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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As with every fuel-burning appliance, proper venting is always critical, and to enjoy your solid (wood/coal) fuel-burning stove, you have to get it right.

This guide is just an explanation of basic safety / code requirements for the solid fuel burning stoves. However, you should always consult stove installation with your local building code enforcement division, owners / installation manual, and / or local Certified Chimney Sweep (www.csia.org)

Every solid fuel-burning stove must have a chimney. Sidewall venting or through the roof venting using the interior type stovepipe is not acceptable and extremely hazardous.

The most popular chimney types for the solid-fuel burning stove are masonry, and a UL approved Stainless Steel Class “A” Insulated Chimney.

Solid-fuel burning stove (and fireplace insert) connection to the existing masonry type chimney.

1. The chimney should be examined first to ensure that it is in a good shape, and lined for this particular application. Older chimneys often lack the liner and might be heavily deteriorated, or even blocked. In addition, there is usually a heavy creosote buildup involved with a wood/coal burning stoves, which will be very hard to remove from the brick chimney without the liner. The most common type of the liner used for a wood/coal burning stove would be a UL approved stainless steel system.

Masonry chimney installed on the house exterior wall is much harder to warm-up so it can provide proper draft. The liner (hopefully an insulated one) dramatically improves draft, and benefits discharge of the combustion products to the house exterior.

The minimum height of the wood/coal stove or the fireplace chimney (masonry or metal) would be 3’ above the roof surface and 2’ above anything within 10’ – of course, you can go higher, which usually improves draft.

2. If you have an existing liner in the masonry chimney:

• it has to be at least the size of the stove collar
• must be less or equal 2X of the stove collar diameter for the exterior chimney
• must be less or equal 3X of the stove collar diameter if the only exposed chimney section is above the roof

3. You cannot just stick the stovepipe into the existing chimney or into the brick fireplace firebox and expected it to vent properly. It has to extend into the chimney flue liner, should be sealed at the liner connection, and below the smoke chamber . Otherwise, gaps around the pipe will cause your house air to dilute combustion products inside the chimney, lower their temperature and compromise proper draft.

4. If you have a manufactured, metal type fireplace, and your idea is to connect a wood/coal burning stove to its chimney, contact the fireplace manufacturer.

5. Remember to keep the stovepipe at a ¼” per foot rise towards the chimney entrance. Maintain at least 18” clearance (for single wall pipe) between the stovepipe and combustible materials, and 6” for a double wall (double wall pipe has a stainless steel interior pipe and black exterior surface). If your chimney is behind the paneling (or any other combustible) wall, you’ll have to remove this combustible surface to provide required spacing. It is possible to reduce those clearances by using approved shielding materials / clearance-reduction system.

UL approved Stainless Steel Class “A” Insulated Chimney

There are basically two types of insulated chimney installations:

1. Typical for a single story house – stovepipe runs straight up from the wood/coal stove and terminates below the ceiling level where it connects with an insulated chimney. From here, the chimney continues through the attic and roof or just through the roof if you don’t have an attic.

2. For any type of the house, especially if your stove is located close to the exterior wall – stove pipe runs from the appliance and terminates at the side wall (remember at least ¼” rise per foot towards the wall), where it connects with an insulated chimney. Special insulated connector passes through the house sidewall and angles up continuing to its required termination point. Do not use interior type stovepipe for the exterior and passing through the wall applications!

Minimum solid fuel (wood/coal) stove clearance to combustible material, unless otherwise listed and labeled by the manufacturer.

Always compare your appliance manual and local code with those minimums below.  Your particular stove design might require different clearances. They may be also reduced if an approved type of clearance-reduction system is used.

• Front clearance of the solid fuel-burning stove – 18” min
• Sidewalls and rear clearance – 36” min
• Top of the stove clearance – 48” min

Bottom clearance for solid fuel burning stove:

• with legs >6” tall – 2” thick masonry base with a metal shield
• with legs providing 2”-6” clearance – 4” hollow masonry base + metal plate
• if less than 2” clearance – entire floor underneath the stove must not be combustible (no hardwood floor under the masonry base permitted)

Some other requirements for the solid fuel burning stove installations

• No garage installations
• Fuel storage minimum  36” from the stove
• No installation in an alcove or an enclosed space with a total area < than 512cu. feet

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Plastic vent pipe types listed below have been certified by the American Gas Association for use in furnaces classified as Category 4.

