Question about combustion air input on a high efficienct furnace.

[modervador]

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.

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.

[Wayne Whitney]

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

[modervador]

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%.

[modervador]

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.

[modervador]

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.

[nicksanspam]

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.

Nick

[Wayne Whitney]

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

[Mike O'Donnell]

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

[nicksanspam]

You may be right.

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.

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.)

Nick

[modervador]

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.

[modervador]

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.

[nicksanspam]

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

now testing a commercial "dynamic breathing wall" product in Scotland, Holland, Italy and Dubai. Coming soon to the US (in February in Houston.)

Nick

[nicksanspam]

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

Nick