Changing TRV on radiator in loft

[andrewparker]

Hi guys, this is my first post on DIYnot. I'm a total novice so I hope you'll be kind on me!

I have a few questions I hope someone may be able to answer, all concerning a radiator in my attic room.

The radiator has a TRV fitted to it but it seemingly either kicks out full heat, or no heat at all, even when the dial is set at medium. Is this a faulty TRV?

If so I will need to replace it, and use the opportunity to decorate behind the radiator.

I have read tutorials on how to replace the TRV, which looks fairly straightforward, however I am unsure as to how to drain the system, or whether I need to.

We have a combi bolier and I'm pretty sure there are no bleed off valves, however there are two isolating valves on the pipes that leave the boiler.

Any help is much appreciated!

 

[D_Hailsham]

The radiator has a TRV fitted to it but it seemingly either kicks out full heat, or no heat at all, even when the dial is set at medium. Is this a faulty TRV?

Has it always been like this or is it a new problem?

If it's ongoing, the system may not be properly balanced; but, if a new problem, the TRV may be sticking.

Remove the TRV head - usually a knurled nut connects it to the valve body; no water will come out! This will expose a pin in the top of the valve. You should be able to push the pin down easily with a piece of wood and it should spring up when you release the pressure. If it sticking a drop of WD40 may help. If it is stuck down, a tap on the side of the body may release it; failing that use a pair of pliers to lift the pin - be careful not to bend it!

If the pin is free, the problem lies with the thermostatic head. You may be able to buy just the head on its own - depends on make.

 

[andrewparker]

Hi, I'm fairly sure it is a new problem. We used the room 12 months ago when we moved in but we have recently renovated the loft space and noticed the problem when we turned the radiator back on. Could excessive dust etc cause the problem you described with the pin?

Thanks for your help!

 

[D_Hailsham]

Could excessive dust etc cause the problem you described with the pin?

Very unlikely. Have you checked the pin is free?

Pins tend to stick after the summer because the hot weather will close the TRV and keep it closed. This usually happens if the TRV is left on 3 or 4, which is about 16-20°C. The solution is to turn the TRV to max (about 30°C) when you turn the heating off.

 

[gavinda]

a trv will only kick out full heat or none at all. try shutting the door to the room and seeing if a different setting on the trv alters the overall temperature of the room. ie try an hour on 1, then on 2 and so forth. some folks have the misunderstanding of trv operation and are led to believe/ believe that the trv controls the temperatur eof the radiator.

 

[homes]

while warming up to the setting required it will get quite hot ,even set on 1
as this is the hot water getting in to radiator,then when demand temp. is reached it will close the valve .










DAVE

 

[Aquaheat]

The radiator has a TRV fitted to it but it seemingly either kicks out full heat, or no heat at all, even when the dial is set at medium. Is this a faulty TRV?

Thats exactly what a TRV should do.

People seem to have this misconception that TRV's will control the temperature of the water in the radiator when this is actually a phallacy.

A TRV picks up the temperature of the room and mid point is normally around 21 degrees. So if your TRV is numbered up to 5 then number 3 will be about 21 degrees.

If the room is below 21 degrees then the radiator will be on. The temperature of the radiator will be dictated by the boiler stat.

When the room reaches 21 degrees then the TRV will shut down and turn the radiator off until it senses that the room temperature drops below 21 again.

 

[andrewparker]

Ah right, I did say I was a total novice!

So the room is still getting incredibly hot, even when the TRV is set to number one. I've checked the pin and it does moves in the valve. I'm able to push it down with my finger and it pops back up again. Would buying an identical TRV and just replacing the head work?

It's really important that I can control the temperature in the room as it is soon going to be used as a nursery

 

[gavinda]

as long as you can get an identical head then possibly. buy an identical valve, put just the head on and see if that works. if not, it'll be the body thats knacked and you'll need to do some wet work. remember if you drain the system you'll need to add some inhibitor.

 

[andrewparker]

The TRV is a Pegler Terrier II. On Screwfix they come in 10mm and 15mm Angled variants.

How do I know which I need and what does this measurement refer to?

 

[Steve69the69Gas]

10 and 15 refer to the pipe diameter they are connected to

 

[Softus]

The TRV is a Pegler Terrier II.

Call Pegler Customer Services and ask them how long the warranty is. They will send you a new head if the valve is not very old.

 

[andrewparker]

It's possibly not worth it. We've only owned the house for 12 months and the TRV might have been on that radiator for many years. They're only £8.22 at Screwfix.

 

[gavinda]

dont ask. dont get.

 

[D_Hailsham]

a trv will only kick out full heat or none at all. ... some folks have the misunderstanding of trv operation and are led to believe/ believe that the trv controls the temperature of the radiator.

You are suggesting that a TRV acts like a switch, it's either on or off; but, according to info from the Honeywell web-site, this is not correct.

The medium passing around the sensor of the thermostat causes the sensor to expand when the temperature rises. The sensor acts onto the valve spindle and this causes the TRV to close. When the temperature falls the sensor contracts and the spring-loaded valve spindle is opened. The TRV opens in proportion to the temperature of the sensor. Only the amount of water required to maintain the temperature set on the thermostat can flow through the valve.

Drayton TRVs quote a sensitivity of 0.22mm/°C; i.e. the valve spindle will move 0.22mm for every degree change in temperature.

The temperature difference between open and closed will depend on the differential pressure across the valve. If the DP is low, the difference will be about 1°C; but if it is high, the difference will be 2°C or more. This means that if we set the TRV to 20°C the valve will be fully open at 18°C and closed at 22°C. One of the reasons for the poor perceived performance of TRVs is that the differential pressure across the valve is too high, so they have poor control over the room temperature. The differential pressure is determined by the pump setting and the lockshield valve.