These particular types of furnaces use positive pressure vent systems (exhaust gas temperature less than 140⁰F above the dew point temperature) and are always equipped with an induced draft motor which removes exhaust gases from the appliance and discharges them to exterior.

The dewpoint temperature is the temperature at which the air can now longer hold all of its water vapor, and some of the water vapor must condense into liquid water. (definition by weatherquestions.com)

Proper installation of the vent system is essential for the condensing furnace (that’s another name for category 4 type furnace) to achieve maximum efficiency and to drain condensate efficiently from the system.

Make sure that your  installation complies with National, State and Local codes. Some of the furnace installation guides may be available online or obtained directly from the manufacturer.

There are 3 plastic types of pipes certified by the American Gas Association for use in high efficiency, condensing, category 4 furnaces: PVC, CPVC, and ABS

PVC  pipe

CPVC Pipe

ABS Pipe

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Furnace and / or water heater chimney requires proper placement and termination to safely discharge exhaust gases from the appliances combustion (fuel burning) process. Improper chimney placement and termination is often responsible for back-drafts, excessive corrosion of the chimney pipe walls, condensation, and Carbon Monoxide contamination of your house.

The rule explained on the pictures is just one of the several that have to be applied when installing the chimney. Make sure that you have a licensed professional performing gas water heater & furnace flue sizing calculations while replacing or adding any of those appliances in your home.

Natural (gravity) and motor induced draft type gas burning appliances chimney installed closer than 8’ from the vertical wall, must extend a minimum of 2’ above the roof placed over that wall.

Termination (height above the roof surface) of the gas-appliance chimney installed further than 8’ from the vertical wall are listed in the table below (relation to the roof slope). Based on International Residential Code and Uniform Mechanical Code

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A brick chimney crown – also known as a chimney termination cap, cement wash, or just a chimney cap – seals the chimney top opening around the flue and sheds rainwater / prevents moisture from deteriorating the chimney.

For the chimney crown to fulfill its purpose, it must be properly shaped and installed.

Unfortunately, after looking at hundreds of brick chimney on new and old houses, I’m wondering if the actual purpose of the chimney crown is ever taken into consideration. The overall appeal of the chimney crown and its complement of the house’s structure from the moment it is build seems to be the only motivation.

There are 3 basic types of materials used to create a brick chimney crown:

1. concrete: it is the most popular and long lasting if properly installed
2. brick covered with mortar: used often but extremely poor in performance
3. stone, which outperforms all materials if one solid block is used and properly installed – usually the most expensive type

The most common material used to manufacture the brick chimney crown is concrete. Such crowns are either pre-manufactured / delivered to the building site, or created on site by pouring concrete directly on top of the chimney into the assembled form – this method is called “cast in place”.

A brick chimney crown made out of concrete should be at least 2” thick and have a 2 & 1/2” overhang (recommended by the Brick Institute of America) to protect the face of the brick wall it sits on.

The top surface of the chimney crown should slope away from the flue wall towards the crown exterior edge at a 3:12 pitch (3” rise for every 12” of the crown surface width). It would be ideal to have a crown shaped this way when it is created, because mortar added on top of the flat crown will always separate, crack and fall apart.

There should be a gap between the chimney crown and liner, filled with a flexible sealant, which allows for liner expansion. Without that gap and during the cold season, exhaust gasses warm-up the liner which expands, pushing against the much colder chimney crown, and causes it to crack.

The cheapest type of a brick chimney crown is a design called a chimney wash. It is a layer of mortar, hopefully pitched properly (3” slope for every 12” of a flat surface) / applied over the top course of bricks. One winter season is capable of destroying it and by the time you’ll find out, the top section of the chimney requires rebuilding.

Stone crowns (usually limestone) on brick chimneys are usually very long lasting but under two conditions – a single piece of a stone has been used and the flue penetration has some expansion space. Without the expansion joint, it will crack – just like the concrete crown. If the chimney crown has been assembled out of several pieces of a limestone (much cheaper than one solid piece), it will separate at mortar joints and cause chimney structure deterioration.

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The following guide is based on Peoples Gas “Construction Guide for Gas Usage” in the city of Chicago and National Fuel Gas Code. Before applying any of the following rules, you must check your local building code requirements, and appliance manufacturer installation instruction.

• Have you checked your house brick chimney lately… or whatever type of the chimney you have?
• Did you have a new furnace or water heater installed / was your house chimney flue recalculated for those new appliances?

