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Direct vent chimneys or exhaust for heating appliances:
This article describes side wall vent systems for conventional & mid-range-efficiency heating boilers, furnaces & water heaters. We include for comparison, low temperature side wall vent systems used by high efficiency or condensing boilers, furnaces & water heaters.
We explain the difference between side wall or direct venting for conventional/mid-range efficiency oil or gas burning heaters and side wall vented high efficiency condensing heating appliances.
Direct-venting or side wall vent chimney and flue systems are a method of venting the exhaust gases from a heating appliance directly out through the side wall of a building while eliminating the need for a vertical chimney of any sort. Gas or oil fired side wall power venters are provided by several manufacturers listed at the end of this article.
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For safe and clean sidewall venting of oil fired heating equipment the vent system incorporates a power vent fan to assure that gases leave the building side wall with sufficient velocity to avoid sooting or otherwise harming the wall.
The system, such as Tjernlund's SIDESHOT® series of power vents draws outdoor air thorough the outer passage(s) of a multi-walled vent plenum, cooling the exhaust gases as they pass through the vent.
One look at the "do-it-yourself" attempt at side wall venting (photo at left) makes clear why the proper equipment is needed to vent an oil fired appliance directly though the building wall. Avoid a building fire with do-it-yourself though-wall chimneys & flues
Our soot stained wall photo (photo at left ) shows what happens at a direct-vented oil-fired heating system when there are multiple errors and unsafe conditions including:
The heater is not working properly and needs immediate service as it is blowing thick dense sooty smoke
The through-wall metal flue vent and "chimney" appears to be a home-made adaptation rather than a listed and approved direct-vent device.
The effects of this home made direct-vent "chimney" are quite visible: the siding on the building has been thickly coated with soot. If you (click to) enlarge the photo you will also see some interesting reverse thermal tracking effects marking the wall studs. This is an unsafe installation that needs immediate repair.
Watch out: the photograph above illustrates an unsafe thorough-wall vent for an oil fired heating appliance.
See SOOT on OIL FIRED HEATING EQUIPMENT for more information. We have received or read building owner complaints that sidewall venting has "ruined the building siding" or "stained the siding" but in our OPINION such problems occur because of an improper installation or improper heating equipment maintenance.
Our photo above shows an Energy Kinetcs oil fired heating boiler vented using an OEM direct vent system.
Shown below is the exterior wall of the same installation. The stained wall photo (photo at left ) shows what can happen at a direct-vented high-efficiency heating boiler if the vent is not properly sloped through the wall. Condensate accumulates in the vent pipe, dissolves flue gas deposits, and ultimately leaks both outside and back into the equipment.
To repair this mistake the installer will have to disassemble the entire vent system, and either remove a bit of masonry block from the bottom of the present wall opening or change the interior flue vent connector piping to slightly raise the inside end of the through wall vent - one or the other - to obtain proper condensate slope and condensate handling on this equipment.
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Exhaust or venting of heating appliances may be horizontal, direct through a building side wall, or vertical, up through the building roof. But what is the difference between Direct Exhaust and Direct Venting ? Weil-McLain makes the following important distinctions: 
Direct Venting uses a power ventilating blower or fan (and in some models a heat exchanger plenum to cool outgoing gases passing through the building wall (sidewall direct exhaust) or roof or through an existing unused chimney through which a vent pipe is passed (vertical direct exhaust). Combustion air for heating boilers or furnaces is drawn from outdoors through a dedicated air intake pipe or duct (the small diameter pipe in the pair at left of our photo below).
Direct exhaust venting draws combustion air from the utility room or boiler room around the heating appliance and vents appliance exhaust out through a building sidewall or through the roof using an approved or listed B-vent, metal chimney, or similar materials. Combustion air is provided to the heating appliance from the space surrounding the equipment.
Sidewall direct exhaust uses a B-vent or other listed or approved metal or even plastic flue vent connector and metal chimney materials to vent outgoing combustion gases through a building side wall (sidewall direct vent).
A blower or power vent draws combustion air in to the heater and a power vent pushes exhaust gases out through a separate or dedicated exhaust flue.
The heating appliance vents directly out through a building side wall, powered by natural draft provided by the heating equipment, typically using a single wall metal flue or chimney or a B-vent. This venting method, typically for gas fired boilers, can be used only by certain heating appliance models such as Weil-McLain's CGs boilers excluding the CGs-4E model.
Vertical direct exhaust is a similar installation to the sidewall direct exhaust vented vertically, typically up through the building roof. This heater venting system, typically for gas fired boilers, is used only by certain heating appliance models such as Weil-McLain's CGs boilers.
