Thermostatic mixing valve
In plumbing · also called TMV
A valve that blends hot and cold water to a set temperature close to an outlet, and shuts the hot off if the cold supply fails. It lets stored water be kept hot enough to control bacteria while the water at a bath, basin or shower arrives at a temperature that is safer against scalding.
Using Thermostatic mixing valve in a plumbing system
Updated
A distinction worth remembering
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.
Where Thermostatic mixing valve fits in the learning path
The Hot Water Systems section covers these topics:
- Hot Water Cylinders
- Combis and Instant Hot Water
- Hot Water Temperature Control
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- Hot Water Temperature ControlHot Water Systems
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Bacteria found naturally in water that can cause Legionnaires' disease when people breathe in fine droplets, for example from a shower. HSE guidance describes them multiplying in water at roughly 20 to 45 °C. In Great Britain, landlords and employers have legal duties to assess and control the risk, and no certificate is involved.
Legionella
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Legionella
Bacteria found naturally in water that can cause Legionnaires' disease when people breathe in fine droplets, for example from a shower. HSE guidance describes them multiplying in water at roughly 20 to 45 °C. In Great Britain, landlords and employers have legal duties to assess and control the risk, and no certificate is involved.
Read the Legionella entryDead leg
A length of pipe where water sits still because the outlet at its end is rarely or never used, or has been removed and the pipe capped. Standing water in it can grow bacteria such as Legionella, so dead legs are cut back to the main run where possible and little-used outlets are flushed.
Read the Dead leg entryBefore 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.
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