Short answer: a booster set service checks that each pump is ready to run, the mechanical seals are not leaking, the pressure settings are right, the non-return valves close, and the diaphragm vessel holds the right precharge. Grundfos says to check the vessel precharge and pressure switch settings at least once a year. Wilo recommends checking the vessel every 3 months. Where the set has expansion vessels, HSE's HSG274 adds flushing them at the frequency in the site's Legionella risk assessment.
What a booster set does
A booster set raises water pressure where the mains cannot reach the top of a building. HSE notes that larger systems, or those that need higher pressure to reach the top floors, "often include break tanks and booster pumps, in place of direct mains water" (HSG274 Part 2, para 2.21). A typical commercial set has two or more pumps on a common manifold, non-return and isolating valves on each pump, a pressure sensor or switch, a diaphragm vessel and a control panel, often with frequency converters.
What the manufacturers ask for
| Item | Grundfos Hydro Multi-S (multi-pump, pressure switches) | Wilo SiBoost Smart 1 (single pump, frequency converter) |
|---|---|---|
| General | Pump bearings and shaft seals are maintenance-free | Regular inspection and maintenance recommended; a maintenance contract with a specialist company is advisable |
| Ready to run | Not listed | Check the set is ready for operation |
| Mechanical seals | Replace if leaking (fault chart) | Inspect; replace if leakage is noticeable |
| Vessel precharge | Check at least once a year | Check every 3 months (recommended, marked optional) |
| Pressure settings | Check pressure switch settings at least once a year | Check settings when faults point to them (fault table) |
| Frequency converter | Not covered in this manual | Clean the fan inlet and outlet filters if very dirty |
| Frost or long shutdown | Drain manifolds, pumps and vessel if not used during frost | Drain the pump through the drain plug after a long shutdown |
Before you touch it
- Power off and locked off. Both manuals say to switch the power off and secure it against being switched back on before work. Grundfos says to lock the mains switch with a padlock. Wilo warns the set may start by itself after a power cut (Electricity at Work Regulations 1989, regs 12 and 13).
- Water pressure released. Wilo's shutdown steps: power off and secured, shut-off valves upstream and downstream of the set closed, the diaphragm vessel shut off at its fitting and drained, and the set drained completely if needed.
- Stored gas pressure. The vessel keeps its nitrogen charge when the water is drained. Wilo warns that too much nitrogen can damage or destroy the vessel and injure people, and asks for the safety rules for pressurised vessels and technical gases to be followed.
- Protective equipment. Wilo lists safety footwear and cut-resistant gloves for maintenance, and hearing protection from 85 dB(A).
Checking and setting the vessel precharge
- Isolate the vessel from the water side and drain the water out of it. Grundfos says to measure the precharge "in a pressure-less system"; Wilo says to close the valve to the vessel and let the residual water out through the drain.
- Remove the cap from the gas valve on top and read the pressure with an air pressure gauge.
- Work out the target from the manual for the set (table below).
- Top up with nitrogen if low, or let gas out if high. Grundfos recommends nitrogen. Wilo says nitrogen only, and refers topping up to its customer service.
- Reopen the vessel, and watch a few pump starts and stops.
| Manual | Rule |
|---|---|
| Grundfos Hydro Solo-S and Hydro Multi-S | Precharge = 0.9 × cut-in pressure |
| Wilo SiBoost Smart 1 | Pump switch-on pressure minus 0.2 to 0.5 bar, or the value in the table on the vessel |
Expansion vessels and Legionella
HSE's HSG274 Part 2 treats expansion vessels as a risk: in systems at steady temperature and pressure they "may have long periods without exchanging any significant amount of water" (para 2.38). Table 2.1 says to flush them through and purge to drain where practical, monthly to six-monthly as the risk assessment indicates, and to change bladders as the manufacturer's guidelines or the risk assessment say. HSE also notes that bladders are often synthetic rubber that may support microbial growth, and suggests checking they are approved to BS 6920 (Info box 2.1).
A hydraulic accumulator on a booster set, correctly installed, partly fills and empties between pump runs, which HSE says reduces the risk of stagnation (Info box 2.1).
The break tank feeding the set is a cold water storage tank. Table 2.1 asks for it to be inspected yearly, with remedial work where needed, and for the stored water temperature, taken away from the ball valve, to be compared with the incoming mains yearly in summer or as the temperature profiling indicates. If a water hygiene contractor does this, note who and when.
Fault finding at a glance
| Symptom | Check |
|---|---|
| Frequent starts and stops | Pressure switch setting, vessel precharge, a failed diaphragm, a non-return or foot valve not closing tight |
| Water from the vessel's gas valve | Diaphragm leaking: replace it or the vessel |
| Pumps run backwards when stopped | Faulty non-return or foot valve, or a leak in the suction pipe |
| Pumps run but deliver no water | Blocked suction or pumps, a valve stuck shut, air in the pumps, wrong direction of rotation |
| Pump does not stop | Big swings in inlet pressure, a blocked or undersized inlet, air in the inlet, a leaking non-return valve, cut-out set too high |
| Pump does not start | Low-water protection tripped (break tank level or inlet valve), motor protection tripped, start pressure set wrong |
| Noise | Cavitation: check suction conditions, valves and strainer |
Worked example: an office block
The plant: a six-storey office with a Grundfos Hydro Multi-S twin-pump booster set fed from a break tank in the basement plant room, and an expansion vessel on the hot water system. The pressure switch is set to cut out at 5.0 bar with a 1.5 bar differential. The client wants a visit every 3 months. The client's Legionella risk assessment sets expansion vessel flushing at six-monthly.
