Rad TRV seems to have only 2 settings - on or off

[nigelad]

My bedroom rad currently has a [bi-directional] valve on the return side. The TRV effectively has 2 settings - on full or [set to 1] off.
I've checked the pin isn't sticking and tried a different TRV head with no difference.
Any idea what the problem is?

Would swapping the rad feed pipes over so the TRV is on the flow side make any difference to this problem?
Thanks for advice.

 

[CBW]

It might be the trv itself, or the head, some heads are compatible some aren’t.

 

[ericmark]

I had problems in mothers house, the hysteresis was huge, the valve did work, but when it called for heat it over shot the mark by a good margin. I had electronic heads, so they reported both target and current in °C so easy to see some thing wrong.

I tried different spacers, and pound coins on the pin, non seemed to work, then I tried adjusting the lock shield valve, and as I closed it the over shoot reduced, so set at 20°C I closed the lock shield until over shoot just reached 21°C, and I repeated for other three heads, there seemed to be no problem hitting 20°C even on very cold days, so I closed just a tad more.

From that point all the four radiators with electronic heads were spot on the setting, so I removed one head and fitted it to all other radiators in the house, adjusting the lock shield valve until target and current matched, then replaced with original wax head, once the lock shield was set, the standard wax head kept the temperature very close to setting around half way 3 to 4 gave me 20°C.

After the event the reason was easy to see, with the TRV on the return the radiator gets stinking hot before the heat reaches the head. The cheap eQ-3 electronic head has 4 error codes, first is battery symbol which shows battery needs changing, but the other 3 are really handy working out when a valve is faulty, F1 valve drive sluggish so warning it's starting to stick, F2 actuating range too wide, likely not screwed home on valve, and F3 adjustment range too small likely valve has sized solid.

With a °C setting so no guess work, it is so easy, I fitted blue tooth version at £15 each, the non bluetooth found at £10 each, at that price why fit wax? I have bedroom temperatures set so radiator not running in the day. I have 9 brand new never used wax heads, never likely to be used. OK those I brought from mothers house were far more expensive at around £45 each, the cheap ones only show target they don't show current.

However the point is likely it needs lock shield adjusting with the wax or liquid heads you have no idea if working or not, with the electronic you can hear them adjusting, no loud, but you know they are working, and you also know if jammed with cheap ones, oddly the expensive do not tell you if jammed or if reached end of travel, in many ways the cheap ones work better.

 

[stem]

They can fail. If it's the type that uses a wax capsule to close the valve off and some wax leaks out, then it won't close the valve off when up to the set temperature. Do you have another radiator with the same TRV that is working? If so swap the heads over. If the bedroom radiator now acts as it should then you know the TRV is faulty.

Having said that if your room is cool (ie below the temperature represented by '1' on the dial) then the radiator will come on full on that setting. A TRV is a thermostat, not a heat regulator, it operates according to the room temperature, not the radiator temperature.

 

[nigelad]

Thanks all. It's not the head cos I bought a bluetooth one, but had the same problem. Cheers ericmark - I'll try experimenting with the lock shield setting, so long as it doesn't throw the balancing of system out.
I might also go for swapping the flow and return on that rad, so valve is on flow.

 

[ianmcd]

You need to understand how a TRV actually works, the valve is simply that it is a valve controlled by the temperature of the air in the room, if the air in the room is below the temperature that the valve is set at , the valve will allow the water from the boiler in to heat the rad, once the AIR temp is reached the valve will close the valve and the rad will go cold, it can not heat or cool the water going into the rad , it can only allow it in or stop it, nothing else, if you had 5 rads and each one set differently from 1-5 and all rooms were cold enough, every rad would be at the same temperature until the air in the room reached a temperature that the valve was set at then the rad would go cold

 

[CBW]

Also bear in mind, it senses the air temperature around the valve head, so any curtains, furniture etc can affect how it operates.

 

[D_Hailsham]

My bedroom rad currently has a [bi-directional] valve on the return side. The TRV effectively has 2 settings - on full or [set to 1] off.

That's typical of a system which has not been correctly balanced; the TRV acts like an on-off switch.

Balancing instructions tell you to adjust the lockshield until there is a 20C drop across each rad (or 10C/11C if it's a older system). That's fine if each rad is precisely sized for the heat loss in the room; but in many cases the rad are oversized so the rad is giving out way too much heat, meaning the TRV has to close right down to maintain the desired room temperature. Ericmark's method of adjusting the lockshield to give the required room temperature seems a good idea; but I'm not sure how the TRV should be set while balancing.