If vents are in attic or in walls that leak out the attic or a cold
crawlspace yes, you loose a bit yes. Dont run it 24x7 and insulate , I
insulated ducts and blew in foam in the walls above in wall registers, I
blew out a few walls too with the expanding foam.
I think you miss the point. A gas furnace requires
1 square inch of opening for a certain number
of BTU's depending on the exact installation. If
you have a furnace siting in the open in a very
large space you don't have to have this
ventilation because infiltration would be
sufficient. But in normal situation you need the
ventilation. In my case it is per 4000 BTU but
not less than 100 square inches each for
combustion and ventilation. The combustion air
opening is required to be within 18 inches of the
floor and the ventilation air is require to be
within 18 inches of the ceiling. These vents are
passive, you may not open and close them, they are
open all the time as safety measures to prevent
build up of CO and to eliminate build up of gas
There is essentially nothing you can do to reduce
the infiltration through the combustion opening
and not way to reduce the heat loss through the
ventilation opening. My contention is that this
heat loss is never calculated in comparing
electric heat with gas heat. The result is a bias
in favor of gas heat because the electric heat
will not have these losses. If you live in an
especially cold climate, and the cost calculated
for electric and gas heat are about equal, the
real world cost of the electric heat will be lower
than for gas heat.
OK you are right to a point I think, Thats why I put in a condensing
unit with cold air intake outside, but I would not have optioned or
considered a window open type set up logical or necessary. To be sure I
understand you every time the furnace runs you are sucking in cold air
on a regular unit, but you say through a vent is necessary, or as in a
window open. If your chimney won`t draw this could be nessecary right,
but then the option is powervent exuast only. Or to be sure a blower
door test is done to calculate air exchanges per hr. I had to go direct
power vent everything after I put in new windows and all tyvek. unless
its a million or 2m btu commercial boiler in a small room I dont see
having a vent open to air as being an idea id even consider and I dought
you could get a house tight enough to not draw in air through
infiltration, it comes in every where, even the attic. Real tight houses
need fresh air recirculators for good oxygen and reducing humidity, but
heating systems through negative pressure generated by exuast get makeup
air through everywhere even closed windows have infiltration ratings,
because they leak. Direct vent condensing furnaces with outside air
intake are common I think all condensing furnaces offer this option
I don't know how the most efficient furnaces are
set up, mine is down a bit as are most peoples.
And I'm not going through the whole Installation
But lets just take the water tank which has an
unpowered flue and a pilot light. The tank has a
hole in the center above the burner, there is a
gap between the tank the funnel which is the
bottom of the exhaust flue which is about 2-1/2 in
diameter. There is a louvered vent in the ceiling
to the attic and a louvered vent in the floor to
the crawlspace. When the burner is on, air is
sucked from the adjoining rooms but mainly from
the crawlspace in to the burner and goes up the
exhaust pipe. Now what do you think the air flow
is like when the burner is not on. Warm air
naturally rises so warm air goes through the
burner area but probably mostly through the gap at
the funnel and goes up the flue to the outside.
In doing this, it pulls air not only from the
living space but also from the crawl space. Of
course warm air also rises through the vent in the
ceiling and it is much larger than the flue. All
of these areas are much warmer than the outside so
heat is lost and cold air from the outside is
pulled in. BTW, the furnace is essentially the
same except that it is a higher efficiency unit so
it has an inductor fan.
Now if the system were completely sealed, and
maybe high efficiency units are but I don't think
they are, you wouldn't have these losses.
Who requires this opening of venting 18 " off the floor and ceiling, not
my furnace lennox. Ive never heard of a house having this open vent to
outdoors set up. My apt building with a 1.6 million btu boiler has a
window cracked 1" and the flame is adjusted by a tech with a Bachrach
set up and he says oxygen, color and burner efficiency are tops. boiler
is in a small room, heats 16 units steam heat. Infiltration does the
I wonder how much, if any, difference it would be with a larger vent
instead of infiltration. I run a large boiler in one of our buildings and it
has a motorized louver. You need a winter coat if you stand in front of the
boiler when the blower is running for combustion. The boiler room is
otherwise closed off by a door. This is in an old building with plenty of
drafts and infiltration, but spread out over four floors and distances of
200 feet or so.
Yours is cold, I packed a foot of insulation and its still real warm,
Yes I need a vent but now I use a window 15 ft away .The proper way is
exterior air or like what you have a motorised air set up is better than
a window, its a Kewanee 2 pass
More precisely, 1 ft^3 of gas has a heating value of about 1000 Btu, and
burning it takes about 9.6 ft^3 of air, so a 50K Btu/h furnace burns about
50 ft^3 per hour with about 9.6x50 = 480 ft^3/h or 8 cfm of combustion air.
An average 2400 ft^2 US house leaks about 0.5x2400x8/60 = 160 cfm, 20 times
more than 8 cfm, so vents for combustion air or gas leak dilution seem
completely unnecessary, unless you install the furnace in a tight closet.
I'm not going to go through all the calculations,
so I will assume they are correct, but see below.
I don't think 2400 square feet is the average
house. Besides average means nothing, the median
size house tells a lot more and I am sure it is
well below 2400 square feet. Anyway my house is
1500 square feet and I don't think that it leaked
anywhere near that amount of air (before
installation of the gas furnace and water heater.
The rules as I read them are for a confined space
and an unconfined space. Mine furnace is in fact
in a closet, but even if it were outside the
closet or the closet door was removed, it would
still be categorized as a confined space and has
to follow the confined space rules. That is
because unconfined space is defined as that part
that directly communicates with the furnace and is
not equipped with doors. Much of my house would
be behind those doors even though they are
normally all open. It doesn't matter what the
actual air flow facts are or what I think of the
rules, the installers apparently followed the
rules and the result is a lot more air
infiltration and passive heat loss than before the
electric furnace and water heater were replaced
with gas appliances.
Just as a side note, I also would never again have
a gas water heater because the thermostat has way
too much range compared to an electric water
heater. My electric water heaters always held the
water within a very narrow range of temperature
(say 3-4 degrees). My gas water heater can't keep
water within a range of less than 10-15 degrees.
Heaters have two different types of efficiency ratings. One is called steady
state efficiency, and one is called seasonal efficiency.
The only way to rate a heater is under laboratory conditions. If you take
the exact same heater, and install it in two different homes, the season
efficiency will change, depending on run time. Natural draft through a
vented heater will cool it down when it isn't running.
The draft you are talking about is considered "infiltration", and is taken
into account during a heat loss/gain calculation. It has little to do with
the efficiency of the heater.
Yes, but my understanding is the furnace
manufacture calculates the heater efficiency
without taking required ventilation (draft) into
account. So when you compare a gas furnace which
requires a certain amount of ventilation to an
electric furnace which requires no ventilation,
The results are skewed in favor of the gas furnace.
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