About three years ago we moved into a 40 year old house. Sometime during
the last few years prior to us buying it, the previous owners replaced the
forced air gas furnace with a Rheem high efficiency model (92, I believe).
The exhaust line is standard 2 inch PVC pipe vented out the side of the
house.
The question I have is regarding the input for the combustion air. There's
no piping for this, just a two inch hole in the side of the furnace.
The furnace is in a full unfinished basement, so it's not starving for
combustion air. The house is not sealed as tight as some newer homes, so I'm
not worrying about oxygen levels in the house, but I would think that by
using the interior air for combustion it would cause cold air to be pulled
into the house via leaks. Also, it's sending the warmer house air out the
exhaust.

Would it be of any benefit to run a PVC pipe to the inlet of the furnace?
The total length of the run would be about 12 feet, and would involve one 90
degree bend and a 45. The pipe would come out of the house about 30 inches
away from the exhaust (is that sufficient?). Also, would I (or should I)
put a final fitting (90 or 45) tilting downward on the end of the pipe where
it sticks outside to keep rain & snow out? The current exhaust pipe has
nothing on the end, it's just the pipe sticking out about a foot.

As I said, I'm working on other issues to help the heat in and the cold
out, but I'm thinking every little bit I can do to keep the cold out would
help.
I did some internet searching, most of what I found seemed to indicate it
would be a good idea. Most concerns I found were about NOT having piped
input air, but it seems to be more of an issue the tighter your house is
sealed.

Mike O'Donnell

Im no pro but believe outside air is what it was set up for and you
would benefit greatly taking outside air. Also get it cleaned for full
eficiency. Talk to Rheem for pipe specs and location of inlet vs exaust

start with installing a carbon monoxide detector in the basement with a
digital parts per million display. then continue reading....

your answer is not in just the manual you need to read for your
specific model, but also in what the installer did, and add to that
what the previous homeowner might have done.
if it is presently ok by the installation manual to be sucking on your
stale basement damp radon air for its combustion, you might be happy to
let it continue doing so.
if on the other hand your basement doesn't have sufficient make-up air
and the furnace is sucking on the hot water chimney flue and causing
exhaust gas corrosion of the flue pipe atop the gas hot water heater,
you have a needed repair.
you will need to know the requirements for that furnace model.

....if the CO alarm has not rung, we're safe for now...

"Mike O." wrote in message ...
| About three years ago we moved into a 40 year old house. Sometime during
| the last few years prior to us buying it, the previous owners replaced the
| forced air gas furnace with a Rheem high efficiency model (92, I believe).
| The exhaust line is standard 2 inch PVC pipe vented out the side of the
| house.
| The question I have is regarding the input for the combustion air. There's
| no piping for this, just a two inch hole in the side of the furnace.
| The furnace is in a full unfinished basement, so it's not starving for
| combustion air. The house is not sealed as tight as some newer homes, so I'm
| not worrying about oxygen levels in the house, but I would think that by
| using the interior air for combustion it would cause cold air to be pulled
| into the house via leaks. Also, it's sending the warmer house air out the
| exhaust.

You are sucking "warm" air out of the house which will be replaced by leakage thru the walls -- Workable but not good. would be better with a 2" intake for outside air. One thing not usually done is to seal all pleneum joints. Sheet metal workers will cringe, but, their joints do leak and there is no reason to add this inefficiency to your furnace load. An excellent use for duct tape - the good stuff, not the cheap paper kind.

|
| Would it be of any benefit to run a PVC pipe to the inlet of the furnace?
| The total length of the run would be about 12 feet, and would involve one 90
| degree bend and a 45. The pipe would come out of the house about 30 inches
| away from the exhaust (is that sufficient?). Also, would I (or should I)
| put a final fitting (90 or 45) tilting downward on the end of the pipe where
| it sticks outside to keep rain & snow out? The current exhaust pipe has
| nothing on the end, it's just the pipe sticking out about a foot.

