Oil burner oil line switch (C) Daniel Friedman Guide to Oil Burner Fuel Units
Heating Oil Pump Installation, Diagnosis, Repair

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Oil burner fuel unit installation & maintenance guide: this article describes the function, diagnosis, adjustment, and repair of oil burner fuel units or "oil pumps", and we provide related oil burner fuel unit safety, heating system efficiency and heating cost savings advice.

We include a discussion of the oil pressure settings, one line vs two line oil piping connections, the location of the air bleeder valve, and the use of an oil delay solenoid valve or quick-stop valve or oil safety valve on some fuel units.

We also describe common maintenance & repair topics & procedures for oil burner fuel units such as drive shaft coupling failures, drive shaft leaks, and internal check valve leaks.

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.

Oil Burner Fuel Units or oil pumps: installation, inspection, diagnosis, maintenance & repair

Webster M34DK Fuel Unit

[Click to enlarge any image]

Article Series Contents

The "oil pump", properly called the fuel unit in most oil heating texts, draws heating oil from the oil storage tank, pressurizes the oil to high pressures of 100 to 125 psi (typically on modern retention head oil burners), and delivers oil to the oil burner nozzle where the combination of high oil pressure, combustion air, and turbulating devices (in the nozzle and/or at the end of the oil burner) atomize the oil and spray it into the combustion chamber.

Danfoss / Sunstrand / Suntec & Webster Oil Burner Fuel Units

Webster single stage fuel unit © D Friedman at Two common brands of widely used fuel units for oil burners are Danfoss Sunstrand or Suntec (photo at page top) and Webster (images at left and above-left). At left is a Webster single-stage 3450-rpm fuel unit.

Both companies make a range of excellent products that in our experience have proven durable and simple to install and maintain.

[Click to enlarge any image]

The single-stage Sunstrand J-pump and the two-stage Sunstrand H-pump were among the most widely used fuel units we encountered when servicing heating equipment in the Northeastern U.S. Webster's M-series fuel units are also widely used as OEM equipment on a variety of oil burners found on residential heating boilers, furnaces, and water heaters.

Manuals for oil burner fuel units are found at OIL BURNER MANUALS

Single stage & two stage fuel unit operating capabilities

Quoting from Webster's description of their current product line we can read the typical capabilities of modern heating oil fuel pumping units:

Single and two stage models with 1725 and 3450 rpm operation feature compact design and field-proven reliability. These units, rated @ 3 to 25 gph (11 to 57 1/hr.) @ 100 psi (6.9 bar), can be used on one or two pipe installations. ... Operating pressure [on heating oil fuel units] is factory [typically] set at 100 psi.

Commercial capacity oil burner fuel pumps

Larger, commercial-application fuel pumps are of course also available, such as Webster's R-series fuel pumps that can deliver up to 75 gph and even V-pumps capable of firing equipment at up to 260 gph (gallons per hour) - far larger than any home application.

Oil Burner Fuel Unit Rotation Direction - CW vs CCW or left hand vs right hand

Fuel units for oil burners are sold in "left-hand" and "right-hand" rotation to meet different electrical motor and oil burner configurations. The direction of rotation is interpreted by viewing the fuel unit from the shaft end.

Installation & Adjustment Procedures for an Oil Burner Fuel Unit

What are the Connections or Ports on a Heating Oil Fuel Unit?

Suntec basic ports & connections - courtesy Suntec Fuel Units

As we describe and illustrate (Suntec ports and components are illustrated at left, courtesy Suntec - click to enlarge) in this article a typical fuel unit or "heating oil pump" will have these connections and controls:

  1. An inlet port - feeds oil from the oil tank into the pump or fuel unit
  2. Oil delivery pressure to the burner (adjustment screw)
  3. Oil return or outlet port or external bypass port - can be used in a two-pipe hookup to return oil to the oil tank
  4. Pressure gauge port
  5. Nozzle line port - high pressure oil delivered to the oil burner nozzle
  6. Heating oil inlet port
  7. An internal bypass port or cover - possibly accessed through the bypass port, an internal plug can be installed or removed
  8. Air bleeder valve

These features will be reversed if the fuel unit rotation direction is reversed, and they may be in different positions on different models and brands of oil burner fuel units. Not directly accessible from outside the fuel unit are additional important parts found on most heating oil pumps (or if you want to sound like a pro, call it a "fuel unit").

