High and low combustion air vents For airflow to move through a duct, there must be a pressure difference across it created by an external source. Insufficient air supply can lead to incomplete combustion, the buildup of harmful gases such as carbon monoxide, and compromised air quality within the boiler room. You would probably need some vents out of the closet to maintain this, but they can be relatively small, so the enclosure would still largely contain the cold air. Can anybody explain why you needed one high and one low duct for combustion air if done that way? I understand that you need x amount of square inches for btu input of the equipment. High vents allow hot air and gases to esc Dec 19, 2020 · As for the high and low vents my guess has always been an attempt to br sure all incoming air does not get blocked. This article explores types of combustion air vents, installation considerations, building codes, and tips to optimize furnace combustion air supply, helping homeowners and installers maintain safe Feb 23, 2024 · Ventilation in boiler rooms begins with ensuring an ample supply of fresh air for combustion and ventilation of combustion byproducts. 1/NFPA54 as "an appliance that operates with a non-positive vent static pressure and with a vent gas temperature that avoids excessive condensate production in the vent", which require careful attention to exhaust venting and combustion air details for safe operation. Ventilation systems depend on the water heater installed. Jun 20, 2025 · Direct Venting Systems Direct vent water heater uses a single pipe to take the combustion air outside and another pipe to release the gases. 3, (Air for Combustion and Ventilation) of the National Fuel Gas Code - ANSI Z223. . May 23, 2023 · By far, the most common problem that occurs with combustion air ducts is that they get blocked. Failure to provide access to enough air leads to inconsistent combustion that leads to carbon monoxide buildup in your home. 1, or Sections 7. Both high and low combustion air vents in an attic are critical for maintaining proper airflow and ensuring safety. Would it not be better to put high and low openings on common hall/furnace door? A residence can lose combustion air through kitchen and bathroom exhaust fans, clothes dryer vents, and woodstoves or fireplaces that lack an outdoor air connection, which can diminish the oxygen levels within the home. 1. The Input BTUH of appliance (s) box tells you that a maximum of 16,800 btuh's Both high and low combustion air vents in an attic are critical for maintaining proper airflow and ensuring safety. Oct 23, 2010 · If the combustion air vents are located on the wall, two are required, one high and one low. One way is to make the exhaust terminate higher than the intake Feb 11, 2011 · The key is that you have to install air inlets so that the furnace or water heater gets the combustion air it needs when it’s running. Combusting appliances need access to a fairly substantial volume of air for safe and efficient combustion. Typically, the vented sections are fitted with slats The low profile vent termination kit allows both the intake for combustion air and the exhaust vent to pass through a standard sidewall. The ventilation system removes these products from the home, making it safe. Combustion Air Duct The combustion air duct solves the problem. A double vent system promotes a more substantial and effective airflow. Since the pressure outside is higher than the pressure inside, outside air gets inside through tiny leaks. 3, or 7. Mar 4, 2020 · A balance to this is installing a Heat Recovery Ventilator (HRV), again this doesn’t appear often but should be discussed on some new homes that already have high air sealing done. When a combustion air duct is blocked, air needs to leak in to the house through many different undesirable pathways. May 14, 2011 · When the theory was that the high vent/intake would provide the dilution air for the old furnaces and boilers, and the low vent/intake would provide the air for the actual combustion. Upper and lower combustion air vents behind furnace that go to small closet in the unit. This article considers just how much air is required for proper combustion, where air intake louvers are located, and how they are sized. Without adequate airflow, boilers, especially non-condensing types, may not receive the fresh air needed for efficient combustion. A two-pipe (direct-vent) furnace has an intake pipe that draws in outside air for combustion air. ” There are two basic ways for your furnace to get its combustion air. The temperature could get high enough to compromise the temp ratings of any electrical equipment. He told me thats why they want a high and low duct- one for intake for the boiler (combustion) and the second for boiler exhaust (room heat?) in which I disagreed. As we are now using an air in-take this pipe can be prone to blockage from snow, pest or child play. ACBI / Grants to Green project, incorporated a combustion closet with separate High‐Low vents to provide combustion for about half of the (80%) furnaces; the remaining furnaces and a standard gas tank water heater were in a vented attic mechanical space with similar combustion