Great idea. Just remember that every 90 degree bend adds about 5 ' to the effective run. The less bends, the better. The outside end of the pipe can be either a 90 degree elbow, a "T" junction with the open ends vertical or , at worst, nothing. There are also screens with a fairly open mesh to help keep rodents and leaves out of the intake. I once had a leaf get into the intake and completely shut down the furnace.

|
| As I said, I'm working on other issues to help the heat in and the cold
| out, but I'm thinking every little bit I can do to keep the cold out would
| help.
| I did some internet searching, most of what I found seemed to indicate it
| would be a good idea. Most concerns I found were about NOT having piped
| input air, but it seems to be more of an issue the tighter your house is
| sealed.

True, with a really tight house, the furnace will not work, with a leaky house the living area will constantly be trying to offset the colder air sucked in to replace what the furnace uses. Remember that having a really tight seal on the ducting causes more air to be pushed out through the registers and less wear and taear on the furnace blower. Above all, keep your filters clean as this eases the blower effort and increases the effective efficiency of the furnace.

|
| Mike O'Donnell
|
|
|
|

--
PDQ

--

"Mike O." wrote in message
...

Would it be of any benefit to run a PVC pipe to the inlet of the furnace?
The total length of the run would be about 12 feet, and would involve one
90 degree bend and a 45. The pipe would come out of the house about 30
inches away from the exhaust (is that sufficient?). Also, would I (or
should I) put a final fitting (90 or 45) tilting downward on the end of
the pipe where it sticks outside to keep rain & snow out? The current
exhaust pipe has nothing on the end, it's just the pipe sticking out about
a foot.



There would be some benefit, how much? Don't know, but I would recommend it
too..
Before you attempt this I would get a manual for the furnace. I work for a
Ruud dealer, which is the same furnace, different label. The air inlet
requires a condensate catch "T" to keep moisture in the fresh air from
making it's way to the furnace. Also the air inlet is supposed to be close
to the exhaust, within a few inches, generally slightly below, with an elbow
pointed down. All this info will be in the manual. find one, read it, and
follow the directions to the letter! It may look easy, and it really is, as
long as the instructions are followed. I have be out on calls to solve
problems with furnaces that are not running properly. Often it is just a
problem with the install. Too much pipe, too many elbows, wrong diameter of
pipe, and so on.

What size of furnace is it? Larger furnace require larger pipe. Longer runs
require larger pipe. It may be possible that your furnace is not installed
properly right now!
I can e-mail you a manual if you give me the model of the furnace.
E-mail me if you like.
Greg

"Greg O" wrote in message
...
"Mike O." wrote in message
...

Would it be of any benefit to run a PVC pipe to the inlet of the furnace?
The total length of the run would be about 12 feet, and would involve one
90 degree bend and a 45. The pipe would come out of the house about 30
inches away from the exhaust (is that sufficient?). Also, would I (or
should I) put a final fitting (90 or 45) tilting downward on the end of
the pipe where it sticks outside to keep rain & snow out? The current
exhaust pipe has nothing on the end, it's just the pipe sticking out
about a foot.



There would be some benefit, how much? Don't know, but I would recommend
it too..
Before you attempt this I would get a manual for the furnace. I work for a
Ruud dealer, which is the same furnace, different label. The air inlet
requires a condensate catch "T" to keep moisture in the fresh air from
making it's way to the furnace. Also the air inlet is supposed to be close
to the exhaust, within a few inches, generally slightly below, with an
elbow pointed down. All this info will be in the manual. find one, read
it, and follow the directions to the letter! It may look easy, and it
really is, as long as the instructions are followed. I have be out on
calls to solve problems with furnaces that are not running properly. Often
it is just a problem with the install. Too much pipe, too many elbows,
wrong diameter of pipe, and so on.

