Debris in a stack pipe (C) Daniel FriedmanSooty Boilers, Furnaces, Water Heaters: Cause, Problems, Cure

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Sooty oil fired heating equipment: this article explains the significance of soot on, around, or inside oil fired heating boilers, furnaces, and water heaters.

We explain how much oil burner soot is normal and when soot production is a problem. Thick soot build-up inside of a flue, the chimney, or inside of the boiler or furnace heat exchanger or inside of a domestic water heater is a problem that causes higher fuel bills, equipment operating problems, and potential fire and safety problems.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

How Soot Causes Problems with Heating Boilers, Furnaces, Water Heaters

Debris in a stack pipe (C) Daniel Friedman

Watch out: Safety warning about soot: Sooting can occur with both gas fired and oil fired systems.

Soot coming from a gas fired heater is probably indicating a very dangerous condition risking carbon monoxide poisoning

. If a gas fired appliance is producing soot, it should be turned off and Immediate service is needed.


and GAS FIRED WATER HEATERS for safety warnings about soot at heating appliances.

Soot coming from an oil fired heater warns of improper operation and risks a destructive puffback.


Significance of Sooty Debris visible in the flue vent connector or on top of equipment around the heating boiler

If you see soot, rust flakes, and debris in the flue vent connector (photo at page top) or chunks of black and brown sooty crud on top of horizontal surfaces near your oil fired heating equipment, this means that the heating system needs to be cleaned and serviced.

Our photo at page top shows what was probably several years of accumulated soot, rust flakes, and debris in the flue vent connector of an oil-fired horizontal furnace in a wet moldy crawlspace. The owner thought that his system, which was almost impossible to access, had "just been cleaned".

Our photo (left) shows soot and crud that has blown out of the boiler flue vent connector and sprinkled itself on every horizontal surface in the boiler room and in a nearby garage.

We wiped these surfaces clean and even vacuumed every thing in sight each time the boiler was serviced. That made it easier to convince our heating service tech that the system was running "dirty".

A little oil burner soot after service is normal: Because cleaning an oil fired heater disturbs soot and some of that leaks into the utility room, it's normal to see a very light coating of soot dust in the 24 hours or so after your heating system is cleaned and serviced.

But if you clean that stuff up it should not quickly reappear. If the heating system has "just been serviced" and you see soot and crud blowing around, especially if it's new soot production, this debris means that service was incomplete.

Watch out: don't use your household vacuum cleaner to vacuum oil burner soot unless you're willing to risk sacrificing that vacuum cleaner - it may become so dirty you won't want to use it in the home for general cleaning. Worse, if the vacuum cleaner is leaky you may end up blowing soot into the living area.

A proper service procedure for oil fired heating equipment includes removal of the flue vent connector and thorough cleaning of all debris from these components as well as a visual inspection of the condition of the chimney to which the flue vent connector joins to send combustion products outside.

Opening this damper and looking inside meant we literally "hit pay dirt". The dirt meant that the system needed to be cleaned, that the owner was paying for heat, but the heat was going up the chimney, not into the home -- as we explain a bit more below.

A hard to access heating system in a cramped nasty area rarely receives thorough cleaning and service. That was the case for this system.

Why is soot or crud in the oil fired boiler or furnace flue a problem?

A flue vent connector ((also called the "stack pipe" or "flue pipe") is the metal pipe that connects the oil fired heating appliance to a chimney in order to safely vent combustion gases outdoors. That's a place where we can see some helpful clues about how the oil fired heating equipment is operating without disassembling the oil fired boiler, furnace, or oil burner itself.

How to open and inspect a barometric damper and flue vent connector (C) Daniel Friedman Soot in oil burner flue damper tee (C) Daniel Friedman

When we look in to the flue close to the heating boiler or furnace, such as at the barometric damper shown at above left, it's normal to see a thin coating of soot on the interior of the metal flue pipe.

Because soot acts like an insulating coating, too much soot in a heating system causes problems. So we do not want to see soot accumulating in depth in the flue vent connector or chimney (above right) nor blowing around the boiler room, accumulating on top of the boiler, or blowing into and staining other areas of the building.

Technical note: the seasonal efficiency of oil fired heating equipment declines between service calls - that's the normal result of soot accumulation. If we run oil fired heating equipment too hot, to produce "zero" soot, we're sending too much heat (and money) up the chimney. So a modest amount of soot accumulation in the boiler, furnace, or flue vent connector is normal between annual service calls.

More examples and photos of soot in and on oil burners and oil fired heaters are

Watch out: if your oil fired heating equipment can't make it from one annual service call to the next one without blowing soot into the building, sooting up, clogging, and becoming noisy or smoky, something is wrong. Just how much soot is ok? We answer that just below.