Make sure that you chimney flue diameter is correct; sizing is one of the parameters that will help to achieve proper draft – the water heater and a furnace exhaust gases will be discharged safely to the house exterior – your life may depend on it. Some of the consequences of an excessive or insufficient chimney draft are:

• poor burner performance
• excessive combustion (burning process) noise
• improper combustion which could result in Carbon Monoxide spillage into the living area
• soot buildup inside and around the burner chamber of your appliances

The Seven Times Rule (applies to natural draft and / or fan assisted type appliances vented into the same chimney) When two or more appliances are connected to a chimney, the flow area of the largest section of that chimney cannot exceed seven times the smallest flue collar, or draft hood outlet area, unless designed with approved engineering methods.

To make it simple – if you have a gas water heater with a 3” draft hood, and a furnace with a 4” flue collar connected to the same chimney flue, you’d calculate that chimney flue size based on the 3” draft hood – the smaller of two. So let’s calculate… or why don’t we skip those calculations and I’ll give you the answers:

The Seven Times Rule application results:

Q. Are those numbers sufficient for calculating you chimney flue size / diameter?

A. No, they aren’t! There are several other factors like the chimney placement, height, lateral length of the vent pipe, type of the vent pipe, number and type of the connectors along the entire chimney flue run, size and category of the vented appliances… So it’s always smart to have a professional performing your chimney flue sizing. But, using the tables above will at least prevent many wild, way off the chart installations. Always have Carbon Monoxide detectors installed according your local jurisdiction requirements!

If you’re looking for a gas water heater and furnace chimney flue sizing tables / guides covering most of the possibilities, try this publication by Hart & Cooley.

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Proper furnace / water heater venting is an extremely important issue, which depends on the performance of the house chimney / chimney flue or other exhaust assemblies. The conditions explained in this post also apply to the wood burning appliances.

Consequences of improperly assembled vent pipes, problematic chimney flue, deteriorating chimney walls can be sometimes fatal – fire and Carbon Monoxide poisoning are topping the list.

Therefore, combustion products from your furnace, water heater, wood burning fireplace, and any other appliance that requires some kind of an exterior vent / chimney, have to be properly properly discharged to the exterior.

Certain requirements have to be met for the appliance chimney to draft properly. The most obvious one is unobstructed surroundings, in particular the roof itself. Depending on chimney placement / distance to the surrounding walls, there are different height rules.

For oil and wood burning appliances there is a very simple formula that regulates this chimney height requirement, it is called “3-foot, 2-foot, 10-foot rule”. What it means is that top of the chimney should be 3 feet above the roof surface through which it protrudes and 2 feet above anything within a 10′ radius measured horizontally (those are minimums) – look at the simple diagram of this rule application at FireplacesNow.

This chimney rule can be applied to wood burning fireplaces, heating stoves and any oil fueled appliances, but remember that manufacturer requirements have to be incorporated into it.

For gas venting appliances, the chimney / venting rules are more complex, simply because of the variety of products on the market. For a double wall type vent (required in cold climates) I’ve prepared a separate post – Gas appliance vent terminations. 

Below are some requirements for the furnace / water heater single wall vent pipe / chimney (warm climate):

• Chimney must terminate min. 2′ above the roof
• Chimney must terminate min. 2′ higher than building withing 10′
• Chimney must terminate min. 5′ above the flue collar (e.g. water heater installed close to the roof framing would need at least 5′ long vertical vent pipe)
• Single wall vent pipe requires minimum 6″ of clearance to combustible materials

General rule -  the chimney should not be outside a window or other house opening.

There’s several other requirements which apply to certain types of furnaces and water heaters venting. Always follow manufacturer guidelines, if you have any concerns about your chimney, call a heating contractor or other chimney specialist and have the chimney / heating system examined.

I’ll try to explain just a few basics that apply to most common types of appliances used in Illinois ((for more detailed information about gas appliance venting regulations in Illinois, go to this website: Illinois Peoples Gas and North Shore Gas , navigate to “Construction Guide“, fill-up simple form and download this free booklet – very helpful information), or contact me.

1. NEVER USE CHIMNEY FLUE TO VENT CLOTHES DRYER, KITCHEN / BATHROOM VENT

2. DO NOT USE SINGLE FLUE CHIMNEY TO VENT GAS AND SOLID FUEL (WOOD BURNING FIREPLACE OR HEATING STOVE) – you can have one chimney but multiple flues for different fuel types

3. READ MANUFACTURER INSTALLATION GUIDE and ask your licensed HVAC or plumbing contractor / installer if you have any concerns – installation guides are usually very clear, with simple diagrams explaining all required clearances, type of required materials, etc.