What size of furnace is it? Larger furnace require larger pipe. Longer
runs require larger pipe. It may be possible that your furnace is not
installed properly right now!
I can e-mail you a manual if you give me the model of the furnace.
E-mail me if you like.
Greg
Thanks for the response. I've replied via email with more info, but in case
you don't receive it, the furnace is a Rheem "90 Plus" upflow natural gas
furnace. I think it's in the85K to 90K BTU range.

The previous owners didn't leave any documentation. I've done some
searching, and have found a spec sheet and a "use and care guide", but
haven't located anything relating to the inlet air piping, so if you can
point me to site with more info, I would appreciate it.

Mike O'Donnell

Call Rheem

"Mike O'Donnell" wrote in message
...

"Greg O" wrote in message
...
"Mike O." wrote in message
...

Would it be of any benefit to run a PVC pipe to the inlet of the
furnace? The total length of the run would be about 12 feet, and would
involve one 90 degree bend and a 45. The pipe would come out of the
house about 30 inches away from the exhaust (is that sufficient?).
Also, would I (or should I) put a final fitting (90 or 45) tilting
downward on the end of the pipe where it sticks outside to keep rain &
snow out? The current exhaust pipe has nothing on the end, it's just
the pipe sticking out about a foot.



There would be some benefit, how much? Don't know, but I would recommend
it too..
Before you attempt this I would get a manual for the furnace. I work for
a Ruud dealer, which is the same furnace, different label. The air inlet
requires a condensate catch "T" to keep moisture in the fresh air from
making it's way to the furnace. Also the air inlet is supposed to be
close to the exhaust, within a few inches, generally slightly below, with
an elbow pointed down. All this info will be in the manual. find one,
read it, and follow the directions to the letter! It may look easy, and
it really is, as long as the instructions are followed. I have be out on
calls to solve problems with furnaces that are not running properly.
Often it is just a problem with the install. Too much pipe, too many
elbows, wrong diameter of pipe, and so on.

What size of furnace is it? Larger furnace require larger pipe. Longer
runs require larger pipe. It may be possible that your furnace is not
installed properly right now!
I can e-mail you a manual if you give me the model of the furnace.
E-mail me if you like.
Greg
Thanks for the response. I've replied via email with more info, but in
case you don't receive it, the furnace is a Rheem "90 Plus" upflow natural
gas furnace. I think it's in the85K to 90K BTU range.

The previous owners didn't leave any documentation. I've done some
searching, and have found a spec sheet and a "use and care guide", but
haven't located anything relating to the inlet air piping, so if you can
point me to site with more info, I would appreciate it.

Mike O'Donnell


Re-send the e-mail. I did not get it, or it got deleted by mistake. I NEED
model numbers!! The numbers will be found on the inside of the furnace.
There are stickers with the info on the right or left side.
Send it to Hotmail addy!
Greg
goo1959 @ hotmail.com (leave out the spaces!)

Mike O. wrote:
About three years ago we moved into a 40 year old house. Sometime during
the last few years prior to us buying it, the previous owners replaced the
forced air gas furnace with a Rheem high efficiency model (92, I believe).
The exhaust line is standard 2 inch PVC pipe vented out the side of the
house.
The question I have is regarding the input for the combustion air. There's
no piping for this, just a two inch hole in the side of the furnace.
The furnace is in a full unfinished basement, so it's not starving for
combustion air. The house is not sealed as tight as some newer homes, so I'm
not worrying about oxygen levels in the house, but I would think that by
using the interior air for combustion it would cause cold air to be pulled
into the house via leaks. Also, it's sending the warmer house air out the
exhaust.

Would it be of any benefit to run a PVC pipe to the inlet of the furnace?
The total length of the run would be about 12 feet, and would involve one 90
degree bend and a 45. The pipe would come out of the house about 30 inches
away from the exhaust (is that sufficient?). Also, would I (or should I)
put a final fitting (90 or 45) tilting downward on the end of the pipe where
it sticks outside to keep rain & snow out? The current exhaust pipe has
nothing on the end, it's just the pipe sticking out about a foot.

