Furnace and water heater vent pipes require clearance to combustible materials – why is that necessary?

Gas burning appliances vent pipes produce heat when discharging exhaust gases to exterior.

Temperature of the vent pipe is transferred to the surrounding area and all the materials that are in the vent pipe vicinity. Problems start arising if those materials are combustible and located to close to the furnace, water heater vent pipe.

If you apply heat to a combustible material for some period of time (that varies between materials), its point of ignition will gradually get lower and eventually little heat will be required to start a fire – that’s why you need a clearance between the vent pipe and a combustible material.

Flash point / temperature at which wood will ignite is 572F (300C).

For all of us not aware of this condition, here are the required / minimum furnace, water heater vent pipe clearances and locations where the single or double wall vent pipes can be installed.

Furnace / water heater vent pipe clearance – single wall

Minimum distance between the single wall vent pipe and combustible materials is 6”

Single wall vent pipe cannot be concealed inside the wall or floor. Even if you keep the clearances at the entrance and exit from that space, whatever section is inside must be double wall type pipe.

Reasoning is simple – you can’t see what is happening inside the wall or floor, and if the single wall vent pipe starts corroding, separates, leaks exhaust gases, you’ll have no control over it. Also, concealed spaces have no way of discharging heat and 6” clearance around the vent pipe wouldn’t apply.

The furnace, water heater single wall vent pipe cannot start in or pass through the attic. Also, it cannot be used in the crawlspace or other unconditioned space. The exceptions are unconditioned basements and garages within the exterior walls if local 99% winter design temperature equals or is larger than 5F.

Winter design temperature is the temperature that is exceeded during either 99% or 97.5% of the hours in December, January and February. Heating systems through the US are designed / calculated based on winter design temperature. 97.5% winter design temperature values are typically used by local codes. (Simplified Design of HVAC systems by William Bobenhausen)

A conditioned space is an area where temperature and humidity are controlled directly or indirectly.
An unconditioned space is an area isolated with doors and / or partitions from a conditioned space. Typical examples of an unconditioned space are unheated attics, crawl spaces or garages.

• Whenever the single wall vent pipe penetrates the roof surface, it requires thimble which must extend 18” above and 6” below the surface. Thimble must be open at the bottom and sealed around the vent pipe above the roof.

• Horizontal section of the furnace, water heater single wall vent pipe cannot be longer than 75% of the vertical vent pipe length

Furnace, water heater vent pipe clearance – double wall

• Typical listed and labeled double wall vent pipe clearance to combustible materials is one inch, but you should always check before installation.

• Maximum horizontal section of the furnace, water heater double wall vent pipe cannot be longer than 100% of the vertical vent pipe length.

Need some additional information about appliances exhaust? Visit furnace /water heater venting, chimney flue sizing and chimney terminations for more.

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Water heater PVC vent pipe information below does not apply to tankless type water heaters.

PVC vent pipes can be used only with power-vent /exhaust motor equipped water heaters also known as an induced draft gas water heaters (also CPVC and ABS but some manufacturer and local restriction may apply to all 3 types).

For the rest of this post I’ll be using “PVC vent pipe” just to make it easier. When installing a new or altering your existing water heater vent pipe, always check your particular water heater model for the manufacturer requirements and contact local jurisdiction authority.

There are two types of PVC pipe water heater venting systems – single (single wall and concentric) and double pipe.

Water heater PVC pipe venting system (single wall PVC) discharges exhaust gases through that single pipe. However, it requires combustion air for fuel burning process and this must be supplied in sufficient amounts from the house interior or exterior.

You cannot install the water heater that uses a single wall PVC vent pipe for venting in a closet, behind the solid doors / no air supplying vents or any type of room that doesn’t conform to the combustion air requirements for fuel burning appliances.

Inadequate amount of air will cause incomplete combustion, and one of the bi-products of that is Carbon Monoxide, which could be deadly.

