Guide written by:
Jeremy, construction site supervisor, Cardiff
Whether you want to drain your pool, clear a flooded cellar or simply water your garden, a motor pump is a great portable one-piece solution. Simple to use, reliable and robust, it'll serve you well. To take the pressure off while you're choosing, follow our buying guide!
- Type of motor
- Total head
- Type of water
- Duration of use
What is a motor pump?
The operating principle is simple: a petrol engine drives a hydraulic pump (either impeller, turbine, or blade), creating a vacuum at the suction point and positive pressure at the discharge point. And before you ask - no - it's not the same as a submersible pump!
The engine can be diesel or petrol and 2- or 4-stroke.
A given pump will suck water from a set depth and discharge it at a set height under the action of pressure.
Portable and self-powered, motor pumps are the most versatile and easy-to-use water pumping solution.
Motor pumps can be installed in a fixed location, but this is uncommon - be careful not to confuse with the more typically fixed booster pump.
Wear parts are few, maintenance is simple, and product lifespans are generally high if you use your pump with care.
Why is water type important?
Although the motor pump's basic function is simple, it's important to understand what differentiates them so you can choose exactly the right one for you. Really, the type of water being pumped is the most important factor when choosing.
If you're looking for a motor pump that only has to deal with watering plants or emptying yourpool, a clean water pump will do the trick. Its maximum grain size (or granulometry) is very small, meaning that suspended particles greater than 5mm diameter tend to clog up the pump.
If you need to pump away floodwater, wastewater, or any other type of water that may contain SM (suspended materials), look instead at wastewaterpumps. The granulometry of these models is generally around 10–20mm.
Heavily polluted water
For heavily polluted waters, you can get specific motor pumps with a larger allowable grain size (> 25mm). If you're planning to pump mud, heavy wastewater (containing paper, wipes, etc.) or external floodwater (small pieces of wood, rubble, small stones, etc.), you can even get a specialist pump equipped with a shredder and a very large grain size.
What power source to choose?
The engine is a crucial part of a motor pump, and special attention must be paid to it. Whether diesel or petrol, the engine characteristics will influence both the performance and the maintenance of your pump.
Petrol engines are best for less frequent use, and are quieter than the diesel alternative. Maintenance is the same as for any other engine – regular checkingof oil level, air filter and spark plugs.
Start-up is easy, even after a long period without use - especially compared to a larger 4-stroke engine. In terms of cost, the petrol engine works out less expensive than the diesel. It's also lighter to handle.
Diesel engines are recommended for intensive and regular use. They're generally found on higher-power motor pumps (especially those used in firefighting), and are much noisier. Start-up is more involved, requiring an electric starter motor for the biggest models.
Diesel models are more expensive than petrol, and more demanding in terms ofmaintenance(starter motor,spark plugs, filters, oil level, fuel injectors). Diesel pumps are often mounted on a wheeled chassis because they're quite heavy to manoeuvre.
Flow, pressure, TH: some explanations
These are essential characteristics of every motor pump, significant when making your choice.
This is the key technical characteristic of a motor pump! Flow (or flow rate) corresponds to the amount of water pumped in a given time. It's given in either l/min (litres per minute) or m3/h (cubic metres per hour). For the sake of simplicity, you can assume that 6m3/h corresponds to 100l/min. However, when choosing your pump it's important to remember that flow will vary depending on the suction depth and the discharge height you're operating at.
More precisely, for a given pipe diameter, the same pump will give a lower flow for a greater difference in height. Conversely, the closer in height your suction and discharge points, the greater the flow rate.
This technical characteristic represents the force of the water at the discharge point as a result of the pipe cross-section, and is expressed in B (bars). Some manufacturers also report pressure in CMW, or Column Metres of Water: 1 B = 10 MCE. Pressure goes hand in hand with flow. This is a basic law of hydraulics: for a given flow, with a pipe of greater cross-section you get less pressure, whereas with a pipe of smaller section the pressure is stronger.