air provided. Most high efficiency furnaces get their combustion air directly from the outside. Nov 21, 2014 · In this post we’ll discuss combustion air, including what happens if we don’t have enough and what the requirements are for “enough. Size of vents are determined by combined BTUs of all appliances in the room. The article continues with photographs and comments on an un-safe home-made vent control intended to avoid cold drafts at High vents allow hot air and gases to escape, preventing dangerous heat buildup, while low vents provide fresh air for combustion, ensuring efficient operation of heating systems. Would it not be better to put high and low openings on common hall/furnace door? May 30, 2025 · Proper ventilation in a boiler room is essential for safety and efficiency. Apr 12, 2008 · Solid door. Inadequate combustion air can lead to dangerous problems. Jul 31, 2017 · To understand the operation of combustion air openings, it’s a good idea to review some airflow basics. May 16, 2023 · In short, houses need combustion air ducts to gives combustion appliances like furnaces and water heaters enough air to work properly. This is a closed combustion system that prevents the burner and the indoor air to mix, thus it is suitable to be used in tightly sealed houses or installations in the living area. The louver door your refering to, would be drawing air from the basement itself. Sketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. The requirements for combustion and dilution air for gas-fired appliances shall be in accordance with Chapter 24. The products of combustion are exhausted through the roof via the vent pipe. Category I furnaces are low- to standard-efficiency, non-condensing (meaning the vent gas temperature is hot enough to avoid excess condensation in the flue), open combustion/non-direct vent (meaning they draw combustion air from the room they are in) appliances with either a natural draft or an induced draft. Here you can see a drawing of my house and where the furnace is, it’s in the center of the house. Follow these instructions as well as the basic furnace installation instruction for installation of the intake/vent pipe(s) and all furnace Apr 12, 2008 · Solid door. It generates heat, moisture, and exhaust gasses. When your furnace burns natural gas to heat your home, it produces combustion gases as byproducts that need to be vented out of your home for it to be safe and efficient. Without a combustion air duct, you’ll have uncontrolled air leakage, and your combustion appliances have the potential to burn improperly or backdraft, both of which are safety issues. This article explains the need for adequate combustion air for fuel The requirements for adequate combustion air for both gas-fired and oil-fired combustion appliances depend to some extent on the number of air changes per hour (ACH) that take place within a home. Their use of in-door air for combustion increases household energy use since the air used for combustion must be replaced by outdoor air. Provisions for combustion and ventilation air in accordance with Section 5. APPROVED VENT MATERIALS The CFR Boiler is a fan-assisted Category I appliance, defined in the National Fuel Gas Code ANSI Z223. It’s important for the exhaust gas to not get sucked back into the furnace, so manufacturers will usually give several ways to do this. 2, 7. Even with the closet containing the cold air, if you have a gas clothes dryer, it still needs adequate combustion air. This is not the area to cut corners. Water heaters burn liquid propane or natural gas. This guide explains how condensing furnace venting works Jan 9, 2023 · All gas-fired appliances must have both a flue/chimney to exhaust the leftover products of combustion (outlet) and combustion air to provide the oxygen for burning (inlet). Nov 27, 2020 · The air to support the combustion enters the water heater or furnace through the vents near the burners. This balanced ventilation prevents moisture accumulation, reduces the risk of mold and wood rot, and Mar 12, 2024 · But in your case, if there is one, and there are gas appliances in the same room, IMO combustion air should be priority number one, many times the easiest way to provide it is to add a vent so air can freely flow between the laundry room and the rest of the house. Traditional installations depend on natural forces to create this pressure difference. Mar 10, 2022 · Determining the amount of combustion air needed for a gas appliance also depends on the type of appliance. Here are the rules for the two-permanent-openings method: If total BTU’s are higher than listed, then combustion air is to be provided as equipment is considered to be located in a confined space. 