Single-line Heating Oil System Fuel Unit Oil Piping Connections, Hookups

Single line heating oil hook up © D Friedman at Where an indoor oil storage tank is installed on or close to the same level as the oil burner, it is common for a single heating oil line to bring oil from the oil storage tank to the fuel unit. In a single-line installation, an internal check valve found in the fuel unit prevents oil backflow into the oil lines when the fuel unit stops spinning.

In our fuel unit photo (left) the green arrow marks the incoming oil from the oil tank and filter (under vacuum when the pump is running). The red arrow indicates the high pressure oil being delivered from the fuel unit into the oil burner nozzle line.

This oil burner pump unit includes an oil delay valve , an electric solenoid switch connected through the burner tube over to the oil burner motor run circuit by an electrical wire. (yellow arrow).

The typical fuel unit used on modern oil burners does not have a high lift capacity - perhaps 6 feet would be a safe lift level for a one-line oil pipe installation.

Watch out: we have found single-line fuel unit hookups where a low or even a buried oil tank was installed. Such systems may "work" successfully for many years even though they violate good practice. Or the single-line hookup may work only if the oil level in the oil storage tank is kept pretty high (reducing the total lift required).

But on occasion we visited a property where there was recurrent loss of heat - a problem that was traced to the oil burner losing prime in the oil line. The "fix" was to convert the system to a two-line oil pipe hookup described below.

Two Line Fuel Unit "Heating Oil Pump" Unit Hookups

Two line heating oil fuel unit hookup © D Friedman at A two-line fuel unit hookup simply means that there are two oil pipes between the fuel unit and the oil tank.

One line is the delivery line - from oil tank into the fuel unit, and the second line is the "return line" through which excess oil pumped by the unit flows back to the oil tank.

A two line oil piping system, required for buried oil tanks or oil tanks located below the oil burner, increases the "lift" capacity of the fuel unit and also nearly eliminates oil piping priming problems.

Our photo (left) shows a two-line oil piping hookup on a Suntec fuel unit - it's pretty easy to see since both the supply oil pipe and the return oil pipe connections are visible at the bottom of the fuel unit.

My pen is pointing to the air bleeder fitting on this fuel unit.

The oil inlet hookup is on the left and the oil "bypass" or "return" hookup is on the right. The label on the heating oil fuel unit face also will identify the various ports on the device.

A third oil pipe, the high pressure line from the pump outlet to the oil burner nozzle assembly is not clear in this photo, though you can make out a small brass tubing loop at the upper left in our photo - a part of the high pressure oil line.

Where a buried oil storage tank is installed or where the oil storage is significantly lower than the oil burner a two-line oil piping hookup is used to give added lift capacity to the fuel unit.

Watch out: check the fuel unit installation instructions and be sure that the internal check valve is set to the proper condition on the fuel unit when changing from a one-line to a two-line oil piping hookup, or vice versa.

Watch out: as we discuss
at OIL LINE SAFETY VALVES, OSVs, when a two-line oil piping system is in use, the fusible link oil safety valve should be installed only on the oil supply line. A check valve without a fusible link is typically installed on the oil return line.

Internal Fuel Unit Bypass Control - does the bypass plug go "in" or should it be "removed" ?

Webster oil burner fuel unit © D Friedman at Watch out: standard factory-set-up usually delivers a fuel unit with an "internal bypass" in the fuel unit "open". That's the set-up for a one-pipe oil piping hook-up.

When installing a two-pipe oil piping hookup to a fuel unit, an allen screw or plug is inserted into the fuel unit to close the internal bypass. The access to the internal bypass will be found beneath a service plug and will usually also be marked on a label on the fuel unit body.

A single-pipe heating oil system requires that the bypass plug is removed. Leaving the plug in the fuel unit will result in a blown seal, and oil leak, and loss of heat.

A two-pipe heating oil system requires that the internal bypass plug is inserted - in place. But kinked oil piping or piping blocked by sludge in the return line can still result in a blown seal in the fuel unit, an oil leak, and loss of heat. [8]

If a novice heating tech is having trouble remembering what to do with a bypass plug or port, just remember that we want only one excess pumped-oil return path - it's either inside the pump itself (an internal bypass is open on a one pipe oil piping system) or it's outside the pump and sends excess oil back to the oil tank (the internal bypass is then shut).