To clarify, here's an example: a pump rated by the manufacturer as having an operating pressure (at the pump outlet) of 2.5 B or 25 MCE gives 2.5 B of output pressure if the discharge point is at the same height as the pump. If the discharge point is 10m higher, output pressure is only (25 - 10)/10 = 1.5 B.
TH (Total Head)
Now this might be a little trickier to get your head round! But since some manufacturers provide a TH value, it's worth a bit of explanation. To obtain the TH you need to make the following calculation: TH = suction depth (between water and pump level) + discharge height (between pump level and discharge point) + desired pressure at discharge point (in CMW, 1 Bar = 10m) + intervening pressure drops.
Example: suction height = 3m (e.g. the bottom of a swimming pool, with the pump placed on the ground); discharge height = 2m (e.g. between the pump and wastewater outflow); desired output pressure = 2 B (i.e. 20 CMW); 10% pressure drop through pipe lengths and fittings. Our minimum required total head (TH) is therefore (3 + 2 + 20) x 110% = 25 x 1.1 = 27.5m. However, be aware that if suction height is 8m or more, you'll need to change pump and opt for a submersible model as this is unfeasible for a motor pump of any size.
Note: because the relationship between flow, pressure and TH isn't exactly second nature to most of us, motor pump manufacturers often provide an easy-to-read operating curve to illustrate it. This can be really helpful if you're scratching your head at what you've just read!
What other characteristics are important?
Once you've determined roughly what size and type of pump you'll need, there are a few other points to consider before you commit to a certain product.
This means that the pump takes in no air at the suction point – and can thus propel the water optimally without unwanted heating. Some pumps need to be primed manually (a tank being provided for this purpose) before every use. These days, most motor pumps are self-priming for improved ease of use and to save time at start-up.
An important criterion, especially if you plan to move your motor pump between locations. Always bring an extra can of fuel!
Especially for intensive use requiring a lot of manoeuvring, give some thought to weight of the machine!
What components are important for optimal pumping?
A decent motor pump is one thing, but with the right components, so much the better!
The suction strainer is located at the bottom of the intake pipe and provides the first stage of filtering of water taken in by your pump.
Fittings connect the pipes to one another and to the pump as required.
Intake pipes are available in a range of diameters.
Seals ensure a watertightness across pipe connections and throughout the system.
Top tips for pump usage
Whatever type of motor pump you choose, to keep it in good working order, always be attentive to enginemaintenance and follow the manufacturer's advice.
Before each use (whether it's a petrol or diesel engine), check oil level, air filter condition and fuel level.
If your pump isn't a self-priming model, refill the priming tank every time you start up (otherwise you risk overheating and engine jamming).
Also check the sealing of the pump fittings, especially on very powerful models!
Accidents caused by "dancing pipes" are common. If a pipe coupling slips, cut the pump engine in the first instance.
Learn more about household water installations...
To find out more about household water installations, follow our editors’ advice and check out their other guides:
- How to choose your booster pump?
- How to choose your rainwater collector?
- How to choose your swimming pool?
- How to choose your pool cover?
- How to choose your pool heat pump?
Guide written by:
Jeremy, construction site supervisor, Cardiff, 131 guides
Electrician by trade, I first worked in industrial estates where I installed, wired and fixed a large number of electrical installations. After this, I managed a team of electricians for this type of work. 10 years or so ago, I turned to building and construction. From the modest family home, to gyms and theatres; I have been able to coordinate, audit and organise all sorts of construction sites. for 4 years now, I am restaoring and bulding an extrension to a bungalow in the heart of the welsh countyside. My experience in manual work and my knowledge means I am proud to be of service. Terraces, interior design, roofing, plumbing, electricty, anything goes! We have, my wife, daughter and I, built almost everything we have from scratch! So to answer all of your questions, and to orientate and advise you on coosing your tools? Easy!