I wondering if anyone has any information on venting a furnace to an
exterior wall and removing the existing chimney. The house is a 1920's
house and I'd like to remove the chimney to add a bathroom. Also, any
idea on the cost associated for a job like this would be greatly
A chimney has a draft that draws out burnt gas, a non powervent furnace
or regular furnace needs the draw of a chimney. Now would be a good time
to consider a 94% high efficiency unit that can vent with Pvc, but then
what about your water heater, where does it vent.
replace the furnace and hot water tank with the direct vent 90+ types.
this will elminate the need for the chimney and save lots of energy
remember the existing fornace and hot water tank flues exhaust heated
air from your home 24 / 7
How much they will save depends entirely on what they have now. To
replace an existing 80% furnace and hot water heater that have a lot of
life left to try to save money usually isn't going to be cost
effective. And going from 80 to 90 hardly qualifies as lots in my
book. Sure, if they need a new furnace, then I would go with the 90,
but if they just want to remove a chimney for asthetics, this is all
gettting very expensive.
On 16 Mar 2006 13:48:29 -0800, email@example.com wrote:
When I took geometry in the 9th grade, the book had a question that
involved calculating the height of a chimney by using congruent
triangles, I believe, probably by using the shadow of the chimney to
determine the triangles.
It was a story question and started off by saying that a tall chimney
Norman asked Why.
The teacher (whose name I shall not give) got perturbed, and
blustered, and didn't answer him, but a week or two later he brought
in a book and made Norman stand at the front of the room and read it.
It said that taller chimneys drew better, but it still didn't say
why!!! Obviously the teacher didn't know either.
I've got my guess, but I don't know for sure either.
But they do. And I'm sure you can't vent the furnace you have now
through an outside wall, unless maybe you built another chimney
outside that wall, and even then the greater distance might mean that
it didnt' work as well. Maybe if you made it taller it would work as
well. I'm sure Norman knows all about this by now.
Insufficient chimney height
(from http://www.chimneysweeponline.com/hodraft.htm )
Chimneys usually draw at least a small amount of air, even when there is no
fire below: this phenomenon is called ambient updraft. Ambient chimney
draft occurs because the top of the flue extends upward several feet, into
a lower density atmosphere than exists at the bottom. Thus, air is drawn up
the chimney in much the same way as liquid is drawn up a soda straw when
you reduce the air density inside your mouth.
Most wood stove manufacturers require a minimum stack height (stovepipe and
chimney) of at least 16 feet. Code requires that all woodstove and wood
fireplace chimneys must extend at least two feet above any part of the roof
within ten feet. Given that topographical and atmospheric conditions can
vary considerably from house to house, it is possible for a chimney to
comply with these minimum specifications but still fail to extend upward
into air of low-enough density to establish ambient updraft. It is not
uncommon for fireplaces and stoves in houses that are surrounded by hills
or trees, for example, or that are located in the high-density air that
often surrounds large bodies of water, to need more chimney height than the
minimum required by code.
Updating the furnace (as suggested) needs to be looked at carefully
because payback can take a long time. Of course, if the furnace will
need replacing in the next few/5 years, replacing can make sense.
My oil-fired boiler has a powervent and vents out the wall, which if it
meets code in your area is an option. What I don't like about it, is
the noise (maybe a newer one would be quiter than mine), and the fumes
do have some smell, and are now distributed near ground level.
You could have a new flue put in on the exterior that takes up a lot
less space than the chimney.
If you go forwards with this, you might be able to save a fair bit of
money removing the chimney yourself, or contracting that out as a
seperate job (after the furnace is plumbed to a different exhaust!).
Two things to keep in mind.
First, your furnace and hot water heater need to be specifically
designed for direct venting. As mentioned in a post above it might be
a good time to consider two new high efficiently units (furnace and hot
Second, you need to be careful as to where the vent exits the house.
There are limits as to how close the vent can be to any upstairs
windows. Think of the vent gases as washing up the side of the house.
You cannot have the vented gasses draft back into the house thru
windows above the vent point. You many need to relocate the two new
units to another part of the basement to accommodate this very
important design criterion.
