At today’s inspection I observed an intake flue that did not come in from the outside but rather had an elbow coming off the top of the furnace with a perforated screen glued into the opening. This is on a townhouse.
There is also the standard flexible 6’’ combustion air brought into the room but I thought both the intake & outtake had to go outside? Any problems with this?
Many manufacturers say run to outside and it is best practice.
Look up the manual.
I’ve heard arguments for doing it both ways. By taking the combustion air from around the furnace it is tempered and not drawing super cold air into the unit which is though to make it less efficient and shorten it’s life. It also draws some fresh air into an area that might not get allot of natural air changes.
The people against this state that drawing air from inside a conditioned space or connected to a conditioned space is not good because your sending air that has been heated through the unit to outside. If the area the furnace is drawing air from is a damp basement or crawl the moisture has been know to cause some of these units to rust prematurely.
I just look up the manufactures recommendation and go by that first.
I have seen them like that. Its less efficient but generally not a problem. The PVC has always been facing the other way though.
No it is a problem …damn you guys make me work but no time to paste.
You tell him it is OK and you are wrong.
Screw the HVAC tech that says it is OK.
The installation is generally not a problem in and of itself Bob. Manufacturers may suggest otherwise.
However, combustion air requirements and contamination problems still apply.
I don’t know about the direction its facing though. It shouldn’t be restricted like that.
WTF?
Any trained HVAC tech would know this needs to be totally outside.
Its supposed to be a CLOSED combustion/exhaust system,
designed NOT to pull air from INSIDE the home.
If that pulls air from inside the home, its going to cause negative pressure in that space where its located. Wrong wrong wrong.
Every 90%er should be BOTH pipes to the outside.
I generally include something similar to this, more or less, in your situation.
Recommendation: High efficiency condensing furnaces may be installed with outdoor combustion air piped directly to the furnace. This is referred to as “direct vent” or “two pipe” installation.
Installation with air taken from around the furnace is sometimes referred to as “one pipe” installation - i.e. only the vent (exhaust) pipe is provided. (The furnace in this house is “one-pipe”) An important consideration in selecting one or two pipe installation is the quality of the combustion air. Indoor air is sometimes contaminated with various household chemicals which can cause severe corrosion in the furnace combustion system. Some common sources of these chemicals are detergents, bleaches, aerosol sprays, and cleaning solvents.
If the furnace is operated without adequate air for combustion and ventilation, it may not perform properly. Excessive exposure to contaminated combustion air will result in performance related problems. The recommended source of combustion air is outdoor air. Check with manufacturers installation requirements.
Unless indoor air is known to be free of these materials, two pipe installation is recommended.
Also:
Outdoor air is required as the source of combustion air when the indoor air is contaminated with chemical substances and in the following types of installations:
_ Furnaces installed in laundry rooms
_ Furnaces installed in hair salons
_ Furnaces installed in commercial buildings
_ Furnaces installed in buildings with indoor pools
_ Furnaces installed in hobby or craft rooms
_ Furnaces installed near chemical storage areas
Follow the combustion air requirements for the air to be taken from indoors. (Sq. ft. - building tightness - dackdrafting issues with other equipment etc.)
As air leakage is the biggest weatherization issue, it is always best to take it from outdoors. The air volume the furnace uses bust be replaced with outdoor air that enters from somewhere else in the house (witch may not be desirable air to be bringing into the house).
As of using cold air, that it total BS.
The colder the air the more O2.
As for moisture drawn from a damp space: more BS.
Do you have any idea what happens to water that is introduced to the flame temperature? H20! That’s a hint…
Can you tell me what the dew point temperature or relative humidity of 100%rh outdoor air at 32F that is heated to 2,000 F? (I’m not picking on anyone except those that think the internet information is flawless).
Hydrogen is a fuel and O2 is what makes it combust.
Are you saying someone told you it would rust the heat exchanger?
That is not possible. It’s against the law of physics and thermodynamics.