4. If you’re replacing an old water heater and a furnace (or any combination of those), with new appliances that also require regular chimney, make sure that your chimney flue is evaluated. Flue gases temperature in older, low efficiency furnaces (78% and less efficient), was sufficient to elevate chimney flue temperature to the point which guaranteed proper draft.

When connecting a new, regular type (known as category 1 type gas appliances) furnace /water heater, with efficiency slightly below 83%, old chimney flue size might need to be decreased with a new liner. This is extremely important, and serves various purposes – properly sized chimney flue:

• prevents acidic condensate from forming on the furnace / water heater chimney flue walls
• colder gases in large size chimney flue will not have enough energy to travel all the way to the  top and discharge safely beyond the property (becomes critical during cold season), they will condensate on cold chimney flue walls, cause deterioration and corrosion of appliance metal vent pipes
• allows furnace / water heater combustion process gases to draft efficiently beyond the property
• prevents flue gases from returning into the house, which would create potentially hazardous conditions, possibility of Carbon Monoxide poisoning

5. If one appliance has been disconnected from the chimney previously serving 2 or more appliances, make sure that a professional re-evaluates your chimney flue. It may need to be down-sized to prevent flue gas condensation and to provide proper draft (typical condition when upgrading to a high efficiency furnace) – check this chimney flue sizing guide.

This video below explains in detail how to install a new liner inside the chimney. It is 8 minutes long, but you’ll get a good idea about the entire procedure – this is an extremely important and often necessary procedure.

6. If you’re replacing natural draft furnace (no motor assistance for exhaust gases removal), with an induced / forced draft motor equipped appliance, and your natural draft water heater remains connected to the same chimney flue pipe, make sure that water heater vent pipe connection is “Y”, and not “T” type.

“Y” type vent pipes connection is necessary to prevent exhaust gases from mechanical draft type appliance vent pipe, from being pushed back into the natural draft type vent pipe, and contaminate living space.

Possibility of such contamination is greater during the cold season and on taller, exposed chimney / for example – attached to the side wall of the house. Before the hot exhaust gases warm up the chimney flue and create proper draft, they often discharge through the water heater’s draft hood.

7. When upgrading to, or having a high efficiency furnace installed, exhaust gases will be discharged through the PVC pipe, and not the chimney.

There are two types of venting associated with those furnaces – single pipe and two pipe systems.

Because high efficiency furnaces venting system must be air tight, PVC vent pipe connections should be monitored periodically during furnace operation for any moisture / condensation. If such occurs on PVC joints, you should have this condition evaluated by a licensed HVAC contractor – connection will have to be resealed.

PVC vent pipes should not be inserted into the chimney flue if it is still used for other appliances venting.

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Don’t forget to take care of the house brick chimney, even if your furnace, water heater or fireplace doesn’t require it any more!

House chimneys are one of the most important, and at the same time neglected components of our homes, and I think… for a very simple reason; because we don’t understand them, and they are not blocking our way to the kitchen or bathroom, we just take them for granted and forget about them… just like many other things. Because majority of problems I’ve encountered during inspections have been associated with older, usually brick / masonry chimneys, first section will cover this particular type.

Huge number of the Chicago-land properties have been equipped with brick chimneys (I’m pretty sure that also applies to other parts of US), and many of those are dating all the way back to the late 1800 and early 1900. With properties exterior maintenance and improvements, brick chimney walls often received nice dressing, or maybe not as nice patches of various sources, just to keep them standing and to avoid costly rebuilding process.

But rarely anyone even considers checking interior of the house chimney, what’s worst, with furnace / boiler and water heater replacement, professional / licensed installers sometimes “forget” about old chimney evaluation and often required modifications, putting in stake customers life.

Chimney Safety Institute of America reported in 1992 that “all unlined chimneys, irrespective of fuel used, are very liable to become defective through disintegration of the mortar joints.”

Deterioration of mortar joints and bricks is a natural, unavoidable process. However, proper maintenance, and replacement of mortar whenever is needed and necessary, protects bricks surface, and prevents, or at least slows spalling / delamination.

