Hot Water for Three Bathrooms: Combi, System or Unvented Cylinder?
Added a rear extension and a loft ensuite? Great space, bigger demand. Three bathrooms push hot water beyond what many combis can deliver, especially at peak family times. Long runs to a loft also reduce flow and temperature.
This guide compares high‑output combis (including storage combis), system boilers with unvented cylinders, and heat‑only upgrades. We explain real‑world flow rates, cylinder sizing and recovery times so you can choose with confidence.
If you’re planning or already have a new ensuite, explore our related services: loft conversions, home extensions and bathrooms.
Start Here: Measure Your Mains Flow (Bucket Test)
Do a quick bucket test to see what’s possible from your cold main:
- Fill a 10‑litre bucket from the kitchen cold tap fully open and time it.
- Flow (L/min) = 10 × 60 ÷ seconds. Example: 10 L in 25 s = 24 L/min.
- Repeat at a busy time and again with another tap running to see the drop.
What to aim for:
- Showers: most use 8–12 L/min (large rainfall heads can be higher).
- Two showers at once: typically 18–24 L/min total.
- Three showers at once: often 28–36+ L/min total.
- Dynamic pressure: 2.0–3.0 bar at the point of use helps mixers stay stable.
If your test shows under 15–18 L/min at busy times, improve the incoming main (larger MDPE, reduce restrictions) or consider an accumulator/booster before choosing a hot‑water setup. For more detail, see our guide: size hot water and pressure for a loft ensuite.
Option A: High‑Output Combi Boilers (Including Storage Combis)
A 35–40 kW combi typically gives about 14–16 L/min at a comfortable temperature rise in real homes. Storage combis include a small internal cylinder to provide a short boost (useful for the first shower), then revert to normal combi flow. Preheat modes reduce waiting but use a little more gas.
Best for: light, staggered use where one strong shower at a time is fine. Two showers at once means compromise; three will disappoint, especially with long loft runs.
- Check your incoming main can supply at least the combi’s quoted flow (otherwise it’s throttled).
- Confirm gas pipe sizing (often 22–28 mm) and viable flue routes early in design.
- Expect priority to hot water when a tap opens; radiators may pause briefly.
This image was generated with AI and may not always represent the product or service exactly.
Option B: System Boiler with an Unvented Hot Water Cylinder
This setup heats a cylinder and delivers mains‑pressure hot water to several outlets at once. A 210–300 L unvented cylinder with a 24–30 kW boiler typically reheats in roughly 20–35 minutes after a run of showers, giving strong simultaneous flow and balanced hot/cold on mixers.
- Typical sizing for three bathrooms: 250–300 L cylinder, good 22 mm distribution (28 mm near the cylinder), and balanced hot and cold feeds.
- Works best with a solid incoming main (aim 25–30+ L/min at 2+ bar dynamic for three showers).
- Requires space for the cylinder and a compliant safety discharge route. Annual servicing by a G3‑qualified engineer keeps it safe and efficient.
It’s future‑friendly and ideal for busy families. If you’re planning a plant space, see utility and plant room planning for neat layouts.
Option C: Keep a Heat‑Only Boiler, Add an Unvented Cylinder
On a traditional heat‑only boiler with loft tanks? Replacing the tanks with an unvented cylinder brings mains‑pressure hot water without changing the boiler immediately. You free loft space, simplify pipework and boost shower performance for a new loft ensuite.
- Needs decent incoming mains flow and pressure; where weak, consider a mains upgrade or an accumulator.
- Controls are updated (e.g. S‑Plan valves, cylinder stat) to manage hot water and heating safely.
- A practical step for older homes moving from vented to modern, balanced pressure.
Choose the Right Cylinder Size: 210 L, 250 L or 300 L?
Match size to habits, not just bedrooms. Example: a family of five taking three showers at 10 L/min for 8 minutes each uses about 240 litres of mixed hot water. Big rainfall heads or back‑to‑back showers push demand further.
- 210 L: smaller families or lighter use.
- 250–300 L: larger families, spa‑style heads or frequent overlap.
- Recovery: a well‑matched boiler can reheat a 250 L cylinder in ~30 minutes, so latecomers still get a good shower.
