Photograph of an outdoor heating oil tank Guide to No. 2 Heating Oil Waxing, Gelling, or Clouding

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Waxing or jelling heating oil: Here we define and explain heating oil waxing, gelling, or pour point problems which can lead to loss of heat when oil tanks are located outdoors in freezing climates.

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Heating Oil Waxing, Gelling, or Clouding: a cause of oil piping or filter clogs and loss of heat - diagnosis and prevention of heating oil problems

Outdoor oil tank in Maine exposed to waxing troubles (C) Daniel FriedmanWhen heating oil does begin to gel, and before it has become actually too viscous to flow at all in the heating system oil piping, wax particles (wax platelets or little spheres of wax or in some articles, alkane "wax crystals") have already begun to form in the fuel.

The wax platelets form first from the long hydrocarbon chains which are a component in the heating oil (or diesel fuel). It is these waxy particles that can clog an oil line or even a oil fired heating boiler, furnace, or water heater.

Our photo (above left) shows an outdoor heating oil storage tank at a Maine home - certainly in an area where heating oil waxing in an outdoor tank would be a real risk.

This article series answers most questions about all types of heating systems and gives important inspection, safety, and repair advice. Criticism and content suggestions are invited from heating service technicians, home inspectors, and home owners.

At what temperature point does home heating oil or No. 2 heating oil begin to cloud, wax or gel?

A Definition of the Pour Point for Home Heating Oil - No. 2 Heating Oil?

The pour point for heating oil is the lowest temperature at which the oil or fuel will continue to flow, that is it's viscosity has not increased or it has not become clogged with waxy particles.

Products sold as additives for home heating oil are called "pour point depressants" because they push the pour point temperature lower - that is, you can continue to use the treated heating oil or diesel fuel down to lower temperatures than otherwise.

An alternative to using pour point depressants is to purchase a cold-weather mix of fuel such as a mix of No. 2 diesel combined with less viscous Kerosene, or one can purchase a fuel or heating oil which has already been treated.

A Definition of The Cloud Point for Home Heating Oil - No. 2 heating oil:

Heating oil or diesel fuel waxing or clouding or gelling begins to occur at about -9 degC or about 16 °F.

The cloud point for heating oil is defined as the temperature at which a cloud of wax crystals first appears in a fuel sample that is cooled following the procedure in ASTM Standard D2500. In more practical words, home heating oil will probably flow through the oil piping, filter, and oil burner without an operating problem down to about -9 degrees Centigrade or 16 degrees Fahrenheit.

A Definition of the Wax Point or Gel Point for Home Heating Oil - No. 2 Heating Oil (or diesel fuels)

The gel point for home heating oil (No. 2 Heating Oil or Diesel Fuel) refers to the fuel or heating oil becoming so viscous that it "gels" and will no longer flow at all by gravity nor by pumping through the oil lines or heating equipment.

What we call the gel point or wax point of home heating oil is either a temperature lower than the cloud point for heating oil, or it may occur at a higher temperature if the oil is exposed to that temperature long enough.

The wax point or gel point of heating oil means that there has been sufficient formation of wax platelets or crystals that heating oil piping lines or oil filters will become clogged.

Here are the gel point temperatures for heating oil or diesel fuel: Heating oil or diesel fuel gelling or waxing to the point at which the fuel will no longer flow is 5? to 10?F below the cloud point.

The pour point, cloud point, gel point or wax point temperatures given for No. 2 home heating oil, which is similar to diesel fuel used in vehicles, are estimates.

The actual gelling or waxing of home heating oil will occur as a function of not only the temperature to which the heating oil (and oil tank) are exposed, but the duration of that exposure. That is to say, if temperature dips below freezing for a few hours and then rises, you may not see any troublesome waxing or gelling.

We have observed that an outdoor home heating oil storage tank exposed to about 10 degrees F for several hours or more may produce gelling or waxing in the heating oil which can stop the heating system from working.

Where in the oil tank, storage, piping, and filter does waxing or gelling first occur and how does this lead to a loss of heat in a building?

The clouding and then waxing of the heating oil will occur where the oil has the least thermal mass and is most exposed to cold conditions.

Often this is at the oil piping line exiting at the bottom of an outdoor heating oil tank since this small diameter pipe has much less volume and mass than the larger and perhaps mostly full tank of heating oil.

All petroleum distillate products contain waxy materials, which, at low temperatures, can crystallize and plug fuel filters. So a heating system may go out of service because it's filter has become clogged with waxy particles even though there was still oil flow in the heating oil lines.

At longer exposure to cold temperatures or exposure to lower temperatures the amount of wax forming inside the heating oil in the oil tank can prevent heating oil from flowing out of the tank. Some writers refer to the observation that waxes settle to the bottom of the oil storage tank.

