Heating oil filter at the oil burner (C) Daniel Friedman Guide to Oil Filters on Oil-Fired Heating Equipment

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Oil burner oil filters:

Here we explain the installation, use, maintenance, and repair of oil filters used on oil-fired boilers, furnaces, and water heaters. We discuss tracking down air and oil leaks at or near the oil filter canister and we describe proper oil filter location and problems to check for during oil filter cartridge changes.

We include considerations of oil pipe leaks out (fuel oil leaks), oil piping leaks in (air in the system), clogged, damaged, noisy, or mis-routed fuel oil piping, and oil fill and vent piping size and location requirements. We also discuss the need for and location for heating oil or fuel oil filters and safety valves.

Discussed: . Common sources of oil leaks around the heating oil filter. Reasons to install double parallel heating oil filters on oil fired equipment - two oil filters? Best locations to install heating oil filters.

Proper placement of service valves & check valves on oil lines at oil filters. Oil leaks and air leaks at oil filters & oil filter fittings, connections - where to leaks occur, how are they diagnosed, how are oil line leaks corrected & prevented.

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.

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Guide to Oil Filters on Oil-Fired Heating Equipment

Oil burner oil line switch (C) Daniel FriedmanWhy do we need an Oil Filter at Oil Burner?

This oil burner fuel piping article series describes defects in heating oil piping, filters, safety valves, and oil tank fill and vent piping. All of these oil storage tank and piping installation defects can easily be found by visual inspection.

[Click to enlarge any image]

Photo at left: the red-capped heating oil filter is installed between the oil burner and the incoming single oil line. In this particular installation the oil safety valve (OSV) is on the "wrong side" of the heating oil filter.

While the valve can provide its fire-safety function in this position, this location makes servicing the oil filter (changing the cartridge) more difficult for the service technician.

To open this oil filter cartridge the heating service tech will have to find the heating oil tank where s/he will hope to find a second shut-off valve on the oil line to permit opening the filter cartridge without spilling oil all over the floor. A better location for the OSV would have been on the left hand or inlet side of the oil filter cartridge cap.

Beyond the costly problem of leaky oil piping, this document lists other important safety or oil-fired equipment operational defects in home and light commercial heating oil storage and piping systems.

Modern heating oil quality varies depending on the oil fields from which the crude oil was refined. In much of North America a significant portion of heating oil comes from Venezuelan crude which produces a more tarry product than product from the Arabian oil fields.

A shift to alternative oil sources began in the 1970's oil crisis. A result is that oil fired heating equipment that had worked fine for decades with no oil filter installed began failing due to oil fuel unit (oil pump) strainer clogging and oil burner nozzle clogging.

As a result, today all oil-fired heating equipment such as home heating boilers, furnaces and water heaters should have an oil filter installed.

Best Oil Filter Location

Sometimes the oil filter is installed at the oil storage tank outlet but the most common and recommended installation location is shown in our photographs at page top and above: the oil filter is installed close to the oil burner where it is easily accessible for service and where it filters oil immediately before the heating oil enters the oil burner assembly.

Location of Fire Safety Controls at the Oil Filter

The best place for the fusible link oil valve (Fire-o-Matic Safety Valve™ for example) is on the oil supply line just before the inlet to the oil filter canister (red arrow, below left), not between the canister and the oil burner as shown at below right (orange arrow). This allows the service tech to shut off oil just before the filter canister in order to open the canister and change the oil filter cartridge.

Heating oil filter at the oil burner (C) Daniel Friedman Oil burner oil line switch (C) Daniel Friedman

With the shutoff valve between the filter canister and the oil burner (above right), changing the oil filter in the canister will require the service tech to go to the more distant oil tank to find and close a valve in that location (if one is even present).

11 May 2015 NHFireBear said:

Regarding "the best location of an oil filter", NFPA 31 (2011): 7.5.8, for indoor tanks up to 330 gallons, requires that a thermally activated shutoff valve be placed inline as close as practical to the outlet from a tank and that a proper filter or screen be installed downstream and WITHIN SIX INCHES of the required thermally actuated valve. If it's required in the code, it doesn't have to "the best" - it just has to comply with the code.

Many oil heat technicians sensibly want to install a fusible-link oil supply line valve at the oil burner, not only because this makes servicing the oil burner easier but because it also recognizes that the most-likely location of a fire is at the oil burner rather than possibly at a more distant oil storage tank.

