What size pump do I need
Sizing a house pump comes down to two numbers and one decision. The numbers are flow, how much water you need running at once, and head, how far up and along the water has to travel before it reaches a tap. The decision is where the pump sits relative to the water, because that sets the type of pump, and the wrong type will not work at any size.
Below is how we work it out over the counter, using the real pumps we stock and the figures off their spec sheets.
The short answer
How many showers and taps could run together on a bad morning. Allow about 9 L/min each, then take 20 percent off, because they never all run flat out at the same instant.
The climb from the pump up to the highest tap, plus friction in the pipe run, plus about 30 m for decent pressure once the water gets there.
At or above the pump, below it, or deep enough that the pump has to go down into it. That decides flooded suction, suction lift or submersible.
Get the type right before you worry about the size
Picking the wrong type of pump is a worse mistake than picking the wrong size, and it is the one we see most.
An undersized pump gives you weak showers. A pump of the wrong type gives you nothing, because it cannot get hold of the water in the first place. So the first question we ask on the phone is never how many bathrooms. It is where the water is compared with where the pump is going to sit.
Flooded suction
The water sits at or above the pump, so gravity feeds it and the pump never has to suck. This is the easy case and it opens up the whole range, including the multistage and variable speed pumps that cannot lift. Pump at the tank, or pump down the hill from the tank. If you have a choice, choose this.
Suction lift
The pump sits above the water and has to pull it up to itself. Only self priming and jet pumps will do it, and there is a hard ceiling of about 7 m. Tank below the house, creek down the bank, shallow bore with the pump at the top.
Submersible
The pump goes down into the water and pushes rather than sucks, so there is no suction limit at all. A drilled bore with the water more than about 5 m down leaves you no other option. On a tank it is a choice, and the reason to make it is silence.
Why the 7 m matters
A pump cannot really suck. Atmospheric pressure pushes water up into it, and atmospheric pressure runs out. The theory allows around 7 to 8 m at sea level and we work to 7 m, because friction in the suction line and warm water eat into it. Past 7 m the pump cavitates, gets noisy and wrecks itself.
| Where the water sits | Type | What we would fit |
|---|---|---|
| Pump beside the tank, or below it | Flooded suction | Anything. Davey HM Series, HP Series, DynaDrive, EvoDrive |
| Tank below the pump, up to 7 m down | Suction lift | Jet pumps only. Davey Dynajet or Grundfos JP Series |
| Tank below the pump, more than 7 m down | Neither works well | Move the pump down to the tank, or put a pump in the tank. Ring us |
| Inside the tank | Submersible | Grundfos SBA or DAB E.SYDIVER 55/120 |
| Drilled bore, water more than about 5 m down | Submersible | Grundfos SQE down the casing. See bore pumps |
| Drilled bore, water within about 5 m of the surface | Suction lift | A jet pump at the headworks, sized like a normal house pump |
| Creek, dam or river | Suction lift | A jet pump on the bank with a foot valve and a strainer, plus treatment before anyone drinks it |
There is a fuller walk through of each install in our flooded suction install guide, suction lift install guide and submersible install guide.
How much flow do you need?
Flow is measured in litres per minute. The number that matters is not how much water the house uses in a day, it is how much it wants at the worst moment, which is everybody getting ready at once.
We count outlets, not bathrooms, because a bathroom might be a shower and a basin. An outlet is one thing running: a shower, a kitchen tap, an outside tap, a washing machine filling.
Our starting figures for a single outlet, before any trimming.
| Outlet | Allow |
|---|---|
| Shower | 9 L/min |
| Bath filling | 12 L/min |
| Outside tap or hose | 12 L/min |
| Washing machine filling | 10 L/min |
| Kitchen tap | 8 L/min |
| Basin | 6 L/min |
| Dishwasher | 6 L/min |
| Toilet cistern refilling | 4 L/min |
Add up what could genuinely run together, then take 20 percent off the total. That last step is deliberate. People always overstate how much runs at exactly the same second, and sizing for the theoretical everything on gets you a pump that is bigger, louder and dearer than the house needs.
Counting every outlet as a shower, which is the quick way to do it in your head.
| Outlets running at once | Allow roughly | What that usually looks like |
|---|---|---|
| 1 to 2 | 9 to 14 L/min | A cabin, a sleepout, garden watering |
| 3 | About 22 L/min | A one bathroom house, couple or small family |
| 4 | About 29 L/min | The common two bathroom family home |
| 5 | About 36 L/min | Two bathrooms plus an outside tap or a washing machine |
| 6 | About 43 L/min | Three bathrooms, or two bathrooms and a busy laundry |
| 7 or more | 50 L/min and up | A large home, or a house plus a second dwelling off one tank |
This is a guide, not a calculation. It is where we start the conversation, not where we finish it.