Our photo (above left) shows a modern single-stage 3450 rpm Webster fuel unit. If you are replacing the fuel unit on an oil burner you will need to note and match the following specifications:

At the bottom of the label in our photo above you can see a reference to the internal bypass plug along with a warning note and an arrow pointing to the threaded "plug" that is either installed (internal bypass closed, two pipe installation) or removed (internal bypass open, one pipe installation).

Dirty oil burner pump fuel unit © D Friedman at

Our fuel unit shown above, also a Webster unit, tells a story without having performed any tests. It appears from the dirt and cobwebs that no one has touched this device - a clue about its service history.

Typical Pressure Settings on Heating Oil Fuel Units

Fuel unit pressure adjustment screw on an oi lburner (C) Daniel Friedman at

When Carol Schnierer and I (DF) serviced heating equipment back in the dark ages, typical fuel unit pressure factory setting was to 100 psi.

The pump output can be adjusted on most fuel units, typically within a range of about 100 psi up to 150 psi.

At oil burner school we learned to set the fuel unit pressure up to 110 or even 120, adjusting the nozzle size down to a smaller gph nozzle to keep the overall fuel delivery rate at the intended design point for the oil burner. That higher pressure gave better, finer oil spray delivery and more efficient oil burner operation.

Currently we see new oil fired heating equipment using 125 psi as a "standard" pressure setting. Modern fuel units such as the Webster M34DK fuel unit is able to maintain its 3 gallon fire size throughout the 100 to 150 psi pressure range.

Our heating service tech Bob, from Bottini Oil, recently installed a replacement fuel unit on our lab's oil burner assembly.

In setting up the system, a 15 year old cast iron Peerless boiler with a Beckett oil burner, Bob set the oil pressure up to 140 psi, and the smoke was set first to "trace" and then (with a slight combustion air boost) down to "zero" - what he regards as current optimum settings for contemporary high speed oil burner units on newer heating equipment.

Watch out: check the labels or tags on your oil burner: the manufacturer will often specify the oil pressure at which the burner is designed to operate, as we illustrate just below.

Do not guess at the oil pressure setting. Unless you install an oil pressure gauge on the nozzle line side of the oil burner you will have no idea what the fuel unit's outlet pressure actually is.

Oil burner fuel unit pressure setting tag on a Beckett oil burner (C) Daniel Friedman at

This oil burner fuel unit pressure data tag was fixed to the oil burner assembly just above the high pressure nozzle line inlet connector on a Beckett oil burner.


Bleed air out of oil piping and oil burner fuel units


Annual Fuel Unit Service & Repair Guide

Change-out of the Internal Filter Screen on Heating Oil Fuel Units

Oil burner fuel unit filter screen © D Friedman at Oil burner fuel units typically use an internal filter/screen to help protect the pump internal parts and the oil burner nozzle.

While a proper heating system installation will include a canister-type oil filter on the oil piping ahead of the oil burner, for several reasons debris can escape the filter and enter the oil pump body.

This screen should be replaced as part of annual oil burner service. The screen comes with a new gasket that should also be used.

Watch out: if you fail to properly torque the four bolts holding the fuel unit cover over this screen assembly, air leaks in and oil leaks out at that location will prevent proper fuel unit operation.

Debris-clogging at the oil burner check valve

Watch out: when the fuel unit screen is never replaced, debris entering the oil pump can cause two common operating problems, both leading to improper and even unsafe oil burner operation:

Debris can clog the oil burner check valve (discussed just above) causing the valve to fail to stop oil delivery quickly when the oil burner stops running. The result can be a sloppy oil burner shut-down, oil burner nozzle clogging, and loss of heat or an oil burner puffback

Debris that prevents the internal check valve from closing may also allow oil from an oil storage tank to feed through the fuel unit by gravity, flooding the combustion chamber and risking a dangerous oil burner puffback, explosion or fire when the system re-starts.

During a service call on a very old oil burner in Poughkeepsie, NY in the 1980's, we found a low-speed oil burner that would keep right on going even after we turned off electrical power to the oil burner motor.

It was scary. The combination of heating oil feeding past the failed check valve and into the combustion chamber along with very hot combustion chamber liner components was enough to keep a "fire" going in the combustion chamber of the old boiler.

