Megaflo unvented cylinder low hot water pressure into shower

danlightbulb

danlightbulb

danlightbulb

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Hi all.

Any ideas appreciated.

Girlfriend's flat has a megaflow unvented hot water cylinder which feeds a thermostatic shower and the hot water taps.

The flow from the shower is barely a trickle when set to a warm/hot temperature and the shower is 'pulsing' as if it is trying to seek a temperature, but when set to cold the pressure and flow seems fine.

Hot water flow from the other taps is ok but not great.

The water is hot enough, the electric immersion elements work fine.

The system is so simple I'm really not sure what could possibly be the issue. Mains pressure in, mains pressure out surely. Could it be the thermostatic shower unit?

Apparently it's been getting worse since Severn Trent were working outside in the road, but cold pressure and flow seems fine (not measured though).

I'm trying to do the procedure to restore the air gap, but nothing is coming out the tundish. I can hear faint gurgles and clunks coming from the cylinder though. No idea if this would be a symptom of low/no air gap or not.

Any ideas appreciated.

Thanks.
 
Last edited:
danlightbulb said:
Hot water flow from the other taps is ok but not great.
.
Click to expand...
How does hot flow from say the kitchen tap compare to cold flow from the same tap then? Notably worse or just slightly?

If not the same, the problem may not be the shower valve and it points more to the cylinder / associated gubbins.

Has the problem existed since the cylinder was installed or developed since? Or has she just moved into this place?

It could be the inlet combi valve. If these systems aren't serviced properly every year (repressurised, valves and filters checked and cleaned) they do go wrong. If there's an insufficient air gap pressure then when the hot water expands the pressure relief valve will let by and it should then be changed.

If it's rented I bet the landlord hasn't bothered.
 
All taps in the flat are mixer taps. I would say the hot and cold flow is similar.

The bath tap and thermostatic shower valve is all one unit. Flow on cold is excellent but on the middle and hot setting the flow dies and the valve pulses water.

Girlfriend says it was ok before the water company were doing work in the street a few months ago. She's lived there over 10 years.

I saw the shower when I first met her and it was ok but not great, now it's barely useable, so it has for sure got worse.

Originally we thought air lock but this is mains pressure system so I doubt that.

As far as I can see there is no inlet filter.
 
danlightbulb said:
All taps in the flat are mixer taps. I would say the hot and cold flow is similar.

The bath tap and thermostatic shower valve is all one unit. Flow on cold is excellent but on the middle and hot setting the flow dies and the valve pulses water.

Girlfriend says it was ok before the water company were doing work in the street a few months ago. She's lived there over 10 years.

I saw the shower when I first met her and it was ok but not great, now it's barely useable, so it has for sure got worse.

Originally we thought air lock but this is mains pressure system so I doubt that.

As far as I can see there is no inlet filter.
Click to expand...
Anything that looks like this?

Caleffi - 22mm Inlet Control Multibloc Valve Group 533002CST https://www.amazon.co.uk/dp/B016W7I5OM/ref=cm_sw_r_apan_i_RKP7YSRNHZGWRY12HKJE?_encoding=UTF8&psc=1

If she's renting tell the landlord and if not best calling a G3 plumber, as these things are not to be messed with unless repressurising fixes it.
 
Participant said:
Anything that looks like this?

Caleffi - 22mm Inlet Control Multibloc Valve Group 533002CST https://www.amazon.co.uk/dp/B016W7I5OM/ref=cm_sw_r_apan_i_RKP7YSRNHZGWRY12HKJE?_encoding=UTF8&psc=1
Click to expand...

No.

The cold inlet goes straight into the bottom of the cylinder, and the hot comes out the top.

I can't see what could possibly be wrong, there are no mechanical parts to break.

It's not a rental, it's her place. She's never had the system serviced though, but like I said it's just simple pipework.
 
Have you tried draining the tank completely?

I forgot the exact proceedure, it's written on the side of ours, but something like close the inlet valve, run the hot water with the tundish open until the water has completely gone, close the taps and open the inlet valve.

Please check what it says though rather than following the above, I can never remember and have to read it every time.

I agree it's really odd though.
 
BUDFORCE said:
Have you tried draining the tank completely?

I forgot the exact proceedure, it's written on the side of ours, but something like close the inlet valve, run the hot water with the tundish open until the water has completely gone, close the taps and open the inlet valve.

