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Discussion Starter · #1 ·
Hi! Today's question is about pulling out a furnace that vents through a shared chimney, and how to cap the hole and make sure the whole decommisioning thing goes smoothly. I don't know a whole lot about HVAC, so I figured it's best to ask. :)

I've got an old house, with a central chimney, and we're decommissioning the old gas furnace. The furnace is old-old; the paperwork for the beast has a six-digit phone number. It says it has normal input of 100,000 BTU/hour, with a bonnet capacity of 80,000. The furnace vents through a 4-inch pipe, into a shared chimney which also vents a gas water-heater and a woodstove.

The chimney's in good shape, and lined, and has two older penetrations which are sealed up.

We're not going to install a new furnace--the woodstove does a fine job of heating the house--and so I'm wondering: how do I seal up the vent hole, and do I need to line the chimney where the vent used to be? Will I need to do anything else other than remove the vent and cap the hole?

Possibly silly questions, but I'm having a devil of a time finding any information on the web that isn't about standalone furnace vents, or talking about replacing an existing furnace with a new one.

Thanks much!
El Barbón
 

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Will you have any other source of heat besides the wood stove after you remove the furnace ?
If not, be careful. Some homeowners insurance won't pay a claim for water damage if you don't have a central heating system (furnace,boiler,electric, heat pump) and a wood stove doesn't qualify as central heating.
 

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Backups--sorry, not directly an answer, but I would suggest you pick up a cheap electric heater or two to act as backups. If you happen to have something happen where you can't be home for a few days (relative in a car accident, delayed at an airport, whatever) and you get a surprisingly cold stretch, you don't want to have to go heater hunting.
 

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don't think you are allowed to share a flue with a wood stove and any gas appliance.
IIRC there are safety issues--maybe the possibility of a backdraft, or of issues if you get a chimney fire from an overfired wood stove or cracked lining.

(There's a farm I know where chimney fires are so common the fire department has stopped responding to them... they really are a thing.)
 

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Might want to consider converting the water to electric and dedicating the chimney to the wood stove. If you decide to put in a new furnace the HE units vent with its own PVC line.
 

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Depends on how cold your area gets, which influences the local codes. Normally, here, you'd have to install a new chimney liner just for the hot water heater when all the other common appliances are removed. Condensate from the flue gases will destroy the masonry. This is why many places have codes that try to prevent it.

Otherwise, sealing up the unused vent with sheet metal is adequate. (a metal end cap usually works perfectly)

Cheers!
 

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I've got an old house, with a central chimney, and we're decommissioning the old gas furnace. The furnace is old-old; the paperwork for the beast has a six-digit phone number. It says it has normal input of 100,000 BTU/hour, with a bonnet capacity of 80,000. The furnace vents through a 4-inch pipe, into a shared chimney which also vents a gas water-heater and a woodstove.
Could I ask why you're decomissioning it?

If it still works why not keep it as a backup or if you sell at some point or need to leave the house unattended.
 

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Discussion Starter · #11 ·
So, foolish me, I posted this and then went on vacation. :p

Anyway, to answer a couple of things: The chimney liner is a six-inch square, the water heater is 24,000 BTU, we've got a couple of space-heaters for backups (and like six cords of wood drying or already stacked in the woodshed :p), we're in Southwest Washington and our winters aren't terrible, and we're taking out the furnace 'cause it needs a new crankshaft and it's sixty years old and the ducts are leaky and the woodstove does a bang-up job heating the house. :)

I *was* wondering about the size of the liner and the appliances vented into it, and then I realized that, eight months out of the year, the water heater is the only thing actually venting in there anyway, and we haven't used the furnace since the first winter we lived here. So it's pretty much just for the wood-stove and the hot water.
 

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You'll probably have to get the chimney re-lined or better yet put in a direct vent water heater.

The old style furnace drafting into it even when not running may provide enough airflow to prevent condensation problems.

24k is low even for a water heater.
 
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Discussion Starter · #13 ·
Well, hell. The water-heater is, of course, right in the middle of the house, next to the chimney. In the basement. It *is* a rather small water heater, but it works like a champ; I don't think we've ever run out of hot water unless we're doing laundry and every single person takes a long shower. Huzzah for gas appliances!

I suppose I'm going to have to move it over to the wall and vent it out all by its lonesome, then? Good thing I've got plenty of pipe dope left! :p
 

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You can't re-use a natural draft water heater without the right liner.

can't sidewall vent a natural draft appliance.

odds are new water heater won't last half as long as the old one. if you don't want to bother with liners, new water heaters, leave the furnace alone but just shut the gas.

There's no harm in leaving it there unless the space is required.
 

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Just a thought..that is unlikely to get much support here.

You live near me in a weather temperate area where furnaces are used much less than other areas and last longer than normal.
If you've lived in that house for a while without using that furnace, you can usually subtract those years from the wear factor for how old it is.

A crankshaft (new main shaft with replacement bearings thrown in) is about a $25.00 part cost plus an hours labour..( Get him to check the integrity of your heat exchanger first.)

....Just in case you want to keep it as a heating backup and save yourself the trouble of all those reno changes.
 

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Discussion Starter · #17 ·
It's a natural-draft water heater, and I *really* don't want to replace it. And we do, unfortunately, need the space that the furnace is taking up.

The chimney liner is smaller than six inches, actually. It's closer to around 5.25" or 5.5", and the sweeps are always pretty happy with the way it looks when they come to clean it. But it sounds like taking out the furnace--even if we're not using it--might change the draft enough that the liner would be too big for the water heater? Maybe it's a dumb question, but could I take out the furnace, close up the hole, and then just leave the wood-stove door open during the off-season? Would that make up for the lessened air-flow?

Failing that... well, what else could I do?

Thanks again to everybody. HVAC and gas appliances are all pretty new territory for me, and I'm pretty sure that they work through sorcery, so all the advice is GREATLY appreciated. :)
 

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You should hire a pro to do the job, at least the gas line disconnection and deal with the chimney liner.

Hanging on to the old water heater could end up costing you more than a new one if the chimney needs relining.

Only an experienced tech who knows the codes in the area can tell you if the existing water heater can be used without re-lining.

I wouldn't hang on to a 25+ year old natural draft water heater if it means throwing good money after bad; wastes energy venting air up the stack continuously. Better to go for direct vent, 100% combustion air from outside, safer too.

Chimneys are going obsolete. New liners are a waste of money.
 
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Discussion Starter · #19 ·
I'm not too worried about doing the gas plumbing, but it *does* make sense to call a pro and get an expert opinion on whether or not I could leave the water heater connected to the chimney. I hate to give up the old water heater, though, 'cause even with five people living here and washing our clothes with hot water, we never seem to quite run out and our gas bills are surprisingly low (even with a gas range/oven and a gas clothes-dryer). Sounds like I may have to retire it, though, if I can't keep it plugged into the existing liner.

Thanks, all!
 

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Yup - key for a new tank is checking recovery rate.

You can go power vent or direct vent. Power vent doesn't work with electricity, has more to break, direct needs a larger opening in the wall.

 
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