HMO Hot Water Cylinder London
Cylinder sizing, install and annual service for licensed HMOs across London. Multi-bathroom design, plate heat exchanger systems, twin-cylinder builds and TMV2 commissioning — every job documented for your HMO licence pack.
210L installs from £1,895, 300L from £2,595, plate heat exchanger systems from £3,495. Building Regulations Part G3 notification included. Same-day surveys across all 32 London boroughs.
An HMO hot water cylinder is sized to the licence-stated maximum occupancy and bathroom count, not the current head count. A 5-bed mandatory HMO with two bathrooms wants a 300L indirect unvented cylinder. Every bath outlet must have TMV2 scald protection at ≤43°C, every job needs G3 install certification under Approved Document G3, and the Benchmark logbook must be kept with the HMO licence file. London HMO installs run £1,895–£4,495 depending on size and complexity.
What an HMO cylinder install actually involves
Hot water for an HMO is a different engineering problem to hot water for a family home. The cylinder has to deliver peak flow to multiple unrelated occupants on overlapping schedules, the licence holder carries documented compliance obligations the local authority will inspect, and every shared bath and basin outlet needs thermostatic scald protection at ≤43°C. Sizing by occupant count alone fails — the inputs are bathroom count, simultaneous-use loading, mains flow at the inlet, and dead-leg distances to the furthest outlets.
The cylinder spec must match the HMO licence-stated maximum occupancy, not the current head count. A 5-bed mandatory HMO that today houses three sharers must still be sized for five, because under-sizing is a licence breach at the next renewal inspection. The same logic applies to bedsit conversions and sui generis HMOs licensed under additional or selective schemes — every London borough now operates at least one, and the cylinder is the single most-inspected item in the building services portion of a renewal.
Most London HMOs land on a 210L–300L indirect unvented cylinder fed by a system or regular boiler, with TMV2 valves on every shared bath outlet and ≤41°C basin protection where basins are used by unrelated tenants. Larger HMOs (6+ sharers, 3+ bathrooms) move into twin-cylinder or plate heat exchanger territory because peak morning demand outpaces any single storage volume. The decision tree below maps occupancy to storage to recovery time to controls choice.
HMO cylinder sizing guide — London 2026
Our HMO sizing rule: 45 litres per sharer + 50 litres per bathroom + 50 litre peak-overlap allowance. Tested against simultaneous-use loadings from the Heatrae Sadia, Gledhill and Telford flow curves. The table below maps real London HMO configurations to the recommended storage volume and recovery time.
| HMO configuration | Bathrooms | Recommended storage | Recovery |
|---|---|---|---|
| 3 bedroom HMO — 3 sharers | 1 bathroom, 1 separate WC | 210L indirect | 26 min from cold (system boiler) |
| 4 bedroom HMO — 4 sharers | 1 bathroom + ensuite | 250L indirect | 32 min from cold |
| 5 bedroom HMO — 5 sharers | 2 bathrooms | 300L indirect | 40 min |
| 6 bedroom HMO — 6 sharers | 2 bathrooms + downstairs WC | 300L + 50L buffer or 2x 210L | 38–44 min |
| 7+ bedroom large HMO | 3+ bathrooms | 400–500L (twin 250L) or PHE system | Continuous via PHE |
| Bedsit conversion (sui generis) | Shared bath + en-suite cluster | 300L indirect + secondary circ | 40 min + pumped return |
Sizing notes: figures assume a 28kW+ system boiler and dynamic mains flow above 18 L/min. If either input falls short, the bottleneck shifts from the cylinder to the heat source or the supply — both worth verifying at the survey before specifying the cylinder. Larger HMOs almost always want a plate heat exchanger system rather than a single bigger cylinder; see the dedicated twin-coil cylinder page for solar / heat-pump pairings.
Plate heat exchanger vs storage cylinder — when to choose what
A storage cylinder is the right answer for the overwhelming majority of London HMOs up to 6 bedrooms. It is simpler, cheaper to install, cheaper to service, and the parts inventory is universally stocked across our fleet. The 300L indirect Megaflo Eco fitted to a 5-bed mandatory HMO will outperform any plate heat exchanger system you could fit for the same money.
