BPW Module 4: Water Supply, Mains & Tank Guide
Water Supply — Mains, Tank & Flow
Your machine is only as good as the water supply feeding it. This module covers everything you need to know about mains connection, tank systems, flow rate requirements, and what happens when your supply falls short.
Learning Outcomes — Module 4
- Understand what flow rate a pressure washer requires from its water supply
- Test and assess mains water flow rate before connecting a machine
- Explain when a tank system is necessary and how to size it correctly
- Set up a tank-fed system safely and efficiently on site
- Identify the signs of water starvation and how to prevent pump damage
- Manage water supply on sites with no mains access
Why Water Supply Is a Pre-Job Decision, Not an Afterthought
The most common cause of pump damage in professional pressure washing is not mechanical failure — it is water starvation. A pump running without sufficient water supply overheats, cavitates, and fails. It is entirely preventable, and it is almost always the result of an operator who did not check their water supply before starting.
Water supply assessment is a pre-job task, not something you troubleshoot when the machine starts losing pressure mid-clean. This module gives you the knowledge to assess every site before you connect anything, and the setup options to handle any situation you encounter.
What Your Machine Actually Needs
Every pressure washer has an inlet flow rate requirement — the minimum volume of water it needs from its supply to operate correctly. This figure is listed in the machine's technical specifications and is expressed in litres per minute (LPM). It is not the same as the machine's output flow rate, though the two are related.
The inlet requirement is always slightly higher than the output flow rate because the pump needs a positive head of water — meaning water must arrive at the pump faster than it is being expelled, to keep the pump primed and prevent cavitation.
Rule of thumb: Your water supply must deliver at least the machine's rated output flow rate, and ideally 10–15% more. A machine rated at 15 LPM output needs a minimum supply of 15 LPM — and 17–18 LPM is more comfortable. Never assume mains supply is sufficient without testing it first.
Mains Water Supply
How to test mains flow rate
Before connecting your machine, test the flow rate at the tap or bib you intend to use. The method is straightforward: use a bucket of known volume and a stopwatch. Fill the bucket from the tap fully open and time how long it takes. Divide the bucket volume in litres by the time in seconds, then multiply by 60 to get LPM.
For example: a 10-litre bucket fills in 45 seconds. 10 ÷ 45 × 60 = 13.3 LPM. If your machine requires 15 LPM, this supply is insufficient for direct mains connection and you need a tank.
Factors that reduce mains flow rate
- Long supply hose runs from the tap to the machine — each additional 10 metres reduces flow
- Small bore hose — standard garden hose (12–13mm bore) restricts flow significantly compared to 19mm or 25mm supply hose
- Flow restrictors fitted to outdoor taps — common on modern installations
- Shared supply — if other taps or appliances are running simultaneously, flow to your machine drops
- Low mains pressure at the property — common in rural areas and older housing stock
Never use a kinked or undersized supply hose: A kinked garden hose between the tap and your machine inlet will starve the pump even when mains pressure is adequate. Always use the largest bore supply hose your machine inlet will accept, kept as short and straight as possible.
Supply hose sizing
The supply hose connecting the mains tap to your machine inlet is not a high-pressure hose — it operates at mains pressure only. However, bore size matters significantly for flow rate. Use a minimum 19mm bore (¾ inch) supply hose for machines up to 15 LPM. For higher output machines, 25mm bore (1 inch) supply hose is preferable. Keep the supply hose as short as possible — ideally under 10 metres.
Tank Systems — When and Why
A tank system uses an intermediate water storage vessel between the mains supply and the pressure washer. The tank fills slowly from the mains at whatever rate the supply can provide, and the machine draws from the tank at whatever rate it requires. This decouples the machine's demand from the mains supply rate — meaning even a slow mains supply can sustain a high-output machine, as long as the tank is large enough.
When a tank is necessary
- Mains flow rate at the site is lower than the machine's inlet requirement
- No mains water access at the site at all
- The job requires more water than the mains can supply continuously — large commercial sites
- The client has requested no mains connection — some commercial clients specify this for billing or hygiene reasons
Sizing your tank
Tank size is determined by two factors: how long you need to work continuously, and the difference between your machine's demand and the mains supply rate. A machine using 15 LPM with a mains supply providing 10 LPM has a deficit of 5 LPM. Over a 2-hour job, that deficit accumulates to 600 litres. You need at least 600 litres of tank capacity to complete the job without running dry — plus a safety margin.
| Machine Output | Mains Supply | Deficit per Hour | Tank Required (2hrs) |
|---|---|---|---|
| 15 LPM | 12 LPM | 180 litres | 360 litres minimum |
| 15 LPM | 8 LPM | 420 litres | 840 litres minimum |
| 20 LPM | 0 LPM (no mains) | 1,200 litres | 2,400 litres minimum |
| 20 LPM | 15 LPM | 300 litres | 600 litres minimum |
Tank Types and Setup
IBC tanks (Intermediate Bulk Containers)
The standard choice for most professional operators. IBC tanks hold 1,000 litres, are sold new or used, fit on most flatbed trailers and larger vans, and have a standard 2-inch outlet fitting at the base. They are cost-effective, widely available, and easy to clean between fills. A single IBC is sufficient for the majority of domestic jobs when combined with mains top-up.
Bespoke van-mounted tanks
Operators who regularly work without mains access often fit purpose-built tanks in the load area of a van — typically 500–800 litres in rotationally moulded plastic or stainless steel. These are more expensive than IBC solutions but integrate more cleanly into a professional vehicle setup and carry better with the van's payload capacity.
Tank-to-machine connection
The tank feeds the machine by gravity or via a small booster pump. Gravity feed works when the tank outlet is above the machine inlet — the IBC on a trailer is typically high enough to gravity feed most machines. If the tank is at the same level as the machine, a small centrifugal booster pump between tank and machine ensures consistent positive pressure at the inlet.
Float valve top-up: On longer jobs with mains access, fit a float valve to the IBC inlet connected to the mains tap. The tank fills automatically as it empties, maintaining a constant buffer. This is the most efficient setup for domestic jobs — you use mains water where available, with the tank as a buffer rather than a finite supply.
Signs of Water Starvation — What to Watch For
If your machine is not receiving sufficient water, it will show characteristic symptoms before the pump fails. Recognising these early allows you to stop and correct the supply before damage occurs.
- Pulsing pressure at the lance — pressure surges and drops rhythmically, indicating the pump is intermittently losing prime
- Unusual noise from the pump — a rattling or knocking sound indicates cavitation — water vapour bubbles forming and collapsing inside the pump head
- Reduced cleaning performance — the machine is running but output is noticeably lower than normal
- Machine cutting out — some machines have thermal protection that triggers if the pump overheats due to starvation
If you see any of these signs, stop immediately: Turn off the machine, identify and correct the supply issue, and allow the pump to cool before restarting. Continuing to run a starved pump will cause permanent damage to seals, valves, and the pump head — a repair cost that far exceeds the value of finishing the job.
Water Supply on Sites With No Mains Access
Rural properties, construction sites, car parks, and some commercial yards have no accessible mains water. In these situations, your options are a pre-filled tank brought to site, a bowser hired locally, or water sourced from a nearby standpipe with the relevant authorisation.
For operators who regularly work on no-mains sites, a trailer-mounted IBC with a dedicated filling point at your base is the standard solution. Fill at base, drive to site, work from tank. For very large sites or multi-day jobs, a water bowser hire is often more cost-effective than multiple tank runs.
Always factor water sourcing into your quote on any job where mains access is uncertain. The time and cost of sourcing and transporting water is a legitimate job cost — it should not come out of your margin.