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.
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.
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
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.
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.
Extremely small, like a submarine :-) Most US houses leak about 10 times
too much air to save any energy with winter humidification.
That seems unlikely to me, in a typical US house with 0.7 ACH. Then again,
is there any such need?
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 :-)
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.
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?
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.
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
On 2005-11-10, email@example.com wrote:
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.
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
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
increase the fraction of water vapor which condenses from the flue
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
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
I've been emailing Greg O quite a bit and he has given lots of good advice.
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.
I've run into that kind of problem when thinking about how to preheat the
air that flows in through the wall with outgoing air, using a large poly
film air-air heat exchanger built into the wall. Maybe the heat exchanger
and incoming air filter need to be inside a 4' cube filled with poly film
"plates" on 1/2 inch centers, or several cubes or valances near ceilings.
http://www.cibse.org/pdfs/8cimbabi.pdf has an equation for the dynamic
metric U-value of a breathing wall, as corrected:
Ud = VRhoaCa/(e^(VRhoaCaRs)-1) W/m^2K, where
V is the air velocity in meters per second,
Rhoa is air density, 1.2 kg/m^3,
Ca is the air's specific heat, 1000 J/(kg-K), and
Rs is the wall's static thermal resistance in m^2-K/W.
Using V = 1/3600 (1 meter per HOUR :-), and Rs = 5.7 m^2K/W (a US R32 wall),
Ud = 0.058 W/m^2, like a US R98 wall. A more typical V = 10 meters per hour
makes Ud = 1.7x10^-8 W/m^2K, like a US wall with an R-value of 334 million :-)
Ud becomes infinite as V becomes 0, which seems odd. And Ud approaches 0 as
V increases, no matter what kind of porous insulation is used. I'm surprised
to see 40 mm (about 1.6 inches) in the pdf. That seems very thick. I thought
Scandinavian breathing walls used something closer to 1/4 inch felt. Would
a single layer of Tyvek or Typar wind barrier work, with a replaceable air
filter? Typar seems to have too much pneumatic resistance. Maybe perforated
"builder's foil." Bucky Fuller wanted to minimize building materials...
With an air-air heat exchanger, we might move, say 240 cfm through 4000 ft^2
of walls and ceilings of a 40x60' house at V = 240/4000 = 0.06 fpm, ie 3.05
x 10^-4 m/s, giving a US R20 (metric R3.5) wall Ud = 1200V/(e^(1200V3.5)-1)
= 0.141 W/m^2K, like a US R40 wall. Doubling V might make it an R93 wall, if
the house were otherwise airtight (a big if, in the US.)
http://www.cibse.org/pdfs/8cimbabi.pdf has an equation for the dynamic
No... it approaches 1/Rs, like it orta, as e^x --> 1+x :-) For instance,
Rs = 5 and V = 10^-8 makes Ud = 1.2x10^-5/(e^(6x10^-5)-1) = 0.19994...
Profs Imbabi and friends at http://www.environmental-building.com
are now testing a commercial "dynamic breathing wall" product in
Scotland, Holland, Italy and Dubai. Coming soon to the US
(in February in Houston.)
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.