How oil line piping design affects cold weather operation of outdoor heating oil storage tanks

If the oil piping is connected to the bottom of the oil tank it will certainly be blocked by oil waxes settling at the tank bottom. The oil piping in this location is also easily blocked by sludge in the oil tank -


If oil piping is taken off of a tapping at the top of the oil tank, 

as is good practice with outdoor installations in freezing climates, the piping may still include a "low loop" or bend down towards the tank bottom, depending on where the heating equipment is located with respect to the oil tank position and elevation.

A low loop in the oil line may accumulate waxes pulled out of the heating oil tank.

Alternatively, oil in the oil piping may gel and cause a blockage because of the line's exposure to very cold conditions because the oil piping has less thermal mass than the oil tank itself.

Wax settlement in heating oil tanks: 

Even if the oil line itself is not fully blocked by waxed or gelled heating oil, if enough wax sphericules or platelets flow into the heating oil filter the filter may become blocked by waxes and thus starve the oil burner for oil until it goes off on reset.

This can be a tricky problem to diagnose since with the heat off for a while but with the oil filter canister sitting indoors in a location that is still much warmer than outdoor temperatures, it's possible that the service technician will arrive and successfully simply re-start the oil burner since the waxes in the filter canister may have returned to their liquid state.

Waxing causes change in heating fuel BTU's:

Waxing of fuels in an oil storage tank (or in a vehicle fuel tank) also causes an operating difference in the boiler, furnace, water heater, or vehicle, since the chemistry and thus the energy capacity of the remaining liquid fuel will have been changed by loss of the longer hydrocarbon molecules which have left it to form settled wax spheres.

How does Biodiesel differ from Petroleum Based Fuel with respect to gelling and waxing?

Biodeisels have a higher gel point than petroleum-based diesel because they contain molecules that form large spheres or crystals of wax at temperatures below 40 °F.

Anti-gel additives (also called pour point depressants) have not been shown to be effective with biodiesel fuel.

Therefore oil companies in areas where biodiesel is sold recommend that biodiesel (B100) should not be used at temperatures below 40 Deg. For that a blend of B100 biodiesel and 50% petroleum-based diesel be used in the vehicle's fuel tank.

Portions of this article paraphrase a biodiesel article at]

How pour point depressants work:

How pour point depressant and anti wax-settling additives work is not clear, but some writers including in patent applications for pour point depressant and anti-waxing heating oil and diesel fuel additives have argued that they work by matching alkanes in the heating oil or vehicle fuel fuel to alkyl or alkylene chains in the pour point depressant additive in order to affect the formation of wax particles.

[Paraphrasing patent description for a pour point depressant at

What are the Different Petroleum-based Fuels and What are their Characteristics?

The differences among these fuels, arranged in our list from "lightest" to "heaviest" are in the types of hydrocarbon chains that are distilled out of the crude oil during refining (and also that some of these fuels may contain other additives.

What this all means is that the heavier petroleum based fuels (higher numbers) have longer hydrocarbon chains than the lower number fuels, they have more BTUs per gallon, they will be more viscous (and often dirtier or will contain more contaminants including environment-polluting sulphur).

It is not helpful to order and burn Kerosene #1 over #2 fuel oil except in outdoor aboveground oil tanks in areas subject to temperatures below 16 °F.

[From the following reference, edited:]

Problems With Heating System Reliability When Heating Oil Additives are Used or Low-Level Oil Tanks are Filled

When we serviced and installed heating equipment we often recommended use of heating oil additives to remove small amounts of water or sludge in oil storage tanks, or to act as a pour point depressant for outdoor aboveground oil storage tanks.

But while these are good products, things didn't always go well.

See details at OIL TANK SLUDGE

We have seen problems with rapid clogging of heating oil filters and thus loss of heat from sludge that was brought out of an old oil tank and into the filter when we added a pour point depressant to our heating oil.

The product also claimed to break up sludge - which sounds good if the oil lines are old and perhaps partly blocked with sludge.

But in this case the combination of use of a heating oil additive with a "de-sludger" combined with the sludge agitation up from the bottom of the oil tank during filling of a nearly empty oil tank led to loss of heat from filter clogging.

Oil companies may recommend that if you have to fill an old maybe sludgy oil tank that was nearly empty, turn off the boiler or furnace for a few hours to let the sludge return to the bottom of the tank (and be sure your heating equipment is fed fuel from the tank top tappings not from the tank bottom tapping.

In emergency when no home heating oil is available, one can use diesel fuel or kerosene to run the boiler or furnace.

An example of an oil additive to prevent gelling is at

OR you can purchase a kerosene/heating oil mix (more costly) to lower the waxing temperature of your heating oil


Continue reading at OIL TANK TREATMENTS, ADDITIVES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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Publisher - Daniel Friedman