Remove unused stack or leave it during roof job?

The roofers are one day into my roof job and I was deliberating having them remove the top portion of a stack that's no longer used. Finally, I said to remove it and they've put 1/2" CDX plywood over it, but it's not too late to change my mind.
It's around 5.5" external diameter and I think it's ceramic and after going through the roof it attaches down the side of the stuccoed house, and goes straight down into the ground. About 2 feet above grade level, it has a section at right angles to the wall that goes under the house into the crawl space. A foot or two from there somebody attached metal ducting, evidently to a now-disconnected gas furnace. Most of that ducting has been removed - there's a few feet they left there.
So, my guess is that they used an abandoned sewer drain and vent to vent the heater. Is this correct? I mean, a heater vent wouldn't have a portion going straight down into the ground, right?
I don't have a central heating system installed here yet, and I'm thinking maybe I should leave this stack protruding through the roof since it's already there - maybe I can use it in the future. Or should I figure I'll install central heating venting when I cross that bridge and have them continue roofing over where the top section of that stack was and eventually remove the pipe going down the outside of the house wall? And if "remove" is the answer, what do I do about the section going underground? Don't I have to plug that or something? How? Thanks for any help on this.
Dan
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That's strange. Couple things about your future furnace plans -
1) If you get a gas or oil (does anyone do new installs with oil ?) furnace that needs a chimney this existing thingy does not sound like something the contractor would want to hook into, based on code, etc.
2) Better yet - if you get a super-efficient gas furnace, you don't need a chimney, the exhaust goes out pvc thru the wall.
(If you're in a moderate climate you may do heat pump which obviates the whole chimney issue.)
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Posted and emailed too (by mistake), however here's the newsgroup post:
On 29 Oct 2005 09:47:50 -0700, in alt.home.repair you wrote:
:That's strange. Couple things about your future furnace plans -
There's a few strange things about this house, not the least of which is the additions that were made, which I still wonder about - what they did and how and why. Some people have ventured opinions but none of them have stuck with me. I guess either I'm dense or they're wrong.
:1) If you get a gas or oil (does anyone do new installs with oil ?) :furnace that needs a chimney this existing thingy does not sound like :something the contractor would want to hook into, based on code, etc. : :2) Better yet - if you get a super-efficient gas furnace, you don't :need a chimney, the exhaust goes out pvc thru the wall. : :(If you're in a moderate climate you may do heat pump which obviates :the whole chimney issue.)
Thanks!
To answer the questions:
I live in a pretty moderate climate. In the winter, nightime temps get down to the low 40's, occassionally into the 30's and very very rarely into the 20's (once a decade, maybe).
I don't think they use oil furnaces here. I presume I'd get gas.
So, you're saying I won't need that roof exhausting vent.
The m.o. will probably be to excavate a bit in the crawl space to make more room for a furnace and install ducting in the walls, upstairs and down (2 story 1925 square foot house). I don't know my heaters/furnaces or what they're installing around here. Efficiency seems like a good idea and I'm hoping to insulate my walls if and when I get the foundation replaced and siding at the same time. Big projects needed here. Thanks again!
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Well if you do nothing you will have sewer gas accumulating in the attic where the vent stack ends if I understood your description. Someone familiar with plumbing issues and codes can say whether it can be capped off or must be removed entirely. Left as it was it would probably have been fine as long as that presumably illegal chimney stuff was removed.
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Dan - I would say you are Ok as long as its not used. It running into the ground has me a bit puzzled. it could be a vent for a sewer line or septic system of some sort. If you are sure its not needed then go over it.
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