Direct or indirect cylinder: choosing the heat-source arrangement
A direct cylinder normally heats stored water with electric immersion heating. An indirect cylinder uses a heat exchanger connected to another heat source, often with an immersion as a secondary option. These descriptions concern how the water is heated, while vented and unvented describe a different part of the system arrangement. Choose the configuration around the available heat source and how hot water is used, not the number of switches beside the cupboard.
Loyal Plumbers · Updated
Discuss a cylinder replacementSeparate the two sets of cylinder terminology
Direct and indirect are frequently confused with vented and unvented. A product description can contain one term from each pair because they answer different questions. When asking for a replacement, provide the exact model and current heat source rather than assuming that a mains-fed cylinder must be indirectly heated.
An immersion heater does not automatically make the complete installation direct-only. It may provide backup on a cylinder normally heated through a coil. Ask the installer to identify the normal heating route and the purpose of each user control. This avoids comparing an electric-only proposal with a boiler-heated proposal as though the difference were merely brand.
Map the heat sources available now
List the working heat source, the electrical arrangements serving the cylinder and any equipment that is being removed. If there is no external source available to heat a coil, an indirect cylinder needs a clear reason for selection and a defined means of heating water from the first day.
Conversely, choosing electric-only storage where a suitable external source already exists changes the operating arrangement. Ask what that change means for electrical work, controls and the way heating periods are scheduled. The decision should be based on the complete installation and your use pattern, rather than assuming either configuration is inherently cheapest to run.
Compare recovery against the actual operating pattern
A household using most hot water in one morning period has a different requirement from a property drawing smaller amounts throughout the day. Describe the sequence of showers, baths and other use, including occasions when demand overlaps. The installer can then compare storage and reheating together.
An indirect cylinder’s published performance depends on the conditions under which its coil is supplied. An electric immersion has its own rating and control arrangement. Do not compare a rapid indirect recovery figure with a direct heat-up time without checking what each measures. The useful question is whether the proposed system replenishes water when your household needs it.
Understand the electrical supply and tariff assumptions
Electric heating needs an assessment of the appropriate circuit and controls, whether it is the primary source or backup. Explain any separate off-peak or boost controls and provide the tariff schedule you actually use. A cylinder model name does not establish how your electricity account is configured.
Ask which heating periods the proposal assumes and how additional demand would be met. Do not treat a lower advertised unit price as a complete annual-cost calculation: water use, heating schedules and other tariff charges matter. Electrical alterations should be scoped by a competent person, with user settings explained separately from installer-only adjustments.
Compare the configurations without oversimplifying
The choice often turns on what already exists and what will change in the property. A clear proposal states the normal heat source, backup arrangement and controls. A description that simply says new cylinder leaves too much unresolved.
Use the following comparison to prepare questions for the survey. It describes common arrangements; the selected product documentation remains the reference for the actual installation.
| Arrangement | What supplies the heat? | What to confirm |
|---|---|---|
| Direct electric storage | Immersion heating within the cylinder | Circuit suitability, heating periods and replenishment |
| Indirect storage | A compatible external source through a heat exchanger | Coil suitability, source performance and controls |
| Indirect with immersion backup | External source normally, immersion when required | How backup is selected and what performance to expect |
| Solar-specific configuration | A designed combination of solar input and other heating | The exact model, source connections and control strategy |
Keep future changes specific
If a heat pump or solar installation is planned, name that project and its likely timing. A spare connection or unused coil is not proof that the cylinder will suit every future source. Gledhill’s solar range, for example, distinguishes electrically heated solar models from indirect versions using a separate boiler coil.
Ask the future system designer to confirm compatibility before paying for features chosen only on expectation. Sometimes a coordinated installation is clearer than buying a cylinder now around an undefined later project. If the later specification is uncertain, record that uncertainty and compare the cost of retaining flexibility with the cost of unnecessary complexity.
Make the proposal and handover unambiguous
The written scope should identify the exact cylinder, normal and secondary heat sources, control changes and electrical work. It should also state what old equipment is removed and what remains active. That gives you a useful comparison between contractors and prevents assumptions about a backup heater being included.
Loyal Plumbers can assess direct and indirect cylinder replacement across London. Tell us the current heating arrangement, when hot water runs short and any confirmed future upgrade. We can discuss a configuration that fits the available services and explain the operating controls and maintenance requirements at handover.
Common questions
- Does indirect mean there is no immersion heater?
- No. Many indirect arrangements include an immersion for another operating mode. Confirm the exact model and how the installation uses it.
- Is direct always unvented?
- No. The terms describe different characteristics. Read the full product specification and have the existing arrangement identified.
- Can one type always replace the other?
- A change may be possible, but it can require different heat-source connections, controls and electrical work. It is a system change to assess, not just a matching-size swap.
Sources and further reading
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