A plate heat exchanger (PHE) system comes into its own when peak simultaneous-use outpaces any reasonable storage volume — typically 7+ bedroom HMOs, sui generis bedsit conversions, or properties with 3+ bathrooms in heavy overlap. Heat is transferred on demand from the boiler primary circuit to the hot water through a stainless brazed-plate exchanger, so output is effectively continuous as long as the boiler can deliver heat. A 42kW system boiler with a PHE will run two showers and a basin together indefinitely, where the same boiler with a 300L cylinder is limited by recovery.
The downsides of PHE in an HMO context are the higher install cost (£3,495+ vs £2,595 for a 300L cylinder), the controls complexity (modulating pump, buffer tank, sensors), and the need for a system boiler matched to the design flow rate. We recommend PHE on a case-by-case basis after the survey — most HMOs are better served by a properly-sized cylinder and a secondary circulation pump where dead-legs warrant it.
HMO cylinder cost London — 2026 prices
Every quote is a fixed figure given on site after the sizing survey. Prices below include VAT, Building Regulations Part G3 notification, TMV2 outlet commissioning where applicable, the Benchmark logbook and removal of the old unit. No call-out fee on quoted work.
| Job | What's included | Typical cost |
|---|---|---|
| HMO sizing survey & flow assessment | Dynamic mains test (L/min at 1.5 bar), simultaneous-use load calc, written sizing report against the HMO licence schedule. | £195 (refunded against any install) |
| 210L indirect HMO install — 3 sharer | Supply & fit 210L cylinder, full G3 inlet group, T&P discharge to outside, TMV2 outlets on bath, Benchmark logbook, Part G3 notification. | £1,895 fitted |
| 250L indirect HMO install — 4–5 sharer | Supply & fit 250L cylinder, G3 group, TMV2 on every bath/basin in shared rooms, commissioning, Benchmark, Building Control notification. | £2,195 fitted |
| 300L indirect HMO install — 5–6 sharer | Supply & fit 300L cylinder, G3 group, secondary circulation pump option, TMV2 chain, full commissioning, Benchmark, Part G3. | £2,595 fitted |
| Twin-cylinder install (2x 210L) | Two paralleled 210L cylinders on common primary, hydraulic separation, balancing valves, single Benchmark per cylinder. | £3,895 fitted |
| Plate heat exchanger (PHE) system | Stainless brazed-plate HE on system boiler return, small (50L) buffer, modulating pump, controls, commissioning. | From £3,495 fitted |
| Vented loft tank → unvented conversion | Strip vented system, fit unvented cylinder, expansion vessel, G3 discharge route, Building Control notification. | From £2,995 fitted |
| Annual HMO G3 service (per cylinder) | Expansion vessel pre-charge, T&P witnessed lift, TMV outlet test (every TMV2 in the property), Benchmark refresh. | £185 first cylinder, £125 each additional |
| TMV2 cartridge replacement (per outlet) | Replace TMV2 cartridge or full valve, retest outlet to ≤43°C, written confirmation for licence file. | £145 per outlet |
| Secondary circulation pump install | Grundfos UPS bronze pump, time-clock, balancing, isolation, commissioning to BS 8558 dead-leg rules. | £695 fitted |
* All prices include VAT and Building Regulations Part G3 notification. Full price list on the pricing page. Hub page for cylinder replacement options: Hot Water Cylinder Replacement London.
Need an HMO sizing survey?
Call 0207 046 1363Our HMO install process — 5 stages
HMO licence + survey call
20 min phone, 60 min on siteCall 0207 046 1363 with the property address and licence type (mandatory, additional, or selective). Engineer attends, measures dynamic mains flow, counts simultaneous-use outlets across all bathrooms, takes pressure readings at the lowest and highest outlet, and writes a sizing report against the HMO room standard schedule.
