a radon vent, but I didn't search about their size.
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Most likely a furnace or hot water heater. Could be an air intake or an exhaust vent. Wait till winter and stick your hand over it and see if the air is going in or warm and coming out. Usually these make some noise because the use a fan to exhaust the air.
Electric v. gas water heater could explain why some have them and some do not.
Also, if it is only on the nicer/larger units it could be a gas fireplace.
Unlikely to be a dryer.
Could also be a very poorly designed radon system, but that's very unlikely.
Find someone planting flowers or something and go ask them.
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wrote:

Do these ever share the same exhaust?

Well, I walked by again today, and noted that the whole row where no one had the pipe had thin metal chimneys coming out of the roof, and no one on the row where everyone had a pipe (there were a few where I didn't see the pipe, but it may have been behind their bushes.) had the thin chimney.
Then I found another row where no one had a pipe in the front, although there might have been one in the back. (The floor plan of these houses had to be a bit different because they faced downhill and the entrance door was in what would have been an underground wall in most of the other houses.) So I saw one house with 2 adults and a kid outside and stopped to ask. They didn't have a pipe in back and hadn't even noticed that others have a pipe (These people live close to the main street, and might not ever even pass those any other houses but their own string of 8.) But she did say that the houses were originally built to be all-electric, and the prospective owners complained and they put in gas also. They had to dig up some stuff in the process.
So, yes, that could why they didn't have a pipe and another reason some have them and some don't.
Is it only gas that works with its own plastic exhaust pipe? Or oil too? In my case, the oil furnace is the only thing using its chimney, so if I didn't use the chimney at all, I guess I could store contraband in it or something?

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high-efficiency furnace exhaust woul be 2 or 3 inch pipe depending on length and number of bends. it must be from a water heater.
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mm wrote:

... Could also be a bathroom exhaust vent. Maybe some other exhaust vent. Downflow cooktop vent? Fireplace air intake? I assume that you know that it is not a dryer vent? If you have propane and your water heater or furnace is above that level, it could be a propane "drain" (a safety "feature" required by some building codes). It could also be something that is no longer in use.
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mm wrote:

Clothes dryer vent maybe.
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On Fri, 25 May 2007 22:19:49 -0400, mm wrote:

Back up sump pump.
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