Combi vs System Boiler with a Storage Tank
Instant hot water from a combi, or stored hot water from a system boiler and tank? Compare flow rate, space and simultaneous demand.
When choosing how your home makes hot water, the two main routes are a combi boiler that heats water on demand, or a system boiler paired with a storage cylinder. Both are excellent — the right one depends on how much hot water you use at once and how much space you have.
Combi boilers
A combi heats domestic hot water instantly as you draw it, with no storage tank. That saves space and means no waiting for a tank to reheat. The limit is flow rate: a combi can only heat so much water per minute, so running two showers at once can stretch it.
System boiler with a tank
A system boiler heats a storage cylinder (often an indirect tank). You get a large reserve of hot water that can feed several outlets at once, ideal for busy households with multiple bathrooms. The trade-off is the space the cylinder needs and a brief recovery time after heavy use.
Choosing for your home
- Smaller home or condo, one main bathroom, limited space: combi.
- Several bathrooms or high simultaneous demand: system boiler with a tank.
- Either way, correct sizing to your real usage is what delivers good hot water.
Key takeaways
- Combi: instant hot water, no tank, space-saving — limited by flow rate.
- System + tank: large stored volume for simultaneous demand — needs space.
- Match the choice to how much hot water your household uses at once.
Frequently asked questions
Can a combi boiler supply two bathrooms?
A well-sized combi can handle a busy single-bathroom home and modest second-outlet use, but two simultaneous showers can exceed its flow rate. Homes with high simultaneous demand are usually better served by a system boiler and tank.
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