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I'm told by some HVAC companies via mail flyers and websites (example) that it may be very expensive to retrofit new PVC pipes to update an 80% efficient furnace to a 90% one. Therefore, the logic goes, it's better to install a new 80% furnace now before the new mandate takes place on May 1, 2013. Does that ring true?

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up vote 8 down vote accepted

PVC is relatively inexpensive so the main cost is installing it. If your furnace is near an exterior wall, they'll simply make a hole in that wall and run the lines directly out. If your furnace isn't close to an exterior wall, then you need to run the lines through the ceiling, preferably in the direction of the joists, until you reach an exterior wall.

If that ceiling is already finished with drywall and has other utilities running through the space, then the expense will be to open the ceiling and get the vents properly routed. Drywall repair can be a DIY job, though mudding the joints tends to be an art if you want it to look perfect. And most HVAC installers would likely outsource that job to another contractor. The second contractor is where the price on the job can start to jump.

That said, the added cost is only in that specific scenario, which is a minority of the installs. And you'll recoup that cost over time with a more efficient HVAC. Therefore, I'd avoid prematurely replacing a furnace since that will be the most expensive option of all. And I'd also recommend against using contractors that are sending out fliers or knocking on doors to get business. The good ones don't have to look for work, they've got a waiting list for their services.

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I'm pretty sure there are also minimum clearance requirements between furnace exhaust and windows/doors, so that can obviously affect the route the pipes need to take. – gregmac Dec 20 '12 at 23:09
@gregmac, I stopped adding details after your fine answer. Great point, and +1 to you. – BMitch Dec 20 '12 at 23:58
+1 for the awesome comment on fliers. – Paul Dec 21 '12 at 2:35
Quick! Buy this nearly-obsolete, less-efficient furnace while you still can! Save on installation now and lock in 10 years of higher heating bills! – mac Dec 21 '12 at 15:26

Is it expensive? Well that depends on your definition of "expensive", the amount of work you need to do, and who you get to do the work.

90% efficient furnaces require PVC venting directly outside, as opposed to using a chimney like the older 80% furnaces do. The extra expense is entirely dependent on how easy it is to route these pipes from your furnace to a suitable location outside (which I believe has minimum clearance requirements from windows and doors).

If your furnace is in the middle of a finished basement it's going to be much more expensive than if it's on the outside wall, simply because it requires opening up drywall, routing around obstacles in the walls, etc.

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In some cases, direct venting can be a little cheaper than chimney venting. This was the case for us when replacing our 40-year-old oil-fired boiler, because our chimney was unlined and code now requires a chimney liner. So it was $1000 to install a chimney liner or a couple hundred bucks of labor to have the plumbers run PVC vents. Granted, this was in an unfinished basement, and we didn't have any difficulties with vent location, and we were paying more up-front for a more efficient boiler. – Shimon Rura Dec 21 '12 at 15:32

My new house was built with a high efficiency condensing furnace and the pipes they use look just like lawn sprinkler pipes. Low pressure PVC pipes like these are very cheap for materials and very easy to work with. Almost any tool can cut them and joints are just glued.

In my case, the pipes just ran straight to the nearest exterior wall, outside and then terminated with a down facing 90 degree elbow. If there were a window that was too close, I'm sure they could just run the pipe up until it was far enough above the window.

I've never had a finished basement so I don't know how important it would be to hide the pipes in that case. Do you guys actually hide the ducts too? Doesn't that make for a very low ceiling?

Aside from that, I would not stay awake at night about exhaust pipes.

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This seems like a clever marketing strategy; as mentioned by Gregmac and Bmitch, the extra cost for the 90% install can be quite variable; you might want to start with an estimate, so you have the facts.

In general, I would only consider this if you think your existing furnace is at the end of its life; otherwise, you might as well wait until you reach that point. Put the money you'd spend today to replace the furnace aside for a future replacement, or use it to improve insulation, weatherstrip, or similar to reduce your energy bill.

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