Plumbing Question

I have bare pipes coming through the floor for my radiators how do i figure out which pipe needs the thermostat on

14 Answers

Best Answer

the one that gets hot first is the flow
most modern stats will go flow or return

Answered 2nd Dec 2011

apex plumbing

Member since 7 Jan 2009


As a rule of thumb most thermostatic valves work on the return of the system. Check the valve itself and there should be a little arrow near the outlet. If the arrow points away from the valve it needs to go on the return side of the radiator, if it points into the valve it should be on the flow side.
How to tell flow or return?. Turn off the boiler and let the system go cold, turn
it on again whilst feeling both pipes into the radiator the one that gets hot first is the flow, the cooler one is the return.

Happy fixing.


Answered 3rd Dec 2011

photofinish contracts

Member since 6 Oct 2011

When you turn your heating on, the pipe which heats up first is the flow and your valve should go on this side of the radiator. However, you can now purchase bi-direction or reverse flow valves which can go on either side of the radiator.
Regards, Alex.

Answered 3rd Dec 2011

AC Plumbing

Member since 4 Jan 2009

Hi, Most thermostats are bi- directional and will work on either flow or return.
Hope this helps.

Answered 4th Dec 2011

Gs gas services

Member since 29 Jan 2009

Nearly all thermostatic valves are now bi directional so both they can go on either end now.

Answered 2nd Dec 2011


Member since 10 Oct 2007

Radiator thermostats nowadays are universal and work on flow and return

Answered 2nd Dec 2011

sovereign plumbing and heating

Member since 23 Nov 2011

use a plumbase trade valve which can flow either way

Answered 3rd Dec 2011

smg plumbing and heating

Member since 19 Nov 2010

No need to worry most thermostic radiator valves work in both directions flow or return, you can see on the valve body that the arrow will go in both directions.
I hope this helps Dave

Answered 3rd Dec 2011


Member since 13 Oct 2007


Most valves are dual flow, so it does not matter so much where you install your TRV. I suggest you put it on the side that will not be obstructed by furnitur, curtains ect and can be easily adjusted.

Dont forget to clean your system when you empty it and then top up with an inhibutor.

Answered 4th Dec 2011

Deans Heating

Member since 22 Jan 2010

you should put the thermostat on the flow pipe to the rad ,,you can find this out by when you turn on your heating the pipe on the rad that gets hot first is the flow pipe ie the water is travelling through that side of the radiator first then through rad and out of the other end pipe ,,return pipe ,, so basicly the hotest pipe to the rad is normally the flow side ,, hope this helps regards richard taggart

Answered 3rd Dec 2011

richard taggart building company

Member since 3 May 2010

buy reversable TRV's, these go on either the flow or return and will work perfectly. They are only a little extra and the pin wont stick as much as time passes. Good luck David Oliver

Answered 2nd Dec 2011

Build It 24/7

Member since 1 Dec 2011

Turn on your central heating and see which side heats up first. That side will be your flow and the other side will be your return. By putting the thermostatic valve on the flow side you can accurately control the temperature through your radiator.

Answered 3rd Dec 2011

Recommended Plumber Ltd

Member since 2 Dec 2011

Hi Adam, Today you can fit a thermostatic radiator valve to either pipe, they are reversible, just ask your plumbing merchant for the correct type. Otherwise you can switch the heating on and go around each radiator and feel the pipes. The one that gets hot first is the flow. This is the one to fit the thermostat to. Good Luck


Martin Hislop

Answered 3rd Dec 2011

Connect Construction Management Ltd

Member since 30 Jul 2010

The one that get's hot first.

Answered 3rd Dec 2011

No Lipstick Works

Member since 29 Mar 2008

Need help with your project?

We have tradesmen ready to help you. Post a job, read reviews and hire today.

Post a job

Need some help?

Post a job on MyBuilder to find quality, local Plumbers who can help you with your project.

Search all questions