There are also single / concentric PVC vent pipe systems used by a high efficiency and high efficiency condensing storage / tank type water heaters. They have combustion air supplied from the exterior (some water heater PVC vent pipe systems might be 2 pipes starting at the water heater and converted into one concentric pipe before wall / roof penetration).

Water heater PVC pipe venting system based on 2 pipes (direct vent) uses one PVC pipe to supply combustion air from the house exterior and second to discharge exhaust gases beyond the house. Both PVC vent pipes MUST terminate on the outside.
Advantage of the 2 PVC pipe water heater venting system (and a concentric / single pipe), is that you can install this type of the water heater in the closet / behind the solid door, or crawlspace with no vents, etc.

Water heaters using double PVC vent pipe also come in high efficiency and condensing high efficiency flavors.

Water heater PVC vent pipe cannot be connected to the regular chimney or share venting with other appliance (e.g. power vent water heater and a high efficiency furnace cannot be venting through the single PVC pipe)

Types of pipes that can be used for power vent equipped water heater

Typical power-vent equipped water heater venting utilizes (unless otherwise specified by the manufacturer and/or by your local jurisdiction) 2″, 3″ or 4″ diameter, schedule 40 PVC, CPVC or ABS pipe. Size of the vent pipe depends on model, BTU rating of your appliance, length of the entire horizontal and vertical pipe section, and the number of installed elbows.

Example (based on A.O. Smith Power-Vent Water Heater requirements):
GPVH-40 and GPVH-50 model (40 and 50 gal / 40,000 and 50,000 BTU) water heaters.
Max water heater PVC vent pipe length would be:
40’ for 2” pipe
120’ for 3” pipe
160’ for 4” pipe
However, for every installed 2”&3” elbow you have to deduct 5’ from the maximum distance, and for a 4” elbow – 8’ of the pipe.

Always check water heater PVC vent pipe requirements that apply to your particular model. If you are missing the water heater PVC vent system installation manual, contact the appliance manufacturer / check online.

• Power-vent induced draft gas water heater PVC pipe can be installed vertically (through the roof) and horizontally (through the wall). For horizontal installations is should be supported every 3’ – 3 ½” to avoid sagging along the run (or per manufacturer instructions).
• All connections on the water heater PVC vent pipe must be air tight (require primer and cement for PVC/CPVC/ABS pipe)
• Slope of the pipe – always check manual, but for non-condensing installations (not high-efficiency water heater) level or slightly up-slope towards the exterior should be OK.

Power-vent induced draft gas water heater PVC pipe exterior terminations – again check the manual, but some of the most common requirements are:

• Horizontal exhaust outlet 12” minimum above the ground or expected snow level (for cold climates)
• Vent riser – 24”(if pipe penetrates the wall lower that 12” from the ground)
• Minimum 18” above the highest point where it penetrates the roof
• Minimum 6” away from the vertical wall
• Exhaust pointing down with rodent screen at the end
• min 36″ above the forced air inlet if closer than 120″ horizontally to the inlet
• min 9″ -12″ from the window / door opening (depending on BTU rating)

Below are links to some of the manuals available online showing different types of installations.

• Power direct vent (water heater with 2 PVC vent pipes)
• PowerVent Induced Draft Gas Water Heater by RUUD / single vent pipe
• A.O. Smith single PVC pipe Power-Vent water heater
• SUPERFLUE - ultra-quiet  power vent equipped water heater

Don’t be afraid to ask …

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All types of fuel burning appliances in our homes require air to properly burn that fuel. It is called Combustion Air and it actually has three functions:

1. It  is required to properly burn fuel
2. To dilute exhaust gases that are being vented to exterior
3. To vent and cool appliances

The most popular fuel used in our homes is of course natural gas. For every one cubic foot of gas, there are 10 cubic feet of air needed for clean burning in addition to dilution and ventilation air.

