Megaflo unvented cylinder low hot water pressure into shower

[danlightbulb]

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.

 

[danlightbulb]

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]

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

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.

 

[danlightbulb]

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.

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]

 

[danlightbulb]

Is there a shower pump in the loft?

No, it is a complete mains pressure system, hot water only (no central heating).

 

[danlightbulb]

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).

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.

 

[danlightbulb]

Here is a close up of the device just downstream of the isolation valve:

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.

 

[danlightbulb]

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.

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.

You said hot water flow from other taps is not great - is it as good as the cold flow from those same taps?

Yeah its about the same, none of the taps flow great but I think its because they are mixer taps.

 

[danlightbulb]

It will be one of the valves blocked. Get it serviced by someone that knows what they're doing and can check them.

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.

 

[danlightbulb]

Do you have a model number for the shower?

It physically looks the same as this one but I don't know the exact model number. https://www.victorianplumbing.co.uk...vcGOoM3spL_dA6owblxoC82AQAvD_BwE&gclsrc=aw.ds

The shower valve can be removed from the wall and will most likely have integral strainers on both inlets.

Although, as I asked above, do the taps have poor hot flow compared to cold, or is both hot and cold flow to the taps poor?

All that comes out from the wall is two pipes, and the shower unit like the above just (by the looks of it) compression fits onto these tails. This is a combined shower/tap unit so the tap function has the same issue as the shower function.

The kitchen and bathroom sink taps are both mixer units, so are fairly low flow anyway. But as far as I can tell, whether on hot or cold the pressure/flow is similar.

The bath tap (combo thermostatic unit), if switched completely to cold, has a very powerful flow and pressure. It is when the hot water is introduced that the flow dies and pulstates.

 

[danlightbulb]

You can test this pretty easily by turning the hot water tap partially off.


Have you messed with anything recently OP? Like switched the tap off? Could be you've opened it 'too much'.

What tap do you mean? If you mean restricting the hot water flow from the cylinder, it can't be done - there is no valve on the inlet or outlet of the cylinder that doesn't also close off the cold water supply.

The only thing that my girlfriend says has happened in the last few months is that Severn Trent were working in the road (major diversion works going on in the area right now) and as a result of this the water supply was cut off on a few occasions over several months. She thinks the pressure is lower than it was before (but I think still adequate for the shower as the cold water flow on its own is strong).

We've not messed with the PRV on the inlet (if that's what it is).

Girlfriend is going to estimate the hot water temperature tonight using a basic thermometer held under the kitchen tap.

 

[danlightbulb]

I discounted an air lock as its a mains pressure system, and hot water flows from the other taps.

Maybe its just a faulty thermostatic shower unit. The only mechanical moving part on the whole system is the thermostatic shower valve.

 

[danlightbulb]

The issue was the shower inlet strainer/filter mesh on the hot supply inlet to the shower valve.

Was clogged up with greeny blue residue. Cleaned and reassembled, shower is back to full power.

I also removed what seems to be a flow restrictor/aerator in the bathroom sink tap which has improved things massively.

Was hard to tell what was going on when all taps were so poor flow. Bloody strainers and restrictors fitted no wonder things were poor.

Nothing wrong with cylinder at all.