Venting for New Furnace & Wtr Heater

[gw2008]

Hello All,

I'm planning on replacing our 20 year old gas furnace and gas hot water heater that have common venting through a metal lined (?) chimney on the side of our house.

Our HVAC contractor recommended:
1. a natural gas Trane XV90 condensing furnace (direct vent I gather) that will require 2 new PVC vents through an exterior wall of the house (one for fresh air in, and one for combustion air out); and
2. a standard natural gas John Woods water heater (not high efficiency), to be vented through the existing chimney flue after installation of a lessor diameter pipe (metal?); as well as installation of a new fresh air supply from the outside of the house.

Forgive what seems like a simple question, but I'd appreciate knowing whether (assuming proper sizing and material) there is some form of common venting that could be installed in the existing chimney (if chimney size permits), perhaps with a HE water heater instead of a standard water heater.

From another perspective, the question is simply whether installation of a new high efficiency gas furnace and a gas water heater (HE or not) requires installation of two (2) separate sets of venting.

While I'll certainly use and rely our HVAC contrator for design and installation, I'd appreciate some getting some background information on this, as most posts seem to related to single appliance venting or orphaned water heaters. TIA.

[comfortdoc]

....
From another perspective, the question is simply whether installation of a new high efficiency gas furnace and a gas water heater (HE or not) requires installation of two (2) separate sets of venting. ....

Yes. A 90+ furnace requires it's own vent system. The water heater must be vented seperately.

[4l530]

I guess if they both vent in PVC you could try to get them to use the masonary chimney as a chase for the flues. If there's room. Not too sure whether you'd really want the combustion air pipe up there also, it would have to be lower and extended out away from the exhausts.

[dave davis]

You might ask the contractor though if both appliances can share the chimney. Almost certainly code will preclude a standard water heater's vent sharing the chimney with the pvc.

But you asked about high efficiency water heaters...it might be that those two vents can share the same chase-which is essentially what your chimney would become. You would abandon the metal liner, or remove it.

Be interesting to hear what the pros say. In that vein, I'd like to know why so many of the condensing furnaces vent out the sill or side and not out the roof, having seen many vents exhausting directly onto cars, the sidewalk, or neighbor's property.

[4l530]

Originally posted by dave davis
In that vein, I'd like to know why so many of the condensing furnaces vent out the sill or side and not out the roof, having seen many vents exhausting directly onto cars, the sidewalk, or neighbor's property.

With a furnace in the basement it's way easier. Plus the pipes can only be so long and the furnace such and such a size before you have to use a larger pipe or find a shorter distance.

[comfortairtech]

I guess if they both vent in PVC you could try to get them to use the masonary chimney as a chase for the flues. If there's room. Not too sure whether you'd really want the combustion air pipe up there also, it would have to be lower and extended out away from the exhausts.

Could never quite understood why some heating contractors in Wisconsin vent high efficiency furnaces through the masonry chase through roof. Why not just eliminate it completely or level it below roof. Suppose you might miss the possiblity of climbing up a ladder only to slide off the icy shingled roof trying to clear a snow covered or an internally iced intake pipe.

[4l530]

Originally posted by comfortairtech
Suppose you might miss the possiblity of climbing up a ladder only to slide off the icy shingled roof trying to clear a snow covered or an internally iced intake pipe.
Lol, yeah
There's a quick easy solution for that that's probably frowned upon, but better than breaking your arse:p

[bonafide]

Go variable speed 90 furnace and replace the water heater with an instantanious diect vent type. Get rid of the chimney and open a new savings account.

[allstar08]

You can vent a 80% furnace with standard hwt in a common flu. Depending on the size of your home and the btu requirements. A 30,000 btu tank and a 60,000 btu furnace will vent in 4 inch pipe, that should fit in any chimney. Usually a 5 inch liner will handle the venting load for most homes, ie 80,000 btu furnace and a 40,000 btu tank. This is a way to save money on your install, but not a good way to save money every month on you gas bill. It would take quite a while to get a pay off from changing to 90% direct vent furnace and a direct vent tank or tankless.

[rifter1]

http://www.gamanet.org/gama/inforesources.nsf/vAllDocs/Safety?OpenDocument

Download and run "Venting Done Right" (I looked at the "installer" version)
this should answer a lot of questions.

[BaldLoonie]

Just remember that a power vent water heater is NOT a high efficiency model. If you look at the house air sucked out by the power ventor, it is really a very inefficient way to heat the house. I commend your dealer for following code and proposing to put a smaller metal liner in the chimney just for the water heater. Too many hacks out there don't bother which is going to haunt the homeowner due to chimney damage from condensation.

Demand heaters are nice but watch for capacity issues. Some of them draw up to 200,000 BTU vs 40,000 BTU for a standard heater. That would require a new gas line run which adds to an already expensive install.

[davidr]

Make sure your contractor test his installation after it is complete.

Just because it is installed according to code doesn't mean it is going to automatically work like it should.