If you a looking to select the best water pump for your rainwater tank then you will see specifications like rated and maximum flow rates
or head. What do they mean? What should you look for? Will the pump be adequate for your needs? The focus of this article will explore the question of rated and maximum “head” in water pressure pumps.
What is Pump “Head” and “Total Head”?
When talking about how pumps work, or looking over pump specifications, you will come across maximum and rated “head” distance. If you don’t know much about water pumps, then you are probably just wanting to know how much pressure it can output and whether such is adequate for your needs. So, what is head? Why is it even mentioned when you just want water pressure!?
Pump head is about water pressure your pump can handle, only vertically. If you were to attach pipes to your pump and extend them vertically upwards, how far can your pump pressurise water against gravity? This is important to know, especially since your water source and pump will likely be lower than its access points on your property. Intuitively, a pump that exerts more pressure should be able to pump water higher and therefore have a higher head.
With a general understanding of “head” there is another term you may have heard about – total head. What is “total head”? It is important to understand that the height a pump can pressurise water to is also affected by how full your tank is. A pump will be able to pressure water higher when received from a fuller tank than one that is almost empty. “Total head” is therefore a much more useful figure which removes the height of water in your tank from the equation.
“Rated Head” and “Maximum Head”
Now we understand what pump “head” is, there are normally two figures listed for this in pump specifications: “Rated Head” and “Maximum Head.” How are we to understand these two different values?
It is best to examine what it would mean for a pump to have a maximum head of 35 metres. This means that under ideal install conditions, the pump can pressurise water to this height. Surely, this would be suitable for most home owners. Perhaps it would suffice, but there are two missing questions that must be answered before going with such a pump.
The first question is the actual water flow rate once pressured to the maximum height? The point at which it is zero pressure is generally considered the “maximum head”. This means your access points will need to be much lower than the maximum head or water just won’t flow from your taps.
The second is that maximum head represents ideal install conditions. The pipes plumbed around your property and into your home often have bends in them, t-joins, small tubing, distances and the like. The pipework can impact upon the “maximum head” pressure, and this is where “rated head” is better to observe. The figure under “rated head” represents the ideal height you should consider its use for – most pumps will quite simply not deliver above their rated head.
Finally, you will want to know what pressure you might expect at certain heights. This is where pump manufacturers often provide line charts mapping head to expected water flow. If you measure the height from the location of your pump to your highest water access point, then you should have a good idea of what pump you will need.
To make it easier, most manufacturers recommend the number of taps a pump is good for. You should now be in a good position tell if one manufacturer is overstating, although most are generally adequate reflections if you take into consideration not all access points will be used at once, water saving taps and aerators or the like might be getting used.