# Please explain how TRVs work on Return

#### YamiPrem

Hello all,

I'm new to the forum and joined to specifically ask this burning question as I couldn't get my head round it.

I understand that bi-directional TRVs exist nowadays that can work on either flow or return side of a radiator.

Now, I totally can visually imagine how TRV works when on the flow side. The TRV simply checks the temp around it and if it's lower than the temp set on the valve, it opens up the flow so the hot water that is going round the system, enters the rad to heat it up as the as the water passes it and exits on the return side. So this makes sense as it essentially stops hot water ever entering the radiator.

However, when TRV is on the return and say a lockshield is on the flow, how does it work? So say the boiler comes on and pumps hot water round the system. Since the lockshield will be open and on the flow side, does that mean the hot water enters the rad no matter what even if the TRV is on off position? Does that not mean the radiator will heat up and if TRV senses the room is hot enough, it will close but... the flow is still going as the lockshield is still open so how is the TRV stopping the hot water from entering at all? And what happens to the hot water? It just goes around the radiator and back out the flow pipe?

I'm no plumber, just very interested in how these sort of things work. And as you can tell, I am likely not understanding some basic things here (since no one else on the net seems to have this question).

Thanks for any insight!

Because it is simply a valve, if bi-directional doesnt matter which end it is on the water cant flow through the rad, when it closes

Because it is simply a valve, if bi-directional doesnt matter which end it is on the water cant flow through the rad, when it closes

That's the bit I don't understand. How is it stopping the flow if it's on the return end? The flow end will still allow hot water to enter the radiator, no?

the hot water is stopped from leaving the radiator and so hot water cannot go into the radiator, because its stopped from flowing around the radiator

The flow end will still allow hot water to enter the radiator, no?
NO the radiator is FULL of water , NOT an empty vessel , so the flow end cannot allow water in because the radiator is full and it cannot leave the radiator

thats why you have a bleed valve on the radiator to remove all the air so the whole system is full of water
and then that flows around the system and as it passes the boiler gets hotter

the hot water is stopped from leaving the radiator and so hot water cannot go into the radiator, because its stopped from flowing around the radiator

NO the radiator is FULL of water , NOT an empty vessel , so the flow end cannot allow water in because the radiator is full and it cannot leave the radiator

thats why you have a bleed valve on the radiator to remove all the air so the whole system is full of water
and then that flows around the system and as it passes the boiler gets hotter

Okay, now I'm catching on. I keep forgetting there is cold water sitting in the system when heating is off.

So are radiators built in a way that if return end is shut off on a radiator, the water can not leave on the flow valve? The water must only travel one direction through the radiator? Otherwise I can't see how hot water cannot enter the flow end and 'mix' with the cold.

Thanks!

Okay, now I'm catching on. I keep forgetting there is cold water sitting in the system when heating is off.

So are radiators built in a way that if return end is shut off on a radiator, the water can not leave on the flow valve? The water must only travel one direction through the radiator? Otherwise I can't see how hot water cannot enter the flow end and 'mix' with the cold.

Thanks!
Think of a circuit,flow goes out your boiler into one side of your rads, it then leaves your rads through the other side via the return pipe and goes back to the boiler, if it’s shut then it won’t flow thus not heating your rad.it’s a constant flow of water through the circuit.

A length of hose pipe imagine as the Radiator
Tap at one end allows water to flow in
Spray gun at the other end , allows the water to come from the Tap , through the hose and out of the spray gun
Turn off the spray gun and water does not enter into the hose , even though the Tap is open
The flow around a CH system is pushed round by the pump, and water takes the path of least resistance
the Radiator at the flow end may get slightly warm due to convection from the hot water
If you turn a radiator off depending on the layout of the pipework , you can still feel one side of the pipework is hot

The water does not actually flow from point to point it circulates.
If a valve is closed there can be no circulation in that circuit..

Think of the radiator as a hose pipe, with a tap at the house end and the sprayer at the far end of the hose.

If you turn the water off at the house (which represents the valve at the entry to the radiator) new water does not enter the hose. But also the water in the hose does not leave (even though far end is open) because to do this it would have to form a vacuum at the house.

If you turn the water off at the spray (the valve at the exit from the radiator) again new water does not enter the hose at the house because the water cannot be compressed to allow more water in.

So it makes no difference which valve is shut off, when either is switched off, the water entering and/or leaving the radiator will stop flowing.

Sfk

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Turn the TRV off and the wife complains "its cold in here", turn it on and she finds something else to complain about.

C'mon....
We can simplify it better than that...

Beer in = beer out.

Jeez, I'm dying for a pi\$\$!

The flow end will still allow hot water to enter the radiator, no?
No! Water is incompressible. If it can't get out of the rad it won't go in. It doesn't matter where you throttle it, the effect is the same.

Here's another analogy. One we can all relate to. Think of it as being like a busy 3 lane road leading into a tunnel. At the entrance to the tunnel roadworks reduce the lanes down to 1. Because of the restriction, the flow of cars through the tunnel is reduced.

The roadworks progress, and now it is at the exit of the tunnel where the 3 lanes reduce down to 1. However, the number of cars travelling though the tunnel remains the same, because there is the same restriction allowing exactly the same number of cars to pass through the tunnel.

And that's how a TRV works on a radiator, whichever end it is on, as it closes down it restricts the flow of water through the radiator. Simples

With the heritage railway I work on yes the boiler does as name suggests and boils the water, and the steam is fed to the carriages, and through the radiators in each carriage and water drips out of last carriage and steam escapes everywhere giving ambience specially at Halloween.

But in a home the boiler does not we hope at least boil the water, and it is not a total loss system, in fact the temperature of the return water is used to control the boiler with modulating gas boilers.

So if the water can't flow through radiators it opens the by-pass valve and returns to boiler hot and that turns down the flame height. (modulates)

However there is a problem fitting the TRV on the return, we want the air to circulate so the TRV is sensing the cool air returning, but it also senses so heat direct from the radiator, better versions have two sensors one for air and one for water and the latter compensates for heat from the radiator, so on the feed side it senses the temperature faster than on the return side, so on the return we want the flow to be slower than on feed side to give the TRV chance to adjust.

I fitted electronic heads in mothers house, and slowly bit by bit adjusted the lock shield valve on the supply side so the TRV recorded temperature did not over shoot, since the TRV showed both target and current it was easy enough to adjust, but when we came to sell the house, and fitted the non electronic heads back on, we found now lock shield set, the standard TRV head worked very well. However when setting we found those on the supply side were far easier to set to those on the return side, with this house many of the TRV heads only show target, this head works well, but since it only shows target you also need a thermometer to set the lock shield valve, these as said do work, but the *12345 does not really help when trying to set up, as your not sure if the TRV or lock shield needs tweaking a little.

WHAT ?

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