- Mixed hot yield: storing at 60 °C and blending to ~40 °C increases usable litres versus the stated capacity.
- Thinking ahead: if solar or a future heat pump is likely, choose a well‑insulated twin‑coil cylinder now.
Strong Showers Upstairs: Supplying a Loft Ensuite
Protect flow and temperature to the top floor with sensible pipework:
- Run a 22 mm main hot and cold up to the loft, then tee to short 15 mm branches for each outlet.
- Keep runs short, minimise elbows, use full‑bore isolation valves and avoid restrictive flexi tails.
- Feed thermostatic mixers with equal‑pressure hot and cold. Fit a secondary return loop if hot‑water wait times are long; control it with a timer or smart schedule to cut heat loss.
- Insulate pipes thoroughly, especially in loft voids.
This image was generated with AI and may not always represent the product or service exactly.
Essex Realities: Hard Water, Noise and Building Control
Essex water is hard. A softener or effective scale control protects showers, boilers and cylinders, keeping flow rates healthy. Keep plant quiet with anti‑vibration mounts, lined cupboards and thoughtful siting away from bedrooms.
Unvented cylinders have multiple safety devices and must be installed and serviced by a G3‑qualified engineer. Installations over 15 litres are notifiable to Building Control, who will check the tundish and discharge route. For what inspectors look for, see our Building Control checklist.
Space and Costs: What to Expect and Where to Put It
A 210–300 L cylinder fits in a standard cupboard or small plant room. Allow roughly 600–700 mm width and 1.8–2.0 m clear height, plus front access for servicing. Plan shelving and access around it to reclaim storage.
Cylinders cost more up front than combis but serve multiple bathrooms better at peak times, reducing queuing and lukewarm showers. Save energy with thick pipe insulation, smart controls and well‑timed secondary loops. If you’re reshaping the ground floor, build a tidy plant space into your layout—our guide to plant room planning shows neat, quiet solutions.
Quick Picker: Which Setup Suits Your Home?
- Poor mains flow/pressure: improve the incoming main first or add an accumulator/booster; accept staggered showers if not upgrading.
- Want dependable three‑shower performance: choose a system boiler with a 250–300 L unvented cylinder and generous pipe sizing.
- Light, staggered use and limited space: a high‑output or storage combi can work, but don’t expect three simultaneous showers.
- Planning a future heat pump: specify a larger, well‑insulated twin‑coil cylinder now.
How Cube Installations Can Help
We start with a friendly survey and on‑the‑spot flow and pressure checks. You get a clear specification for boiler, cylinder and pipework, plus practical routing for quiet, tidy results. We handle neat installation and certification for unvented cylinders, and provide sensible aftercare.
Talk to our design team about bathrooms, plant rooms and layouts that work day to day. Ready to get started? Contact us to book a visit.
FAQs
Can a big combi run three showers at once?
Not reliably. High‑output combis can run one strong shower, sometimes two with compromise. Three at once usually disappoints, especially with long pipe runs.
Do I need a 300 L cylinder for three bathrooms?
Not always. A 210–250 L suits lighter use. Choose 300 L for larger families, rainfall heads or regular overlap. Match size to habits and reheat time.
What if my mains flow is poor?
Test it first. Consider a larger incoming main, removal of restrictions, or an accumulator/booster before spending on hot‑water kit. Without enough flow, any system will struggle.
Will an unvented cylinder work in a loft conversion?
Yes—if the incoming main is strong and pipework is sized well. The cylinder can be on a lower floor; what matters is mains flow, pressure and sensible distribution to the loft.
How long does a cylinder take to reheat?
About 20–35 minutes for a 210–300 L cylinder with a well‑matched boiler. Controls can prioritise hot water during busy periods.
Do unvented cylinders need special certification?
Yes. They must be installed and serviced by a G3‑qualified engineer with correct discharge pipework, and the installation is notifiable to Building Control.
Can I keep my existing boiler and still upgrade hot water?
Often, yes. Pair a heat‑only boiler with an unvented cylinder to gain mains‑pressure hot water and remove loft tanks.