Draft hood on a gas fired heating boiler (C) Daniel FriedmanGuide to Draft Hoods on Gas Fired Heating Equipment

  • DRAFT HOOD, GAS HEATER - CONTENTS: Draft Hoods & Vents: Guide to Draft Hoods on Gas Fired Furnaces, Boilers & Water Heaters - Purpose, Inspection, Repair. Troubleshooting heating system drafts, vents, and chimneys - combustion air and carbon monoxide production
  • POST a QUESTION or READ FAQs about draft hoods on gas fired boilers & furnaces: inspection, troubleshooting, repairs, hazards

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Draft regulation for gas fired heating equipment: draft hoods.

Here we explain the purpose and function of draft hoods or vents on gas fired boilers, furnaces, water heaters or other gas fired heating equipment. This article explains the purpose of these key venting devices on gas fired appliances.

We list and give photo-examples of common operating and safety defects found at draft hoods and draft regulators on gas fired heating equipment.

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Guide to Draft Hoods on Gas Fired Furnaces, Boilers, Water Heaters - Purpose, Inspection, Repair

Photograph of a gas fired furnace draft opening (C) Daniel FriedmanThis article describes draft regulation on gas fired heating equipment.

[Click to enlarge any image]

The draft hood or draft regulating device we are discussing here is normally used only on gas-fired heating equipment, not on oil-fired equipment. If your heater is oil-fired or you are using a wood or coal stove, see DRAFT REGULATOR, DAMPER, BOOSTER for draft regulation and barometric dampers on oil fired heating equipment.

What is a Gas Appliance, Furnace, Water Heater, or Boiler Draft Hood

This photo of a York gas fired furnace displays a conventional draft hood opening - the large horizontal opening space shown in the middle of the furnace.

The purpose of this opening is to permit additional air to flow into the flue vent connector (stack pipe) and chimney when the gas burner is operating. This additional air flow avoids excessive draft at the gas burner.

Too much draft at the gas burner could result in improper gas combustion. (The gas burner will be below this opening and behind the cover with the louvered openings. The louvers provide combustion air to the gas burner.)

Defects & Trouble Signs in Gas Appliance Draft Hoods

Improper gas appliance draft hood location or size is dangerous

Photograph of a flue gas spill at a water heater (C) Daniel Friedman

Dome type gas appliance draft hood clearances:: Dome type draft hoods are commonly installed on gas fired heating boilers.

For dome type draft hoods such as the funnel-shaped device shown at the center of this photo (air enters at the under-side of the dome) the manufacturer of the boiler specifies the required distance from the bottom edge of the hood to the top surface of the boiler.

Usually this clearance required for gas fired appliance draft hoods is given in inches, embossed right into the lower edge of the draft hood itself.

Look for the draft hood clearance specification and measure what is actually installed. If the draft hood is installed too close to the boiler top, or too high, too far above the boiler top, it will not work properly and the system may be unsafe.

Modification or removal of a draft hood can be very dangerous, and also the presence of rust or debris on top of the boiler below the draft hood may indicate a dangerous condition such as a blocked chimney - risking dangerous combustion gas or carbon monoxide spilling in the building.

We describe a case history where this occurred at DANGEROUS CHIMNEY CASE STUDY.

Rust or damage at the gas heater or water heater draft hood and what it means

Photograph of debris at a furnace draft hood (C) Daniel Friedman Photograph of a draft hood on a gas fired appliacne (C) Daniel Friedman

These photos show an unusual accumulation of debris at a gas fired furnace draft hood opening.

Rust at this location at a furnace draft hood could indicate an unsafe condition. If the chimney draft is inadequate or if the chimney is blocked, or if the heater has been damaged by flooding or other wet conditions, you may observe rust and debris on and around the gas operated furnace, boiler, or water heater draft hood. Further inspection by an expert is needed.

Soot at the Draft Hood or Elsewhere on Gas Fired Heating Equipment

Watch out: the presence of fragments of soot or soot staining on gas fired heating equipment may indicate a very dangerous condition in which improper combustion, probably inadequate combustion air or a blocked chimey or flue, can cause the production of fatal carbon monoxide gas in the building.

The safest step if you see soot formation at gas fired heating equipment is to turn it off immediately.

In all events be sure that you have working carbon monoxide detectors properly installed and located in the building as well, of course, as smoke detectors and alarms.


Stains or Debris at the Gas Hood

Photograph of a flue gas spill at a water heater (C) Daniel FriedmanWatch out: The black stains around the draft hood on this water heater were an indication of something seriously wrong with the installation.

Tracing the flue vent connector from the draft hood atop the water heater (shown in this photo) to its connection at a chimney (not shown) we found that while the water heater was a gas fired appliance it was sharing a flue with an oil fired heating boiler whose chimney was blocked.

Backpressure at the chimney was sending the oil burner's exhaust back down the gas-fired water heater's flue and into the building.

Unsafe Flue gas spillage at a water heater draft hood

Our client is pointing to the draft hood on the gas fired water heater in the home she was buying.

Photograph of a flue gas spill at a water heater

We could see two things: first, foam insulation on the hot water pipe was melting and second, we suspected that there was excessive flue gas spillage from this appliance.

Watch out: This is an unsafe condition that needs investigation and repair.

Dangerous Blockages at a water heater draft hood

Watch out: Look for signs of debris or any other material that blocks clear flow of air into the gas heater's draft hood. Blockage can cause improper venting, improper combustion, and even an increase in heating costs.

Photograph of a flue gas spill at a water heaterThis photo (at left) of a draft hood on the gas fired water heater in the home lets us see two concerns:

First, the gas fired water heater's draft hood was partly blocked with hair - so the water heater may not be burning its fuel nor venting its combustion gases safely.

Second, the previous owner of the home must have kept a very hairy and shed-prone dog in the basement where it's hair was so thick in the air that the draft hood was blocked.

If this home were going to be occupied by someone with dog allergies, extensive cleaning would also be in order.

More photos and descriptions of gas water heater defects including draft hood issues can be seen at GAS FIRED WATER HEATERS.

This article series answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects.

The articles at this website describe the basic components of a home heating system, how to find the rated heating capacity of an heating system by examining various data tags and components, how to recognize common heating system operating or safety defects, and how to save money on home heating costs. We include product safety recall and other heating system hazards.


Continue reading at DRAFT MEASUREMENT, CHIMNEYS & FLUES or select a topic from the More Reading links shown below.

Or see DRAFT REGULATOR, DAMPER, BOOSTER for draft regulation and barometric dampers on oil fired heating equipment

Suggested citation for this web page

DRAFT HOOD, GAS HEATER at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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