Heating on, but some radiators switched off and still getting warm?

ThisIsWeird

Modern TRVs are 'bi', so can be fitted to either end of a rad.

ThisIsWeird

Remove the TRV head and check:
1) that the valve pin can be pushed downwards smoothly (tho' needs a firm push), 'bottoms out' with a firm thunk, and pops back up without hesitation when released.
2) that the centre part inside the head goes up and down when the head is rotated - that's the bit that acts on the pin.

Gazelle1985

@ThisIsWeird thanks, will give it a go

Section62

Gazelle1985 said:

Veteransaver said:

Are the trvs turned right off? And not just to the frost setting (snowflake symbol).
If they are right off then it will be a knackered valve or maybe some debris in the valve allowing a little bit of hot water through so you could try to dislodge by turning the valve on and off whilst the heating is pumping.
Which pipe to the rad is getting warm, the one with the trv on it or the other one?

They're turned off as far as they can go (which is the snowflake - all radiators are the same, but most work fine).

Weirdly one of the radiator pipes which is faulty is hot on the TRV side and the other is hot on the non-TRV side.

The symptoms don't necessarily mean your TRV's are faulty (radiator heating isn't always quite as simple as a binary on/off as some of the previous posts have suggested).

Firstly there is a possibility the TRV's are working as designed.  Unlikely, but possible.  Before replacing them it would be worth considering how long it will take for the saving on your gas bill by having these radiators fully cold will pay back the cost of replacing the valves. The savings from turning these radiators off fully may not be as much as you'd hope, as some heat is likely to be 'leaking' into the unheated rooms from the adjacent heated ones.

Secondly (and I think possibly the more likely case) the issue could simply be due to a thermosyphon effect - that rather than heated water entering via the flow pipe (the one usually controlled by the TRV) you've got heated water getting into the radiator via the return (or non-TRV) connection.  If so, that is usually due to a design fault in the system, or just bad luck.

It would be worth trying to get to the bottom of why one of the radiators is hot(ter) on the non-TRV side.  This could be because the installer has simply put the TRV on the return side of the radiator, or it might confirm you've got a thermosyphon situation going on.

FreeBear

Section62 said:

Gazelle1985 said:

Veteransaver said:

Are the trvs turned right off? And not just to the frost setting (snowflake symbol).
If they are right off then it will be a knackered valve or maybe some debris in the valve allowing a little bit of hot water through so you could try to dislodge by turning the valve on and off whilst the heating is pumping.
Which pipe to the rad is getting warm, the one with the trv on it or the other one?

They're turned off as far as they can go (which is the snowflake - all radiators are the same, but most work fine).

Weirdly one of the radiator pipes which is faulty is hot on the TRV side and the other is hot on the non-TRV side.

The symptoms don't necessarily mean your TRV's are faulty (radiator heating isn't always quite as simple as a binary on/off as some of the previous posts have suggested).

Indeed. I have noticed that as the room temperature reaches the set point, the TRV partially closes and still lets water flow through. Only when the temperature has risen above the set point does the TRV shut down fully, and even then, it does so very slowly.

Gazelle1985

Thanks - yes it occur to me that the potential savings from not heating those radiators are probably less than the cost of getting them fixed!

Albermarle

Secondly (and I think possibly the more likely case) the issue could simply be due to a thermosyphon effect - that rather than heated water entering via the flow pipe (the one usually controlled by the TRV) you've got heated water getting into the radiator via the return (or non-TRV) connection.  If so, that is usually due to a design fault in the system, or just bad luck

Just a wild guess but if this is happening, would it likely to be more pronounced if the pump was running at full speed/faster than necessary?

prowla

I have radiators where both inlet and outlet are connect to the same pipe, with the tap at the top one; turning it off does mean that hot could possibly get in from the other.

I don't think they would get really hot though.

Ganga

I have just changed a radiator in our hall ,turned the lock sheild off by counting the turns then turned the TRV down to closed ,snowflake sign then tried to drain the radiator ,i was getting hot water out of it even though it had been closed down for a few hours ,checked the TRV was not fully turned off by a miniscule amount ,worth checking they are fully closed.

Section62

Albermarle said:

Secondly (and I think possibly the more likely case) the issue could simply be due to a thermosyphon effect - that rather than heated water entering via the flow pipe (the one usually controlled by the TRV) you've got heated water getting into the radiator via the return (or non-TRV) connection.  If so, that is usually due to a design fault in the system, or just bad luck

Just a wild guess but if this is happening, would it likely to be more pronounced if the pump was running at full speed/faster than necessary?

Honest answer is I don't know, I've always put thermosyphonic effects into the dark arts category, and whether or not it is a problem depends very much on the specifics of each case.  For example it could make a difference whether the radiator is connected from the branch of a 'T' or one of the ends.

But as a gut-feel guess, I'd probably expect the effect to be greater (in the single branch pipe case) where the flow in the main pipe is relatively calm (or zero).  The reason for that is the forces involved in thermosyphonic flow are relatively small, and if the CH pump is creating a relatively massive flow in the circulation pipe then it wouldn't take much for the syphonic flow forces to be cancelled out.  Just a guess though.  For a real answer we'd probably need to find someone trained in CFD and a finite element model of the OP's heating system.... which would definitely cost more than just leaving the radiators getting unwantedly warm.

The solution for unwanted thermosyphonic effects usually involves rearranging the pipework in a way that experience suggests will work.... and following the plumbing 'rules', even where the reason for them isn't always apparent.