There are two products out of a perfectly balanced gas combustion – carbon dioxide and water vapor, which are perfectly safe. However, if we limit the amount of air for the combustion process, there might be a third bi-product, unfortunately a deadly one – Carbon Monoxide.

Example:

Imagine a gas water heater in a small closet, behind the solid door and without any openings in walls providing that necessary combustion air. After using all of the closet’s air for its combustion process, the air pressure in the closet will become lower than the one inside the water heater vent flue pipe.

The water heater closet will pretty much act now like a vacuum and the exhaust gases may be sucked back into the closet space instead of being discharged to the exterior. Since the oxygen starving combustion process is now producing that deadly Carbon Monoxide, it will eventually leak through the gaps around the closet door into the house…

The areas designated for fuel burning appliances are described as confined space and un-confined space.

The water heater closet from my example could be a confined space – an area with a closed, solid door, and walls / ceiling that create a barrier between the appliance and the rest of the house.

A confined space is a room that has less than 50 cubic ft of air volume for every 1000 BTU/hr of appliance input .

If the water heater has a 40 000 BTU/hr rating, the room would have to have over 2000 cubic feet of volume to provide enough air (without additional sources) for proper combustion process.

Un-confined space is a room that has more than 50 cubic ft of air volume for every 1000 BTU/hr of appliance input (BTU/hr input should be on the appliance label / information tag).

Assuming that our example water heater closet itself is a confined space, and it is located in a room being an un-confined space, we would require two openings in the closet door or wall. One opening should be located within 12” from the floor, and second within 12” from the ceiling of the enclosure.

Openings between confined space and unconfined space must have at least 100 sq. inch area each and provide at least 1sq. inch of a free airflow area per 1000 BTU/hr of the appliance input (1/4” min. or larger mesh screen if installed over the openings, metal grills and louvers provide 60%-75% of their net free area, wood louvers only 20%-25%).

With the 40000 BTU/hr water heater input, the combustion process would require only 40 sq. inches of an opening at top and bottom, but you still have to obey the minimum 100 sq. inches per each opening.

Combustion air opening sizes between confined space and un-confined space (inside of the house air)

Up to 100K BTU/hr – 100 square inch openings
125K BTU/hr – 125 square inch openings
150K BTU/hr – 150 square inch openings
…

More about combustion air coming soon.

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To fully understand house water heater venting I highly recommend reading House Brick Chimney Problems & Gas Water Heater and Furnace Chimney Flue Sizing posts – several of the water heater vent pipe conditions described below depend on properly operating and installed chimney!

There are two common types of gas water heater vents:

• natural draft – explained below
• induced draft / water heater PVC pipe venting / power venting

Natural is still dominating, and as long as the property has a chimney dedicated for use with gas burning appliances, you can use this type of venting.

Typical connection of the gas water heater vent pipe would look more or less like one on the picture (one of the problems with this connection – missing screws at draft hood). There might be several variations… some good, and some bad of course.

Assuming, that you don’t know anything about this subject, this is how the gas water heater venting should be installed:

On top of gas water heater, in its center section, right above water heater vent, you should have a draft hood installed. It sits on 3-4 short legs, sometimes secured to the water heater top plate with screws, or (depending on design) has its legs shaped like pins or hooks at the end, inserted into the holes in water heater top cover.

For the gas water heater vent to operate properly its Draft Hood Has to be Centered over the vent hole, and its legs must be straight – any displaced or deformed draft hoods should be serviced / replaced if necessary. They serve a very important purpose and if not installed correctly, carbon monoxide gases may be expelled into the living space. I’m pretty sure you’re aware how dangerous Carbon Monoxide can be…

1. Gas water heater draft hoods provide additional air for the combustion process gases, to be properly pulled out from the burner chamber (base of the water heater), into the water heater vent pipe and the chimney
2. Gas water heater draft hoods act as a device, which in case of down-draft (a condition which forces air / wind back into the vent pipe / chimney), prevents air from extinguishing the gas burner.