As I said, I'm working on other issues to help the heat in and the cold
out, but I'm thinking every little bit I can do to keep the cold out would
help.
I did some internet searching, most of what I found seemed to indicate it
would be a good idea. Most concerns I found were about NOT having piped
input air, but it seems to be more of an issue the tighter your house is
sealed.

Mike O'Donnell

This is Turtle.

get with Grego and check this job out.

Yes put a air intake from outside will be a big help in heating cost by
burning out side cold air and not the houise room air that is heated
already. if you are going to run a intake air for a 92% afue Rheem
furnace. I suggest you run it with 3" PVC pipe to make sure your not
starving it in some way. i looked at the chart sometime back when
installing one and I had on a 100K btu heat furnace to run the vent out
and in with 3" pvc pipe and could have 2 -- 90 degree elbows and 90
foot run.

Now you need to get a Rheem Venting chart to see the size, length, and
btu rating for all to work.

TURTLE

wrote:

Yes put a air intake from outside will be a big help in heating cost by
burning out side cold air and not the houise room air that is heated
already...

If 100 cfm of 130 F air leaves the flue in either case, you can either
a) heat 100 cfm of house air from 30 to 70 F with about 4K Btu/h from
the furnace, then heat it from 70 to 130 with 6K Btu/h, total 10K Btu/h, or
b) heat 100 cfm of 30 F outdoor air from 30 to 130 with 10K Btu/h.

Where is the energy savings?

Nick

Nick the obvious energy savings is in not pulling in inside air to the
furnace, which the house makes up by sucking in cold outside air, you
need to learn some basics.

m Ransley wrote:

Nick the obvious energy savings is in not pulling in inside air to the
furnace, which the house makes up by sucking in cold outside air, you
need to learn some basics.

I disagree. There is no obvious savings.

Nick

Nick, yes the uninformed would disagree,

m Ransley wrote:

Nick, yes the uninformed would disagree,

You seem to be in arrogant dimwit mode again :-)

Where is the energy savings?

If 100 cfm of 130 F air leaves the flue in either case, you can either
a) heat 100 cfm of outdoor air from 30 to 70 F with about 4K Btu/h from
the furnace, then heat it from 70 to 130 with 6K Btu/h, total 10K Btu/h, or
b) heat 100 cfm of 30 F outdoor air from 30 to 130 with 10K Btu/h.

Nick

wrote:
wrote:

Yes put a air intake from outside will be a big help in heating cost by
burning out side cold air and not the houise room air that is heated
already...

If 100 cfm of 130 F air leaves the flue in either case, you can either
a) heat 100 cfm of house air from 30 to 70 F with about 4K Btu/h from
the furnace, then heat it from 70 to 130 with 6K Btu/h, total 10K Btu/h, or
b) heat 100 cfm of 30 F outdoor air from 30 to 130 with 10K Btu/h.

Where is the energy savings?

The outside air is "dry" so you'd have to add moisture to get it within
the "comfort zone", which requires additional energy. Using outside air
for combustion reduces or eliminates this energy sink. It also
decreases the potential for air infiltration in certain areas which
would make those areas "feel" cold, inciting the occupants to increase
the thermostat setpoint.

But I do believe you are correct that you must heat the combustion air
from 30 to 130 F overall, whether it comes through a pipe from the
outside or by way of the interior space. No free lunch. I'd be curious
if anyone could come up with an reasonable explanation otherwise.

Nick you are the dimwit, call any heating equipment manufacturer to
learn the benefits of using outside air, your math is irrelevant again.

The air in the house was heated at 90 percent efficiency (by the
furnace
heat exchanger) before it is sucked into the combustion chamber to
burn. If
you draw outside air directly into the combustion chamber, it is heated
at
100 percent efficiency (no losses in the heat exchanger). Also you
don't
have the effect of drawing humidity out of the house into the flue and
you
improve comfort by reducing drafts.