Details about the proper location for OSVs are

Watch out: do not install an additional OSV on the return line of a 2-line oil piping system. Use a check valve instead.


Bad, Dangerous, or Stupid Locations for Heating Oil Filters

Improper use of two oil fusible link valves (C) D Friedman

Notice in our photo at above left that there is no fusible link oil shutoff valve installed on the oil return line exiting at the bottom of the fuel unit. A second valve in this location is a fire hazard.

Use a check valve on the return line instead. Details about this hazard are

Details about the selection, use, and installation of fusible link oil line safety valves are found

Remote oil filter locations such as at the bottom of the oil tank located across the basement or in another room. Anything that makes the job more difficult for the heating service technician increases the chances of this important oil heat maintenance detail being ignored or delayed.

Make sure that your oil canister installation is not leaking oil nor air

Heating oil filter at the oil burner (C) Daniel Friedman

Notice that there's a heating oil leak below the connection to the filter outlet? This leak will suck air into the oil burner when it's running, leading to improper and possibly unsafe operation, risking a puffback.

After any service procedure that disturbs oil line, filter, or burner oil piping fittings we recommend checking for evidence of oil leaks again after the system has been in use for a day or so, because slow leaks and small leaks in the oil piping system may not show up immediately.

As we report and illustrate in detail just below and as we also cite at at
at OIL LINE PIPING LEAKS, oil leaks anywhere in the oil piping and filter system may be hazardous.

Keep in mind that an oil leak out is an air leak in to the oil piping system as well, leading to improper oil burner operation, a dangerous puff-back, or loss of heat in the building.

Common Heating Oil Leak Points at the Heating Oil Filter

Details of where leaks are found at heating oil filter canisters & their fittings are now

Heating oil pipeline leaks (C) Daniel FriedmanOur photo at left shows a pretty severe oil line leak that seems to be coming from the oil safety valve just ahead of the oil filter. We discuss this leak and other heating oil piping leaks in detail

After any service procedure that disturbs oil line, filter, or burner oil piping fittings we recommend checking for evidence of oil leaks again after the system has been in use.

Oil piping leak at the copper piping flare to iron piping connection.

As we explained earlier, oil leaks anywhere in the oil piping and filter system may be hazardous. That's because even though the absolute amount of oil that drips out may be trivial, an oil leak out is an air leak in to the oil piping system as well.

Air may be drawn into the oil line when the fuel unit is pumping leading to improper oil burner operation, a dangerous puff-back, or loss of heat in the building.

Where Do We Usually Find Leaks at the Heating Oil Filter Canister Assembly?

Here are more illustrations of places where we often find leaks at heating oil filters: at the canister air bleed screw (not tightened, stripped threads, or failure to install a fresh gasket), at the canister top center bolt (same), and at the joint between the oil filter canister lid and base.

The oil filter assembly shown in the photos below was in deed leaking. But it took a bit of work to find out exactly where the problem was. Because oil flows from a leak anywhere on the canister top down the canister sides, you can be fooled about exactly where the heating oil leak is occurring. That's why an expert cleans and dries all of the surfaces meticulously. That makes watching for leaks easy.

Heating oil leak points at the oil filter (C) Daniel Friedman Heating oil leak points at the oil filter (C) Daniel Friedman

We examined the connections at the inlet side of the oil canister, the flare itself and the IPT threaded fitting entering the canister lid (above left). We examined the filter mounting bolt at the center of the canister lid (above right).

Other tricks for finding an oil leak include using tracer powders on the cleaned surface, pressure tests, and even a paper towel on the floor below suspected drip points.

Next we examined the air bleed screw (below left) - ultimately we found both the center top bolt loose and the air bleed screw loose - the heating service tech had just forgotten to check & tighten them.

Heating oil leak points at the oil filter (C) Daniel Friedman Heating oil leak points at the oil filter (C) Daniel Friedman

At above right we indicate where you will often find wet heating oil - in the gap between the canister lid and canister base. But this is a confusing "wet oil" location. It could be wet in this spot for the following reasons:

Below at Frequently-Asked Questions (FAQs) about Heating Oil Filters on Boilers, Furnaces, Water Heaters we describe these oil filter leak points and how they occur in still more detail.

How to Change a Heating Oil Filter

Oil filter cartridgeThe oil filter cartridge should be changed during oil heating equipment annual cleaning, service, and tune-up. You should not have to change oil filter cartridges more often.

If the cartridge clogs and leads to service calls or heat loss between annual service calls then we suspect the oil tank is contaminated with water or sludge - problems that need to be corrected at the oil tank.