Written quote + works schedule
Same dayFixed-price quote sent the same day. Schedule built around tenanted occupancy — most HMOs need staged works to keep at least one bathroom live each day. Building Control notification is filed by us under the Competent Persons Scheme.
Strip-out & first fix
Day 1 (4–6 hours)Old cylinder isolated, drained, removed. New cylinder positioned, structural support checked (300L full = 350kg). First-fix pipework: 22mm cold inlet from mains, primary flow/return to boiler, G3 inlet group, T&P discharge in 22mm copper to outside.
Second fix + TMV chain
Day 2 (3–5 hours)Cylinder filled, vented, pressurised. Every bath outlet across the HMO retro-fitted with TMV2 at ≤43°C. Basin outlets where used by shared tenants get TMV2 at ≤41°C. Each TMV individually balanced and the outlet temperature recorded.
Commissioning + licence pack
Day 2 (1 hour)T&P witnessed-lifted, expansion vessel pre-charged to manufacturer spec, energy cut-out tested, Benchmark logbook signed. Compliance pack issued: Part G3 certificate, TMV2 outlet log, legionella risk assessment refresh, cylinder warranty registration.
G3 compliance & Benchmark commissioning
HMO cylinder work sits on a small stack of statutory rules. Every job is documented against them and the paperwork forms part of the licence file the local authority will demand at renewal.
Building Regulations Part G3
Every unvented hot water storage system over 15 litres is a controlled service. Installation, commissioning and modification must be carried out by a G3-qualified engineer (BPEC HWSS, LCL Awards or City & Guilds 6035) and notified to Building Control. We self-certify via our Competent Persons Scheme so the local authority is notified within 30 days and the Compliance Certificate is posted to the freeholder.
Benchmark commissioning logbook
The HHIC Benchmark logbook ships with every cylinder and must be completed at install and at every annual service. For HMOs we keep a digital copy on file alongside the property licence — it is the document the manufacturer will demand if you ever make a 25-year tank warranty claim, and it is the document the local authority will ask for at HMO licence renewal.
HSE HSG274 / L8 — Legionella control
HMOs sit in a higher-risk band for legionella because of variable occupancy patterns and longer dead-legs in retro-fitted bathrooms. Stored water is held above 60°C, distribution above 50°C within one minute. We refresh the written legionella risk assessment with every annual service and flag any TMV cartridges showing thermal drift.
TMV2 scald protection (NHS D 08)
Every bath outlet accessible to shared tenants must deliver water at ≤43°C, basins at ≤41°C in bathrooms shared between unrelated occupants. We fit and commission TMV2 thermostatic mixing valves, log each outlet temperature, and supply written confirmation for the HMO licence file. Annual TMV2 retest is mandatory.
Housing Act 2004 — HMO room standards
Mandatory HMO licensing applies to any property of 5+ unrelated occupants forming 2+ households. Additional and selective licensing schemes operate borough-by-borough (Newham, Waltham Forest, Brent and most inner-London boroughs cover all rental property). The cylinder spec must match the licence-stated maximum occupancy — under-sizing is a licence breach.
Gas Safety (Installation and Use) Regulations 1998
Where the cylinder is fed by a gas boiler, that boiler must hold a current Gas Safe CP12 inspection — annually and renewed at every change of tenant. Most landlords book the CP12 and the G3 cylinder service together; we run both in one visit at a discount on PPM contracts.
Real London HMO jobs — worked examples
Every HMO is different. Five anonymised jobs from the last twelve months — diagnostic, work carried out, final figure.
6-bed mandatory HMO, Tower Hamlets — undersized cylinder
Inherited 180L direct cylinder with six sharers and two bathrooms. Tenants reported cold showers from 07:30 onwards. Stripped and replaced with a 300L Heatrae Sadia Megaflo Eco indirect on the existing system boiler, added a Grundfos bronze secondary circulation pump on a time clock (06:00–10:00, 18:00–23:00) to kill the 4m dead-leg to the rear bathroom. TMV2 retro-fit on both baths. Two-day install, tenanted throughout. Final invoice: £3,295 inc. VAT. Outcome: zero further hot water complaints across the next two tenant cycles.
