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AGE of CHIMNEYS & FIREPLACES
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AIR CONDITIONING & HEAT PUMP SYSTEMS
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ASBESTOS IDENTIFICATION IN buildings
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BLUE vs YELLOW COMBUSTION FLAMES
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CHIMNEY INSPECTION DIAGNOSIS REPAIR
CHIMNEYS & Flues - Asbestos Transite Pipe
COOL OFF HEAT, Thermostat Switch
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COMPLETE COMBUSTION, Stoichiometric
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CONVECTOR HEATERS - HYDRONIC COILS
CREOSOTE FIRE HAZARDS
Curved Brick Chimneys - Sulphation
DEFINITION of Heating & Cooling Terms
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DIRECT VENTS / SIDE WALL VENTS
DIRECTORY of OIL TANK EXPERTS
DRAFT HOODS - gas fired
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DUST, HVAC CONTAMINATION STUDY
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FLOODED HEATING EQUIPMENT REPAIR
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FLUE VENT CONNECTORS
FREEZE-PROOF A BUILDING
FUEL OIL TYPES & CHARACTERISTICS
FUEL UNIT, HEATING OIL PUMPS
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HEAT TAPES & CABLES on Roofs for Ice Dams
HEATING COST FUEL & BTU Cost Table
HEATING COST SAVINGS METHODS
HEATING LOSS DIAGNOSIS-BOILERS
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HEATING OIL CLOUD WAX GEL POINT
HEATING OIL EXPOSURE HAZARDS, LIMITS
HEATING OIL - OLD, USEABLE?
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HEATING OIL SHELF LIFE
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HEATING SMALL LOADS
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HOT WATER IMPROVEMENT
INSULATION INSPECTION & IMPROVEMENT
LOW VOLTAGE BUILDING WIRING
LOW VOLTAGE TRANSFORMER TEST
LP & Natural Gas Safety Hazards
MANUALS & PARTS GUIDES - HVAC
MIXING / ANTI-SCALD VALVES
MOTOR OVERLOAD RESET SWITCH
NATURAL GAS COMBUSTION PRODUCTS
NO HEAT - BOILER
NO HEAT - FURNACE
NOISE / SOUND DIAGNOSIS & CURE
NOISE AIR CONDITIONER / HEAT PUMP
NOISE, DUCT VIBRATION DAMPENERS
NOISE CONTROL for HEATING SYSTEMS
NOISE, PLUMBING CHECKLIST
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ODORS & SMELLS DIAGNOSIS & CURE
ODORS FROM HEATING SYSTEMS
OIL BURNER FUEL UNIT
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OIL BURNER NOISE SMOKE ODORS
OIL BURNER NOZZLE & ELECTRODES
OIL BURNERS, RETENTION HEAD
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OIL FILTERS on HEATING EQUIPMENT
OIL FILTER MISSING
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OIL LINE CLOGGING FIX
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OIL FILL PIPE LEAKS
OIL PUMP FUEL UNIT
OIL SPILL CLEANUP / PREVENTION
PLASTIC HEATER VENT
PULSE COMBUSTION HEATERS
PRESSURE REDUCING VALVES
PUMPS, PONY PUMPS
RADIANT HEAT Floor Mistakes to Avoid
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RADIANT SLAB TUBING & FLUID CHOICES
REFRIGERANTS & PIPING
RELIEF VALVE LEAKS
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RELIEF VALVE LEAKS
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THERMAL IMAGING, THERMOGRAPHY
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THERMAL TRACKING & HEAT LOSS
THERMOSTATS, HEATING / COOLING
TRANSITE PIPE CHIMNEYS & FLUES
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WATER HEATER SAFETY
WATER HEATERS for HOME HEATING USE?
WATER HEATER NOISES
WATER HEATER SCALE - De-Liming Procedure
WATER HEATER SCALE PREVENTION
WINTERIZE A BUILDING
WOOD, COAL STOVES & FIREPLACES
WOOD STOVE SAFETY
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.
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Also see FIRE SAFETY CONTROLS wHere we explain the proper and improper use of fusible link oil line valves - an improperly placed valve can cause leaks or even an explosion or fire at an oil-fired heating device oil pump fuel unit. 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. If you don't know what kind of heat your building uses, we explain how to figure out the answer at HEATING SYSTEM TYPES. If your heating system is not working properly, see NO HEAT - BOILER or NO HEAT - FURNACE.
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.
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.
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.
Quoting from Webster's description of their current product line we can read the typical capabilities of modern heating oil fuel pumping units:
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.
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.
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:
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").
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.
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.
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 FIRE SAFETY CONTROLS and at OIL LINE SAFETY VALVES, 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.
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. 
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).
Our fuel unit at left, 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.
When we (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.
Beckettpoints out in their oil burner installation instructions that it is essential to vent air out of the oil piping and out of the fuel unit itself during installation. You'll see a rounded fitting that looks suspiciously like an automobile brake system bleeder valve installed on fuel units for that purpose.
We attach a clear plastic tube to the fitting, put the other end of the fitting into a clear jar where we can watch for the end of air bubbles, then run the system to pump oil and air through the fuel unit until no more air bubbles are occurring.
This same procedure may be necessary if an oil fired heating system has run completely out of oil.
There are two reasons why this air bleeding operation is essential:
Two pipe oil burner systems do not normally need to be air- bled since the fuel unit will return any excess air along with oil down the return line to the oil tank.
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.
Annual service for the fuel unit also includes a vacuum check for proper pump operation and to check for leaks in the input line.
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:
We provide an article series that explains the installation & use of
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.
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.
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.
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Frequently Asked Questions (FAQs) about heating oil fuel units (oil pumps)
Question: What causes pulsating oil pressure at the oil burner?
Since I asked for one, my oil burner has a pressure gauge mounted on the unit showing the output pressure on the system. Usually the gauge is steady at 120 psi but now I see the needle is waving all over the place. What might be wrong. .
I would be a little nervous about leaving a gauge on the outlet side of a fuel unit for heating oil equipment. There is a risk of blowout of the gauge and a horrible oil leak mess. That said, look for:
Other leaks and problems besides those air leaks above can also lead to oil burner shut-down or improper oil burner nozzle cutoff problems, including:
Incidentally, Suntec also offers these additional diagnostic suggestions regarding heating oil pressures: 
If oil pressure at the outlet side of the fuel unit is low, check the accuracy of the pressure gauge, then check that the nozzle capacity is not greater than the capability of the fuel unit. We add: check that someone has not misadjusted the fuel unit pressure screw.
Improper oil burner nozzle cut-off diagnosis - here is a diagnostic procedure from Suntec:
Questions & answers or comments about diagnosing, adjusting, repairing, or replacing heating oil fuel units or oil pumps on oil-fired heating equipment: boilers, furnaces, water heaters
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.