Details of a step by step procedure for changing the heating oil filter are now
at OIL FILTER CHANGE STEPS The procedure for changing the oil filter cartridge is simple, but if it's performed improperly you could be left with no heat.


Installing Duplexed Parallel Oil Filter Canisters - A Pair of Oil Filters Handles Oil Tank Sludge

In some difficult cases we found that we could not make it from one annual service call to the next without debris clogging in the oil filter and canister.

Duplexed oil filters (C) D FriedmanA solution that was far less costly than changing out the sound but sludgy indoor oil storage tank was the installation of duplexed oil filter canister cartridges.

As shown in our photo at left, we installed the filter canisters side by side and routed piping so that they operated in parallel, not in series. (In series the first filter clogs and shuts down the system and the second filter is not helping.)

The copper oil line feeds a fusible-link oil valve shown at the left side of our photo. The oil piping then feeds a tee that in turn connects to two high-capacity oil filter canisters (center of the photo).

The outlets of each oil filter canister feed back into a common line that exits through a second tee, passes by an oil pressure (vacuum) gauge, and then connects by a flexible line to the oil burner inlet port.

Once installing this system we had no more heat outages due to oil filter clogging between annual service calls.

Watch out: generally it's preferable to remove sludge from an oil tank rather than add oil filters. In this case we had our oil company service technician check the indoor oil storage tank soundness before we elected to install the duplex oil filter system shown here. Had the tank not been sound it would have had to be replaced.

See OIL TANK SLUDGE for details about methods for correcting this problem.

Where & Why Leaks Occur at Heating Oil Filters - & How to Fix Heating Oil Filter Leaks

Mitco oil filter canister (C) D Friedman and E.S.Watch out: leaks in heating oil appliance piping or filters can be much more serious than just a drip spot on the floor.

The same leak that allows oil to drip out of the oil filter or piping connections allows air to be drawn into the system when the oil fuel unit (oil pump for the water heater, oil fired boiler or furnace) is running.

That air leak into the system results in improper oil burner operation, soot clogging, and even a loud bang at oil burner start-up or worse, a dangerous puffback. Detailed explanation of why an oil line leak causes a bang or puffback at the oil burner, and loss of heat is
at OIL LINE LEAKS - Oil Line leaks found

Reader Question: Why is my oil filter canister leaking oil - apparently from the bottom of the unit?

I recently replaced my oil filter (I actually replaced the entire oil canister with a new oil canister that included the filter already inside of it). The burner is working properly, the flame is on,and I'm getting hot water.

The problem is that I seem to have a small/minor leak from the oil canister itself. Oil is dripping from the canister (I think it's leaking from the bottom of the canister). I've checked all the fittings to make sure they're not loose. They're all tight, as are all the connections in and out of the oil canister.

Do you know why the oil canister could be leaking? Was I supposed to tighten the bolt/screw that's on the bottom of the canister (underneath the filter) as well? I assumed the oil canister came ready to install and that I wouldn't have to adjust the top or bottom bolts on the canister.

Any idea why the oil canister is leaking oil (from the bottom)? - Thanks, Ed Salva

Thanks to Mr. Salva for the Mitco oil filter canister photo (above left). Mr. Salva's photo illustrates an important detail: place a suitable container below the oil filter canister to catch any spills before you attempt to service the unit.

Reply: Check for missing or damaged fiber washers and o-ring as well as leaky flare fittings

Oil filter canister top bolt (C) Daniel FriedmanEd, The oil filter canister does not normally leak through its bottom as that is solid steel on the models I am familiar with. Only an older canister with rust perforation would leak there.

The oil filter canister is installed with a bolt that goes down through the red top and taps into threads in the bottom of the canister.

In our photo (left) my pencil points to the top bolt that secures the oil filter canister base to the top.

There are two seals: a (usually red) fiber washer that mounts under the bolt head and an O-ring seal between the lip of the canister and its top.

There is also a tiny fiber washer underneath a small air bleeder screw found on the top of the oil filter canister.

Above at Bleeding Air from the Heating Oil Filter Canister you can see a closeup of this oil filter air bleed outlet on another oil filter canister brand, and there you'll see the fiber washer too.

If any of those three seals were omitted or damaged, that would make an oil leak that might begin at the edges of the canister or even its top but then run down the device and appear to be dripping off its bottom.

Tightening the bolt won't fix it.

Where to leaks occur at heating oil filter canister assemblies?

