Suntec basic ports & connections - courtesy Suntec Fuel UnitsOil Burner Fuel Unit Diagnostic FAQs
Questions & answers about diagnosing & fixing the oil burner pump

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Diagnostic questions & answers for a heating or water heater oil burner fuel unit.

This article reports common questions & answers about the oil burner fuel unit or "oil pump" used on oil-fired heating equpment suchj as the oil burner for a heating boiler, furnace, or oil-fired water heater.

This article series explains the inspection, diagnosis, & repair of oil burners used in heating appliances such as hot water boilers, steam boilers, & water heaters. We describe how to inspect, troubleshoot and repair heating systems to inform home owners, buyers, and home inspectors and students of heating service methods about common heating system defects. The articles at this website describe how to recognize common oil-fired heating appliance operating or safety defects, and how to save money on home heating costs

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Oil Burner Fuel Unit (oil pump) Diagnostic Questions & Answers

Single line heating oil hook up (C) D FriedmanThis article contains questions and answers regarding oil burner fuel units and is a companion to our lead article on this topic: OIL BURNER FUEL UNIT

Also see OIL BURNER WONT RUN for a flowchart of diagnostic steps for an oil burner that won't start or won't keep running: what to check in what order.

Question: trouble bleeding air in oil lines

(Aug 5, 2012) roberto said:

I cant bleed the line


Roberto check the air bleed instructions in the article above and let us know if / where you get into trouble

Question: parts of an oil pump

(Aug 5, 2012) Anonymous said:

what parts are in the fuel pump


Anon: if you check out some of our references/ citations you can dig into a fuel unit manual and see details, cross sectional drawings etc. There are plenty of parts inside.

Opening up the fuel unit side by removing the 4 bolts - a service tech job - reveals a replaceable filter screen that should be changed at annual service (though many techs ignore it). Incoming oil passes through the screen, through a pump, and at high pressure out to the oil burner.

A check valve stops oil flow when the motor stops spinning. An internal bypass valve allows excess oil to circulate inside the pump or if that valve is closed in a 2-pipe system the excess oil flows back through the 2nd oil line to the oil tank. The pump drive shaft is operated by a coupling that connects the pump through the oil burner blower assembly over to the electric motor that drives the whole assembly.

Question: fuel unit cross reference on parts

(Mar 15, 2013) Tom said:

Good reading - learned a lot! I need to replace the oil burner fuel unit on a Lennox OHP31C-7 burner. Original appears to be a Sunstrand pump (from physical looks) but label is unreadable. Where wuld I find a cross reference to Suntec or something else suitable?



You want to replace the unit with another of the same brand and model type to be sure it'll work with your oil burner, though there may be some cross references between Sunstrand and Webster.

The critical features are the direction of rotation of the drive shaft for the fuel unit (LH rotation or RH rotation) and its capacity.

Try taking the old unit into your HVAC supplier who can probably match it with a replacement.

You could also simply take note of which way your oil burner drive motor shaft spins by watching briefly through the burner top - the shaft rotation direction is usually quoted when viewed from the connecting-end or exposed end of the drive shaft but as long as you can clearly state what you saw your supplier can be sure you're getting the correct-rotating replacement pump.

Question: oil burner won't stay lit

(Mar 24, 2013) Jason said:

My oil furnace will not stay ignited and running. It will fire up but will immediately trip the breaker due to lack of fuel. I am able to manually prime the pump with oil, then reconnect all of the lines and fittings, start the furnace, and it will run for about one to two minutes before cutting out and tripping the saftey relay. I have not discovered another way to start the furnace with some, although minimal, success.Is this a common symptom of a failing or falled pump?

(Mar 24, 2013) Jason said:

My single-stage oil furnace will start up but does not ignite and immediately trips the relay breaker as a consequence from the absence of fuel. I am able to manually prime the pump with fuel by pouring a couple ounces directly into the pump and into the "inlet line", next reconnect the line and fitting, start the furnace, quickly bleed the line until I have a solid stream, and the furnace will instantly ignite the fuel and run for about one to two minutes before tripping the breaker from the absence or lack of fuel being supplied. I have about 15-20 gallons of fuel in my tank. Is this a symptom of a failing or failed pump?



I'm not sure what's wrong - could be a bad electric motor or wiring short - that's the sort of thing that trips a circuit breaker.

If on the other hand you mean the primary control is tripping and shutting down the burner, say for lack of solid flame, it's doing what it's supposed to.

If you only have 15-20 gallons in an oil tank it's techically empty - and is either sending air or sludge over to the oil burner.

Question: how to change from 2 line to 1 line fuel unit hookup

(Apr 2, 2014) Anonymous said:

dear sirs, i purchsed a miller mobile home oil funace (aprox 12 yrs old) with a dual line hook up,we capped of the return line for a single line useage, tuned it on for 20 sec. oil leak somwhere?, do i need to use a special plug or change something in the pump, thanks Bob



Watch out: When converting from 2-line to 1-line oil piping the fuel unit internal bypass must be opened or you risk dangerous pump damage and leaks.

We describe 1-line vs 2-line oil piping hookups and the internal bypass plug in the article above.

Question: Wikola burner won't keep burning

(Dec 19, 2014) Edward said:
We are visiting Germany for Christmas and have a Wikola burner.
When switching it on it does not burn continuously but only 3 secs at a time with a pause of about 20 secs.
It is cold outside (and inside) and it does not heat up as it usually has done.