Unfortunately, with older brick chimneys, several other factors accelerate this natural process, and without taking care of them, exterior maintenance provides only temporary and rather cosmetic solution. Because, older chimneys often lack

chimney liners (metal pipe, clay tile or ceramic material surrounded with exterior bricks),

chimney crowns (usually piece of pre-manufactured or made on site concrete block sealing top of the chimney and preventing water / moisture penetration), and

chimney rain caps (made of concrete, metal or clay, sitting above the liner section protruding from the crown), acidic exhaust gases from fuel (gas or oil) burning appliances, and water / moisture destroy interior surfaces, and without proper action, this progressing process might create fatal results.

Notice on the pictures showing chimney interior missing bricks and mortar, partially missing liner sections. All that material has naturally fallen inside the chimney flue, and could be blocking it, creating extremely hazardous conditions – with blocked venting, CO (Carbon Monoxide) might contaminate your house air.

I wouldn’t suggest you going up on the roof and under no circumstances trying to claim up the chimney – don’t ever attempt to lean the ladder against the chimney!!!, leave examination of a inaccessible chimney to the professionals!

There are a few simple steps, you can take to get some basic idea about the condition of your chimney:

1.Check the house brick chimney from exterior, look for any deteriorated areas, missing mortar / gaps or holes between the bricks, separation from building structure (side chimney with 3 exposed walls / one shared with the building), leaning

2. Step further away from the building, so you can see upper section of the chimney, you should be able to tell if top has a crown, or if there’s a flue pipe / cap installed. Sometimes, the flue pipe might be even with chimney crown, or have only small piece installed on top of the chimney to support the cap… if you can only see the edge of the last, top layer of bricks, get a professional because your chimney might look just like one from the pictures in this paragraph.

3. If you have access to the base section of the house chimney (in the basement, crawlspace, or sometimes house exterior), look for a small metal door, it might be heavily corroded, but if you manage to open it, get a powerful flashlight and a mirror.

You should be able to see at least part of the chimney interior, up to the appliance vent pipes penetration, and hopefully determine if it is lined – don’t be surprised, if after opening of the door all you can see is bricks, mortar, clay tile pieces, carefully pull it all out if possible to get a better look at interior (use gloves or some tool to remove debris to avoid injury). You may also notice chimney wall cracks, which pose serious hazard, especially if there’s no liner installed behind the bricks, or if existing liner is also cracked.

Water leaking from under the chimney clean-out door, or at the chimney base usually has 3 sources:

- condensation on chimney interior walls often caused by poor drafting

- leaking underground section of the chimney – just like old home foundations, older chimneys underground sections have no damp / waterproofing installed, and are subject to deterioration / water penetration

- missing chimney crown and / or cap

4. If section of your house brick chimney is accessible from the attic, use it to your advantage, but be careful in the attic, many have no floor boards, you have to step on ceiling joists, sometimes covered with insulation – drywall or plaster will not support you!.

You might find missing chimney bricks, holes, improperly connected appliance vent pipes, cracked or just falling apart chimney. You might also see a chimney built on an angle (this is called corbelled brick chimney), which is normal and as long as it’s in a good condition (look for displaced bricks, cracks, missing mortar), there’s nothing to worry about – it was designed this way for purely aesthetic reasons, to come out at the center of the roof. However, installing a new liner in corbelled chimney might be difficult.

If you’re planing furnace and water heater replacement (or maybe you’ve already replaced it), or just one of those appliances, have your chimney inspected, make sure that its flue dimensions comply with new appliance requirements, and if necessary install a new liner. This is critical when replacing one of the appliances using chimney venting with a high efficiency appliance.

For example, you used to have a furnace and water heater venting through an old chimney lined with a large diameter clay liner. Even though an older appliance’s efficiency was lower, they wasted more fuel in heating process, discharging significant amount of hot exhaust gases through the chimney, elevating chimney flue temperature to the level which guaranteed proper draft.

If you’ve eliminated chimney vented furnace / installed new high efficiency furnace using PVC pipe for venting instead of a chimney, and now have only water heater hooked up to that old, liner-less or with oversized liner chimney, you’re creating a pretty hazardous situation.

Exhaust gases from the water heater will not be hot enough to raise chimney interior temperature to such level which would guarantee adequate draft force. During cold months, gas exhaust flow rate through the chimney might be reduced to the point, where poisonous gases return into the house. The conclusion is a very simple one -

never vent appliances (also applies to fireplaces) into an oversized chimney – if there’s any doubt – have it inspected by a professional, and check this chimney flue sizing guide!

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