Other stored and instant hot water
Stored Hot Water in Depth · Plumbing
Not every home stores its hot water in a cylinder. This topic covers the other ways water is stored, kept moving and heated, and which parts of each job belong to someone else.
A thermal store holds hot heating water and warms mains water as it passes through a coil, so the hot water at the taps is as fresh as the mains. Many are open-vented, but some are unvented, so check the store and follow its maker's instructions.
On a long run, a secondary loop keeps hot water moving out to the far taps and back, with a pump and a check valve on the return and short branches to each tap. HSE guidance asks for taps to deliver water at 50 °C or hotter inside about a minute.
Point-of-use heaters make hot water at the tap, and their electrical supply is for an electrician. Solar thermal and waste-water heat recovery are covered for awareness: MCS is a certification scheme for small renewable systems, not a licence.
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Know which water is stored, and keep hot water moving
A thermal store holds heating water and warms the mains as it passes through; an unvented cylinder holds the drinking water itself. On long runs, a secondary loop keeps hot water moving, so the far taps run hot quickly and water does not sit lukewarm.
Questions to take further
- How quickly should hot water reach a tap?
- HSE guidance on Legionella asks for taps to deliver water at 50 °C or hotter inside about a minute, with the store kept at 60 °C or hotter. A secondary loop with short branches is one way to manage that on a long run.
Sources
- HSE: Legionella and Legionnaires' disease — How Legionella risk is assessed and controlled in Great Britain, including the duties of landlords and employers.
- GOV.UK: Approved Document G — Guidance for England on hot-water safety, including stored hot water; guidance is not itself the legislation.
Key numbers
- 50 °C
- HSE guidance looks for hot water at outlets to reach this or above within one minute.
Common mistakes
- Assuming every thermal store is open-vented.
- Leaving long branches off a secondary loop.
- Taking on an instantaneous heater's electrical supply instead of calling an electrician.
Lessons on the plumbing path
- Thermal stores and combi-stores
- Secondary circulation and long runs
- Point-of-use and instantaneous heaters
- Solar thermal and waste-water heat recovery
Terms to know
Before you start: some plumbing work is regulated or notifiable
Some water fittings work needs notice to, and consent from, the water supplier before it starts. Check the rules where you live.
Gas work is for competent workers covered by a Gas Safe registered business in Great Britain. Northern Ireland has separate gas regulations. Do not work on gas pipes or appliances yourself.
In England, installing an unvented hot-water system over 15 litres is notifiable building work: use a competent installer registered with a competent-person scheme, or tell building control before work starts. Leave this work to a suitably skilled person.
Wales, Scotland and Northern Ireland have their own water and building rules; check which apply before work starts. Electrical connections belong with an electrician.
This guide does not qualify you or make you competent, approved or registered for plumbing, gas or unvented work.