Unvented cylinder reduced flow rate

[GQuigley67]

When you first open a tap pressure and flow rate is good, it then dies down like a combi would reducing from approx 20 l/min to about 10 l/m. Balanced cold doesn't have this issue, expansion vessel replaced due to being burst, couldn't clean filter in combination valve so temporary bypassed it and same issue, I can only think there must be a blockage in the cold feed?

 

[Best]

If it is happening on all hot taps then I guess it must be either the cold feed or the hot outlet blocked partially. Connections at tank need checked for blockage also.
Is there any iso valves on hot outlet?

 

[GQuigley67]

alL hot outlets, balanced cold ok. Can only think tank connection at cold feed, possibly rust deposits from the old combination valve?(valve was rusty as hell its old and bulky) my first diagnosis of filter was wrong so don't want to start draining tank etc to find out I'm wrong again

 

[Best]

Yes, I was thinking that you were trying to avoid wasting time.
If there is no check valve on hot outlet and no isolating valve on hot outlet that could be blocked, then I would drain unit and check cold in connection and then hot connection out until I found a blockage.

 

[BradleyBrittan]

Possibly could be air lock. If you have a mixer tap in your kitchen you could try running the cold main back down the hot pipe for 30 seconds or so (may need to repeat) to try and clear the air out. Very simple to do, just hold a cloth tightly over the mixer tap, turn on the hot then the cold main slowly (so you don't splash everywhere) and hold for 30 secs. May not be the problem but certainly worth a shot, especially if you have no valves to worry about. No partially closed gate or isolation valves anywhere?

 

[Best]

Unlikely to be an airlock as the unvented cylinder is mains supplied, so pointless.
Really we shouldn't be putting mains at kitchen sink into the hot unvented supply anyhow, because the mains at kitchen is usually direct off the main stopcock - with no pressure regulator and could be at a much higher pressure than the 3bar max for stainless steel unvented cylinders. You would be bypassing the units combination valve in effect, however momentarily and pressure would build up unless a second hot tap was left open.
This might apply also to other mixer taps in a home, as not all unvented installs use the balanced cold (3bar max) from combination valve connection to every, if any taps.

Note - Putting the mains cold into the hot side of a mixer tap can only be done in most modern mixers by removing the spout and holding your hand over the tap body, as the spout is double pipe and won't allow the hot & cold to mix, unlike the old mixers that had a single chamber spout.

 

[BradleyBrittan]

And that's is why you shouldn't reply to a post when half asleep the UN part of unvented didn't register at all, apologies. In that case I concur with your above posts - debris

 

[GQuigley67]

Double check valve is after stopcock so can't be it as balanced cold is ok, will let you know how I get on.

 

[GQuigley67]

Just an update folks...

Turns out that it's a cylinder made of ferrous metal, it had a magnaseum anedode in it which was meant to be checked every year, cylinder has never been serviced, so it is full of sludge!!

Replacing cylinder in near future

Never came across a cylinder like that before, everydays a school day!!

 

[Best]

Nice wee job for you soon!
Glad you have found the problem.

 

[GQuigley67]

Client noticed the magnesium aneode before me, he remember what it was for from his chemistry days... I was clueless oops! Then again he is a senior A&E doctor in local hospital so he's no dummy haha.