Past about six outlets we stop trusting a table at all and want to talk, because that is usually where a single house pump stops being the right answer and a second dwelling, an irrigation line or a header tank turns out to be in the picture. Past 100 L/min of genuine sustained demand you are out of domestic pump territory altogether.
How far and how high: working out head
Head is pressure written as a height. It is measured in metres, and it is the honest way to talk about a pump, because a metre of head is a metre of head whether the water is going up a wall or down a paddock. Three things add together.
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The climb
Straight up from the pump to the highest tap. A single storey house is about 4 m, a two storey about 8 m, and a house up a bank above the pump can be 15 m or more. If the water has to come up out of a bore or up out of a low tank first, that lift is added on top.
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The friction
Every metre of pipe eats a little pressure, and narrow pipe eats a lot. We size the pipe so friction costs no more than about 10 percent of the head, which on a normal house run means 25 mm or 32 mm poly, not the 20 mm that gets used because it was on the trailer.
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The pressure you actually want at the tap
The bit people forget. A tap that merely dribbles at the right height is no use. We add about 30 m of head for a comfortable shower, which is close to 3 bar, and that single figure is usually the biggest number in the sum.
The trap in every spec sheet
A pump is advertised with a maximum flow and a maximum head. They never happen together. Maximum flow is what it shifts with nothing in the way, and maximum head is what it reaches when almost nothing is moving. A 50 L/min pump does not run 50 L/min of taps.
That is why a Davey Dynajet XJ50 is rated at 50 L/min but we only put it on two or three outlets. The rating is the top of the curve. Your house sits somewhere along it.
We want a pump at least 15 percent clear of what the job needs, on both flow and head. Sitting right on the number is how you end up back on the phone.
Once the sum runs past about 80 m of total head, a single domestic pressure pump is no longer the right tool and we would rather spec it with you than sell you something that will disappoint.
Where the water sits, and the numbers that trip people up
More than 15 m below the tank
The standing water pressure sitting on the pump gets too high for an ordinary electronic pressure controller to switch on and off properly. The pump itself is fine. The control gear needs a tech to spec, so ring us before you buy.
More than 10 m away from the tank
Every metre of suction line costs the pump pressure before it has pushed anything. Move the pump closer to the tank if you possibly can. If you cannot, run the suction line one size larger than the pump's suction port.
Bore water drops while you pump
The level in a bore falls as the pump runs, which is called drawdown, and the 7 m suction ceiling has to cover the resting level plus that drop. We allow 2 m of drawdown for a surface pump at the headworks, which is why the water needs to rest within about 5 m of the surface before a jet pump is on the table. For a submersible we allow about 6 m.
A bore pump has to fit, and not be too greedy
Get the casing diameter and the bore depth off the drilling report first. The Grundfos SQE is a 3 inch body built for a standard bore, but nothing goes down a hole we have not confirmed. Past about 80 m deep, or on a bore with a low yield, it is a tech job. Over pumping a slow bore damages the hole itself.
Town supply that just needs a boost
Mains water arrives with pressure already in it, so there is no suction lift to worry about. The pump only has to make up the difference to the top tap. That is a smaller job than most people expect.
Off grid and solar
Power changes the answer as much as geometry does. A pump started direct on line draws several times its running current for a moment, which is what trips a small inverter. Tell us it is going off grid when you ask and we will point you at something with a soft start. See what a VSD pump is.
Three worked examples
Same method every time. Flow first, then head, then type, then find a pump with 15 percent in hand on both.
Two bathroom house, pump at the tank
Flooded suction
Flow. Four outlets at once, so about 29 L/min.
Head. Single storey, 4 m up to the top tap. About 20 m of 25 mm pipe to the house, so roughly 1 m of friction. Add 30 m for pressure at the tap. Call it 35 m.
Type. The pump sits beside the tank, so it is flooded and the whole range is open.
Pump. A Davey HM90-08 at 90 L/min and 460 kPa, which is 46 m, clears 35 m with room to spare. Add a Torrium2, or step to a DynaDrive 60-10 at 80 L/min and 68 m if you want held pressure instead of a switch cycling.
Same house, tank 3 m down the bank
Suction lift
Flow. Unchanged. About 29 L/min.
Head. 3 m of suction lift, then 4 m up to the top tap, about 2 m of friction over a 40 m run, plus 30 m at the tap. Call it 39 m.
Type. The pump is above the water, so it has to be self priming. Half the pumps in the first example are now off the list, on 4 m of difference.
Pump. A Davey Dynajet XJ70 at 70 L/min and 480 kPa, which is 48 m, or a Grundfos JP 5-48 at 85 L/min and 48 m.
Two bathroom house on a drilled bore
Submersible
Flow. Four outlets, so about 29 L/min again.
