Hot Water Temperature Control
Hot Water Systems · Plumbing
Hot water has to be hot enough to keep bacteria down and cool enough at the tap not to scald. This topic explains how both aims are met at once.
HSE guidance describes Legionella as dormant in cold water, multiplying between about 20 and 45 °C and not surviving at high temperatures. People are infected by breathing in fine spray, so showers and little-used outlets matter most.
In buildings where someone manages the risk, HSE guidance checks the outlets nearest to and furthest from the source each month. Landlords in Great Britain are legally responsible for assessing that risk and controlling it, and no certificate is involved.
A thermostatic mixing valve blends hot and cold near the outlet and shuts down if the cold supply fails, so the store can stay hot while the bath or shower stays safe. Regular flushing and clean shower heads deal with water that would otherwise stand still.
Hygiene and scald control happen in different places
Updated
How to reason about the system
Stored water is kept hot to control bacteria, and the temperature at a bath or shower is brought down close to the outlet. Treat these as two separate controls, and remember that water standing still in a pipe undoes both.
Questions to take further
- Does a landlord in Great Britain need a Legionella certificate?
- No certificate is required. A landlord has to assess the risk and control it where needed, and sampling is not usually necessary in a home.
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.
Free interactive sample
Choose an option, check it and retry here. No account, payment or energy needed. These public samples are separate from your learning path; progress is not saved.
Original plumbing learning material. For revision only; do not use samples to guide work on site.
Learning content, not a qualification: it won't make you competent, certified or registered. Follow the regulations that apply and a qualified plumber before acting. About our content
Lessons on the plumbing path
- Why hot-water temperature matters
- Outlet temperatures and routine checks
- Thermostatic mixing valves
- Stagnation, flushing and showers
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.