Definitely sounds like you need a service call - short cycling like that risks increased soot and risks an oil burner puffback.

Question: how to get the end cap off of the fuel unit

(Jan 30, 2015) Anonymous said:
how do i get the end cap off pump, ihave all 8 bolts removed


Providing you've removed the connecting piping so that the cover is free to take off, tap it gently on one side. Be sure you've a catch-pan below as oil will spill out.

Question: Install a 3450 rpm fuel pump on a 1725 motor? Weil-Mclain 65 Series Oil boiler with a Weil-McLain burner Unit Model 45 CU-1

(Feb 22, 2015) Konrad said:
We have an older Weil-Mclain 65 Series Oil boiler with a Weil-McLain burner Unit Model 45 CU-1 (obsolete). It has a 1725 RPM motor and not sure what make oil pump (unit). Lately, I have noticed that after running for awhile, it sounds as if the oil pump is making an intermittent bearing failure type noise. I will replace the pump at the earliest opportunity (this is in my second home, a 3.0 hour drive). My question is, can I install a 3450 RPM oil pump to run with a 1725 RPM motor, or MUST the two RPM's coincide. Thank you!


They need to coincide. If you are keeping the boiler but want to upgrade the burner assembly it may make more sense and be more economical to buy a complete assembly. Even then there will be other details to get right: the gun and burner tube specs, the nozzle spray pattern has to match the combustion chamber design, &c.

Question: to what height can an oil burner pump lift oil?

(Apr 13, 2015) Anonymous said:

I just bought a used oil furnace and tank to put in my garage. It is a single line beckett setup. Hooked it up and temporarily put fuel line in a jug of diesel fuel about same height as pump and it works great. Ran line thru wall out to tank which is 4 1/2 ft lower and added filter in line. Put 5 gallons in tank and tried running furnace but it keeps pulling air bubbles in fuel when I bleed. Furnace will only run 10 seconds then shuts off due to air. Tank too low? Not enough fuel in tank? Would 2 line system cure problem?
Thanks, Steve


Anon / Steve

Even a one stage single line J pump can lift the 4 1/2 feet you cite. I suspect that there is a leak in your piping. Leaks can be hard to find. Installing a vacuum gauge on the line can help. When the burner stops, watch the gauge.

Question: Suntec fuel unit oil pump blows pressure gauge

May 22, 2015) paolo said:
what will cause a suntec type ta burner pump to blow the 40 bar pressure gauge on it


Paolo, there are several things that will blow the pressur gauge on a heating oil burner fuel system:

  1. Installing the gauge on the high pressure outlets side of the fuel unit, leaving it always exposed to pressure,
  2. surges in pump pressure, or
  3. blockage in the internal bypass or in the oil return line of a 2-line oil piping system.
  4. Or a damaged or defective gauge.
  5. Or something else we've not considered.

Question: not getting fuel to the oil burner nozzle

(Nov 3, 2015) Bob Bell said:
Just installed a fuel pump (one line), and new nozzle. The problem is that now I'm not getting any fuel up to the nozzle ! Need some help! I have tried everything I can think of!


Bob you probably fixed this by now, but if not I would start by making sure that you have bled air from the system starting right at the oil filter. Then you can confirm that the Fuel unit is delivering oil through the high pressure line that connects the Fuel unit to the burner. If you have oil there than the problem may be our nozzle, nozzle adapter, or even (though less likely) the tubing between oil burner high pressure outlet and the nozle adapter.

Often I see a tech simply disconnect the high pressure line at the side of the oil burner nozzle assembly, directing it into a container. Then she turns on the burner to watch to see that oil is flowing (it will be flowing at 100 psi or more) out of the nozzle line tubing. If no oil is emerging we start working backwards: confirming that there no clogged strainer in the fuel unit, and that oil is entering the fuel unit.

Question: will water damage the oil burner pump (fuel unit) ?

2016/02/20 dean said:
will water damage the pump Oil Burner Fuel Units or oil pumps:


Water in the oil supply system, typically coming from the oil tank, will ultimately damage the fuel unit by causing rust or corrosion inside the pump motor. That may not occur right away but may occur if water remains in the pump over time, such as when the system sits idle out of the heating season.

Search InspectApedia for OIL TANK WATER to see a series of articles on detecting and removing water in the oil fired heater system.

Question: intermittent shut-downof the oil burner, off on safety / reset

2016/02/24 Chris olah said:
My oil furnace runs well but once every couple days it shuts off for no aparent reason. I push the rest button and it starts right up what could be causing this to happen ?


Thanks for the interesting question, Chris. There are several possible causes of intermittent oil burner failure that I can name and probably others that your service tech can suggest but that I've not thought-of.
- An oil burner that needs cleaning and adjustment
- A loose wire
- A cad cell sensor that's loose in its mount or is failing
- Intermittent draft problems
- A fuel unit whose coupling to the drive motor through the blower fan in the oil burner may be developing a bearing or drive shaft failure or the coupling itself may be slipping
- water or debris in the oil tank, supply lines, filter, or fuel unit screen

Really just about any operating problem at a heating boiler or forced warm air furnace system can show up as an intermittent failure, often worsening over time.

Often an intermittent problem gets worse until it becomes a hard failure - not very convenient during the heating season, but easier to diagnose. I would prefer to find and fix the trouble rather than "shotgunning" by just starting to replace parts.


Continue reading at OIL BURNER WONT RUN - Diagnostic Steps - what to check in what order, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.




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