Change of radiator of Hive TRV?

[NorthernLights811]

My ensuite bathroom has 2 external double brick walls and a large window. Naturally, in the winter, it's the coldest room in the flat by around 5 degrees, and very humid. It's also the only room with a single radiator, everywhere else it's a double. I can manage the heating of the entire flat about ok with the manual TRVs but that room stays cold the whole time.
I was thinking of installing Hive TRVs (I already have the thermostat and the hub) so that I can control each room's temp, including the bathroom's, so that it remains at a certain temp. I would get the benefit of being able to control the temp everywhere in the flat, but most importantly in the bathroom. However, to do that, I'll have to replace all the rads pipes, because the Hive TRVs don't currently fit (I tried, and I also called Hive's customer service).
On the other hand, plan B, I can get away with just replacing the radiator with a double one and have a warmer room?
Any suggestions as to plan B would work and save me the hustle of re-doing all the pipework?

 

[stem]

If the room isn't being heated by the radiator its output is not sufficient for the required duty.

Adding a Hive TRV to a radiator won't make it emit more heat, It will just switch the radiator off when the room has reached the selected temperature. Exactly the same as your manual TRV's do.

 

[NorthernLights811]

If the room isn't being heated by the radiator its output is not sufficient for the required duty.

Adding a Hive TRV to a radiator won't make it emit more heat, It will just switch the radiator off when the room has reached the selected temperature. Exactly the same as your manual TRV's do.

I mean it is bring heated, just not sufficiently, so I guess you're right.
I thought manual TRVs just adjust the amount of hotnwater circulating in the radiator, no?

 

[stem]

TRV's start off fully open, provided that the room is below the selected room temperature. As the room approaches the required temperature the TRV will start to close and reduce the flow of water entering the radiator, so the radiator will the start to cool down and emit less heat.

In your case, as you say the bathroom "stays cold the whole time" that would suggest that the temperature set on the TRV has not been reached and therefore the TRV is fully open. Therefore, the radiator is emitting the maximum heat it can, it can do no more.

EDIT
Thinking more about your post:

manual TRVs just adjust the amount of hotnwater circulating in the radiator, no?

I wonder if you are using the correct terminology TRV stands for 'Thermostatic Radiator Valve' which is different to a 'Manual Valve' which is not 'Thermostatic' and works just like a tap that just regulates the flow according to its setting. I had assumed you were using the term manual TRV to refer to the traditional TRV's in contrast to one linked to the Hive app.

In any case, if you were to replace a Manual Valve, with a TRV, provided that the manual valve had been fully opened a TRV will not improve the flow, it will only reduce it as the room warms up.

 

[NorthernLights811]

TRV's start off fully open, provided that the room is below the selected room temperature. As the room approaches the required temperature the TRV will start to close and reduce the flow of water entering the radiator, so the radiator will the start to cool down and emit less heat.

In your case, as you say the bathroom "stays cold the whole time" that would suggest that the temperature set on the TRV has not been reached and therefore the TRV is fully open. Therefore, the radiator is emitting the maximum heat it can, it can do no more.

EDIT
Thinking more about your post:

I wonder if you are using the correct terminology TRV stands for 'Thermostatic Radiator Valve' which is different to a 'Manual Valve' which is not 'Thermostatic' and works just like a tap that just regulates the flow according to its setting. I had assumed you were using the term manual TRV to refer to the traditional TRV's in contrast to one linked to the Hive app.

In any case, if you were to replace a Manual Valve, with a TRV, provided that the manual valve had been fully opened a TRV will not improve the flow, it will only reduce it as the room warms up.

I've got a Siemens TRV, numbered 1-5, I just didn't know that it slowly closes as the room temp reaches the set number. I'm saying this because the rest of the radiators that I set them to say, 2 just heat up less. So I guess it kind of works like a manual tap in the sense that the radiator heats up less, albeit progressively.
How I've used mine is, in a room that I didn't want to be very hot, I must turn the TRV to 2 or 3, which might have the same effect as reducing the flow in a manual tap. I'm also usually boosting my thermostat by 45-60mins so this effect might be more profound, as opposed to having a set temp and constant flow.

 

[stem]

So I guess it kind of works like a manual tap in the sense that the radiator heats up less, albeit progressively.

That's exactly it. As the set room temperature approaches, the TRV will start to close and the radiators heat output will start to fall progressively.

When the TRV reaches a point where the set temperature is reached and the radiators output matches the heat loss from the room it will settle at that point. After that it will only change as a result to a change in the room temperature.