Sealed combustion also reduces standby losses that would otherwise go
up a
chimney. Such systems often require balanced pressures between intake
and
discharge to work properly. Check with your equipment manufacturer.

Stretch

wrote:

If 100 cfm of 130 F air leaves the flue in either case, you can either
a) heat 100 cfm of house air from 30 to 70 F with about 4K Btu/h from
the furnace, then heat it from 70 to 130 with 6K Btu/h, total 10K Btu/h, or
b) heat 100 cfm of 30 F outdoor air from 30 to 130 with 10K Btu/h.

Where is the energy savings?

The outside air is "dry" so you'd have to add moisture to get it within
the "comfort zone", which requires additional energy. Using outside air
for combustion reduces or eliminates this energy sink...

Agreed, altho you don't have to add moisture to be in the comfort zone.
You can also raise the air temp slightly. The energy needed to evaporate
water is several times greater than the energy saved with a lower thermostat
setting, unless you live in an extremely airtight house.

It also decreases the potential for air infiltration in certain areas which
would make those areas "feel" cold, inciting the occupants to increase
the thermostat setpoint.

And if exterior walls are warmer, with less inward infiltration, they lose
more heat to the outdoors. And uniform infiltration through wind barriers
could make them act like Scandinavian "breathing walls" with no heat loss
to the outdoors.

Nick

m Ransley wrote:

Nick you are the dimwit, call any heating equipment manufacturer to
learn the benefits of using outside air...

Call Lennox about winter humidification :-)

Nick

Stretch wrote:

The air in the house was heated at 90 percent efficiency (by the
furnace heat exchanger) before it is sucked into the combustion chamber to
burn. If you draw outside air directly into the combustion chamber,
it is heated at 100 percent efficiency (no losses in the heat exchanger).

Good point. There's a real energy savings, although it is small.

Sealed combustion also reduces standby losses that would otherwise go
up a chimney...

An automatic flue vent might do that better.

Nick

wrote:
wrote:

The outside air is "dry" so you'd have to add moisture to get it within
the "comfort zone", which requires additional energy. Using outside air
for combustion reduces or eliminates this energy sink...

Agreed, altho you don't have to add moisture to be in the comfort zone.
You can also raise the air temp slightly. The energy needed to evaporate
water is several times greater than the energy saved with a lower thermostat
setting, unless you live in an extremely airtight house.

Some people would rather not tolerate relative humidity below a certain
level, even if they "feel" warm enough, so "comfort zone" does not
pertain to temperature alone. We agree that constantly replenishing
water in the air generally requires greater energy than the slight
increase in temperature that would make it "feel" just as warm, unless
the humidity lost to air leaving the house can be made small. Sealed
combustion eliminates one of these losses. It is my observation that
sealed combustion can eliminate the need for supplemental
humidification altogether.

It also decreases the potential for air infiltration in certain areas which
would make those areas "feel" cold, inciting the occupants to increase
the thermostat setpoint.

And if exterior walls are warmer, with less inward infiltration, they lose
more heat to the outdoors. And uniform infiltration through wind barriers
could make them act like Scandinavian "breathing walls" with no heat loss
to the outdoors.

That's food for thought, but I remain skeptical that infiltration
induced by non-sealed furnace combustion will significantly decrease
the overall exterior temperature of conventionally built exterior walls
or make them perform like these Scandanavian walls of which you speak.
If infiltration creates cold spots on the interior surface, they'll
draw heat from the room.

It seems to me that even if unidirectional infiltration (i.e. enter
through wall, exit through furnace exhaust stack) were to reduce the
wall exterior surface temperature to outside temp so as to eliminate
heat loss from that surface, it wouldn't eliminate heat loss to the
outdoors altogether. The heat-losing surface has simply relocated to
somewhere within the wall or the living space.

I could see how the R value for a well designed breathing wall would be
unexpectedly high compared to a conventionally framed and fiberglassed
wall, if one's expectations are based on looks alone.

