# How TRVs actually work?

#### berty3000

Hello,

I have "Center" manual TRVs on my rads.

Some of them seem to cut off the water supply to the respective rads before the room is anywhere near the set temperature. So if I have the TRV set to 3.5 (22C), the radiator has gone lukewarm whilst the room is only at 19.5C. If I turn the TRV to setting 4, I hear water flowing in and it gets hot again.

This proves that the TRVs are not very accurate and common sense explains why. The thermometer is right next to a hot pipe and radiator. Obviously a different position to in the middle of the room. They should really calibrate the TRVs to account for this, but I suppose that would involve guesswork because room sizes vary and objects in rooms like furniture will have an effect.

Anyway, to overcome this I have set the TRVs to more like 4 in the rooms where they were going off too soon and now those rooms heat up to 21C, with the TRV setting being 24C. Fine.

The problem I now have is that people keep saying that TRVs let the radiator heat up until the desired temperature is reached on the TRV, THEN restrict the water flow. This is obviously untrue because if I have the TRV set to 4 and it is hot whilst the room heats up, and immediately increase the TRV setting to 5, I hear a very clear change to the flow. In other words, like non-TRV dials, going from 4 to 5 changes the amount of water going into the radiator. It isn't the case that it is always on full flow until the set temperature is reached.

If this is correct then my question is: If I use my heating for three 2-hour sessions a day, where it doesn't reach the central thermostat temperature until the end of each cycle, isn't it best to have the TRVs on the highest setting at all times? This way, more hot water goes into the radiators and releases more heat into the rooms, return temperatures are reduced as a result, and the boiler operates more efficiently whilst also getting the rooms hotter more quickly? I have the flow temperature of the boiler at 55C.

Why burn gas and have these stupid TRVs restricting the amount of hot water in the rads?

Does having the radiator dial set to fully open mean more of the system's hot water in the radiators and therefore return temperature will be reduced?

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TRVs are dumb, self limiting and have a hysteresis feed backloop.
They are also a dynamic throttling device.

The idea is the heat output is proportional to the delta being felt at the Head.
Temps are indicative only.

For the same reason, I think having the TRV enabled rad "Balanced" is a nonsense.

However, system temp drop is proportional to Q (Flow rate).
Slow rates (flow) means larger temp drops as the fluid has more time to give up its heat).
Which means less output (Higher mean temp drop across the rad).

If you want quick rads, open her up fully. This means higher relative return temps, (or lower temp drops) so you will start getting issues on boiler returns being too high if you don't adjust the boiler output and pump speed to suit.

So in summary, unrestricted flow through the rads will give higher heat outputs and lower temp drops.

It's all related. Temp, flow, pressure.

Hence why trvs have numbers on instead of temps . So people dontt say oh my rad is turning off 1 degree or so earlier

The problem I now have is that people keep saying that TRVs let the radiator heat up until the desired temperature is reached on the TRV, THEN restrict the water flow.
They're right
This is obviously untrue because if I have the TRV set to 4 and it is hot whilst the room heats up, and immediately increase the TRV setting to 5, I hear a very clear change to the flow.
That's what you'd expect to happen. If it's set to 4 and the room is at the corresponding temperature, the TRV is doing its job ie throttling. If you then turn it up to 5 it thinks you want a higher temperature so opens wider and the flow increases.
the central thermostat temperature
The central thermostat should be in a room with no TRV.
If this is correct then my question is: If I use my heating for three 2-hour sessions a day, where it doesn't reach the central thermostat temperature until the end of each cycle, isn't it best to have the TRVs on the highest setting at all times?
No. If you do that the room with TRV might get too warm before the timer or central thermostat stops the boiler, wasting gas. If it's left at the setting you want the room won't heat any more slowly, as the TRV is wide open until temperature is reached.

TRVs are dumb, self limiting and have a hysteresis feed backloop.
They are also a dynamic throttling device.

The idea is the heat output is proportional to the delta being felt at the Head.
Temps are indicative only.

For the same reason, I think having the TRV enabled rad "Balanced" is a nonsense.

However, system temp drop is proportional to Q (Flow rate).
Slow rates (flow) means larger temp drops as the fluid has more time to give up its heat).
Which means less output (Higher mean temp drop across the rad).

If you want quick rads, open her up fully. This means higher relative return temps, (or lower temp drops) so you will start getting issues on boiler returns being too high if you don't adjust the boiler output and pump speed to suit.

So in summary, unrestricted flow through the rads will give higher heat outputs and lower temp drops.

It's all related. Temp, flow, pressure.

Thanks for the response, has really helped with my understanding.

So having the TRVs fully open will mean more heat is given out by the radiator, yes?

But there will also be less temperature drop across the radiator?

This part confuses me. If more heat is given out by the radiator surely the returns should be lower?

For info, my Worcester 25i has the max CH output set to 10KW (range rated), the pump head setting on the lowest one, and the flow temp at 55C. What I have noticed is that there is only 5 or 6 degrees difference between flow and return at the boiler, so possible that lock shields now need adjusting as I've tried everything else. But am wondering if I allow full flow into the radiators with trvs opened right up, might help with more heat given out and lower returns, bearing in mind the settings I'm using and the way I am using the system?

As mentioned previously, I use the heating in three 2hr blocks per day and it reaches the central thermostat temp towards the end of these before switching off for a few hours or over night.

