New unvented cylinder / Replace old vented cylinder and add a ST pump?

[dave100]

Hello,

I am renovating a small bathroom in an apartment on the 5th floor. Heating is electric, no gas or oil. I have four (related) questions about this below.

I had intended to reuse the cold water tank and the cylinder sitting underneath it but my plumber suggested I replace the cylinder as part of the renovation. The tank and cylinder are 28 years old. My cylinder is pictured here, its heated via two electric elements, it is gravity-fed from the cold tank above it and has an overflow pipe back into the cold tank. The drain-off valve near floor level doesn't open and I think I'd damage the tank trying to replace it. The dimensions are 36 inches high and about 20 inches in diameter. To replace that today I'd need to buy a 36 X 18 Direct Vented copper cylinder, I believe.

QUESTION 1:
Do you agree that it would be better to replace the cylinder as part of this renovation?

I will add a Stuart Turner 2-Bar Negative Head twin impeller pump and a thermostatic shower valve when doing this renovation:

This is a two-outlet mixer with separate feeds for a handheld shower head and a ceiling-mounted 'rainwater' shower head. Having the two of those running (if I ever did) would take a lot of pressure which is why I was thinking of going with the 2-Bar pump.

There might be another option though, the cold mains water pressure is really good in this apartment block, this short video shows the pressure at the kitchen tap:

When looking up the kinds of cylinders that are available I discovered that I could get an unvented cylinder which takes its water supply from the mains rather than the gravity feed from the cold tank. The advantage is that I would get good hot water pressure.

QUESTION 2:
In my case, would it be better to put in an Unvented cylinder and get rid of the ST pump?

QUESTION 3:
Is there any downside to using an unvented cylinder? Is there an increased chance of leaks given that mains pressure is now going through the hot pipes?

QUESTION 4:
For the thermostatic mixer shown above, would I need to have a mains cold supply going into that to balance the hot supply? I think those moxers work best with a 'balanced' incoming pressure between the hot and cold?


Many thanks for your help!
Regards,
Dave

 

[fowlerboi]

Unvented cylinders are definitely better as long as the flow rate and pressure are suitable. A quick guess would say yes given your video but its more scientific than that.

They also have to be fitted by someone with an unvented ticket so it's not something you can do your self, at this stage you would ask a plumber to quote it for you and they can do the tests above and look for a way to terminate the relief pipe that has to be fitted with them.

There is a risk of leaks changing over from gravity to pressure. Normally you can gauge what the plumbing is like from what you can see and take an educated guess what will be under the floor but utlimately you won't know until you fill up, and no plumber will guarantee that. Could you just re-pipe the whole lot?

When you put an unvented in they come with a multi-function valve which reduces the pressure as the cylinders want 3 bar maximum, on this valve is a special connection for the colds which you would then just pipe across to the old colds (or re-pipe would be the better option)


If you don't want to go down that route 100% change the copper cylinder as its already out


If it was me I would be putting in an unvented at the extra cost