New shower head cold water only?

[Sunnysuns]

In on suite shower cubicle there is old shower , it’s not electric shower, it’s connected to the main system/ combi boiler.
The old head is dripping and it’s looking old so I ordered nice shiny shower head from Amazon, excellent reviews etc.
Put it on and water was coming out lovely, just like a waterfalls or rain, 5 modes, lots of water in great pressure, it all looked perfect except it was only cold freezing water coming out!
I can’t understand why as I only changed the head.
What happens normally when any hot taps or showers in either bathrooms get turn on, the combi boiler downstairs comes on instantly with blue light and warms the water.
So when only cold water was coming out I checked the other taps,they all had hot water ok.
I run downstairs quickly to see if the boiler came on while I had the shower on and nothing, it seemed the boiler hasn’t registered the tap being on at all, hence freezing water.
I looked online and I read however I still don’t get it.
The old shower head was from Argos or so, just cheap one, now it seems I can’t just get any replacement head, needs to be some specific one?
The old shower head also have no any good pressure, clogged with limescale, not a proper pressure anyway, the new head one was amazing, with lots of water coming out..
I just don’t get it..can anyone tell what I was doing wrong?
I put the old head back on, hot water instantly..

I just don’t get why the new shower head didn’t tripped the boiler to come on, especially with the lots of water coming out, I would say the old head has poor pressur,pe anyway…arrrgh I don’t get it..

Not all shower heads are compatible with all systems (not just electric). Is it one of those water saving ones that increase the pressure whilst using less water for example?

@TheCrenchinglyMcQuaffenBrothers i wouldn’t think so, just an old shower (25 yrs old minimum ) in rental, it’s nothing fancy at all. When we changed the head about 8 yrs ago, it was the cheapest one available.

@TheCrenchinglyMcQuaffenBrothers plus the old shower head it’s really poor pressure, I was so happy when the new head run like a waterfall, however just cold.

What happens if you run the shower with no shower head on the pipe?

I meant is the new shower head you bought a water saving one.

If so then it's probably taking longer to draw the hot water through from your combi system. How long did you leave it running? Does it get warm eventually if you leave it running for a few minutes?

In any case, based on your description of what is happening, it will be something to do with the flow rate of water through that particular shower head (as I said, it may be set up as a water saving device or something) and it not registering with the boiler as needing hot water.
Try running the shower and then running the hot tap in your bathroom a little bit at the same time. Does the shower get hot then?
Look inside the shower head - the end that connects to the hose - is there a little flow controller fitted, if so can it be removed? If it can try using the shower without it - do you get hot water then?

Ours was the flow. It only ran hot for ages. I had to get the plumber to come and adjust the flow but I have no idea how he did it.

@TheCrenchinglyMcQuaffenBrothers it was this one, I already sent it back as I thought it’s faulty . The cold water didn’t come at all, even after long time.
@summeroccupation If I remove the head and only have the tube, hot water comes ok.

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@TheCrenchinglyMcQuaffenBrothers I meant the hot or warm water didn’t come at all, even after long time.
I never used the hose which came with it as only needed the new head.