Head. Water resting 8 m down, plus 6 m of drawdown while the pump runs, gives a 14 m lift before the water even reaches the surface. Then 4 m up to the top tap, about 2 m of friction, plus 30 m at the tap. That is roughly 50 m.
Pump. Here is where the 15 percent rule earns its keep. A Grundfos SQE 3-55 is rated to 55 m, which is only 10 percent above the 50 m the job needs, so it is too close for comfort. The SQE 5-60 at 84 L/min and 60 m is the one we would fit. Same house, same taps, but the depth of the water pushed it up a size.
What we sell, and roughly where each one lands
Most models below sit on one merged listing with the sizes on a picker, so you pick the model rather than hunting for the right page. Figures are the manufacturer maximums, so read them with the spec sheet trap above in mind.
| Pump | Type | Range | Where it lands |
|---|---|---|---|
| Davey Silver Series | Suction lift | Light duty jet pump | Garden watering, sprinklers, a cabin. Up to two outlets |
| Grundfos JP Series | Suction lift | JP 3-42 at 50 L/min and 42 m, JP 5-48 at 85 L/min and 48 m | The 3-42 for up to two outlets, the 5-48 for four to six |
| Davey Dynajet | Suction lift | XJ50 at 50 L/min and 450 kPa, XJ70 at 70 L/min and 480 kPa, XJ90 at 90 L/min and 450 kPa | Two to three outlets, four to six, then six and up. The workhorse when the tank is below the house |
| Davey HM Series | Flooded suction | HM60-06 at 60 L/min, HM90-08 at 90 L/min, both 460 kPa. HM160-15 at 250 L/min and 460 kPa | Multistage, so steady head. The pick for a two storey house or one up a slope |
| Davey HP Series | Flooded suction | HP65-06 at 100 L/min, HP85-08 at 140 L/min, both 330 kPa | Biased to flow over height. Big single level homes spread out flat |
| Davey DynaDrive | Flooded suction | 60-10 at 80 L/min and 68 m, 90-11 at 130 L/min and 59 m, PRO 160-19 at 160 L/min and 57 m | Variable speed, so pressure holds when a second tap opens. Up to six outlets, six and up, then seven and up |
| Davey EvoDrive | Flooded suction | 105 L/min, 70 m head, 0.9 kW | Constant pressure at a friendlier price than the DynaDrive. Four to six outlets |
| Grundfos SBA | Submersible in a tank | 50 L/min at 45 m, 0.55 kW | Sits in the tank, nothing to prime, near silent. Three to four outlets |
| Grundfos SQE | Submersible in a bore | SQE 3-55 at 50 L/min and 55 m, SQE 5-60 at 84 L/min and 60 m. Both 3 inch, 1.15 kW | Four to six outlets, then six and up. Variable speed ready with the right controller |
| Davey Vortex | Submersible, dirty water | D15VA at 140 L/min and 7 m, D40VA at 240 L/min and 7.5 m | Not a house pump. Emptying a sump or a flooded pit, passing 20 mm or 35 mm solids |
Browse the lot in house pumps, bore pumps, submersible pumps and pump parts and accessories.
Sizing the pump is only half of it
A correctly sized pump on the wrong control gear still annoys you daily. How the pump decides to start and stop is a separate decision, and it is where most of the complaints we hear actually come from.
- What is a VSD or variable speed pump, and when a fixed speed pump with a controller is the better buy.
- What is a pressure controller, the Torrium2 and PM PLUS class of thing.
- What is a pressure switch, the older approach with a pressure tank alongside it.
- Why is my pump turning on and off, which is nearly always the tank or the switch rather than the pump.
- Common pump faults and what they mean.
If the water is coming off a roof, out of a bore or out of a creek, filtration is part of the same job. Start with a water test, then the cartridge guide and whole house filtration.
Be honest about where a chart stops working
Everything above will get you close, and close is genuinely useful. It will tell you whether you are shopping for a jet pump or a multistage, and whether you are in the 1 kW bracket or the 2 kW bracket.
What it will not do is see your property. It does not know that the old alkathene line to the house is half scaled up, that the tank outlet is a 25 mm fitting throttling everything behind it, that the top tap is in a sleepout nobody mentioned, or that your bore report says the yield drops off in February. Those are the things that change the answer, and they come out in about two minutes of conversation.
We install this gear around Waihi and Thames every week. Ring us before you order, not after.
Tell us about your place
Four things get us most of the way there: where the water comes from, whether the pump sits above or below it, how many showers and taps could run at once, and how high the highest tap is above the pump. A photo of the existing setup helps too. A real person picks up.
65 Kenny Street, Waihi, Monday to Friday 8 to 5. 74 Kopu Road, Kopu, Monday to Friday 8 to 3.
