Changing an upstairs TRV - risky strategy?

I assume you’ve tried bleeding the rad?

Tried tapping the valve on the side with a hammer?  That fixed mine

Quite a few vids on YouTube. I've done it before, didn't make too much mess.

I almost posted this exact question the other day. Was getting ready to replace the radiator, or a least take it off the wall to check it or the valves weren't gubbed up. Checked a few things first:
- Bled, no air.
- Turned lockshield and TRV off, bled til pressure was lost and stopped, so valves were both working in off position.
- Tried bleeding with TRV open, lockshield closed and vice versa, and found water was flowing fine through both ends.
- rad was still cold, so I turned down all the other radiators a bit to push more through the cold rad.
Now it's fine and our oldest is sleeping better now his room isn't cold!

Edit: I watched a couple of this chap's vids on valve and rad replacement. I'm not a plumber, but he seems to do a good job of covering off a lot of little mistakes you could make, while not doing any more than you need to.

No. Or not quite.

If you have assumed correctly that the TRV is blocking flow and there is pressure in the system, then that pressure will be released onto your floorcoverings when you remove the TRV. You can release this overpressure in a less messy way by running a little water off somewhere more convenient to catch it. Switch off and let it cool. Rads off and lockshields closed upstairs as you say, then take a couple of pints out from a downstairs drain cock if you have one, or from the flexible fill loop of your combi boiler if you have one of them, or even a downstairs radiator bleed valve.

I assume you’ve tried bleeding the rad?

... And maybe a wee bit of careful percussive maintenance in case the valve is just lightly stuck.

Your approach is sound enough but make sure your trvs are actually off, the adjuster on most won't fully close the valve and you'll have to get a spanner or similar on the valve after taking the trv head off.

And make sure you've turned off the fill valve if there's a gravity tank or equivalent!

Is the lockshield valve working?

My first port of call would be to make sure that was fully open too, and perhaps close the valves on the other radiators to see if I could force water through that way.

Thanks gents.

To answer a few of the (quite right to ask) obvious questions.

Yes - the radiators are bled.

I've also tried a little percussive maintenance.

I turned up the rads to full whack last night, and every other one got roaring hot. This one isn't at the end of the circuit as far as I can tell, so I don't think its just badly balanced. I will try closing all the others off and seeing if I can force heat to this one though, that's a good idea. (for a short while at least, don't want to overheat the boiler if no flow)

Lockshield valve is nice and free, but good point about that poss being the blockage.

And finally, yes, I had thought to de-pressurise the system as part of my plan, just forgot to list that line in my method statement!

edit - and no header tank to worry about. Standard combi-condenser set-up.

Boiler won't overheat, it'll just shut off.

You might find the rad is a bit blocked internally. If you can't get flow with everything else off, that'd be my next port of call - close both valves, remove rad, flush it out, then try again.

With the rad off you could also easily assess whether the valves were allowing flow...

if you take the TRV head off the valve body, it should default wide open. That'll help you determine whether the valve is knackered (stuck) or the head (it always thinks its hot)

changing the head without the valve is a 10 second job, changing the valve is a faff, probably involves draindown (I guess you could do it without by freeze spraying the pipe?

Have you tried swapping the TRV head with one from another radiator? 30 second job. Mine seem to be gradually packing up after 8 years use, and need replacing. Worth trying before you do anything drastic...

I'd certainly be turning all the other rads off first to see if there's an air lock somewhere - this worked for my downstairs cloakroom that hadn't had heat for about 4 years!

having tried this before i found draining half the system the best method.

It came out at quite a pace after i removed the valve.

Yes, I've had the head off the TRV, but I've not yet tried to swap it with a known working one.

My assumption about the open position of the pin was the wrong way round - so this has probably already helped me avoid a greater spillage than anticipated!

edit - now I know that taking off the head means the valve is open, that's an easy way to prove the head is faulty or not.

I don't think the valve is open when the head is removed. My radiator wouldn't work until I screwed a working TRV head back on

I don’t think the valve is open when the head is removed

Yeah, that's only true if the valve isn't stuck closed or partially closed. Which is apparently not the case here.

The little plunger does to up and down but there’s no heat in the radiator

If the valve spindle's moving freely with the thermostatic head removed, it's unlikely to be the trv, unless it's completely blocked with shite (that's the technical term for sludge and rust). Normally if the spindle's fully up the valve is full open. They usually come (when new) with a screw cap which replaces the trv head to shut them off completely for maintenance.
Has the trv been fitted at the wrong (outlet) end of the rad? Or did it work until recently?

If the valve spindle’s moving freely with the thermostatic head removed, it’s unlikely to be the trv

I could be wrong but I think the pin exposed by the head removal bears on the top of the thermostat capsule, it's not attached to it. If the actual valve is stuck closed the pin will be free to move, the giveaway is that it will move down freely whereas with a functioning valve there will be resistance because it has to push the valve closed.