Replacing two hot water cylinders with one

[HRP123]

Try this, shut the cold and hot valves on the fast cylinder, shut the hot only on the slow cylinder, open the expansion valve (by twisting the top), cold water should start flowing out, if only hot/lukewarm then problem is on the cold feed, probably the pressure reducing valve.

So I followed your instructions - and assuming you mean turn the red PRV valve when you say "open the expansion valve" - only hot / warm water came out from the slow cylinder discharge. Is it possible the handwheel gate valve can be faulty? Up to the handwheel gate valve on the cold inlet pipe to the slow cylinder, it feels cool to touch, but after the gate valve (in open) it feels warmer all the way until the pressure reducing valve. However on the fast cylinder this section of pipe from the gatewheel to the pressure reducing valve feels cold to touch. Pictures attached.

 

[Johntheo5]

Where is the red handwheel gate valve??, there is a tap like handle valve to the left of the yellow circulating pump, is this not the cold water supply to the slow cylinder?.

 

[HRP123]

Where is the red handwheel gate valve??, there is a tap like handle valve to the left of the yellow circulating pump, is this not the cold water supply to the slow cylinder?.

Yes this is the cold inlet isolator - it is the tap one - sorry I guess it is not called a red handhweel gate valve but it is a tap valve (I don't know the technical terms for each valve!)

 

[fixitflav]

So the diverter valve is connected to the cylinder return to the boiler, rather than the flow into the cylinder. Not sure why it has been done that way.

It's more common to have the diverter valve on the inlet to the HW cylinder, but I don't see any problem with it on the return. Perhaps that was more convenient for the overall layout.

 

[Johntheo5]

OK, no problem, ensure that this is open, (shut it, then reopen it) and shut the HW outlet valve, then slacken the coupling nut approx a 1/2 turn anti clockwise, right to left as you are looking at it, just below the pressure reducing valve which is marked 3 bar in the first photo, above. shake and pull the pipe downwards to break the seal and cold water should come out, if so shut the cold water vale, retighten coupling nut, clockwise, left to right.

 

[HRP123]

OK, no problem, ensure that this is open, (shut it, then reopen it) and shut the HW outlet valve, then slacken the coupling nut approx a 1/2 turn anti clockwise, right to left as you are looking at it, just below the pressure reducing valve which is marked 3 bar in the first photo, above. shake and pull the pipe downwards to break the seal and cold water should come out, if so shut the cold water vale, retighten coupling nut, clockwise, left to right.

Unfortunately I don't have a tool to slacken that coupling nut as its really large, my spanners only go up to 22inch. Will try and test that out. It is remarkable though how the fast cylinder, even when not draining, has a cold to touch inlet pipe, versus this one is luke warm.

 

[Johntheo5]

OK, just below the 3bar pressure reducing valve, top photo. there is a T, with a pipe going vertically upwards (and maybe downwards) can you post a photo of this and see where its going to/from?

Have you checked the obvious, that the tap handled isol valve is actually opened?

 

[HRP123]

OK, just below the 3bar pressure reducing valve, top photo. there is a T, with a pipe going vertically upwards (and maybe downwards) can you post a photo of this and see where its going to/from?

Have you checked the obvious, that the tap handled isol valve is actually opened?

Yep, the tap is opened, have turned it a number of times - and it is currently open in the same position as the fast draining cylinder.

 

[Johntheo5]

With the fast cylinder cold water isolated, you could open a hot tap, remove the plastic cap from the (slow cylinder) pressure reducing valve, there should be a adjusting screw, turn this slowly clockwise and see if the hot tap flow increases, (make a note of the adjustment turns)

 

[HRP123]

You are a star!! I had to do quite a few turns on the slow cylinder PRV and then suddenly I heard some gurgling, felt the pipe go cold, and hot water flow started up... wow I cannot believe you managed to fix it. Thanks for this! Really appreciate it.

 

[Johntheo5]

Very good, next chore is to balance them.
Suggest shutting off the cold feed to the slow cylinder only, get a bucket or something similar, open a hot tap and note the time it takes to fill the bucket. Shut the cold feed to this fast cylinder, re open the slow cold feed and do the same test, then adjust the pressure reducing valve to match the same filling time as the fast one and that should give a reasonable balance. I wonder was the slow cylinder ever commissioned.