The Quick Answer: Which Is Best For A Busy Home?
For one bathroom with modest hot water use, a quality combi boiler is usually the simplest, most space‑saving choice. It heats water only when you open the tap, so there’s no hot water cylinder. Expect it to keep one strong outlet running. As a guide: most showers like 10–12 L/min; baths feel quicker at 15–18 L/min.
For homes with two or more bathrooms, an unvented hot water cylinder generally wins. If your incoming cold main can supply 20–25+ L/min at good pressure, a well‑sized unvented system can feed two outlets at once with less drop‑off. Combis tend to prioritise one strong outlet at a time.
Start with a quick flow test, then match the system to your routine. If you’d like help, we can test and size the right option via our Plumbing & Heating service. Want DIY steps? See our guide to sizing hot water and pressure for a loft en‑suite.
How Each System Works In Daily Life (Plain English)
Combi boilers make hot water on demand. No cylinder, no loft tank. They save space and suit one‑bath homes with typical, one‑at‑a‑time use. While a hot tap is open, most combis pause central heating to prioritise hot water.
Unvented cylinders store hot water at mains pressure and share it across taps and showers. That’s why pressure feels steadier when someone turns on the kitchen tap mid‑shower. Large rainfall heads and body jets behave better on these systems. If you’re planning a new shower room, see our Bathrooms page.
Older vented systems rely on a loft tank. Many families upgrade to clear the loft, improve pressure, and simplify future works. If you’re converting the loft, removing old tanks frees headroom and eases layout planning.
Flow Rate Basics You Can Test In 2 Minutes
Do a bucket test at the kitchen cold tap:
- Use a 10‑litre bucket, open the tap fully and time the fill.
- 10 litres in 30 seconds = about 20 L/min. Test with other taps closed.
Busy homes wanting two strong outlets at once are happiest with 20–25+ L/min available at, or just after, the stop tap. Less will still work, but you’ll notice a bigger dip when two outlets run together.
Pressure is the push; flow is how much water arrives. You need both. In winter, colder mains water reduces a combi’s hot flow because the boiler must work harder. For seasonal tips, see our Winter Plumbing Checklist.
This image was generated with AI and may not always represent the product or service exactly.
Combi Boilers: Benefits, Limits, And Real Examples
Combis are compact, quick to fit, and ideal for flats and smaller houses. They shine when one shower or a sink is used at a time.
Limits are about hot flow. At a 35°C rise, a 24 kW combi gives roughly 9–10 L/min, a 30 kW about 12–13 L/min, and a 40 kW around 15–16 L/min. In winter, expect a small dip as the mains gets colder. A 30 kW combi will keep one 12 L/min shower strong; a second shower at the same time will likely suffer in flow or temperature.
Right‑sizing the boiler and tidy pipework help, but pick the system that suits your routines first. Large baths, rainfall heads and simultaneous use usually point away from a combi.
Unvented Cylinders: Steady Pressure For Multiple Bathrooms
Unvented cylinders deliver stored hot water at mains pressure to several outlets at once. That makes morning showers calmer, even while someone fills a kettle. Homes with large rainfall heads, body jets or a big bath feel the benefit most.
Typical sizes: 170–200 litres for smaller families; 250–300 litres for two or more bathrooms; larger for big tubs or multiple teens. Reheat is often 20–40 minutes, depending on the boiler and controls. Site options include an airing cupboard, utility or a small plant room. Lofts are possible with the right structure and a safe discharge route.
Installation and sign‑off must be by a G3‑qualified engineer. Annual servicing keeps safety valves, expansion vessels and pressure controls working correctly. For neat layouts, see our guide to perfect utility and plant room planning.
Best Option By Project Type: Lofts, Extensions, And Refurbs
Loft en‑suite add‑on: a combi might cope if it’s the only shower running. For steadier pressure upstairs, an unvented system is usually better, provided the mains is strong.
Side‑return or rear extension with a new bathroom: many families choose an unvented cylinder to future‑proof and keep two showers happy. A small plant or utility space pays off in day‑to‑day use and servicing.