2 GAS VENT CATEGORIES The Modulex boiler is approved for a Category IV vent configuration as well as for sealed combustion installations. Mar 10, 2015 · With both the high-low vents and single opening methods, the building codes specify the amount of vent area required. I recently sealed this HVAC closet and am having an AC company vent the door with a high and a low. Should return air vents point up or down? Those are positioned like combustion air exchange vents, which means covering them or repurposing them is a bad idea. Jul 31, 2025 · A combustion air vent for furnace provides fresh air necessary for fuel combustion, directly impacting furnace performance and indoor air quality. The burning of gasses from the water heater is known as combustion. Implementing high and low vent openings enables efficient heat dissipation and allows combusting gases to escape in cases of backdrafts. General The Concentric Vent allows both the intake for combustion air and the exhaust vent to pass through a standard roof or sidewall. Blocked vent. Sometimes low and high return air openings are installed with registers that can be opened and closed. In high-efficiency furnaces, the combustion air is generally piped directly from the outside straight into the combustion chamber. These doors are critical in maintaining proper combustion air, reducing carbon monoxide risks, and ensuring efficient furnace performance. Oct 2, 2009 · The low one is combustion air and the high one is relief air. As with calculating volume when you use indoor air, the vent area required is based on the total capacity of the combustion appliances. Using a single-pipe venting setup creates negative pressure inside your home. f. Learn the step-by-step process of properly venting your furnace and avoid high-efficiency furnace venting problems. This is an alternative to the standard two pipe intake/vent shown in the basic furnace installation instruc-tions. Behler-Young Behler-Young Usage Example: Problem: There are 3 appliances in a 14'x10' furnace room in a house with 8' ceilings whose combined total input is 180,000, can all combustion air be taken from inside this room? Solution: Fill in the Square Feet of Indoor Area with 8' Ceiling box with 140 (14x10=140) and click the calculate button. I dont see how the high and low vent setup would work in an underground basement. The size of the inlets depends on the capacity of the furnace or water heater. High-efficiency furnace venting is different from traditional chimneys. *all fuel burning equipment: heating equipment - furnaces, boilers, ,(including heat producing fireplaces), water heaters, cooking appliances(gas), refrigerators(gas) and clothes dryers(gas). Natural gas requires a minimum of 10 parts air for 1 part natural gas to support combustion. Feb 10, 2025 · Is it better to have air return high or low? Having the return air opening high on the wall will work best for air conditioning since it will pull the warmer air near the ceiling. It uses low-temperature exhaust and sealed air pathways to maximize efficiency and safety. Coats etc blocking vents. They also said they could put in a more expensive system which grabs outside air with PVC pipe running through the attic. In fact, the space above the furnace is very small and it needs to be well ventilated to bring in enough combustion air. Apr 25, 2025 · Combustion air can be supplied via a single opening within 12 inches of the ceiling, connecting directly to the outside environment through vertical or horizontal ducts. Other fuels are similar in their requirements. Fuel-fired furnaces, clothes dryers, and water heaters that are vented to the outside can also pose problems. Single-pipe venting pros and cons Single-pipe venting pros: Increased fresh air ventilation. Modern gas appliances are called “sealed combustion” appliances, meaning that the combustion chamber (where the fire happens) is sealed from the inside of the house, and the required combustion air is pulled through a pipe from the Combustion air openings operated by power louvers or power vent opening mechanisms: This article describes the requirements for safe operation & control of heating systems whose combustion air is provided by a powerd automatically-opening and closing louver or vent system. One thing that surprises many people is that in order to cause natural gas to efficiently combust, you need approximately 10 times the volume of air to the volume of natural gas. They are typically used in high-efficiency furnaces and are beneficial because they do not rely on indoor air for combustion, reducing the risk of indoor air contamination. This article discusses methods for providing adequate, safe combustion air for fuel-burning appliances in tight buildings - how to provide outside combustion air for heating appliances. With a conventional furnace (under 90% efficiency), combustion gases are exhausted through a vertical vent that’s attached to your system. Question does this seem workable if interior closet door was louvered. No louvers on this closet door. Water heaters utilize a pipe or vent duct (flue or chimney) to remove these gasses from the Either way, my combustion air ducts can’t go straight up. Dec 17, 2024 · Vent Terminals: keep them separate Most high-efficiency furnaces have two pipes coming out of the wall; one to bring combustion air into the furnace and the other to blow exhaust gases back out. Makeup air requirements for the operation of exhaust fans, kitchen ventilation systems, clothes dryers, and fireplaces shall be considered in determining the amount of space to provide combustion air requirements Jul 3, 2024 · Garage Wall Vents are very important for 1) Safety 