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A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings, but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure. The word is derived from the Latin valva, the moving part of a door, in turn from volvere, to turn, roll.
The simplest, and very ancient, valve is simply a freely hinged flap which swings down to obstruct fluid (gas or liquid) flow in one direction, but is pushed up by the flow itself when the flow is moving in the opposite direction. This is called a check valve, as it prevents or "checks" the flow in one direction. Modern control valves may regulate pressure or flow downstream and operate on sophisticated automation systems.
Valves have many uses, including controlling water for irrigation, industrial uses for controlling processes, residential uses such as on/off and pressure control to dish and clothes washers and taps in the home. Even aerosol spray cans have a tiny valve built in. Valves are also used in the military and transport sectors. In HVAC ductwork and other near-atmospheric air flows, valves are instead called dampers. In compressed air systems, however, valves are used with the most common type being ball valves.

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  1. R

    Can motorised valves go under floorboards (legally)?

    Hi guys and gals, I'm looking to try to change from a Y-plan to an S-plan-plus this year to have a heated bathroom. While the house isn't that large, a mixture of south-facing brick and north-facing (and Jerry-built) timber frame construction means no single thermostat will cut the muster...
  2. C

    Isolator valves on basin taps

    Hi I have always had a poor water delivery to my bathroom and en-suit basins. The cold is just about acceptable but the hot is pathetic coming from the tank on the same floor. Both rooms have isolator valves fitted in the correct direction and then to flexi tails that just reduce the flow...
  3. prburkis

    Are there any "Recommended" Smart TRV Valves for large houses?

    Hi All, I thought I'd come to the experts and see if anyone is installing and/or recommending the use of any Smart TRV valves? Particularly in large(ish) solid wall houses. I live in a 1920's arts and crafts house - it's about 2000sqft, not massive, but it's approx 25% larger on the...
  4. B

    Kitchen stop valves result and questions

    Hi everyone, Finally I have figured out what the two stop cocks in the kitchen do as they both had to be replaced due to leaks. The one under the sink stops the cold water from coming into the kitchen tap and the one which is in the wall panel is connected to the incoming mains water and needs...
  5. R

    Valve tips and advice

    Hi all! as I am attempting my first plumbing project and the stakes are high, I would greatly appreciate picking your experienced brains regarding valves. I am building an overland truck/motorhome and since I will be traveling to remote areas and underdeveloped countries I'd like my plumbing...
  6. L

    How do I fit Bosch thermostatic valves to old radiators .

    background: I have 15 radiators 20+ years old with no thermostatic valves and Bosch instructions depend on replacing the actuator part only of old thermostatic valves. Surely there is an option for new complete valve replacing? Tom
  7. M

    Help required for replacing union valves.

    Hello, New here. I have some older type single radiators in my home, that I want to keep. The are supplied by 15mm copper pipe, into the valve, that has a 3/4 union type fitting. I want to replace these fittings with TRV's and am having difficulties finding a direct replacement. My floors are...
  8. A

    what is it with Ideal Standard inlet valves

    The usual problem of quarter turn inlet valves not shutting off, so I eventually found replacement diaphragms 32mm... and although they fit properly, the water still does not shut off. If I lift the float up I'd have thought the water should shut off but it makes no difference. So I tried the...
  9. C

    Pressure release valves

    Anyone finding these 'failing' early? I'm in Ireland and have never seen these valves opening under normal conditions. Recently, in the last year, I have had three brand new PRVs opening under normal conditions. One of them was on the mains supply which actually has a pressure reducing valve on...
  10. R

    Faulty Danfoss HPA2 zone valves

    Hi, Seems like both my upstairs and downstairs HPA2 zone valves are on the blink. Every other day one of them doesn't switch off at the thermostat cut off temp and I come home to the Bahamas 😅. Firstly, is it worth trying to repair them or is the fix only likely to last a short time before...
  11. D

    Repressurise Vaillant boiler - no valves

    HI I would like to repressurise our Vaillant EcoFit Pure 625. Normally a job even an ignoramus like me can do. However we have no filler pipe and no valves. Could these be inside the case?
  12. S

    Magnaclean stuck on valves!

    I'm hoping to do a magnacleanse clean shortly but the speedfit connectors on the magnaclean micro valves on the system seen stuck on and will not budge. I would prefer not to drain system and remove compression fittings on the filter but realise I might end up having to. Im in the know on how...
  13. J

    Tap still dripping after valves have been replaced?

    Hi I have a dual lever bath tap / shower mixer and have replaced both ceramic type valves on each tap. Since then the tap continues to drip and was hoping to get some advice, thanks I’m advance
  14. Bettyboo

    shower valves and change of water pressure?

    Hi All Thames Water recently came to fit an outside water meter and since then I have had two 1901 bristan shower cartridge/ valves dripping . Is it possible this was the cause ? When I did up the house I had a 25mm MDPE main water pipe fitted between the external stopcock to a new internal...
  15. C

    Preferred Radiator Valves...

    Hello Everyone Peeps...Would any plumbers like to share their preferred Rad Valve manufacturer? Drayton, Honeywell, Danfoss, Siemens, Peglar? Peglar offer 5 year guarantees, Honeywell for High Tec I have a leaning for Drayton as I live and they manufacture in the West Country. Thank you everyone
  16. Reddwarf4ever

    Changing valves in a Brixton quest kitchen tap

    Have removed the cold valve easy enough, but the hot valve will not unscrew, have used a spanner and mole grips, but both start sliding and starting to damage the nut surface, is it possible that the hot tap valve unscrews the opposite way ? I assumed both would be anti clockwise to unscrew...
  17. E

    How long do honeywell two port motorised valves last?

    Ours are lasting between a year and two years and then I have to replace. Is this normal or should I be investigating our central heating system to find out why? Which motorised valves do you recommend I use?
  18. G

    How do I shut off a bathroom radiator with angles valves?

    My bathroom radiator is very hot in a small room and I would like to shut it off but can’t work out how. There are two valves one at each side at the bottom with a small prong sticking out of each one. I don’t know what the difference is between them or even if that is what I have to turn or how!
  19. 2

    Rad not heating up. Warm water through both valves.

    I have a house with 15 rads in it. In a bedroom upstairs I have a rad that isn't heating up. This rad was moved about 5 years ago with 1/2inch plastic pipe to new location in the same room but was never turned on. Tonight I tried to blead it but then removed the rad, emptied it and when off...
  20. E

    Isolate shower to service valves

    Hi All, new here and need some help. I’ve just bought a house with a built in Mira 915b shower. The valves are playing up so I want to remove them to put new o rings and grease on. Trouble is I can’t isolate the water to it. Hot is fed from the cylinder and the feed has a gate valve which is...