Gas water heater draft hoods come in different sizes, and if you are replacing water heater, make sure, that the draft hood and the vent pipe match the new appliance. If the draft hood that came with your water heater has a top opening diameter designed for 3″ vent pipe, and you have 4″ vent pipe installed, use reducing connector.

However, if the gas water heater draft hood is larger, designed for a 4″ pipe – do not downsize it! / DON’T install reducing connector from 4″ draft hood to 3″ water heater vent pipe just to accommodate an old venting system.

Replace the smaller size vent pipe with a proper / required size (most 30 gallons, 40 gallons, and 50 gallons water heaters use 3″ diameter vent pipe, some 50 gallons might require 4″, 75 gallons and more will call for 4″ and more – follow manufacturer recommendations and local code requirements).

Draft hood and water heater vent pipe connections should be secured with sheet metal screws – three per connection on a single wall pipe are recommended.

Gas water heater vent pipe material - for natural draft type just use galvanized steel pipe and don’t experiment with aluminum pipes, food cans with removed both ends, stainless steel sections, blue stove pipes, high temperature plastic tubing, flexible pipes…, etc.

Gas water heater vent pipe must continuously run upward towards the chimney entrance, raising not less than 1/4″ per linear foot, to provide proper draft. Some installation might be a significant challenge, or even become impossible, due to a water heater height and chimney flue connection level, in those cases, induced draft motor equipped water heater should be used.

Gas water heater vent pipes should be monitored periodically for corrosion and deterioration. Problems with proper drafting, and chimney conditions usually cause galvanized pipes to corrode. Small holes start appearing on their surface – most common areas are along the bottom portion and on connectors. At some point, corroded gas water heater vent pipe wall becomes very soft, and might fall apart when under even slight pressure.

Corroding gas water heater vent pipes should be replaced as soon as possible, to prevent possibility of exhaust fumes / Carbon Monoxide contamination.

Single wall gas water heater vent pipe can not be installed closer than 6″ from combustible materials (floor / wall framing, paper, etc.) such installation could create pyrophoric conditions and a fire-hazard (material will ignite spontaneously at significantly lower temperature if constantly exposed to heat).

Single wall gas water heater vent pipes can not be used in unheated areas like attic or garage (even if only partially penetrating that space) , because such installation will cause excessive condensation on vent pipe walls and compromise proper drafting – double wall pipe / B-vent type is required.

Single wall gas water heater vent pipe can not be used inside the walls, ceilings and any inaccessible areas – double wall type vent pipe / B-vent must be installed.

There’s more about house water heater venting in vent pipe clearances post – enjoy it.

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Illinois remained very conservative, as far as water heater gas connector type (or any other permanently installed appliance gas connection), which used to be black steel pipe only.

However, according to Peoples Gas / Nicor website “Updated Construction Guide” (just fill-up the pup-up from and you’ll be able to open it) the use of CSST – corrugated stainless steel tubing, as a final gas connection for permanently installed appliances, such as water heaters, furnaces and boilers is now permitted.

If you are planning to use CSST tubing as a water heater gas connector, make sure that you consult this installation with your local code enforcement division, and if approved, have a certified contractor perform this work. Because water heater CSST type connector installation is still rare in our state, I’ll explain a couple of most common violations associated with water heater gas connector installation.

Water heater flexible gas connectors are not a permitted type of installation in Illinois. Often, when replacing water heater, its dimensions are slightly different from the previous one, and old gas piping has to be altered, so it matches new connections.

The easiest way is to eliminate old steel pipe, and attach flexible connector to the water heater. Unfortunately, if you’ll have a gas company employee visiting your house, and he’ll notice this installation, you might get a violation ticket and have a gas turned off until proper connection is established.

Drip leg on water heater gas supply line – short piece of a pipe visible on first image, with a cap on the bottom is called “drip leg” (attached to the bottom of a ‘”T” connector).