5-bed additional licence HMO, Newham — vented to unvented conversion
Original 1970s vented cylinder with cold loft tank — failing tank float and persistent low pressure to the top floor en-suite. Stripped vented system, fitted a 250L Megaflo Eco indirect, expansion vessel, new G3 group, T&P discharge through the rear wall to gully. Loft tank removed, primary repipe in 22mm copper from the boiler. Building Control notification filed under WaterSafe. Cost: £2,895 inc. VAT. Compliance certificate posted within 21 days for the licence renewal file.
8-bed large HMO (sui generis), Hackney — plate heat exchanger system
Three bathrooms plus a downstairs WC, peak morning demand outpacing any single cylinder. Designed a plate heat exchanger system: a stainless brazed-plate HE on the system boiler primary, 50L buffer, modulating pump, Honeywell controls. Continuous hot water at 14 L/min as long as the boiler can deliver heat. Two-cylinder fallback retained for the rear en-suite cluster. Total spend: £4,495 inc. VAT. Outcome: simultaneous-use complaints eliminated.
4-bed HMO, Brent — TMV2 retro-fit only
Existing 250L Megaflo, perfectly sized, but no thermostatic protection on bath outlets — flagged at HMO licence inspection. Fitted six TMV2 valves (two baths, four basins), commissioned each to ≤43°C / ≤41°C, supplied a written outlet log with each cartridge serial number. Visit completed in 4 hours, cost £825 inc. VAT, written confirmation accepted by the local authority and licence renewed without further action.
Twin-cylinder install, Wandsworth — 6-bed HMO with split occupancy
Property split front/rear with effectively two flow circuits. Single cylinder solution rejected on dead-leg grounds. Installed two paralleled 210L Telford Tempest cylinders on a common primary loop with hydraulic separation and individual isolation. Each cylinder commissioned and Benchmark-logged separately. Build cost: £3,895 inc. VAT. Outcome: peak-load demand met without secondary circulation electrical running cost.
HMO licence renewal coming up?
Same-week sizing surveys. Two-day installs around tenanted occupancy. Licence-ready compliance pack with every job — Part G3 certificate, Benchmark logbook, TMV2 outlet log, legionella risk assessment.
Areas covered across London
HMO cylinder work carried out across every London borough. Heaviest call volumes from the boroughs with the most active additional / selective licensing schemes: Newham, Waltham Forest, Tower Hamlets, Brent, Hackney, Southwark, Lambeth and Wandsworth.
Frequently asked questions
What size hot water cylinder does my London HMO need?
Do HMOs need a G3 qualified engineer for cylinder work?
What's a plate heat exchanger and when does an HMO need one?
Do I need TMV2 valves on every outlet in an HMO?
How long does an HMO cylinder install take?
Can I keep the loft tank and just upgrade the cylinder in an HMO?
What does an annual HMO cylinder service include?
Do I need secondary circulation in my HMO?
What HMO licence types exist in London and what do they want?
How much does an HMO cylinder install cost in London?
Will my mains supply cope with an unvented HMO cylinder?
Do you handle the HMO licence paperwork for the cylinder?
Can I run two showers at once in a 5-bed HMO?
What's the difference between mandatory, additional and selective licensing?
Is a Mixergy or smart cylinder a good idea for an HMO?
Question not answered? Call 0207 046 1363 and speak to a G3 engineer directly — no call centre, no triage.
Service area
HMO cylinder work — All London boroughs
G3 qualified HMO cylinder engineers based in central London with same-week sizing surveys across all 32 London boroughs. Heaviest licensing activity in Newham, Waltham Forest, Brent and Tower Hamlets.
HMO cylinder undersized, failing, or licence-flagged?
G3 qualified engineers across London. Same-week sizing surveys, fixed-price quotes, two-day installs around tenanted occupancy. Building Regulations Part G3 notification, Benchmark logbook and TMV2 outlet log on every job. £5M public liability.
London 247 Home Services Ltd · Company No. 17120057 · £5M public liability · G3 qualified · 24/7 emergency line