Image at above left courtesy of Carson Dunlop Associates. [Click to enlarge any image]
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Safety controls and power vents for gas fired heating appliances are certified by the AGA, the American Gas Association.
Watch out: don't confuse direct vent heating equipment with the through-wall venting and air intake of high efficiency boilers and furnaces.
Both types of heating systems can vent horizontally through a building wall, but the exhaust products of high efficiency or condensing boilers and furnaces are generally cool and have different venting and combustible cleareance specifications.
In comparing the venting of exhaust gases from a high efficiency furnace, boiler or water heater with the venting of exhaust gases from a conventional heating system it will be immediately obvious that the high efficiency equipment exhaust is produced at a low-enough temperature that it is vented through comparably small-diameter plastic piping rather than a cooled, fire-protected metal heating vent.
What can be confusing is that some mid-range efficiency heating equipment may vent through a (larger diameter, say 4") plastic heater vent referred to as HTPV (high temperature plastic vent) chimneys.
We illustrate an HTPV system at below left and a high efficiency plastic direct vent system at below right (Image courtesy of Carson Dunlop Associates).
More details about HTPV and a product safety recall are
at PLASTIC Plexvent / Ultravent RECALL.
At GAS APPLIANCE DIRECT VENT CLEARANCES we provide a complete list of required clearance distances between the air intake or combustion gas exhaust vents for direct vented heating appliances. Illustration adapted from Thermo Products installation instructions - click to enlarge this or any other image or photo at InspectAPedia. 
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Some highlights include:
We moved this data to a separate table now found at GAS FIREPLACE VENT CLEARANCE REQUIREMENTS Separate Table for Direct Vented Gas Fireplaces
See CLEARANCE DISTANCE, HVAC where a reader commented that his building inspector did not accept the safety of the installation shown below. We agreed.
I googled “carbon monoxide plastic flue roof top side house” and came across your website with relevant information. I have a question below, and please let me know if there is a fee for answer and what is the fee. I would like to address my concern of CO coming out from side of house, even if it is per code.
Breathing in CO is of great concern to me. Comparing it to car exhaust does not comfort me, as we would not a stationary car for the exhaust gas with CO at same place as plastic flue side wall vent. Please assist answer the questions below. Thank you. B.L. by private email, 2016/03/30
Direct vent gas vent clearance required depend in part on the size in BTUs (BTUH or thousands of BTUs per hour input rating) of the heating appliance. We marked on your gas vent photo the two measurements that are typically made: the vertical distance to the window and the horizontal distance to the window from the nearest point of the gas vent.
For a typical gas fired water heater, the distance from the vent outlet to some building features are a bit different depending on whether your home is in Canada or the U.S. or in another country and jurisdiction.
But for operable windows, both countries use the same guideline: the distance shown by the red arrows needs to be 3 feet for heating appliances over 100,000 BTUH.
These clearance instructions were in the General Venting Requirements installation manua for your vent system, in the document that you sent to me (page 65), as
Watch out: Both direct vent chimney specifications (discussed beginning at the start of this article
at DIRECT VENTS / SIDE WALL VENTS) and
condensing boiler or furnace vents (shown at below left and discussed
at DIRECT VENT GAS HEATER IMPROPERLY INSTALLED) must be installed following the manufacturer's specifications.
Otherewise there is risk that condensate either freezes (if you're in a freezing climage) to block the heater's exhausts (you lose heat) or to produce dangerous carbon monoxide gas in the building.
Specifications for direct vent and condensing boiler vents may specify that the vent line slopes to indoors where condensate is to be disposed-of at an interior condensate handling system or drain, or they vent may be required to slope to the outside.
Our illustration at left shows the plastic pipes of two types of direct vent heating appliances protruding through a building sidewall.
We can tell from the height above ground that the heaters are most likely located in the building basement.
The pair of plastic lines in the left of the photo are an air intake (the shorter protruding plastic pipe) and exhaust vent outlet (the longer plastic outlet pipe). The wider single round plastic vent at the right side of our photo is venting a second appliance, perhaps a water heater.
Watch out: We can also see that as with the gas appliance power vent shown in the previous section, this high efficiency heating system condensate is also not being properly drained from the left hand condensing heater - instead of condensate running back into the building and into a building drain, this pipe is sloped so that condensate runs out of the end of the plastic vent line.
The problem with this arrangement becomes evident in cold weather as condensate freezes and the ice formed can actually block the safe venting of exhaust gases.
