I take your point about measuring the water flow out of the head and not the bottom of the valve but the shower doesn't work if I drop the shower head into the bath. It was just easier to do out of the valve.Is that how you start/run the shower usually? A true test would do it at the height that the shower is normally is at with the shower head on, that gives you an accurate representation of L/Min in normal use
Your hot water supply is the problem here, the thermo valve when opened starts with mostly hot to first and then starts to add cold as the hot supply heats up until up to temp. If there isn't enough flow at that point, from the shower head, then it won't start the pump.
The only thing that's changed here is the shower itself, so that's where your problem lies. If you had a bar mixer before chances are it was a thermo too but we'll never know. No or faulty NRV's can allow the pump to drain back to the cylinder when another hot outlet is opened but they can restrict the flow a little too as @steve32 mentions, removing them may allow the flow to increase enough but could also exacerbate the air issue in the hot supply, which is what you are suffering from.
If you can try this test too. Turn off the power to the pump, turn on the shower, turn the pump back on, does it run and keep running?
If I set the shower away without the pump running nothing comes out of the valve in its centre temp position. Unless I bleed the pipes in the loft then it does. Then if I turn the pump on it runs ok. Leave it a short while though and it doesn't work.
I slackened the nuts on the back of the valve with the pump on and obviously water came out of them and then the the pump started. So I tightened the nuts up and the shower worked fine on all temperatures. Left it 5 minutes and it would only start on full cold. I know that if I then left it say overnight it wouldn't work at all without the bleeding to get it flowing.
I've got a 22mm one way valve but I haven't fitted it. It will be a bit of hassle and salamander only show a NRV when the pump is above the shower head height. Do you think it will make a difference if I fit it just above the hot outlet?
Was in B&Q today and they were selling of loads of taps and showers. I bought a basic single lever valve for only £15 and fitted that. It works perfectly however long I leave it and it doesn't need any bleeding. When I first switched it on it sputtered a bit and that was it. It looks sh*t, it looks like a single lever kitchen tap sticking out the wall but at least it operates the pump. I haven't given up with the other shower but at least I can fit that knowing I won't get grief off the wife when she can't have a shower.
I've still got the previous bar mixer, how can I tell if it's thermostatic? It has a button on the temperature knob that you have to depress to go past a certain temperature but I don't know if it's thermostatic or not.
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