On 2005-11-10, wrote:

If 100 cfm of 130 F air leaves the flue in either case, you can either
a) heat 100 cfm of house air from 30 to 70 F with about 4K Btu/h from
the furnace, then heat it from 70 to 130 with 6K Btu/h, total 10K Btu/h, or
b) heat 100 cfm of 30 F outdoor air from 30 to 130 with 10K Btu/h.

Where is the energy savings?

Perhaps using colder combustion air results in colder flue gasses?

Personally, I'm in favor of direct vent gas appliances from a safety
point of view, to eliminate the possibility of backdrafting. Likewise
having a good source of combustion air is critical to the efficiency
of the burner.

Cheers, Wayne

m Ransley wrote:
Nick, yes the uninformed would disagree,

But the informed understand and agree with Nick's math, with the caveat
that the inside air was heated at less than 100% efficiency. Here's a
re-accounting of that original analysis.

Given a 92% efficient furnace, if 100 cfm of 130 F air leaves the flue
in either case, you can either

a) with sealed combustion, heat 100 cfm of 30 F outdoor air from 30 to
130 with 10K Btu/h, or

b) heat 100 cfm of outside air "leaked" into the house from 30 to 70 F
with about 4.3K Btu/h burned in the furnace, then heat it from 70 to
130 with 6K Btu/h, total 10.3K Btu/h.

In case a, 10K Btu/h is being exhausted, but remember that this furnace
exhausts only 8% of the energy it burns. That means the heat output
into the living space is 115K Btu/h, with 125K Btu/h total energy input
to the furnace. 115/125 = 92%

In case b, same 125K Btu/h input, but only 114.7K Btu/h real output
(10.3K is lost to support the combustion process). 114.7/125 = 91.8%.

Wayne Whitney wrote:

Perhaps using colder combustion air results in colder flue gasses?

Which leads to cooler air from the register.

Remember, the only change we're talking about is whether the combustion
air comes from inside or outside. If you can't see that adding cold air
to the fire makes the register air cooler, just imagine what the
register temp would be if the flame were extinguished but the fans kept
running.

Mike O. wrote:
About three years ago we moved into a 40 year old house. Sometime during
the last few years prior to us buying it, the previous owners replaced the
forced air gas furnace with a Rheem high efficiency model (92, I believe).
The exhaust line is standard 2 inch PVC pipe vented out the side of the
house.
The question I have is regarding the input for the combustion air.

Some points to consider in case it has been lost in the mix.

If the old furnace shared a flue with a gas water heater, the old flue
may be too large for the water heater by itself and may not be drafting
properly. Improper drafting may allow spilling of exhaust into the
building, and/or corrosive condensation in the flue. Investigate
whether you need a smaller diameter liner installed in the flue that
serves the water heater.

The heat exchanger of the 92% furnace is more easily clogged than the
low tech design. Inside air can contain dust, vapors from solvents etc.
that react with the flame and increase the rate at which the exchanger
corrodes or clogs.

Outside air marginally improves system efficiency. However, having the
intake and exhaust in the same "pressure zone" keeps their pressures
balanced when it is windy outside and promotes smoother combustion.

Eliminating this source of negative pressure may help control unwanted
drafts and reduce the "too dry" feeling indoors.

wrote:

Some people would rather not tolerate relative humidity below a certain
level, even if they "feel" warm enough, so "comfort zone" does not
pertain to temperature alone...

It does in a narrow technical sense, based on surveys of thousands of people
("Does it feel warm enough now?"), but as you say, some people have quirks.

We agree that constantly replenishing water in the air generally requires
greater energy than the slight increase in temperature that would make it
"feel" just as warm, unless the humidity lost to air leaving the house can
be made small.

Extremely small, like a submarine :-) Most US houses leak about 10 times
too much air to save any energy with winter humidification.