“Tight” buildings (with weather stripping
and caulk to reduce infi ltration),
may require special provisions for
introduction of outside air to ensure
satisfactory combustion and venting,
even though the furnace is located in
an unconfi ned space.
An unconfi ned space is an area including all
rooms not separated by doors with a volume
greater than 50 cubic feet per 1,000 Btuh of the
combined input rates of all appliances which
draw combustion air from that space. For example,
a space including a water heater rated
at 45,000 Btuh and a furnace rated at 75,000
Btuh requires a volume of 6,000 cubic feet [50 x
(45 + 75) = 6,000] to be considered unconfi ned.
If the space has an 8 foot ceiling, the fl oor area
of the space must be 750 square feet (6,000
/ 8 = 750). In general, a furnace installed in an
unconfi ned space will not require outside air for
combustion.
WARNING:
Furnaces installed with combustion
air drawn from a heated space which
includes exhaust fans, fi replaces, or
other devices that may produce a negative
pressure should be considered
confi ned space installations.
-
Exterior air, in the winter, is colder and denser. It therefore has a slightly higher oxygen content and the HE furnaces are designed to use the extra oxygen and burn cleaner, therefore, more efficiently.
-
By taking combustion air from the interior, you lower the air pressure inside and create a negative pressure condition in the house. This strains the furnace to get more air and also leads to more cold air leaks into the house.
Finally, why would you want to take interior air, that you have already spent money heating, and send it out the furnace? Think the old style fireplaces, with the combustion and draft air coming from the interior, vs. the newer type fireplaces with a sealed hearth and an exterior combustion air vent out the back of the chimney.
Air Requirements For One-Pipe
Installation
When air for combustion is to be taken from
around the furnace, a protective screen must be
installed over the combustion air intake opening.
This screen is provided with the furnace installation
instructions and functions to prevent debris
from entering the combustion system. It should
be installed on the combustion air intake collar
or inlet PVC. If furnace location is such that this
opening might be unintentionally obstructed, a 3”
PVC elbow must be installed on the collar, and
the screen placed inside the inlet of the elbow.
See Figure 10.
CAUTION:
“Tight” buildings (with weather stripping
and caulk to reduce infi ltration),
may require special provisions for
introduction of outside air to ensure
satisfactory combustion and venting,
even though the furnace is located in
an unconfi ned space.
An unconfi ned space is an area including all
rooms not separated by doors with a volume
greater than 50 cubic feet per 1,000 Btuh of the
combined input rates of all appliances which
draw combustion air from that space. For example,
a space including a water heater rated
at 45,000 Btuh and a furnace rated at 75,000
Btuh requires a volume of 6,000 cubic feet [50 x
(45 + 75) = 6,000] to be considered unconfi ned.
If the space has an 8 foot ceiling, the fl oor area
of the space must be 750 square feet (6,000
/ 8 = 750). In general, a furnace installed in an
unconfi ned space will not require outside air for
combustion.
WARNING:
Furnaces installed with combustion
air drawn from a heated space which
includes exhaust fans, fi replaces, or
other devices that may produce a negative
pressure should be considered
confi ned space installations.
It is stupid ,you need same air pressure differential.
I think it is not so hard to run a send schedule 40 and if it is then use a concentric.
Why argue …just do it.
Yeah Bob. None of those things say the installation in of itself is wrong. Unless the manufacturer says otherwise its ok.
Like I said, combustion air and contamination still apply as with all units.
Its not ideal but its not a defective installation.
I agree that its absolutely stupid. You pay for a high eff unit and then the idiot installs a one pipe system. Its still not wrong.
Some manufacturers will void the warranty if not from outside and with all furnaces going cat 4 after May 1st now is the time to learn this.
You feel like looking at each and every manual from now on?
Here I took a picture from I think a Rheem cat 4 manual this summer .
Can you guys read it ?
I often look at manuals. If I couldn’t find one, I would write something to the effect that some manufacturers require combustion from the outside and I would recommend they contact the manufacturer. I would not write that its wrong.
I am not sure how many times I have to say it depends on the manufacturer. I have one up now that says it can be either way. Want a picture? :roll: