Thinking about replacing your boiler or upgrading your hot water system?
The big question is usually this: Do I go for a combi — or do I need a hot water cylinder?
With hope I have put this article together for you for you as a good guide to help with the daunting decision
Combi boilers heat water on demand, usually with no storage tank. Hot water cylinders store it, ready to go. They’re completely different systems — and picking the wrong one can leave you with cold showers, half-filled baths that turn cold halfway through, pressure drops, or a boiler that just can’t keep up.

So which one’s better?

That depends on:
How many bathrooms you’ve got
How much hot water you use — and how often
What your mains water flow rate and pressure
How much space you’ve got in the property

This guide breaks it down — combis, unvented cylinders, vented setups, and thermal stores — so you can choose the right hot water setup for your home.

But first — let’s quickly look at how much water your home actually uses, then dive into the pros and cons of each system (whether you’re using a boiler or a heat pump — cylinders only with those!).

Typical Hot Water Usage – What You Actually Need

Combi Boiler

Combi boilers heat water on demand — no hot water tank needed. They’re compact, efficient, and ideal when you want to save space.

They’ll work with any mains flow rate (minimum 1.5–2 L/min just to trigger the flow sensor), but performance depends entirely on how much your incoming mains can deliver. A good 30–35 kW combi with decent mains flow can comfortably run a shower and tap at the same time — or even two showers, depending on your needs and supply.

Some combis have a small built-in hot water reserve (e.g. Vaillant storage combi), which helps with quick demand or multiple outlets. With careful use, these can serve 2–3 bathrooms.

Best for:

Flats or houses with 1–2 bathrooms
Homes with average or above-average mains flow
Properties with limited space and moderate hot water usage

Things to watch:
A combi can only do one job at a time: heat water or run radiators — not both together
Frequent hot water use (e.g. back-to-back showers) will keep interrupting the heating
Hot water flow rate depends entirely on your mains and boiler output — both matter
If you’re interested, check out our Best 30kW Combi Boilers Comparison here.

Unvented Cylinder

Unvented cylinders are the go-to solution for homes that need plenty of hot water at the same time — especially properties with multiple bathrooms.

They store hot water at mains pressure, so you get strong flow from every tap and shower without needing a cold water tank in the loft. A system boiler or heat-only boiler usually provides the heat, but they can also run on solar thermal panels or just an electric immersion heater — meaning no gas or heat pump is needed if set up that way.

In smaller properties or flats, there are compact heat pump cylinders available too — a very efficient option for hot water without a separate heating system.

Benefits:
Mains pressure at all outlets — great for multiple showers and high-flow taps
Perfect for larger homes — handles multiple bathrooms with ease
No loft tank needed — frees up space and avoids frozen pipes
Works with system boilers, heat-only boilers, solar, immersion, and heat pumps
Stored hot water — backup immersion provides hot water even if the boiler fails

Things to watch:
Needs strong incoming mains — ideally 20 L/min or more and 2 bar+ pressure
Most manufacturers require 20 L/min+ — low flow could void the warranty
Always check the product spec or consult the manufacturer before fitting one on low-flow mains
If your flow rate is low, consider:
Upgrading the mains pipe (check with your water supplier if your area has known flow issues before digging)
Installing a mains booster pump
Fitting an accumulator tank
G3 building regulations apply — must be installed by a qualified engineer
Requires a safe discharge route
Takes up more space than a combi
Requires regular maintenance

Extra notes:
Compatible with air source heat pumps — just choose a heat pump-ready model
Also suitable for solar-only or fully electric setups
Correct sizing matters: aim for 45–50 litres of stored hot water per person, or about 45 L per bedroom as a rule of thumb in most UK homes

What options do you have if your mains flow is too low — or you can’t afford to upgrade it?

1. Thermal Store

Unvented cylinders won’t work properly if your flow rate is poor, and combis can only do one thing at a time — heat water or run the radiators. This is where thermal stores come in.

A thermal store doesn’t store drinking water under pressure like an unvented cylinder. Instead, it stores hot water used to heat incoming mains cold water through a coil or plate heat exchanger — giving you mains-pressure hot water without needing 20+ L/min flow or G3 certification.

Benefits:
Delivers hot water at mains pressure — works with any flow rate
No G3 building control regs — not storing pressurised drinking water
No risk of legionella — just like combis
No discharge pipework, tundish, or expansion vessel needed
Works with heat-only, system boilers, combis (though rarely needed), heat pumps, solar, or immersion
No heating pause — hot water and central heating can run at the same time
Suitable for 1–3 bathrooms with a correctly sized cylinder and coil/heat exchanger
Safer and simpler to install in flats, or where you can’t route a discharge pipe — or simply don’t want the hassle of G3 maintenance

Things to watch:
Takes up similar space to an unvented cylinder
Must be sized correctly — too small and you’ll run out of hot water early
Flow rate still depends on your mains — it won’t boost low flow

Bonus Tip:
Thermal stores are a great fit when:
You’re on a shared or restricted mains supply (e.g. flats or converted houses)
Your mains flow is under 20 L/min
You want to avoid discharge pipework completely or can’t route it safely
You’re using a heat pump and don’t have the ideal flow rate for an unvented setup

2. Vented Cylinder

Vented hot water cylinders are the traditional setup found in many older UK homes. They use gravity to deliver hot water from a cylinder (usually in an airing cupboard), which is fed by a cold water storage tank in the loft.

A heat-only boiler (or system boiler) heats the cylinder. The pressure at taps and showers depends on the height of the cold water tank — so unless you boost it with a pump, flow can feel weak, especially upstairs.

Benefits:
Simple, proven system — well understood by most plumbers and engineers
Often cheaper to install if the property is already set up for it
Works even when mains pressure is poor
Compatible with heat-only and system boilers
Can work with solar thermal setups
No G3 regs required — easier installation and fewer compliance issues

Things to watch:
Requires a cold water tank in the loft — takes up space and adds freezing risk in winter
Relies on gravity — pressure is often too low for modern showers unless pumped
Hot and cold water often unbalanced at outlets (e.g. mixer taps may not work properly)
Open tanks are not sealed — water can become dirty or stagnant over time (never drink from it)
More components overall — tanks, pipework, venting — so more potential for maintenance

Final note:
Vented systems still work fine in the right setting — especially if your mains pressure is poor or your home is already set up for it. But in most modern installs, they’re being replaced with sealed systems like unvented cylinders or combis.

Conclusion — So What’s Best for Your Home?

There’s no one-size-fits-all answer — it depends on your water flow rate, how many bathrooms you have, and what kind of system you actually want to live with.
Here are a few key things to check before making a decision:
First, check your mains flow rate.
If it’s low (say 8–10 L/min), even a small 24 kW combi will do — because you’re limited by what’s coming in, unless you upgrade it.
Next, think about your bathrooms and hot water habits.
Even if you don’t use a lot of water at once, frequent usage can trip up a combi — especially when it keeps pausing the heating.
Be honest about what flow rate you’re happy with.
If 5–6 L/min in the shower feels fine, a correctly sized combi might be all you need — even in a house with 2–3 bathrooms (if usage is staggered).
But if you want two rainfall showers running at the same time across 2–3 bathrooms — forget a combi.
Even with amazing flow, a combi just can’t cope with that kind of demand.
And finally: how much hot water do you actually use?
Cylinders are designed for normal shower times — not Hollywood-length soak sessions. If you’re a 30-minute shower person, size accordingly.

Rule of thumb if all that was too much to digest:

1–2 bathrooms with modest demand? A combi will usually do the job.
Anything more? It’s time to start looking at a cylinder.

We’re always happy to help and give honest advice.

If you’re in the Leeds or West Yorkshire area, give us a call or drop us a message — we’ll be there for you.

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• Boiler Installation Guide – Leeds 2025

• Best 30kW Combi Boilers for 2025

• Boiler Service Cost in Leeds

• Solving Mains Water Flow Issues – Case Study

• Smart Thermostats & Heating Efficiency

Attila Boruzs
A Tech Installs Ltd
Gas Safe Registered Boiler Installer — Leeds & West Yorkshire