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Heating & Air Conditioning

high efficiency furnace

17 years ago

I am thinking of changing to a high effieciency gas furnace for my 1920's home and the furnace is in the centre of the house. I have read that high efficiency has to be vented to an outside wall - does that mean that it will not be possible. My basement is completely finished and is used as living space. The current furnace is about 25 years old.

Look forward to hearing responses.

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Comments (8)

blacknumber1
17 years ago
Whats wrong with your current furnace?
aemeeich_
17 years ago
Do you have a gas fired water heater?

If you DON'T have a gas water heater and nothing other than the furnace is currently vented into the chimney, you can use the chimney as a chase to run the pvc pipes up to the roof.

Even though the ceiling is finished, it might be possible to push the pvc pipes through the joist space. My brother's house has a sidewall vented furnace that was installed during construction before the basement was finished, however, looking above the finished ceiling, I can see that there is enough room, and it would be possible to run the pipes over to the exterior wall if needed. Your house may be the same way.
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I have a high efficiency modulating furnace (Rheem) put in in 2006. It has worked well, nothing beyond standard maintenance checks It replaced a 30 year old furnace so who knows what the efficiency was there; but my bills dropped so much the gas company came out 3 times to check the meter. Of course, in a colder climate than yours, ruko37. As I recall, getting the right size not only for your square feet but also number of windows, insulation etc is very important to continued high performance of the furnace. I guess your decision (80% vs95%) is rather like when I was trying to decide whether to buy a Versa or a Prius. I did the math--how much I drove, how much gas cost and could cost in the future (I assumed more), the gas mileage of the vehicles, to calculate how much I might save with the Prius; then compared to the price difference of the vehicles. In that scenario, even with gas pushing $4, the Versa was much more cost effective for me. You need to do a similar calculation. If the difference is small, go for the higher efficiency.
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It is always possible to posit or observe a wind speed and direction for a given structure that will result in different pressures at two separated locations on the structure. How different vs. what the unstated actual requirement is would be hand-waving at this point. It is notably difficult to construct an outside absolute atmospheric pressure gauge that is, within the desired accuracy, unaffected by even low speed wind, so differential pressures between locations where the wind is different should be expected to be different. For reference, 50 ft of altitude is worth 1.4 inches of water column, so compared with the back-draft safety specification of 0.03 inches implies a maximum altitude difference of about a foot, although in this case the lower altitude as an intake would be positive for draft, all else being equal, such as temperature. The point here being that variables such as intake vs. exhaust altitude affect the draft in ways that the manufacturer may not want. If the furnace manufacturer would list a specific differential pressure value not to exceed, then the pressure differences between two candidate intake and exhaust points could be measured by not very expensive means (excluding the labor involved in moving tubing around and in taking measurements over a wide range of conditions). It is probably easier to just follow the instructions because where the pressure difference requirement is low, it is generally safe to assume that a roof port will see a different pressure than a wall port in windy conditions.
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deltag
Original Author
17 years ago
Actually, the furnace works OK, it's just that I don't know how much longer it will keep going since it is 25 years old.
It is a lennox.
blacknumber1
17 years ago
It will work just fine for another 10-15 years with a little TLC. Don't throw away your money on a new furnace which will need a major (meaning expensive) part by it's 8th birthday and will probably only last 14 years in total unless your current furnace is dead. Older furnaces have much less to break, have no overcomplicated electronic components and can be fixed quickly with off-the-shelf parts. Spend the money on better insulation, better windows and the like.
don21
17 years ago
My 2 stage, condensing gas furnace is vented to the attic

A 3 PVC inch combustion air pipe from the attic down to the furnace and a 2 inch PVC exhaust pipe up from the furnace, through the attic to outdoors - In my case, through a gable vent, but it can go up and through the roof as well

Don
airsome
17 years ago
Gotta get the condensate out. Vertical bad, horizontal good.
bk2000ext
17 years ago
I think your current furnace have 6" exhaust, and takes basement air for the combustion. So for new furnace you may use the same shaft(it has a 24"x24" box going out in attic. So you can use that for intake and exhaust.
If you have reasonably good older furnace and you are from mild winter area then just use existing one. About $4K to $7K investment takes more than 15 years to recover cost! 7Kx0.07 = $490 interest, So calculate your total heating bill and 20% to 30% savings over your older furnace!!!
blacknumber1
17 years ago
Too bad new furnaces only last 15 years and will usually need at least one major repair in that time. There goes all the money you saved.

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