I think my primary question is really: is there any reason not to have the trvs fully open during each two hour block if it means those rooms reach a comfortable temperature? Will I be burning more gas or less? Will the boiler be getting used in an efficient way (condensing) bearing in mind the other settings I'm using and the way I use the heating system? Is there anything else I can do to get better efficiency?

Also, how about having the TRVs fully open but closing the lockshields a bit, will this help with heat output from the radiators and reduce return temps?

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They're right

That's what you'd expect to happen. If it's set to 4 and the room is at the corresponding temperature, the TRV is doing its job ie throttling. If you then turn it up to 5 it thinks you want a higher temperature so opens wider and the flow increases.

The central thermostat should be in a room with no TRV.

No. If you do that the room with TRV might get too warm before the timer or central thermostat stops the boiler, wasting gas. If it's left at the setting you want the room won't heat any more slowly, as the TRV is wide open until temperature is reached.

I'm afraid you're wrong and obviously haven't understood my post.

If the TRV is set to 4 and the radiator is in the process of heating up the room, but I then change it to 5 before it has even reached 4, the flow increases. This suggests that the TRVs don't open up fully until the corresponding temperatures are reached. Setting 5 seems to be more open than 4 regardless of the temperature. One of my vertical radiators gets hotter over a greater area if I go from 4 to 5, irrespective of whether 4 was anywhere near being reached.

There is no TRV in the room with the central thermostat. That radiator just has a plastic cap on it that turns.

Your last comment is wrong for the reasons given above. Also, the rooms won't get too hot in my case as the heating only comes on for 2 hour blocks with a 55C flow temp. Which is why I'm asking, is there any harm in just having full flow through the rads and will it actually reduce return temps if more water is being allowed into the rads by the higher TRV setting?

I'm happy to be wrong, and you aren't the only one who has described TRVs as you have, but it isn't what I am observing.

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Thanks for the response, has really helped with my understanding.

So having the TRVs fully open will mean more heat is given out by the radiator, yes?

But there will also be less temperature drop across the radiator?

This part confuses me. If more heat is given out by the radiator surely the returns should be lower?

only if you think that the same amount of heat is being put in.

High flow rate, high heat input. low temp drop
Low flow rate, low heat input. High temp drop

Ish.

Loads of other variables are in effect as well.
It's not linear relationships as there is a physical limit to how much heat the rad can get rid of and how much the fluid can supply and how much the boiler can provide.

Also, the boiler temp control is (usually) on the supply side.
This means as the flow rate goes up, the return temp goes up as well because the boiler works harder to keep the output to temp because the flow rate is faster etc. etc.

This is nothing more than reducing the flowrate through it to make the temp drop across it matches the original design intent
(Well, no, actually, they just turn them all down to a 1/4 turn on the LS in the mistaken belief that this is the same thing).

So think like that, to increase the temp drop, reduce the flowrate.

but I then change it to 5 before it has even reached 4, the flow increases.
I was basing it on the room being up to temperature, but in any case there's not a sharp cutoff, so that could happen. It also depends how far away the room is from set temperature.
This suggests that the TRVs don't open up fully until the corresponding temperatures are reached.
A TRV closes when the set temperature is reached.
Also, the rooms won't get too hot in my case as the heating only comes on for 2 hour blocks with a 55C flow temp.
That's an odd way to control room temperature, what do you think will happen when the weather changes?

But have it your way, you seem to know everything

A TRV closes when the set temperature is reached.
A TRV closes AS the set temp is reached.

It is not a switch, more of a lever.

A TRV closes AS the set temp is reached.

It is not a switch, more of a lever.
OK point taken, what I was getting at was, it closes, not opens.

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It is just an expanding wax that pushes on the needle valve, it has a fuzzy temperature limit by design. Near to the set point the actual temperature you get depends upon many things. Also if the TRV's are all wide open then the one nearest the boiler (in terms of pressure drop) will get all the flow, so the others will heat up slower until the nearest is starting to shut down. So yes the system will operate and eventually get to steady state, but the rooms will be more comfortable sooner if you set them to have a more equal pressure and flow when first starting. the final steady state will arrive sooner if it is a bit more balanced.

So think like that, to increase the temp drop, reduce the flowrate.
So how do I reduce the flowrate?

I have all mine installed horizontally, on the return as I feel they give a more representative room temperature. TRVs have a very small hysteresis of < 1 deg C but are very slow acting and it can take up to 20 minutes to travel full stroke, they also can act a bit erratically once they have throttled down so that the rad return temperature is ~ 25C, say 5C above room temperature which means that once the rad demand falls to ~ 50% from a flowtemperature of 75C or ~ 35% from flowtemperature of 60C, they are then practically closed, once fully closed then there can be a lag before regaining steady control but within reason work quite well IMO, I have 8 TRVd rads, the two other rads are in a combined dining/sitting room where we "live" and are under a basic non TPI digital roomstat set with a 0.3C differential which gives excellent control in these two rooms (oil fired boiler), I have the L/shield valves throttled on these so that the boiler gets decent on times without the stat constantly cutting in/out which results in quite reasonable temperatures in the remaining rooms.

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So how do I reduce the flowrate?
THe TRV reduces the flowrate as the room temperature slightly exceeds the set temperature, which in turn reduces the return temperature resulting in a lower mean rad temperature = less heat emitted to maintain the set room (air) temperature.

THe TRV reduces the flowrate as the room temperature slightly exceeds the set temperature, which in turn reduces the return temperature resulting in a lower mean rad temperature = less heat emitted to maintain the set room (air) temperature.
I'm wanting to get a bigger difference between flow and return ro increase boiler efficiency.

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