Whole‑house refurb: plan a central plant area for the cylinder, boiler and controls. It’s tidier and easier to maintain than scattered kit. If you’re exploring a loft conversion, our Loft Conversions team can coordinate structure, plumbing and ventilation from the start.
Real-World Flow-Rate Scenarios (With Numbers)
3‑bed semi, one bathroom and a downstairs loo: bucket test at 16 L/min. A 30 kW combi is likely fine for one shower at a time. A bath will fill; avoid running a second outlet during showers.
4‑bed family with two bathrooms and a utility: stop‑tap flow at 26 L/min. Choose an unvented cylinder, 250–300 litres. Expect two 10–12 L/min showers at once, while the kitchen tap runs at a steady 3–5 L/min.
Victorian terrace with new loft en‑suite: mains at 20 L/min. An unvented setup gives balanced pressure upstairs; upgrade trunk lines to 22 mm for best results. See similar work on our Projects page.
This image was generated with AI and may not always represent the product or service exactly.
Space, Noise, And Running Costs You’ll Actually Notice
Space: a combi frees cupboards in smaller homes. An unvented cylinder needs a dedicated cupboard or plant room, which pairs well with a utility.
Noise: modern kit is quiet, but placing equipment away from bedrooms is sensible. Rubber anti‑vibration pads and good pipe clips tame noise.
Costs: a combi only heats what you use. Cylinders benefit from thick insulation, timers and smart controls to trim standing losses. Annual servicing protects efficiency and lifespan for both systems.
Low Mains Pressure Or Flow? Practical Fixes
Start with the basics: upgrade the incoming main and internal run to 25 mm where possible, remove old restrictions, and set the stop tap fully open. That single change can lift flow and stabilise showers.
Accumulators store mains water under pressure, giving strong short bursts at peak times. In trickier areas, a break‑tank with a pump set can help (needed where direct mains boosting isn’t allowed). You’ll need tailored advice.
Re‑pipe key runs in 22 mm, keep routes short and tidy, and balance hot/cold supplies. For surveys and upgrades, see our Plumbing & Heating service, and our guide to testing flow and pressure.
Regulations, Warranties, And Choosing A Trusted Installer
Unvented cylinders must be installed and certified by a G3‑qualified engineer. Discharge pipework and controls must meet Building Regulations (Part G3). Where required, installations are notified to Building Control.
Correct commissioning protects performance and keeps manufacturer warranties valid. Keep the benchmark and handover papers safe, and book annual servicing.
One joined‑up team coordinating plumbing, electrics and layout will save time and rework. It also ensures your hot water plan fits your renovation from day one.
Your Quick Decision Guide—And How We Can Help
Use this 30‑second checklist:
- One bathroom and 14–18 L/min available? A good combi may suit.
- Two+ bathrooms and 20–25+ L/min available? Unvented is usually best.
- Under ~15 L/min? Improve the incoming main or consider an accumulator.
At a friendly home survey we test flow and pressure, review your bathroom plans and agree a neat, future‑proof route. Cube Installations will size the system, plan the pipework and leave your space tidy.
Ready to talk? Get in touch via Contact Us, or see our Plumbing & Heating and Bathrooms services for more detail.
FAQs
Can a combi boiler run two showers?
Usually no. Most combis are happiest with one shower at a time. A second outlet often reduces flow or temperature, even on high‑output models.
What size unvented cylinder do I need?
Smaller families suit 170–200 litres. Two or more bathrooms, or teens at home, usually need 250–300 litres. Larger baths or body jets may need more.
Will an unvented cylinder work with low mains flow?
It will still work, but performance drops. Improve the incoming main or add an accumulator (or a break‑tank and pump where appropriate) before sizing the cylinder.
Do I need annual servicing?
Yes. Servicing keeps safety devices working, preserves warranties and maintains efficiency on both systems.
Where should the cylinder go?
An airing cupboard, utility or small plant room is ideal. Lofts are possible with the right structure and a safe discharge route.