Davey Household Pump Selection Guide

Firstly work out what the pump will be used for...

For a pump for the house. (Whole house, like a farm house where no mains is available)

1.Work out how many people you want to cater for at once. (no need to include numbers for parties or christmas dinners when everyone comes around).

Simple way is to allow 15-20 litres per min per person.

Hard way is to work out all the appliances in the house, rate them (i.e. 6 L/min per toilet, 10 L/min per shower, 15 L/min per tap, etc). Then to work out based on number of people, worst case scenario. i.e. I could have two people in the shower, washing machine going, and someone has just flushed the toilet and is washing there hands. so 10 x 2 + 10 (for washing machine) + 6 +15 =51 L/min.

2. Work out pressure required.

Starting pressure we recommend is 300 kpa or 42 psi WORKING PRESSURE. This means pressure at the required flow. (not max pressure). If you tank is at the bottom of the hill and your house is at the top you will need an extra 10 kpa (1.42psi) for every vertical metre (plus a little extra for pipe friction). Alot of people at this point go for 500 kpa. WRONG. (Well mostly misguided). Most people get this figure from the pressure reducers that are on the street. 500kpa pressure reducers. This does not mean you have 500 kpa at your house, it means you are limited to 500 kpa. With pumps, you are paying electricity for all the pressure and double the pressure is four times the power required. So go easy on the pressure. Sure if you like a nice firm massage in the shower or your washing machine to finish quickly, get more pressure. But for 75% of people 300 kpa is fine.

Allow for pipe friction, a rule of thumb is to add 10% to your pressure required, but if you have a lot of small pipes or your pipes are long (tank is a long way from the house), you will need more. 

3.Get your Duty and review.

So now we have our flow rate (51l/min) and our pressure 300 kpa we now have our pump duty. Pump duty or duty point is the designed flow rate. For domestic pumps this often means worst case duty. Our duty for this case is 51 L/min at 300kpa. Once you have this, you need to look at you house plumbing. The pipe sizes installed might dictate that we can't get the desired flow.  A max guide is 1/2" pipe 13 l/min, 3/4" pipe 37 L/min and 1" pipe is 60 L/min (based on max speed of 2m/s).

In my case if i was 51 L/min and i've got 3/4" plumbing (referring to copper, poly can be worse), i'm unlikely to get it. I'm more likely to be stuck to a max of around 37 L/min. Let just assume that's not a issue and we  can use 51 L/min at 300 kpa. You can know use this information to accurately compare pumps. All good manufactures will have published performance of there pumps. You may need to convert units.

i.e. 51 L/min at 300 kpa = 3.06 m3/h at 30 metres. (same but this just different units.)

4. Type 

Above Ground pump

Multistage is typically best (most efficient), however if sucking out of dam, river or well sometimes it better to consider a jet pump. Jet pumps are less efficient but can handle air better and generally suck better. If using of typical tank installation i would always, always recommend multisage. You will not waste electricity and it will be nice and quiet.

 

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