After those two issues are ironed out, it's just a matter of taking
down the chimney brick by brick and covering in the roof, ceilings and
Say he is lucky and has an 80% unit, which without proper maintenance
and its actual design might be 70-75%. A modern 94.5% unit vs an 80% is
a % difference in operating savings of 18% not 14.5%. Factor 18%
savings with the true rapid increases in fuel costs that we have had and
will continue to see and his now 18% savings becomes greater with every
increase in fuel. Add the possibility of his being oversized now and
that a properly sized unit will increase efficiency even more by running
at the units peak design longer. Add to this that a VSDC blower will
easily cut 10-20% off his electric bill winter and summer. Now look at
water heaters, Ng tanks loose efficieny every year due to scale, he has
options of a higher efficiency tank, or the highest efficency Ng
tankless, whick have Energy Factor ratings of 18-25% more than Ng tank.
New units could if picked right easily save him 22% more in utilities.
In those numbers there is payback, a good investment. It will also raise
his houses resale value. Keeping old equipment with present and future
utility price projections easily becomes penny foolish when all factors
How are you calculating this? By my calcs, going from an 80%
efficiency unit to an 94.5% unit will result in saving 15.4% on energy
bills, not 18%. So if his heating bill is $1300 for a season, it would
be a savings of $200/yr. That is completely wiped out by the time
cost of money, which is to say if he just puts the $4500 he could spend
on a new furnace/air in the bank at only 4.5%, he would earn $202 every
year. If he has to finance it, it gets a lot worse. The savings on
the newer motor would certainly contribute some to this as well, but
the biggest other impact would probably be the savings in central air
cost, assuming he has that and is replacing a much older, less
I agree that at some point, in can make sense to upgrade. But if the
furnace is 80% and still working fine, I seriously doubt you will come
out ahead financially, unless you live some place very cold where you
will use a lot of energy, etc. And don't forget to factor in that to
repair one of these new systems can be very expensive. Replacing that
variable speed DC motor or control board is gonna cost a lot more than
a simple $100 motor for the furnace he has now.
The difference of % efficiency gained inbetween two units is calclated
by the difference in percentage of the 2 units. Or 80% is 18% different
than 94.5%. I had a hard time accepting this till I asked Lennox. So
people upgrading to more efficient units really achieve higher percent
savings then they usualy realise. Repairs are always an issue, 10 year
warrantys are what should always be obtained especialy with VSDC
blowers. My numbers show a 6 year payback on a 500$ replacement blower
for me, but that is todays Kwh cost, soon I am going up 15-20%. With a
10 yr warranty I am ahead no matter what happens. Another point is how
many factor in past Ng price increases to reflect future costs into
their payback, there again payback is quicker with a higher efficiency
Well, Lennox doesn't know what they are talking about. Let's say you
spent $100 on natural gas. At 80% efficiency, that produced $80 worth
of heat, the other $20 was wasted. To produce the exact same $80
worth of heat with a 94.5% furnace requires 80/.945, or $84.6 worth of
natural gas. $84.6 X .945= $80. So, instead of spending $100 for
gas, you spend $84.6, which is 15.4% less. It's pretty simple and
I wonder just what the energy savings in $$ is for elminating the
chiney flues going with power direct vent is?
2 things theres the cost of the energy used to heat the air being
constantly exhausted out the chimney
Theres also the cost of heating the replacement air being sucked
indoors to make up the air going up the chimney.
bpth of these must amount to something, on a cold day my chimneys have
a good draft even with the furnace or hot water tank disconnected. I
noticed that one day inspecting my chimneys.
Just think its like leaving a window open 24 / 7 :(
Here's another way of looking at it. You put $100 of gas in, with 80%
efficiency, you get $80 worth of useful heat out. According to
Lennox, by going from 80 to 94.5, you save (94.5-80)/80, or 18.1% in
fuel cost. If that's true, then we can buy 18.1% less fuel, which is
$81.90. Take $81.90 and put it into the 94.5 furnace and we get
81.9x.945, or $77.40 worth of heat out, which is less than the original
However, if you take 15.4% off the $100, you get $84.6 and put that
through 94.5, you get 94.5*84.6, which is $80
What Lennox is doing is just taking the differences in percent
efficiencies, which is mathematically incorrect. But it does give a
higher figure, which I'm sure they like :)
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