Tankless water heaters and combi boilers both give you endless on-demand hot water with no storage tank — so people often confuse them. The key difference is simple: one only makes hot water, the other also heats your home. Here's how to choose.
The Core Difference
A tankless water heater does one job: it heats domestic hot water on demand for your taps and showers. A combi boiler does two jobs: it makes on-demand hot water and provides your home's heating through radiators or in-floor loops. If your home has hot-water (hydronic) heating, a combi replaces both your boiler and your water heater in one unit.
| Factor | Tankless Water Heater | Combi Boiler |
|---|---|---|
| Domestic hot water | Yes | Yes |
| Home heating | No | Yes (radiators / in-floor) |
| Replaces | A tank water heater | A boiler + water heater |
| Best for | Homes heated by a furnace | Homes with hydronic heating |
| Install cost | $3,000–$5,500 | $6,000–$12,000 |
Which One Do You Need?
It comes down to how your home is heated:
- If you heat with a forced-air furnace and just want better hot water, a tankless water heater is the right, simpler choice — it replaces your tank water heater only.
- If you heat with radiators or in-floor heating (a hydronic system), a combi boiler can replace both your aging boiler and your separate hot-water tank in one unit, saving space.
- If you're replacing both an old boiler and a tank at once, a combi is usually the most efficient, space-saving upgrade.
Efficiency and Space
Both are high-efficiency and both free up the floor space a storage tank used to occupy. A combi does more, so it costs more — but for a hydronic home replacing two aging appliances, it's often the best value. We'll look at how your home is heated and recommend the right path — never the more expensive option by default.