Vacuum Checks at the fuel unit

Annual service for the fuel unit also includes a vacuum check for proper pump operation and to check for leaks in the input line.


Internal Check Valve on Heating Oil Fuel Pumps: leaks, repairs, oil delay valves

An internal check valve on the fuel unit is designed to rapidly stop the flow of oil out of the pump and to the oil burner nozzle when the electric motor stops spinning. If the check valve is not working, as the electric motor shaft slows below its full operating speed, oil may be otherwise delivered to the oil burner nozzle at low-pressure.

In turn, low pressure oil delivery means incomplete combustion of the oil in the combustion chamber (it is not fully atomized or broken into a fine spray). Incomplete combustion of heating oil in turn clogs the oil burner nozzle, leading to sooting and even loss of heat or a dangerous oil burner puffback.

Because a failed or "sticking" oil burner pump check valve causes serious operating problems and because this internal part is not easily repaired in the field there are two common repair approaches:

  1. Replace the entire oil burner fuel pump or fuel unit
  2. Install an oil delay valve -
    see OIL LINE QUICK STOP VALVES for details.

    An oil delay valve is an electric solenoid installed on the high pressure oil line between the oil pump outlet fitting and the oil burner nozzle tube.

    The oil delay valve can help assure clean oil burner start-ups and shut-downs by making sure that the electric motor spinning the oil burner air intake blower as well as the oil pump are up to full operating speed before the solenoid opens to allow oil to flow to the oil burner nozzle.

This article series explains the installation & use of

Drive shaft connections for oil burner fuel pumps on boilers, furnaces, water heaters

An electric motor mounted on one side of the oil burner drives a shaft that in turn spins both a combustion air blower (that draws combustion air into the oil burner tube) and the fuel unit. Both of those components are connected in line on the drive shaft of the electric motor using a somewhat flexible rubber or plastic and rubber coupling.

Older "slow speed" oil burners used a motor that rotates at 1725 rpm, a speed that in turn determines the speed of the blower assembly and the fuel unit as well.

Modern "high speed" oil burners use a motor that rotates at 3450 rpm, a bit noisier but because of the additional combustion air that these systems can provide, high speed burners typically operate at a higher efficiency than the older models.

Watch out: a stripped or failed or loose coupling can result in slow burner blower assembly or fuel unit rotation and improper oil burner operation, or in no oil burner operation at all. A bad coupling or bad coupling alignment (everything should be in a straight line, motor, blower shaft, fuel unit shaft) will also be a cause of noise at the oil burner or fuel unit.

Drive Shaft Bearing Leaks on Oil Burner Fuel Units

Leaky shaft at the oil  burner fuel unit (C) Daniel Friedman

Damaged coupling joining the electric motor that drives the oil burner, the combustion air blower squirrel cage fan, and the shaft that turns the fuel pump unit itself.

In our photo, left, the heating service tech has removed the fuel unit (oil pump) from the oil burner to inspect for evidence of leaks around the oil burner's fuel unit drive shaft.

He did so after observing oil and debris and tar inside the burner tube assembly and on the oil burner's squirrel cage fan assembly. Because this fuel unit had a (small) leak around the fuel unit shaft, the entire unit was replaced.

In this CLOSEUP PHOTO of the fuel unit shaft leak you can see a tarry accumulation that convinced the tech (and us) of the leak problem.

What happens if we have even a small leak around the fuel unit drive shaft on an oil burner pump? Oil dripping into the oil burner tube soils the squirrel cage blower fan blades just enough that the blades accumulate and hold dust and debris.

The result is that while the fan may look ok, it is moving less air, leading to inadequate combustion air and a smoky sooty oil burner operation, eventually perhaps a puffback. Crud accumulating in the oil burner tube will eventually clog a small bleeder hole in the tube bottom, leading to still more oil and debris accumulation in this location.

The heating system articles provided at this website explain how to inspect and detect defects and hazards on heating systems, boilers, furnaces, and other equipment. Methods for saving on heating cost and on improving heating safety are included. Heating safety hazards such as carbon monoxide gas leaks, unsafe furnaces, furnace and boiler recalls are addressed.