Watch out: Ice formation at sidewall vents is not the only cold weather hazard for this equipment. Our photo at left shows that the vents are less than 24 inches from the ground. In climates where snowfall may occur at depths capable of covering the air intake or sidewall vent outlet, Vermont Gas and Thermo Products both warn that it is critical to keep sidewall vents clear of snow-cover.
Blocking the combustion air intake or exhaust outlet by accumulated snow, ice, or even shrubs or piled leaves can result in dangerous. potentially fatal carbon monoxide gas poisoning of the building occupants.
You should inspect the exhaust vent and combustion air intake vent for blockage at least annually, and we recommend further inspection in winter for blockage by snow or ice:
The vent and combustion air terminations shall be checked periodically, at least at the start of each heating season, for restriction or blockage from foreign material in the exhaust vent or in the air intake piping. Clean the air intake and vent terminations when necessary. 
The outlet/inlet of the vent and air intake terminations shall be a minimum of 12 inches above highest anticipated snow level. The vent outlet must be installed a minimum of 12 in. above the air intake inlet.
Terminations must also be kept clear of any leaves, weeds, combustible materials, snow, and ice build-up. 
and Thermo Products further recommends:
In geographical areas with considerable snowfall, it is advisable to locate the vent terminal much higher than the minimum 12-inches above ground to prevent blockage by snow accumulation or drifting. 
For at least some heating appliances and manufacturers, and to solve vent clearance difficulties when your installation cannot meet the specifications in the Gas Code, manufacturers' specifications, or local building codes, roof vent termination kits are available.
Notice that the illustration (left) of roof-vent termination of direct-vent appliance air intake and exhaust does not show the necessary flashing & sealing to avoid roof leaks.
Illustration adapted from Thermo Products installation instructions. 
The furnace may be vented vertically through the roof. The outlet/inlet of the vent and air intake terminations shall be a minimum of 12 inches above highest anticipated snow level. The vent outlet must be installed a minimum of 12 in. above the air intake inlet.
The combustion air intake shall be installed upwind of the vent outlet when exposed to prevailing winds. The exhaust vent and combustion air intake can be a minimum of 3 in. and a max. of 24 in. apart. 
Watch out: when chimneys or vents pass through building floors and roofs above, additional fires stopping may be required.
The combustion air intake shall be installed upwind of the vent outlet when exposed to prevailing winds.
Avoid locating the vent terminal on a wall facing prevailing winds and wide-open areas.
When impractical, choose a location that protects the vent from strong winds, such as behind a fence or hedge. 
Comment: Keep hedges, fencing, or other wind barriers far enough away from the air intake vent to avoid obstructing air intake, and keep hedges far enough away to avoid plant injury from the heat of exhaust gases.
Illustration adapted from Thermo Products installation instructions. 
Avoid locating the vent terminal over areas where dripping of condensate, or small pools of acidic condensate, could create a problem. 
Comment: OPINION: an exhaust vent that is dripping condensate to the outdoors in freezing climates risks dangerous blockage by ice formation.
Weil Mc-Lain , Thermo Products  and other manufacturers warn that if the heating appliance is in an area where local indoor-area-supplied combustion air is likely to be contaminated the installer must pipe an outdoor combustion air supply to the heating boiler (or other heating appliance) combustion air intake port.
Watch out: There are critical concerns with combustion air contamination for heating appliances:
Combustion air that is contaminated with corrosives can damage the boiler by corroding the heat exchanger or other components. The result can be worse than damage to the equipment. Corrosion that leads to flue gas leaks can leak potentially fatal carbon monoxide or other gases into the occupied space of the building.
Other combustion air contaminants that are flammable or themselves combustible could lead to an actual fire or explosion.
If your building contains any of these or other corrosive or explosive products and if you cannot remove them from the locale, an outside combustion air supply must be piped to the heating appliance air intake.
Watch out: also make sure that the combustion air supply outdoors is not itself close to a source of corrosive or explosive materials.
Direct sidewall vent heating appliance manufacturers & products can be listed here at no fee. CONTACT US to provide information or technical comment.
Listed special gas vent systems that comply with UL-1738 & UL S636 and in Canada, certified by CSA are the only vent systems that can be used with Weil Mc-Clain's CGs heating boilers. Depending on the brand and model, your heating appliance may have similar restrictions so be sure to read the installation instructions with care.
This topic has moved to DIRECT VENT INSTALLATION, SNAFUS & SPECS - live link given just below.
Continue reading at GAS APPLIANCE DIRECT VENT CLEARANCES - table of clearance distances, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
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