Sealed combustion eliminates one of these losses. It is my observation that
sealed combustion can eliminate the need for supplemental
humidification altogether.

That seems unlikely to me, in a typical US house with 0.7 ACH. Then again,
is there any such need?

It also decreases the potential for air infiltration in certain areas which
would make those areas "feel" cold, inciting the occupants to increase
the thermostat setpoint.

And if exterior walls are warmer, with less inward infiltration, they lose
more heat to the outdoors. And uniform infiltration through wind barriers
could make them act like Scandinavian "breathing walls" with no heat loss
to the outdoors.

That's food for thought, but I remain skeptical that infiltration
induced by non-sealed furnace combustion will significantly decrease
the overall exterior temperature of conventionally built exterior walls

It might move an extra 20 cfm in through the walls, cooling the insides
of the walls a bit and adding fresh air to a house (at a time when it
probably isn't needed :-)

or make them perform like these Scandanavian walls of which you speak.

They are carefully designed for a slow uniform inward airflow. Their
only "insulation" is a 1/4" porous felt with an infinite R-value :-)
It's unlikely a US wall would work that way, even with a wind barrier.
Tyvek and Typar are less porous than that breathing wall felt.

If infiltration creates cold spots on the interior surface, they'll
draw heat from the room.

And lose less heat to the outdoors. A wall with a 60 F interior surface
because of air infiltration will lose less heat to the outdoors than
the same wall with a 65 F interior surface with no air infiltration,
even with room air at the same (eg 70 F) temperature, no?

It seems to me that even if unidirectional infiltration (i.e. enter
through wall, exit through furnace exhaust stack) were to reduce the
wall exterior surface temperature to outside temp so as to eliminate
heat loss from that surface, it wouldn't eliminate heat loss to the
outdoors altogether. The heat-losing surface has simply relocated to
somewhere within the wall or the living space.

I could see how the R value for a well designed breathing wall would be
unexpectedly high compared to a conventionally framed and fiberglassed
wall, if one's expectations are based on looks alone.

I don't see how this has to do with looks. If a uniform sheet of outdoor
air is slowly flowing towards you from a wall (at a velocity less than
a perceptible draft), you can't lose any heat to that wall by convection,
because the warm air won't travel upwind. You can still lose heat by
radiation to the wall, and it still takes power to heat the air that
flows in through the wall, but the heat loss by convection through
the wall is zero, even with 1/4" insulation. Scandinavians have been
building and measuring walls like this for 30 years.

Nick

On 2005-11-11, wrote:

Wayne Whitney wrote:

Perhaps using colder combustion air results in colder flue gasses?

Which leads to cooler air from the register.

That may be, but doesn't lowering the temperature of the flue gasses
increases the fraction of water vapor in the flue gasses which
condense, improving the efficiency? It's not clear to me which effect
would be greater.

Cheers, Wayne

wrote in message
oups.com...

Mike O. wrote:
About three years ago we moved into a 40 year old house. Sometime during
the last few years prior to us buying it, the previous owners replaced
the
forced air gas furnace with a Rheem high efficiency model (92, I
believe).
The exhaust line is standard 2 inch PVC pipe vented out the side of the
house.
The question I have is regarding the input for the combustion air.
There's
no piping for this, just a two inch hole in the side of the furnace.
The furnace is in a full unfinished basement, so it's not starving for
combustion air. The house is not sealed as tight as some newer homes, so
I'm
not worrying about oxygen levels in the house, but I would think that by
using the interior air for combustion it would cause cold air to be
pulled
into the house via leaks. Also, it's sending the warmer house air out the
exhaust.

Would it be of any benefit to run a PVC pipe to the inlet of the furnace?
The total length of the run would be about 12 feet, and would involve one
90
degree bend and a 45. The pipe would come out of the house about 30
inches
away from the exhaust (is that sufficient?). Also, would I (or should I)
put a final fitting (90 or 45) tilting downward on the end of the pipe
where
it sticks outside to keep rain & snow out? The current exhaust pipe has
nothing on the end, it's just the pipe sticking out about a foot.