Water heater gas supply line drip leg is intended to capture any condensed water that may have been transported with gas, and shall be installed as close to the inlet of the water heater as practical (not required on illuminating appliances, ranges, clothes dryers, gas fireplaces and outdoor grills).

Some states might not require drip leg installation on a water heater gas connector, simply because they have a better gas quality than Illinois…, or for other reasons.

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Every water heater (gas or electric) must have a TPR (Temporary Pressure Relief) valve installed, also known as Watts Temperature and Pressure Relief Valve. The pressure relief valve is usually located on top or side wall of the tank enclosure (tankless water heater safety valve, also known as T&P valve is located underneath the appliance).

Some old water heater tanks (1960s or earlier – they are still operating in some houses), were not equipped with pressure relief valves, and I would highly recommend replacement for safety and financial reasons (sediment at the base of old tanks is so thick, that it takes much more gas to heat up the same amount of water).

Water heater TPR valve releases excessive tank pressure if for any reason it reaches an unsafe level, that’s why all tank type units MUST HAVE a pressure relief valve installed, otherwise they could simply blow up. Because some of us are very creative, and you might own a property purchased from such a person,  below are the major no-no’s.

Water heater TPR valve must have a discharge pipe attached to it (blue tubing on this picture is just one of the few permitted types of pipes).  Discharge pipe should extend from the water heater TPR valve and terminate within 6″ from the floor surface (distance and discharge location might slightly vary between some jurisdictions, so check with your local code enforcement division, or local / licensed plumber).

Such installation is required so if the valve needs to relief pressure / hot water, it will be discharged at the floor level, and not sprayed at your face or / and body.

Water heater TPR valve should never discharge into the sink, tub, shower stall, etc. Emergency discharge of hot water / steam might cause severe injury. Keep the discharge pipe low, always pointing down and within 6″ from the floor level – you may also pipe it directly into the floor drain.

Water heater TPR valve discharge pipe must be the same size as the valve’s discharge end, which is 3/4″ in diameter. The reason is simple – it has to be able to relief the same amount of water as the amount coming through the inlet side of the water heater, no reductions should be made or any type of valves installed between the safety relief valve valve and discharge pipe end.

There should be no threads at the end of the water heater TPR valve discharge pipe, because when this valve starts leaking / dripping, and there’s a thread at the end of pipe… some of the home owners cap it, which creates a very dangerous situation. Whenever the safety valve will be required to open because of the increasing temperature / pressure,  that capped discharge pipe might cause the water heater to blow up.

Water heater TPR valve discharge pipe must be made out of same materials that is used for water distribution – galvanized steel, hard-drawn copper or CPVC (regular PVC tubing would not withstand high temperature and it is not permitted). If you are planning to use anything else, make sure that it is approved in your jurisdiction.


Discharge pipe from the water heater TPR valve must always run downhill - if the TPR valve opens occasionally, and water / steam instead of being drained, accumulates / floods the valve, it might eventually cause its corrosion, and prevent it from functioning properly or at all.

Water heater TPR valves should be tested periodically (as recommended by the manufacturer). There is a small lever on the top of the valve, which opens the valve when lifted.

BUT - most of the home owners never do it, and any type of valve tested for the first time after long period of time might not close anymore and will keep leaking. So before you make any testing attempts, first follow this short check list:

1. make sure that you know location of the cold water shutoff valve (main or dedicated for water heater), and that it is functional … just in case you have to use it

2. make sure that there’s a properly installed water heater TPR valve discharge pipe

3. make sure that discharging water will not damage your flooring material (you can place a bucket underneath and even cover the top with a towel to prevent hot water from over-spraying surrounding area)

4. if it’s an old water heater, I wouldn’t even test the TPR valve, but it would make sense to replace – they  are very cheap

Now, go ahead and check your water heater …

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