Calculating Fuel Pump Vacuum

A good rule of thumb for calculating fuel pump vacuum is quite simple. Taking a pump vacuum is a necessity on fuel system when there is a lift situation. To just take the vacuum reading is not enough. We need to have an idea what the vacuum should be first. Think of it like this, what would be the reason to take a tire pressure if you did not know what the tire pressure should be, or blood pressure if we did not have a standard to compare it to. I have had techs take a vacuum reading and tell me it was good just because it was high. We are not looking for a high vacuum we are looking for a vacuum around the number we should have by our calculation.

Before we get into figuring a pump vacuum let's look at the different pumps. In our example we will look at Suntec pumps and all others work the same. The difference if what vacuums the pumps will work at through different manufacturers. Suntec has many pumps, but we will look at just a few new and old pumps. A fuel pump will work as a 1 pipe or two-pipe system. Installing a two-pipe system will give you more lift (high vacuum) if needed. There is also one stage and two stage fuel pumps.

A single stage pump will have one set of gears to pump oil from the tank to the oil burner. If the single stage pump is piped in a two-pipe application, it will return the oil back to the tank with the same set of gears. There are also 2 stage pumps which will in a two-pipe system pump oil to the burner with one set of gears and return oil back to the tank with a second set of gears. A two-stage pump should always be pipe with a two-pipe system, or it is a waste of money as the vacuum chart below shows. The two-stage pump with a single line has no more vacuum than a single stage fuel pump with a single line. We will only be discussing the Suntec Mini-pumps here since they are the most common residential fuel pumps. If you have a different fuel pump you will need to use your fuel pump manufacturer information for other type fuel pumps.

A good rule of thumb calculation is as follows:

1" of vacuum for every foot of vertical lift not offset by vertical drop.

1" of vacuum for every 10 ft of horizontal run

1" of vacuum for the valve and filter combined

Calculate the vacuum for other items in the line

Here is an example. What would the pump configuration be?

Here is the calculation

We have chosen the pump from the chart below. The single stage pump will work as a two-pipe system. It exceeds the one pipe rating of 6" of vacuum but is within the two-pipe single stage pump vacuum of 12". Now we have an idea of what the vacuum should be when checked.

If the vacuum is less than our estimated vacuum by more than 2", we may very possibly have an air leak at one of the fittings, pump cap, or pump plugs.

If a higher reading, I look at the following items but not limited to this list.
1. Look for kinks and flattening of tubing
2. Check the number and types of fittings, too many fittings increase pump vacuum. Make sweeping bends instead of using brass elbows where possible.
3. Condition of filters and strainers, verify they are clean.
4. Condition of valves, make sure they are full open and use full port fuel valves.
5. Contaminate build-up on inside walls of lines, clean lines if needed and possible. Do not blow out underground fuel lines. They may rupture and you will not be aware of it.

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