Hot feed from twin pump stalls when cold feed starts in bath mixer tap

[foxotron]

Check out the fitting highlighted as well sometime.

That's the fitting I assume is the vent, but I could be wrong obviously

 

[John.g]

If the above is a T piece with a pipe coming off it then that will be the vent.

You say the hot only flows OK through the shower, does it flow at the same rate (visually) as the cold only through the shower?.

You might also check the obvious like any isolation valves between the cold water storage tank and the HW cylinder fully open by re closoing them fully and then reopening fully (about 4 or 5 turns on the red wheel gate valves). The vent itself should also be checked clear and ensure no one has fitted a valve on it.

I would also check (may need your plumber) that the elbow and pipework from the HW cylinder Top are clear by removing them.

Ensure HW temperature not excessive.

Can't think of any thing else right now.

 

[foxotron]

Thanks for the advice! appreciate it!

 

[John.g]

If you just run with a hot and cold tap fully open and you then keep closing the hot tap, does it still keep flowing hot water right down to a trickle until almost fully closed?.

One other thing, there may have been a special type of dip tube installed in the top of the cylinder 15 years ago when pump installed, this (if installed) may be partially blocked so something else to check out some time.

 

[foxotron]

Thanks for the advice! appreciate it!

If the above is a T piece with a pipe coming off it then that will be the vent.

You say the hot only flows OK through the shower, does it flow at the same rate (visually) as the cold only through the shower?.

You might also check the obvious like any isolation valves between the cold water storage tank and the HW cylinder fully open by re closoing them fully and then reopening fully (about 4 or 5 turns on the red wheel gate valves). The vent itself should also be checked clear and ensure no one has fitted a valve on it.

I would also check (may need your plumber) that the elbow and pipework from the HW cylinder Top are clear by removing them.

Ensure HW temperature not excessive.

Can't think of any thing else right now.

Thanks for the advice! appreciate it!

I don't think that the marked fitting in the picture above has a pipe coming off it, but it might be hidden behind so I'll need to check. I'll need to look for the vent but it's possible there isn't one (!?)

in terms of hot vs cold flow rate - excellent point - they seem similar visually but I can measure with a bucket.

I'll try the isolation valves opening/closing a few times too.

The hot water temp is about 55c.

 

[foxotron]

If you just run with a hot and cold tap fully open and you then keep closing the hot tap, does it still keep flowing hot water right down to a trickle until almost fully closed?.

One other thing, there may have been a special type of dip tube installed in the top of the cylinder 15 years ago when pump installed, this (if installed) may be partially blocked so something else to check out some time.

I'll check on the first thing.

The initial installation 15 years ago was done by a cowboy as I later found out. I doubt he opened the cylinder if I'm honest. When the original pump died and was replaced by a proper plumber, he may have installed the dip tube but I wouldn't count on it.

I guess we are saying that there may be a partial air block in the pipework above the cylinder and the normal best practice ways to prevent this might not have been followed? If so it seems I need to call in a pro!

 

[John.g]

I suspect there isn't a vent. That fitting looks to me to be a non return valve, have a closer look (maybe photo) you should see a arrow on it but doesn't matter, that shouldn't be fitted either but again it worked for years with it installed so might be partly seized, mind you its surprising that the proper plumber didn't remove it, maybe he installed it!.
Not having a dip tube doesn't necessarily mean it won't work.

Getting a pro in is a very good idea because with (if) no vent and if the isolating valve on the cold feed to the cylinder is shut then a very dangerous situation when cylinder starts heating.

 

[foxotron]

I suspect there isn't a vent. That fitting looks to me to be a non return valve, have a closer look (maybe photo) you should see a arrow on it but doesn't matter, that shouldn't be fitted either but again it worked for years with it installed so might be partly seized, mind you its surprising that the proper plumber didn't remove it, maybe he installed it!.
Not having a dip tube doesn't necessarily mean it won't work.

Getting a pro in is a very good idea because with (if) no vent and if the isolating valve on the cold feed to the cylinder is shut then a very dangerous situation when cylinder starts heating.

Thank you so much. I am now officially out of my depth and will call a plumber tomorrow first thing!