An instantaneous gas heater warms water as it passes through the appliance. A storage gas water heater keeps a supply of hot water and reheats it as needed. Simultaneous demand, available space and daily routines influence the choice.
Boilers intended for space-heating circuits or combined duties must be identified by model and system design. This comparison starts with bathroom and kitchen hot water to distinguish instantaneous operation from storage.
Instantaneous: enough flow during use
There is no hot-water storage tank. When water is drawn, the unit heats the flow within its rated capability. Its size can suit limited spaces, but location, ventilation, flue and connections still need to meet the model’s requirements.
Our instantaneous gas heater page lists the approved models. When comparing them, ask for the flow rate and the temperature rise used for that rating. A litre-based model label is not a tank capacity. Outlet counts also need checking against the actual showers and taps.
An instantaneous heater avoids holding a tank of hot water during idle periods. It does not remove the time water takes to travel through a long pipe to the tap.
Storage: ready water and recovery
A storage gas water heater holds hot water, with the burner replenishing heat and draw-off under its controls. Tank size matters alongside recovery rate and delivery over the usage period.
Stored water helps cover a concentrated draw. For sustained use, recovery must keep pace. Tank litres alone cannot select the system, and storage does not provide unlimited output.
The installation needs a suitable place for the filled tank and access for service. Review that space and the pipe routes before choosing the model.
Compare against your routine
- Intermittent use and limited space: check instantaneous flow at the required temperature rise.
- Simultaneous or large draws: compare a suitable instantaneous unit with storage and its recovery rate.
- Widely spaced outlets: review distribution, insulation and delivery time as part of the system.
- Future space heating: state that as a separate requirement before selecting the heat source.
If you need hot water and space heating
Not every gas appliance can supply radiators or underfloor heating. We review its intended duty, heating load, circuit separation and controls as part of the design. The name “boiler” or a large tank does not establish suitability.
Read how boiler capacity is selected for the information needed for each duty.
Prepare for the choice
Have the property type, simultaneous outlets, available gas supply and proposed unit location ready. If replacing equipment, include the model and what is unsatisfactory about its operation. That keeps the free survey focused on your needs.
Where a suitable gas installation is not possible, we can review a heat pump or solar system against the site and available energy.
Frequently Asked Questions
How do instantaneous and storage gas heaters differ?
Instantaneous units heat flowing water during a draw. Storage units hold hot water and reheat it. Compare flow and temperature rise for the former, and storage and recovery for the latter.
Does instantaneous mean hot water arrives at the tap immediately?
Heating starts during the draw, but water still takes time to travel through the pipe between the appliance and the tap. Pipe length and layout matter.
Which suits several bathrooms?
Both have limits specific to the model. We review simultaneous flow and draw duration, as well as recovery when storage is used.
Can a storage gas water heater run space heating?
The model and arrangement must be suitable for that duty. A tank or gas burner alone does not establish compatibility with radiators or underfloor heating.
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