Soot layers too thick in boilers or furnaces means we spend more to heat the building:

Soot inside the furnace or boiler reduces the transfer of heat into the heating system's water (or air if it's a furnace). Thus the transfer of heat into the building is reduced by soot in the heating equipment. As the soot layer gets thicker less heat is transferred and more of the heat simply continues to go up the chimney instead of into the building.

When a heating service technician measures the "efficiency" of a heating system, the number, say 85%, means that for each dollar you spend on heating oil, about 85 cents is coming into the building as heat, and the remaining 15 cents is going up the chimney as wasted energy.

Soot layers too thick in a boiler or furnace could be unsafe, even a fire hazard:

Soot layers in boilers or furnaces that are too thick means that the equipment is running "hot" and could even be unsafe.

Where a heating boiler was nearly blocked solid with soot, we've measured flue temperatures close to the boiler of over 1000 degrees F.!

Any combustibles too close to a hot metal flue or chimney could catch on fire at this temperature.

So how thick can the soot layer be before we have to clean the furnace or boiler?

1/8" of soot is the limit

So if the thickness of the soot you see in the flue vent connector, looking in at the barometric damper is 1/8" thick or more, the system should be cleaned and tuned.

Warning: even if the soot layer is thin and fine where you're looking, don't rule out other possible boiler, furnace, flue, or chimney problems. For example, debris could be blocking the chimney, or the flue pipe could be blocked with debris further on closer to the chimney.

But seeing that the flue looks clean where we can inspect is good news.

BOILER OPERATING PROBLEMS discusses the signs of improper oil fired hot water at operation.

Warm air furnaces are discussed
and problems with loss of heat are discussed

A discussion of our page top photo and those rusty sooty fragments in view is found at How to Inspect a Barometric Damper. Separately, diagnosing black stains on indoor surfaces in the living space, possibly caused by oil fired equipment sooty operation or puffbacks, is discussed

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.

Troubles with Heat Scavengers & Passive Flue Gas Heat Recovery Devices on Oil or Wood Fired Heating Equipment

Reader Question: 11/29/2014 paul said: Oil Burner Filled House with Smoke

We just installed new oil burner last year in [our] house. We put [a] heat scavenger on pipe going to chimney side ways.

We didn't put a [draft regulator or barometric] damper on [flue] pipe.

We recently we had the furnace blow smoke out - [it] filled house bad[ly] with smoke and co2.

[We] took [the] furnace apart soot was a good half inch thick on top the nozzle, [though it] was clean the [combustion] chamber where it fires. [That area was] was clean as a whistle but the top of the furnace was sooty.

Question are them [sic] heat scavangers illegal in PA? Also do you need [a barometric] damper with them? Also can they be on their side? Also what went wrong?

Reply: flue gas heat recovery device cautions

Paul, various types of heat scavenging devices or passive flue gas heat recovery devices have been around for over 30 years:

I understand the temptation to just use the space where a barometric damper would have fit for a heat scavenger - since you don't need to cut flue pipe nor make other adjustments. But I have two worries with that approach:

Draft regulation is needed on all fossil fuel fired appliances: gas, oil, wood

First, without a barometric damper it is absolutely impossible to set the oil burner for optimum performance - draft conditions vary in and outside of a building for a number of reasons. The service tech needs to set the proper draft in the breech or flue for best performance. And bad draft as you'll hear me gripe next, can also speed oil burner clogging and malfunction.

Heat scavengers can become a source of clogged flues or chimneys, leading to trouble

Second, a heat scavenger on oil fired heating systems is asking for trouble in my OPINION because the multiple passages provide obstructions that invite soot accumulation, clogging, and subsequent oil burner back-pressure in the combustion chamber, overheating or incomplete, sootier-still combustion. Laws aside I personally don't like these devices on oil fired heating equipment.

You need to have an experienced heating service tech help you out with a thorough cleaning of the oil burner and flue, remove the scavenger, install a barometric damper, and inspect the chimney to decide if a more thorough chimney inspection and cleaning are needed.

If you give me the brand and model of your heat scavenger I will be glad to report more about the manufacturer's installation instructions. You might have mounted it improperly, but in my OPINION even a properly-mounted device may add risk fo clogging and venting and safety concerns.

Some state, provincial, or local codes may preclude the use of these heat miser or heat scavenger devices, either explicitly or implicitly by more general provisions. For example:

Chapter 9, Fire Prevention: (e) Boilers and furnaces in which oil burners are installed shall be connected to flues having sufficient draft at all times to assure safe operation of the burner; a suitable draft regulating device shall be inst alled where necessary to prevent excessive draft. Smoke pipe dampers, if any, shall be such that they cannot close off more than 80 percent of the internal cross section area of the smoke pipe. - Charter & Code, Townof Easton, Maryland, retrieved 11/29/2014 original source

I'd give your local fire inspector or building department a call to ask their opinion about the use of this device on wood stoves as well as on oil fired heating equipment. Let us know what you're told.

Additional references suggest that these devices may be more reliable on gas-fired heating appliances.


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