As I said, I'm working on other issues to help the heat in and the cold
out, but I'm thinking every little bit I can do to keep the cold out
would
help.
I did some internet searching, most of what I found seemed to indicate it
would be a good idea. Most concerns I found were about NOT having piped
input air, but it seems to be more of an issue the tighter your house is
sealed.

Mike O'Donnell

This is Turtle.

get with Grego and check this job out.

Yes put a air intake from outside will be a big help in heating cost by
burning out side cold air and not the houise room air that is heated
already. if you are going to run a intake air for a 92% afue Rheem
furnace. I suggest you run it with 3" PVC pipe to make sure your not
starving it in some way. i looked at the chart sometime back when
installing one and I had on a 100K btu heat furnace to run the vent out
and in with 3" pvc pipe and could have 2 -- 90 degree elbows and 90
foot run.

Now you need to get a Rheem Venting chart to see the size, length, and
btu rating for all to work.

TURTLE



I found the installation manual on the net, it goes into great detail about
the venting. Including specifying 3" PVC where the original installer (way
before we purchased the house) used 2". Looks like my next project is lined
up..

I've been emailing Greg O quite a bit and he has given lots of good advice.

Mike O'Donnell

wrote:
wrote:

Sealed combustion eliminates one of these losses. It is my observation that
sealed combustion can eliminate the need for supplemental
humidification altogether.

That seems unlikely to me, in a typical US house with 0.7 ACH. Then again,
is there any such need?

My criteria are that if the house holds 45% RH, there is no such need.
It is possible that the houses in question were atypical, but the
sealed combustion was instrumental in this.

A hypothesis that may interest you: the effect of a very large "stack
effect" air loss through the old furnace flue, now eliminated. For
example the flue was 8"x8" tile liner within a masonry chimney. Has
since been lined with a 4" stainless steel "snorkel" for the water
heater, sealed top and bottom (except of course where the 4" tube
passes through) to minimise air movement. This is a very large
reduction in effective thermal mass. I guesstimate that the water
heater flue cools within minutes of burner shutdown and stops drawing,
as compared to the case when the furnace operation kept the tile and
masonry warm enough on average to draw constantly through the generous
passageway.

Wayne Whitney wrote:
On 2005-11-11, wrote:

Wayne Whitney wrote:

Perhaps using colder combustion air results in colder flue gasses?

Which leads to cooler air from the register.

That may be, but doesn't lowering the temperature of the flue gasses
increases the fraction of water vapor in the flue gasses which
condense, improving the efficiency? It's not clear to me which effect
would be greater.

I think you're trying to ask, doesn't lowering the temperature of the
flue gasses
increase the fraction of water vapor which condenses from the flue
gasses?

As far as I know, the flue gasses which condense are already close to
100% water vapor because the CO2 fraction does not readily condense at
the temperatures in question.

If you lower the flue gas temperature by running the streams through
the heat exchanger in countercurrent instead of the same direction,
then you can increase efficiency and increase the condensed fraction of
H2O. Then you can add more area to the exchanger and condense some
more, up to a point. But those modifications were already done in
switching from a 80% non-condensing to a 90+% condensing furnace.

Now let's take the countercurrent condensing exchanger and lower the
temperature of the combustion air, which lowers the energy input into
the system. The resulting lower temperature throughout most of the heat
exchanger will result in decreased heat transfer there. The question
is, at the coolest part of the exchanger, will there be enough of an
increase in condensation to add enough heat to the airstream to
counteract the lower energy tranfer elsewhere? Somebody else might run
some numbers using some equations for heat exchangers, heat of
vaporisation and such, but I think the best you could hope for is a
wash.

wrote:

Given a 92% efficient furnace, if 100 cfm of 130 F air leaves the flue...

The gas furnace output would be about 500K Btu/h, enough for 10 houses :-)

Nick