Only top of radiator gets hot

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Hi all, I am new to my 2016 flat and only just tried the heating recently.

The radiators all seem to work, except one in the hallway that takes ages to get going.
After some time, the top gets very slowly hot
TRV is at the top right, outlet (?) is at the bottom left.

it's almost as though water is ever so slowly warming up inside, but it isn't evacuating out the other end. (or is super slow)
So I think it must be the valve (?) at the bottom left?
Do I need to open the bottom valve somehow? if so, what should I do? I've only used TRV's before

-not sure if relevant, but this week we had a guy move a radiator along a wall and replace with a new one (more stylish) so some fiddling has gone on with the system, but the one I'm complaining about shouldn't have been touched.

Thanks
 

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Remove the white actuator head of the TRV. There is a pin in the centre of the valve body ,see if it moves up and down freely, leave the actuator removed,and see if rad heats fully
 
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This symptom is indicative of inadequate flow through that radiator. The cause is more difficult to determine at this stage.
Suggest you report back after following @terryplumb 's advice. Useful info is:
1. Power of boiler in btu/hr or kW.
2. How many storeys are heated; assuming one in a flat!
3. How many rads on each floor.
4. Whether or not any other rads are performing unusually well....your fancy new one perhaps?

MM
 
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Regarding the guy who installed our new radiator, is this something he should have checked before signing off? Something I should go back to him with?

-new rad works fine
we are over two floors (in a block of flats 5 high)
1 new rad upstairs
1 rad upstairs turned off (as behind a big book case)
1 rad turned off downstairs (as behind bed)
1 rad downstairs working fine
1 rad downstairs, (this one I'm talking about)
 
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You have 5 rads in total, only 3 are in use.
You have not supplied any boiler information. I'm not asking just to be nosey, it's relevant!
Do you have a hot water cylinder?
Do you have an automatic bypass valve (ABV)? Do you have a manual by-pass valve?
Do you have any other control valves, motorised or not?
What does 'new rad works fine' mean? What is the pipework temperature at the hotter end? What is it at the cooler end?
Poor flow with only 3 rads in use is an extreme case..... try the following.

1. Turn off the new rad. Any improvement to the downstairs rad?
2. Turn off the working downstairs rad. Same question, does it improve the poor rad?
3. Turn both back on, does the poor rad revert to being poor?

MM
 
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Sorry, I missed out those answers. Apologies.

We actually have no boiler.
Our flat is a nice new(ish) build and has centralised hotwater/heating/boilers that is piped in and is metered. (theres a building down the road that pipes it over the estate)
There are two inlets for the house, hot water and heating, each has a big tap that affects flow/pressure. They were on full.
We also have a Honeywell thermostat/timer which also regulates the heating further.
Most rads have a TRV, except the one turned off behind the bookcase.
ABV? that I don't know
New rad heated up in good time, temperature was as expected - (its a new 5ft vertical rad) TBH, only turned it on for the first time last night since its installation, so I don't know if it was "too good" or what too good would be.

As for you 3 points for fault finding I will try them over the next couple of days (I am not home much so testing will be slow). but thanks for the suggestions, will try.

Thanks for your replies though. Much appreciated
 
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Me again. finally had a chance to do testing.
With all rads off, the dodgy rad was slow as hell to warm up.

But I then went and did what Terryplumb mentioned, took the TRV off and noted the tiny metal spike didn't depress easily.
Gave it a bit of effort and it sprung into place. Seems that it has fixed the problem. But if there's anything else I am sure I will be back.
Thanks everyone.
 

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