2) Freshness 3) Health 4) Well Being 5) Cooling 6) Reducing Condensation and 7) Combustion Air Overview The combustion of all fuels requires oxygen, and LOTS of it. Combustion air and vent piping to be pitched back to boiler at minimum 1⁄4” per foot (21mm/m) from intake and vent terminals so that all moisture in combustion air and vent piping drains to boiler. Why your gas appliances need combustion air: Combustion air is a source of oxygen needed to burn fuel such as natural gas or liquid propane (LP). Here in Maine snow can get high or junk can block the low but less probable both would be blocked. Combustion Air – (1) Air that is supplied to combustion appliances to be used in the combustion of fuels and the process of venting combustion gases. Jan 17, 2024 · A single-pipe (central) furnace uses indoor air for combustion air intake. 4 of CAN/CGA B149, Installation Codes, or applicable provisions of Oct 16, 2024 · What is a Direct Vent System? Direct vent systems draw combustion air directly from the outdoors and vent combustion products directly to the outdoors. This duct leads to an air intake vent in the house’s wall, which is basically a hole. Called high-low vents, one must terminate within a foot of the ceiling and the other within a foot of the floor. This can lead to incomplete burning of fuel and results in both energy loss and increased emissions of harmful gases like carbon monoxide. This is most important, an inadequate air supply can lead to poor combustion performance and even to boiler furnace explosions. You can't produce a flame in a furnace without Aug 2, 2019 · An Early Childhood Care center, a 17,000 s. (2) The duct work installed to bring fresh, outside air to the furnace and/or hot water heater. Mar 3, 2003 · There is a vent in the lid that is open to the attic, and anouther vent in the wall witch goes up the stud bay to the attic also. Feb 3, 2020 · Basically you have to have a low level combustion air inlet to the room as well as a high level relief vent. Nov 19, 2008 · The intent, whether through a vented attic or a vented crawlspace, is to provide at least one high and one low vent opening which will allow for air circulation within that confined space and provide more efficient combustion air flow. Air is essential to proper combustion and that is why proper utility room ventilation is essential to make a furnace, boiler, or water heater work properly and safely. Direct Vent Direct vent gas water heaters are a type of gas water heater best suited for use when there is insufficient ventilation within the installation space in your home. A direct vent water heater draws air for combustion from outside your home rather than inside. Up here, I don't need relief air in a furnace room, but I still need it in commercial boiler rooms. Jan 18, 2013 · Where your getting confused, is that the high and low vent would need to be cut in to an exterior wall, outside air. The Code requirements are somewhat based on the basic combustion formulas, but with saftey factors. Maby this keeps gasses ''combustable air'' out? Why high & low vents on your typical water heater door? Thanks BOSS for the attic venting info. Follow these instructions as well as the furnace installation instructions for installation of the intake/ vent pipe(s) and all furnace installation procedures. May 31, 2022 · Vent Terminals: keep them separate Most high-efficiency furnaces have two pipes coming out of the wall; one to bring combustion air into the furnace and the other to blow exhaust gases back out. The methods of providing combustion air in this chapter do not apply to fireplaces, fireplace stoves and direct-vent appliances. Adventitious air Air : gas ratio Air intake Air vent At Risk Balanced compartment Balanced flue Carbon Carbon monoxide Carbon dioxide CO CO2 Combustion air Compartment Complete combustion Convection Cross-ventilation Downdraught diverter Flue Flue type Flued Flueless Free area Gas Heat input Hydrocarbon Immediately Dangerous Incomplete May 8, 2013 · As air cools inside, the air pressure also lowers. Often well intending – but not thinking – home owners restrict the combustion air to their heating appliances by: placing heating appliance in a small utility room or closet covering Aug 29, 2025 · Meta Description: Learn how high-efficiency furnace venting works, best materials, code clearances, condensate handling, and cold-climate tips for safe, reliable home heating. This is the best and most efficient way to do it. The purpose of the relief air vent is to remove hot air from the boiler room. On the other hand, a high-efficiency condensing furnace makes further use of those Aug 14, 2025 · What Is A Vented Door For A Furnace Room? A vented door for a furnace room is a specially designed door that incorporates airflow openings, allowing air to pass freely in and out of the enclosed space. However, there are many ways to pipe in the needed fresh air for combustion. High vents allow hot air and gases to escape, preventing dangerous heat buildup, while low vents provide fresh air for combustion, ensuring efficient operation of heating systems. caon qoxpb nuyq ohdka czeyv bwirgvr ahjbwychl theuucl lzt lesfuoj zlgypjyq yrlbu zhnon lsx ejdkreza