Unsafe through-wall side wall vent chimney (C) D Friedman Side Wall Vent Chimneys & Flues - Direct-Venting Heating Appliance Chimneys & Flues

  • DIRECT VENTS / SIDE WALL VENTS - CONTENTS: guide to choosing, installing, & troubleshooting through-wall or side-wall venting exhaust devices for heating appliances like oil or gas fired boilers, furnaces, or water heaters - Photos & examples of sidewall venting or direct venting installations that are improper and/or unsafe; Side Wall Power Vent Equipment for Gas Fired Boilers, Furnaces, Water Heaters. DEFINITIONS of Sidewall or Vertical Direct Venting compared with Direct Exhaust;
  • POST a QUESTION or READ FAQs about the installation or diagnosis of sidewall vent or direct vent chimney or flue exhaust systems for heating appliances: oil, gas, other fuels firing heating appliances & fireplaces
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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.

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Guide to Direct Vent or Side-Wall Vent Chimneys & Flues

Vent Drip (C) Daniel FriedmanDirect-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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Article Series Contents

Side Wall Vent Equipment for Oil Fired Boilers, Furnaces, Water Heaters

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.

Risk of fire and heater malfunction with improperly installed or site-built side wall vent systems

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.

Side Wall Power Vent Equipment for Gas Fired Boilers, Furnaces, Water Heaters

Vent Drip (C) Daniel Friedman

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.

Vent Drip (C) Daniel Friedman

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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Direct or Sidewall Vent Conventional Oil or Gas Fired Heater Exhaust compared with a High Efficiency Condensing Furnace or Boiler

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: [10]

Definitions of Sidewall or Vertical Direct Venting compared with Direct Exhaust

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).

Definitions of Sidewall or Vertical Direct exhaust

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 heating appliances

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.[10]

Vertical direct exhaust-vented heating appliances

Unsafe sidewall vent (C) Carson Dunlop Associates

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]

Safety Controls at side wall power venters include

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Safety controls and power vents for gas fired heating appliances are certified by the AGA, the American Gas Association.[6]

Vent condensate freeze (C) Daniel Friedman

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.

What's the Difference Between Venting a High Efficiency Condensing Boiler or Furnace or Water Heater with a Mid-range or Conventional Heating Boiler, Furnace, or Water Heater?

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).

Unsafe sidewall vent (C) Carson Dunlop Associates

More details about HTPV and a product safety recall are
at PLASTIC Plexvent / Ultravent RECALL.

HTPV high temperature plastic vent

Clearance Requirements for Direct Vented Gas Appliances

Exhaust Vent Termination Clearance Requirements

Direct vent clearance distances sketch - adapted from Thermo Products & NFGC

In a table below 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. [15]

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Some highlights include:

Table of NFGC Recommended Clearance Distances from a Direct Vent Gas Appliance Terminal to Other Building Features

Direct vent gas exhaust termination item clearance Distance required (U.S.) Distance required (Canada)
Distance above grade (ground surface), balcony, deck, porch, veranda 12 inches [note that in areas of deep snow this clearance may be inadequate and thus unsafe]  
Distance to a window or door that is operable (can be opened) 12 inches [Note 2] or 6 to 36 inches depending on BTUH  
Distance to a permanently closed (or non-operable) window 12 inches [Note 3]  
Distance to a ventilated soffit (roof eaves, overhang) above the vent terminal & within a horizontal distance of two feet from the centerline of the terminal 24 inches [Note 3]  
Distance to an un-vented soffit 24 inches [Note 3]  
Distance to outside corner of the building 24 inches [Note 3]  
Distance from the vent centerline to a building inside corner 3 feet [Note 3]  
Distance to each side of centerline extended above a [gas] meter / regulator assembly 4 feet [Note 3]  
Distance to a [gas] service regulator vent outlet 4 feet [Note 3]  
Distance to a non-mechanical air supply inlet to the building or to the combustion air inlet for any other appliance 12 inches [Note 2]  
Distance to a mechanical [systems] air supply inlet 3 feet above if within 10 feet horizontally  
Distance above a paved sidewalk or paved driveway on public property 7 feet [Note 4]  
Distance below a balcony, deck, porch, veranda 24 inches [Note 3]  
Distances between a gas fireplace direct vent termination cap and other building features or mechanical systems See GAS FIREPLACE VENT CLEARANCE REQUIREMENTS

Opinions on Additional Recommended Cleareance Distances from a Direct Vent Gas Appliance Exhaust Opening to other Building Features or Equipment

Distance from a clothes dryer exhaust vent wall termination to the nearest air inlet point on a driect vent chimney 10 feet [Note 5]
Distance to an air conditioner or heat pump compressor/condenser unit to the nearest point of a direct vent gas heater exhaust 24 inches to 6 feet [Note 6]


1 Authorities/Sources:

  • CSA B149.1, Natural Gas and Propane Installation Code
  • National Fuel Gas Code, ANSI Z223.1 / NFPA 54 [8]
  • Gas Fired High Efficiency Furnace Down Flow & Direct Vent (Sealed Combustion) Thermo Pride Model CMA1-50N & CMA2-75N Installation and Service Manual" [15] [cites & recounts illustrations & clearances from the U.S. National Fuel Gas Code

2. For non-direct vent applications such as the side-wall vent appliance shown earlier on this page, the required distance is much greater: 4 feet to the side or below the opening, or 1 foot above the opening.

