When it comes to shower pumps, the most common question we get asked is 'What Is A Negative Head or Positive Head Pump?'. There can be many complicated terms when installing a shower pump. Getting your head straight is key. Pun intended.
What is Head Pressure?
The concept of "head" in a pump refers to the highest height at which a pump can convey fluid against gravity. For example, a pump with a 5 meter head can pump fluid up a pipe 5 meters from the discharge exit. The greater the pump's head rating, the greater the pressure that the pump can generate.
Considering the diagram above, this is how we can define the different general types of head:
Zero head is generally considered to be 600mm below the water level, as there will still be very poor flow above this level.
Negative head, also known as negative pressure, is where the outlet is above the zero head level or where there is poor very low natural flow of water.
Positive head on the other hand is when the pump outlet is below the zero head level and water pressure can be expected to reflect the distance in height from the water tank.
Overall, as a very quick check, where an outlet is above the zero head level, or natural water flow is less than 2 litres per minute, it should be considered to be in negative head.
How does this work inside my home?
In a traditional home with a gravity system (like the diagram above), the water level in that tank is your start point. Your outlets will be your showers, taps, toilets or otherwise. If any of those are above the water level, they are definitely in negative head. If the outlets are below that water level, they are probably in positive head.
Watch out though - this head designation isn't always permanent. The water level in your tank will fluctuate as and when you use water, which means your outlet could easily and quickly transition between negative head and positive head. Generally, you should consider the water outlet - 600mm the point of zero head.
But as we say, if your flow is less than 2 litres per minute, you should consider your outlet to be in negative head.
So, what's the difference between a Positive and Negative Head Pumps?
First off, simply, a positive head pump is designed to be used in positive head only, and a negative head pump (or universal pump) can be used in all situations.
From a technical standpoint, a positive head pump will use an existing, albeit low, flow rate to kick-start the pump into action. Negative head pumps use a vacuum, or another mechanism, to move water against gravity and so cannot be triggered in the same way. Typically, negative head pumps detect pressure drops, or another method, to know when to start pumping.
When Do I Use a Positive Head Shower Pump?
A positive head shower pump is recommended when your outlet sits at least 600mm below the water outlet. However, generally speaking, if your flow rate is better that 2 litres/minute, use a positive head pump.
When Do I Use a Negative Head Shower Pump?
A negative head shower pump can be used in all applications, but they are designed to be used if you have outlets that sit above 600mm below the water outlet. However, generally speaking, if your flow rate is worse that 2 litres/minute, use a negative head pump.
Frequently asked questions
What is a negative head pump?
A negative head pump is designed for situations where there is poor or no natural flow of water in a home, i.e., when your outlet is in negative head. These pumps create a negative vacuum which draws water through from the tank and pushes it towards the shower.
What is a positive head pump?
A positive head pump is designed to be fed by gravity. The gravity feed serves to kick-start the pump, which then pushes water to the shower.
Do I need a positive or negative head shower pump?
The choice between a positive or negative head shower pump depends on the water system you have in your house. If any of your outlets (showers, taps, or toilets) are in the negative head zone then you would need a negative head pump. If the outlets are below that water level, they are in positive head and you can use a positive head pump.
What are the two types of shower pump?
The two most common types of shower pumps are positive head shower pumps and negative head shower pumps. Positive head pumps are designed to be used in systems where gravity feeds water into the pump, while negative head pumps are designed for systems where there is no or very poor flow of water.
What is the difference between a positive and negative head shower pump?
The main difference between a positive and negative head shower pump is how they are activated and the conditions they are designed for. A positive head pump needs an existing flow rate to kick-start it into action, while negative head pumps typically use a vaccuum to move water against gravity.
When should I use a negative head shower pump?
A negative head shower pump should be used when your shower head is in the negative head zone. Generally speaking this is when it is at the same level or higher than the water tank, but if you are experiencing no flow or very poor flow of water then this is a good sign that you need a negative head pump.
Any more questions?
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