Swapping radiator valve from manual to trv in bathroom.


This is more of a plumbing question than a Tado one specifically.

I’ve installed a bunch of Tado smart trvs throughout our house and have decided now to get one for the bathroom, only to find it’s a manual valve body which I understand will need to be swapped for a Tado friendly one.

I bought a 15mm drayton trv body online with the plan to install myself but the nut is a lot smaller than the one attached to the rad and I have a feeling that i need a larger trv body. I’ve read there are larger 22mm valves on some bathroom rads but looking online i can find any trv bodies that fit that size, only 15mm ones.

So a couple of questions:

Are my assumptions above correct? Maybe a 15mm trv body could work, maybe I dont even need one.

Do 22mm trv bodies exist or are they only manual, if that’s the case are there adapters/alternatives to getting the smart trv attached?

I’m happy to replace the valve to get the Tado trv on, but if it’s a case of replacing the whole radiator I might have use this new smart trv elsewhere... but Im thinking there must be a way.

I supplied some photos to show the rad and current valve

Im obviously no plumber, but happy to try a job like this before getting a pro in.

Cheers, Ed


  • I’m not a plumber either, but it’s my understanding that those kind of towel rails aren’t usually fitted with trvs. That might be why there aren’t many options out there.
  • Thanks @johnnyp78 .

    Yeah I think thats the thing, that this is a towel rail and not really a radiator.

    I might be better of swapping it out after all.


  • GrilledCheese2
    GrilledCheese2 ✭✭✭
    edited November 18

    @decoy the valve currently on your towel rail has a 3/4" union connection, but I guess your new valve has a 1/2" connection. You need to unscrew the old tail from the towel rail and replace it with the new 1/2" tail that came with your TRV. If you don't want to remove the old tail you will have to search for a TRV with a 3/4" union connection and 15mm compression for the pipe. If you're not familiar with plumbing it will probably be easier for you just to purchase a new TRV.

  • Quite often the towel rail in the bathroom is deliberately left without a TRV. If there is a thermostat calling for heat but all the TRVs in the house have reached temperature and closed the radiator or towel rail without a TRV provides a path for the hot water to flow through. It can be very bad for the boiler to be firing and the pump running if there is nowhere for the water to flow.
    Tado should know when its valves have closed and stop calling for heat but commands can take time to be transmitted.
    A lot of new boilers are fitted with an internal bypass valve to cope with this situation but this needs setting up when the system is commissioned.
    In my case the radiator without a valve is in the hall and I would like to put a SRV there but I dont know if the bypass was correctly set up when the system was installed. As they made no attempt to balance the 16 radiators I doubt it was. Boiler service is due in a month so going to ask engineer then.
  • As said, you need to change the tail (short metal pipe goes into the rad, has an olive on it, the trv secures to this) the new trv will come with this.

    It will mean draining the rad.
    Go round, lock off every rad lockshield and trv that is on the same level or above. This will mean you only drain that rad and pipework to it. Not every rad.

    Bedore you do this, you need to know how to refill the system. You may have an integral filling loop etc. Make sure you work this out first. Read instructions for boiler. This will list the options, and you can work out which you have.

    As for should you have a trv.
    The room with the stat typically shouldn't have a trv by plumbing guides. This is two fold. So it's not fighting against the stat, closing before stat wants heating off. And also as a bypass as the other poster said. If the boiler is on, or has been on, and there is no where for the hot water to flow, because every rad is shut. It can damage the boiler. But, it's very common now for boilers to have internal bypass valves to negate this.
    Of you read the installers manual, it wil tell you.

    IE my Greenstar says to have no trv where stat is. And that it also has a bypass valve.

    It's common to not have TRVs on towel rails, purely to be able to dry towels! If the room is warm enough, the rad may never come on with a trv.

    Personally, I have a tado trv on my towel rail, because I don't want an unregulated room blasting heat.
    I also have a trv where stat is (will change when I replace rad, this is how it was when I moved in) but its set to max so doesn't mess with stat operation.

    As for
  • @Rom the rule for no TRV where there is a wall stat does not apply to smart stats. You can group the TRV and wall stat in the same room using the app. The wall stat will be the sole temperature sensor for the room and the TRV becomes a simple on/off valve that is controlled by the wall stat. If the TRV has reached the scheduled temperature the radiator will stop heating, but other smart TRVs can continue to call for heat and will heat their respective rooms.

    This will mean all rooms have a regulated temperature, but as you stated there needs to be a bypass valve somewhere on the system.

  • Another point - radiator systems should have been balanced when they were installed, this is done using the lock shield valve. Water always takes the easiest flow path so if all the lock shield valves were fully open all the hot water will try to flow through the radiator nearest to the boiler and the radiators furthest from the boiler (by pipework length) may never heat up. On our balanced radiators the lock shield valve on the closest radiator to the boiler is open 1/4 turn. The furthest is open 3 turns.
    When you isolate radiators using the lock shield valve it is prudent to take a note of how many turns each valve takes to close so that you can open them back by the same ammount afterwards. Balancing radiators takes a lot of patience, dual thermometers to measure temp into and out of a radiator and a good knowledge of the pipe runs to work out each radiators distance from the boiler to get the order correct. On a well balanced system all radiators should heat up at the same time presuming all the TRVs are open.
  • Thanks for the responses, this is hugely helpful.

    Apologies for the long response but we have a boiler inspection in a few days so I want to be armed with the right questions to ask the engineer about where its all at. I also asked for someone familiar with the Tado system so Im hoping theyll come with a fair bit of knowledge.


    Great! I think this all makes sense. I already have the ½” valve so I might as well replace the tail (I actually couldn’t find different sizes of TRVs out there, perhaps searching the wrong thing, but Im happy to swap out the tail)


    Indeed. My partner is incline to leave the towel rail valve on in these cold mornings hence me wanting to get a tado TRV for it as Im worried about this rad running everytime the boiler turns on.

    I understand my boiler has an internal bypass (Greenstar 37cdi combi). I need to read more about it.


    Draining the rad… I have some questions about this that Ill drop at the bottom.

    I have my stat and 3 trvs in our open plan living/kitchen area. They are all grouped and appears the stat ‘masters’ them automatically so theyre not working against each other. I think this is what @GrilledCheese2 is saying (If I understand correctly?)

    So, slightly off topic but relating to draining/balancing the radiators noted by @Kissarmy and @Rom .

    I actually removed a rad the other day (after watching several youtube vids) to strip wallpaper behind it. Its a large open plan room with 4 radiators, the one I removed has been off (as it was leaking) since we moved in about 6 months ago. As its rusted I dont plan to put the old one back until I replace it.

    Its ground floor, I didn’t lock off the other rads on that floor nor upstairs (as mentioned above), only this one. I stopped the boiler, drained the rad and then closed the valves and capped them.

    I then repressurised the boiler back to 1.5 as per manual and as far as my knowledge goes (again, little) this should be it, that is- if the pressure remains around 1.5 (and rads are heating properly) everything should be fine- this likely a naive understanding. Pressure has remained stable and rads are functioning as expected… But reading the above perhaps this isnt the case(?)

    I eventually intend to strip all the wall paper and repaint downstairs which will require removing all the rads, temporarily, perhaps one by one if wiser, so Im hoping to get a better understanding of all this once i get there.

    Thanks again for any assistance/info

  • @decoy Have you finished the job? Would you mind posting a picture of what it looks like now? I have a towel rail exactly like yours, without a TRV, and am trying to work out of I can do the job myself.

  • @overratedcarbon I might have a go this weekend if I have time. Will let you know the level of disaster it was :)