Another common leak point at oil filter cartridges is the joint between the oil filter canister base and the red oil filter assembly top. If a hasty service tech (or you) forgot to tighten the top bolt the result will be an oil leak and improper oil burner operation. Common leak points at or close to a heating oil filter assembly include:

Leaks between the canister base top edge and the canister top, due to an old, faulty, gasket or due to a loose assembly top bolt

Leaks at the oil filter air bleeder screw due to a faulty or missing gasket or due to a loose bleeder screw

Leaks at the oil filter fittings entering and leaving the canister assembly, particularly at flare fittings that may have been wiggled and jiggled around by a heavy-handed service person who used just one wrench to loosen or tighten the top bolt. Use two wrenches, one on the bolt and one on the rectangular inlet or outlet casting to avoid twisting oil piping and causing still more oil piping leaks.

If you never have opened the oil filter canister, I'd shut off oil at the fire-o-matic valve between canister and oil tank, turn off the oil burner of course, and open the canister - to be sure it contains a filter and to be sure that all of the gaskets and o-rings are in place and un-damaged.

At OIL TANK PIPING & PIPING DEFECTS you'll find more about oil piping and filter leaks, photos, and suggestions.

In the article above (at OIL FILTERS on HEATING EQUIPMENT) you will see photos of the filter canister type I'm discussing.

Send me a photo of your filter/canister as it may be a different brand or model than what I describe above.

Continued from Ed:

Thanks for the quick response. I have to say that this website has a TON of valuable information!

I'm at work now but when I get home I'll try to attach a picture of the oil filter. It actually looks very similar to the one in the picture on this website (the picture under "HEATING OIL FILTER - NONE - No Oil Filter Installed on Oil Fired Equipment"). The silver one.

I replace the filter yearly on my burner. This year, I decided to change the whole canister as I felt it needed to be changed (it's the original canister that was installed 8 years ago). Then I ran into the problem of oil leaking from the oil canister.

When I first replaced the oil canister a couple of days ago, that's when I noticed the oil leak. I thought immediately that I had bought a defective oil canister. So I decided to take it apart. Surprisingly, everything in the canister was intact (filter, o-ring, etc). Then I went back to the hardware store and purchased ANOTHER oil canister and installed this new one. Same problem, oil leak from bottom of oil canister.

That's when I started researching and came across this site and posted my question.

Can the nozzle have anything to do with this leak? It's not a major leak, just a few ounces per day. Nonetheless, any oil leak is not good and I want to stop the leakage if I can on my own.

When I got home last night, I took some pictures of the oil canister.

I don't know how to attach photos to the comments section here, thus I can email them if interested.

I also noticed that the small leaks seems to be coming from the 'OUT' part of the oil canister. I checked to make sure the fitting was tight, and it was. I will take the brass fittings off and put on pipe joint compound on the pipe threads. Then I'll reattach the brass fittings to the oil canister/pipes and make sure that the lines are air tight. Hopefully, this will stop the leak.

I don't want to make them to tight where I risk stripping the threads or the brass fittings.

Continued from DF:

Ed mail photos using the CONTACT link found at top, left, or bottom of our web article pages.

Try using blue LeakLok™ thread sealing compound (available at auto supply stores, plumbing and heating suppliers, or from Argo [1]) on thoroughly cleaned threads at the outlet fitting on your canister. I'd clean the reads carefully on both mating surfaces and then use that product or a similar one rated for use on piping systems for heating oil.

I agree with taking care not to strip threads. Using a sealant can help avoid trying to fix a leak just by tightening. Also FYI iron pipe threaded fittings are tapered so that the fitting gets tighter as it is turned inwards.

But that means that every time you disassemble and re-use a fitting it has to be turned in further than before to get tight. In turn that means on some devices a fitting can bottom out or reach end of threads without being satisfactorily tight. That's why plumbers often will not re-use brass-to-steel fittings such as the adapter on your filter canister inlet and outlet sides. If you disassemble and reassemble, it's cheap to use a new fitting plus leak-lok.

Beware: an oil leak OUT is also an air leak IN under varying operating conditions and an air-leak in causes burner operating problems, sooting, and even risks a puffback.

If your heating appliance fuel is LP gas or natural gas, see GAS PIPING, VALVES, CONTROLS.

Oil Burner Adjustment Guide Books

OIL BURNER MANUALS provides links to manuals for oil burners, controls, fuel units, etc.

Also see


Continue reading at OIL FILTER CHANGE STEPS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

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