Reader Question: leaks at Electro Oil Inter B9B burner in a Heatslave 15/19 boiler - Contacts for Danfoss Oil Burner Pumps

(Feb 21, 2014) Stewart Brown said:

Hi! We have an Electro Oil Inter B9B burner in a Heatslave 15/19 boiler. Oil has started leaking, quite slowly, from the pressure adjustment screw on the front of the oil pump. Is there something I can tweak or tighten to cure this without turning a minor problem into a catastrophe?


Hi Stewart.

What is the name brand on the oil pump? We ought to be able to look at its instruction manual to see what's sealable.
Typically there is a lock nut and a slotted screw that's turned to adjust the oil pressure, or on some units a recessed allen screw.

If yours is a Suntec fuel unit you can also contact Suntec’s
field service department at 800-367-7116.

Keep me posted or send along a photo so I can ID and research your particular unit

Suntec's manuals for their fuel units are at

(Feb 21, 2014) Stewart Brown said:

It has Danfoss on the front. The rest of the description is from the WEC Heatslave installation manual (Fig 24 on page 32). Sorry, can't fathom how to attach photo/manual extract.

(Feb 21, 2014) Stewart Brown said:

More sleuthing on the internet: It is very similar to a Danfoss BFP21. The front plate with the controls/adjusters is identical although the orientation of our main body (solenoid/bleed valve) is different.


Stewart there are a number of Danfoss oil burner pump models and the company provides manuals online - though it sounds as if you've got one. From my scan of the documents I don't see an obvious cap or leak discussion

. If there is a faceted fitting at or around the pressure adjustment screw port you might try gently tightening that to be sure it's secure; I would give Danfoss a call to ask about the leak you are observing: a concern is not just oil leaking out but the possibility of air leaking in - which can cause burner malfunction.

You don't say where you are located so I'm giving just contact information for North America; there is additional contact data for other countries

You'll see that for most of these fuel units there will be more than one way to hook up fuel lines, that may be the difference; also there are LH and RH rotation models that can be confusing to some. There ought to be a label on the unit giving model number, though on occasion the oil burner company or a service tech may have re-labeled.

If you want to use our CONTACT link to send me photos of the unit and a sharp closeup of where you see a leak I may be able to comment further.

(Feb 21, 2014) Stewart Brown said:

I have found the schematic! P1 on: but it doesn't answer the practical problem I have!


give them a call and let me know what advice you receive; if you call a tech s/he will either just tighten something or will replace the unit. We don't know if there is a repairable seal.

(Feb 21, 2014) Stewart Brown said:

There is a slightly clearer document:
It is a deep seated screw with an allen key head.

Function: to adjust oil flow. The screw thread is adjustment rather than seal & it is starting to look as if the diaphragm in our unit has given up the ghost. I can't really complain about longevity but it is a nuisance nonetheless!


Sounds as if a tech would at this point replace the fuel unit.
Be sure to get exactly the right one, including rotation direction; a service tech will know how to measure vacuum, adjust nozzle line pressure, and bleed air.

Danfoss Hago Inc Contact Information

Using Biodiesel B6-B20 vs. B-100 in Home Heating System Oil Burners

Reader Question: B100 and Suntec Oil Pump - rubber component damage & leaks: loss of pressure at Suntec fuel unit (oil burner pump)

12/5/2014 Steve said:

I am running B 100 in my oil furnance with using a Beckett / Suntec oil pump and have had premature failure of the pumps. The pump gradually looses oil pressure and I readjust till there isn't anymore to work with. I've inspected several of the pumps and find them very clean and no signs of wear anywhere.

B100 is destructive to rubber components but I can't find any rubber components other than a material in the tip of pressure regulating component with a small hole through it. do you have any insight as to why I am loosing pressure?
Thank you, Steve


For space and editorial focus we have moved this question and discussion.
See BIODIESEL HEATING FUELS for details about using biodiesel or bioheat fuels in oil-fired heating equipment: oil burners used to heat boilers, furnaces, water heaters, etc.


Oil Burner Adjustment Guide Books

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

Also see


Continue reading OIL BURNER FUEL UNIT DIAGNOSTIC FAQs or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see OIL BURNER FUEL UNIT DIAGNOSTIC FAQs-2 - more questions & answers about the oil burner pump

Or see OIL BURNER INSPECTION & REPAIR home page for oil burner diagnosis and repair



Or see OIL BURNER WONT RUN - Diagnostic Steps - what to check in what order,

Suggested citation for this web page

OIL BURNER FUEL UNIT at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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