air trapped in cold feed to immersion tank

[matt1e]

how high is the cistern above the cylinder? as this will determine the pressure at the hotwater outlets
since you are in a bungalow I suspect not much

about a year ago I re-routed the hot water pipes, which are below the bottom of the hot water cylinder, at the same time I replaced the stop cock feed from the cistern to the cylinder, so the cold water cistern
since that time I've had what I call lower pressure from the hot water pipes

are you sure its not the flowrate thats down as apposed to pressure since then?

Matt

 

[goateater]

its between 3-4 foot above the top of the hot water cylinder
it used to work ok, before I re-routed the pipes to the down stairs bathroom, the height of the pipe did not change I just moved the pipes so they all run next to each other

 

[xr4x4]

For how long did you run the cold back through the hot?

 

[goateater]

about 1min, until all the top of the hot cylinder was cold
I could have it on longer, but due to the weight of the water on the input side of the hot water cylinder, I think the cold water will find the overflow on the hot water output side easiest route

 

[xr4x4]

Mains pressure should be more than your gravity pressure, so it will always override it.

I'd run it a little longer, go up into the loft and check the tank to see if any bubbles come out the outlet in the tank.

Just check the overflow is connected...!

 

[Madrab]

OK, you may have mentioned where the problem may be

it used to work ok, before I re-routed the pipes to the down stairs bathroom, the height of the pipe did not change

If it was ok before and now isn't then changing the pipework route has obviously altered something.
When you re-routed the pipes underneath are there a number of bends and is this in 22mm? You say there are 4 bends in the distribution pipe to the hot water cylinder so all of that is adding restriction/friction to the flow. Given you have low pressure to start with (0.1/2 bar) then won't take many elbows to lower that again.

 

[denso13]

I have thought about replacing the 22mm cold water feed pipe with a 28 mm feed pipe

That's the one