Typical clearance from an operable window to a direct vented gas appliance vent outlet:

6 inches (15 cm) for appliances up to 10,000 Btu/hr (3 kW), 12 inches (30 cm) for appliances between 10,000 Btu/hr (3 kW) and 100,000 Btu/hr (30 kW), 36 inches (91 cm) for appliances above 100,000 Btu/hr (30 kW)

3. This distance requirement may vary in order to be in accordance with local building codes, the gas supplier, and the equipment manufacturer's own product installation specification

Clearance must be in accordance with local installation codes and the requirements of the gas supplier

3.a. for a gas fireplace exhaust termination cap, 3 feet (91cm) within a height of 15 feet (4.5m) above the meter / regulator assembly

3.b. for a gas fireplace exhaust termination cap, 3 feet (91cm) above - if within 10 feet (3m) horizontally

4. In direct vent applications, this clearance distance must be in compliance with local building codes, the requirements of the fuel gas supplier for the heating appliance in use, and the appliance manufacturer's instructions. For non-direct-vent applications (that is, side wall venting only), the required clearance is 7 feet.

5. This is the author's OPINION based on other clearance distances and standards discussed in the article above. Also see DRYER VENTING FAQs

6. This is the author's OPINION considering that heat from the gas exhaust vent blowing onto an A/C compressor unit condensing coil could imact the coil's ability to condense refrigerant back to liquid form, affecting the condenser unit's efficiency and life. I'm considering the situation in which the hot gas exhaust blows onto the air intake side of a condensing coil. (Most modern condenser units draw cooling air in from the unit sides and push it out the unit's top). Also see CLEARANCE DISTANCE, HVAC

Watch out: there could be other more dangerous conditions: if the air intake vacuum created by the A/C compressor / condenser unit actually draws air or gases out from the nearby direct vent gas heater or from a passive-vented (atmospheric vented) gas fireplace appliance, that increased draft may impact the safe operation of the gas burner, risking dangerous or even fatal carbon monoxide hazards. Separately, if a different compressure unit desigh caused the unit's exhaust to blow into the gas heater exhaust vent, that could prevent safe venting of exhaust gases. Where either these conditions are possible I'd increase the clearance distance to 10 feet or 3 meters.

Distances for the Exterior Vent Termination of a Direct-Vent Gas Fireplace

We moved this data to a separate table now found
at GAS FIREPLACE VENT CLEARANCE REQUIREMENTS Separate Table for Direct Vented Gas Fireplaces

Reader Question: is this direct vent gas fireplace outlet too close to my air conditioner?

See CLEARANCE DISTANCE, HVAC where a reader commented that his building inspector did not accept the safety of the installation shown below. We agreed.

Gas fireplace vent too close to air conditioner compressor/condenser unit (C) InspectAPedia.com

Reader Question: is this direct vent outlet too close to my window?

Direct vent wall inlet / outlet clearance to window (C) InspectApedia.com BL

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

Reply: here are typical clearance distances for a direct-vent gas fired appliance vent opening to an operable window

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

Importance of Proper Slope on Heating Equipment Exhaust Vent Piping & Protection from Blockage by Snow, Ice, Shrubs or Wind

Vent condensate freeze (C) Daniel FriedmanWatch out: Both direct vent chimney specifications (discussed beginning at the start of this article
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.[13][15]

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. [15]

Protect direct vented appliance vents from becoming blocked by snowfall

As you can read in our citation of direct vent and sidewall vent clearance distance requirements in the FAQs below,

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. [15]

Rooftop installation of direct vent intake and exhaust vents - Thermo Products, InspectAPedia

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. [15]

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. [15]

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. [15]

Watch out: when chimneys or vents pass through building floors and roofs above, additional fires stopping may be required.

Protect direct vent appliances from wind:

Wind protection for direct vent appliances - Thermo Products Installation Instructions

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. [15]

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. [15]

Avoid locating the vent terminal over areas where dripping of condensate, or small pools of acidic condensate, could create a problem. [15]

Comment: OPINION: an exhaust vent that is dripping condensate to the outdoors in freezing climates risks dangerous blockage by ice formation.

Safety Warnings Regarding Combustion Air Supply Source for Direct Vented Heating Boilers/Furnaces

Weil Mc-Lain [10], Thermo Products [15] 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.

Examples of corrosive contaminants include

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.

List of manufacturers of Side Wall or Through Wall Vent Chimney Systems & Equipment

Rutland sealant for direct vent chinmneys (C) InspectAPedia.comDirect 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.

Direct Vent Exhaust for Heating Flues: Installation & Inspection Questions

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