Please check what it says though rather than following the above, I can never remember and have to read it every time.

I agree it's really odd though.
Click to expand...

That's the procedure to refresh the internal air gap.

If I close the inlet valve, barely anything comes out the lowest tap (to be expected, they are on the same level as each other).

And nothing comes out the tundish.
 
danlightbulb said:
Hi all.

Any ideas appreciated.

Girlfriend's flat has a megaflow unvented hot water cylinder which feeds a thermostatic shower and the hot water taps.

The flow from the shower is barely a trickle when set to a warm/hot temperature and the shower is 'pulsing' as if it is trying to seek a temperature, but when set to cold the pressure and flow seems fine.

Hot water flow from the other taps is ok but not great.

The water is hot enough, the electric immersion elements work fine.

The system is so simple I'm really not sure what could possibly be the issue. Mains pressure in, mains pressure out surely. Could it be the thermostatic shower unit?

Apparently it's been getting worse since Severn Trent were working outside in the road, but cold pressure and flow seems fine (not measured though).

I'm trying to do the procedure to restore the air gap, but nothing is coming out the tundish. I can hear faint gurgles and clunks coming from the cylinder though. No idea if this would be a symptom of low/no air gap or not.

Any ideas appreciated.

Thanks.
Click to expand...

The incoming main pressure reducing valve has a strainer, this may need cleaning. Similarly, the shower itself may have nrv's and strainers, see if either the hot or cold have debris.

I assume the cold to the shower is teed from the same cold inlet to the cylinder? (to rule out a blockage in one vs the other).
 
fezster said:
The incoming main pressure reducing valve has a strainer, this may need cleaning. Similarly, the shower itself may have nrv's and strainers, see if either the hot or cold have debris.

I assume the cold to the shower is teed from the same cold inlet to the cylinder? (to rule out a blockage in one vs the other).
Click to expand...

Is that the device just under the isolating valve? I was unsure what this was.

But yes, the entire supply to the flat is after this valve - so the cold water to all taps is tee'd off the same pipe before it goes to the cylinder. The pipework here is so simple hence why Im struggling to think what the problem could be. Could the PRV be adjusted upwards?

The thermostatic shower is a surface mounted Hans Grohe like this one: https://www.victorianplumbing.co.uk...UT5zm-U07R7I32cqikhoCSEkQAvD_BwE&gclsrc=aw.ds

So I assume it has nothing extra fitted in its upstream pipework because it simply isn't accessible.
 
Here is a close up of the device just downstream of the isolation valve:

VNmxOQi.png

But this PRV (if that is what it is) supplies cold too, and when the shower is set to fully cold the flow is strong. If this device was blocked the cold flow would also be poor.
 
Your thermostatic shower will have strainers (and probably non-return valves) on both the hot and cold inlets. It could be the hot has debris.

You said hot water flow from other taps is not great - is it as good as the cold flow from those same taps?
 
Unlikely to be shower if it's happening at every hot outlet to a greater or lesser extent.

It will be one of the valves blocked. Get it serviced by someone that knows what they're doing and can check them.
 
Uhtred said:
could it be the thermostatic shower cartridge needs replacing?
Click to expand...

This was my thought, had the same years ago

BTW air bubble wont affect the flow, its there to soak up the pressure when the tank is heating, no bubble means no compression available to the expanding water.
If your missing the bubble the pressure relief will kick in on heating, and some water will vent down through the tundish

After a decent heating cycle check the tundish to see if any evidence of water. Or depending where it goes outside you may be able to just stick something like a mug underneath to test it.
 
fezster said:
Your thermostatic shower will have strainers (and probably non-return valves) on both the hot and cold inlets. It could be the hot has debris.
Click to expand...

Is this built into the surface mounted unit itself, or separate parts built into the pipework feeding the unit? If the latter, then this pipework is behind a tiled wall and completely inaccessible.


fezster said:
You said hot water flow from other taps is not great - is it as good as the cold flow from those same taps?
Click to expand...

Yeah its about the same, none of the taps flow great but I think its because they are mixer taps.
 
Participant said:
It will be one of the valves blocked. Get it serviced by someone that knows what they're doing and can check them.
Click to expand...

i dont think there are any valves extra to what Ive already shown. There are no obvious valves on any of the pipework leading off the hot water cylinder supply side. If there are any in the bathroom itself then for sure they are inaccessible behind tiled walls so shouldn't have been put there.
 
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