- Every 3 months (agreed in the contract): seal inspection, inlet and outlet pressure, a test run on each pump, short cycling or noise noted, non-return valves watched on stop.
- Every 6 months: add the expansion vessel flush and purge from the risk assessment, recorded for the client's Legionella log.
- Every 12 months (Grundfos minimum): add the diaphragm vessel precharge and the pressure switch settings, plus the inlet and dry-running protection.
Cut-in is 5.0 − 1.5 = 3.5 bar, so the Grundfos precharge is 0.9 × 3.5 = 3.15 bar. On a Hydro Multi-S each pump has its own pressure switch, with cut-out pressures set 0.3 to 0.5 bar apart and the same differential on all of them, so each pump cuts in at a different pressure; the example uses one switch. The manual does not say which cut-in the precharge follows, so record the switch settings you used and ask Grundfos if in doubt. If the set were a Wilo SiBoost Smart 1, its rule would give 3.0 to 3.3 bar, and Wilo's recommended 3-month vessel check would put the precharge on every visit.
Installing or replacing a set
In England and Wales, anyone proposing to install "a pump or booster drawing more than 12 litres per minute, connected directly or indirectly to a supply pipe" must notify the water undertaker and must not start without its consent (Water Supply (Water Fittings) Regulations 1999, reg 5). The same regulation also covers "the extension or alteration of a water system on any premises other than a house", although an approved contractor does not need to give notice for that item. Check with the undertaker whether a like-for-like replacement needs notice.
Who does what
- The dutyholder (the employer or whoever is in control of the premises) owns the Legionella risk assessment and agrees the service frequency with you.
- The maintenance contractor services the set to the manufacturer's instructions, records readings and reports faults with how urgent they are.
- The manufacturer's service team handles controller and frequency converter faults the manual reserves for them. Wilo asks for pump and control faults to be fixed only by its customer service or a specialist company.
What to record
- Set make, model and serial number, and each pump's running hours
- Inlet pressure, outlet pressure, cut-in and cut-out
- Vessel precharge as found and as left
- Motor current against the nameplate
- Pass or fail for seals, non-return valves, low-water protection, controller and alarms
- Expansion vessel flush date, where the risk assessment asks for one
- Faults found, urgency, parts used, photos
Running booster set contracts in Field Suite
- Equipment register: each set at its site, with make, model, serial number, service interval and job history, and a QR label on the set that opens its page.
- Contracts: visits that repeat every set number of months, each with a form attached, created as jobs to book a set number of days before they are due. Visits are set in whole months, so a 3-month and a 12-month visit fit directly.
- Pump service checklist: a ready-made form in the library. Add your own questions, such as vessel precharge as found and as left, with the form builder.
- Van stock and faults: seals and vessels used on the visit come off the engineer's van stock. A failing vessel becomes a fault marked by urgency, and the office turns it into a quote.
Use our pump service checklist, read about sump and sewage pumping station maintenance, see pump maintenance software and planned maintenance software, check prices, or book a demo.
Field Suite is job management software. The manufacturer's instructions and the site's own risk assessments decide how a booster set is serviced.
Sources
- Grundfos, Hydro Multi-S installation and operating instructions, ss.11, 12 and 13. https://api.grundfos.com/literature/Grundfosliterature-3566070.pdf
- Grundfos, Hydro Solo-S installation and operating instructions, ss.5.1, 5.2 and 6. https://api.grundfos.com/literature/Grundfosliterature-3614.pdf
- Wilo, SiBoost 2.0 Smart 1 / SiBoost Smart 1 installation and operating instructions, 2535459, Ed.05/2025-03, ss.2.8, 2.9, 7, 8, 9 and 10. https://cms.media.wilo.com/dcidocpfinder/wilo735475/9988007/wilo735475.pdf
- HSE, Legionnaires' disease: Technical guidance, HSG274 Part 2 (2024), paras 2.21, 2.38, 2.39, Info box 2.1 and Table 2.1. https://www.hse.gov.uk/pubns/priced/hsg274part2.pdf
- Electricity at Work Regulations 1989, regulations 12 and 13. https://www.legislation.gov.uk/uksi/1989/635/regulation/12 and https://www.legislation.gov.uk/uksi/1989/635/regulation/13
- Water Supply (Water Fittings) Regulations 1999, regulation 5. https://www.legislation.gov.uk/uksi/1999/1148/regulation/5
Last checked: 5 November 2026 against the sources above.
Field Suite is job management software for UK building services firms, not a consultancy. Your own risk assessments, contracts and advisers decide what applies to your work. Spotted a mistake? Email info@fieldsuiteapp.com and we correct it.