Tankless water heater upgrade

[Philip Saunders]

Hi, everyone.

I have a shower room that currently has hot water provided by a 7kw tankless water heater. The room is plumbed for hot and cold water from the domestic hot water (oil fired), but the pipe is so long (more than 60m) that it's barely warm by the time it gets there, hence the heater that is plumbed into the cold line.

The existing unit always struggled to keep the water hot enough, and it's now given up.

I inherited the existing setup which is - feed taken from the CU through a 40A mcb via 10mm cable to an secondary CU which contains an RCD. From there out through the loft space to the unit also in 10mm cable).

The main CU is 3 phase.

I've seen 11kw units that require connecting across 2 phases (ie they are 400v single phase devices).

Given that I have the power available (3 phase CU), how can I implement this correctly? I can't find anything that covers this in the 17th edition regs. Unless I missed it, it's not in the part that deals with installations in bathrooms.

I know it will need an RCD, but how is that implemented? Should/can I use a 3 phase RCD and just not use one of the poles?

I intend to get this all checked out professionally when I'm done, but I live in the middle of nowhere and getting someone to come out just for a bit of advice is a big ask.

I want to do all the legwork before I get a sparky in to finally connect it up.

Thanks

 

[Darkwood]

Hi..
We are no longer using the 17th edition and we are working from the 18th now as of Jan 1st.

As this is DIY and this work is beyond what we would class as advising on for DIY we can only really suggest you get an electrician out, I would discuss your options with the electrician and express you want do some of the work for which you could come to some agreement, every installation is different and to carry out such works a look at the job would be required, this not only serves to understand what is required for the work you are requesting but also allows the professional to ensure there are no other issues that may need addressing before any work commences, such issues could include putting in bonding if it is required yet missing or addressing any earthing issues at the mains as examples.
I think you will be hard pressed to find an electrician who is willing to connect up sign off on work they haven't done unless pre-arranged and with at least some supervision to ensure it has been installed correctly.

There may be a member local to you who could come out and advise, I will finish by mentioning that you may have issues with a 3ph rcd but this will be dependent on how the heater is wired and which RCD you install, I am guessing the heater will not require a neutral which can be problematic with some RCD devices and they may need adapting but this can depend on the brand and how the internals are wired like the test button for example, there are solutions for these devices but like already mentioned, this is not in the scope of DIY advise we offer and you need a competent qualified person who can ensure the correct connection of the RCD and modification if needed as well as verified testing of it.

 

[Leesparkykent]

What about running a secondary return for the HW with a bronze pump?

 

[Philip Saunders]

What about running a secondary return for the HW with a bronze pump?

Yep thought about that - in fact I do have secondary circulation but I'd need to massively extend tbe loop which I was hoping to avoid as it's basically just for 1 shower, and a cable is easier to run. The existing (unused) hot feed is a really long 'spur' off the existing hot loop.

 

[Deleted member 26818]

I know Redring do single phase instantaneous heaters that are supposed to be good for a shower and sink.

 

[DefyG]

Presumably its a mains pressure cold feed, so an 11Kw 'undersink' single phase type water heater positioned nearer or inside the bathroom (depending on zone) would suffice and overcome any complications of 2ph and RCD!

 

[Philip Saunders]

Presumably its a mains pressure cold feed, so an 11Kw 'undersink' single phase type water heater positioned nearer or inside the bathroom (depending on zone) would suffice and overcome any complications of 2ph and RCD!

Yes it's mains cold feed. I was only looking at the 400v unit because the 10mm cable will be marginal at 240v and 11kw - especially given the length of the cable (about 40m). However, it looks like the 3 phase stuff will get complicated (read expensive), so I'll just run a 16mm cable I think. I'll still get it checked out. Somebody mentioned some bonding might be needed - never thought of that - I just assumed the existing unit was wired properly but of course that's not guaranteed.

 

[DefyG]

You will need to get it checked out (formally inspected, tested and certified) as it will be subject to Part P building Regs and therefore notification.
There may of course be other issues such as the bonding you mentioned but without more info difficult to say, (eg 10sqmm over 40m IF clipped direct would be fine, with a volt drop less than 5%)