DAFI 400v Water Heater protection

[Marty21]

I have a three storey building with 3-phase supply, which is split so there is a single phase on each floor. There are no 3-phase DBs in the building, only single phase (one on each floor).

I've recently bought an 11kW 400V Dafi to see how it performs as a hot water heater. It requires two phases, no neutral, and earth.

How to connect this in terms of fusing / MCB / local isolater without spending more than the cost of the heater itself?

There's approximately 15m cable required from the incoming supply to the Dafi - proposing to use YY 3 core (2 x black & 1 x earth, 4mm).

I've found a 2 pole fuse disconnect isolating switch HR17B-40/200 (32A) that could go just after the main incoming switch and then use a 45A 2-pole switch adjacent to the Dafi.

The HR17B has fuses in it. Any thoughts?

Also, anyone know if a 45A 2-pole switch can be used with 2 live phases, rather than L-N?

 

[chivers67]

Can you use a rotary isolator?

 

[flameport]

The HR17B has fuses in it. Any thoughts?

http://www.ebay.co.uk/itm/191925959008
Totally unsuitable and unsafe.
It's designed for use within some other enclosure in a country far away, it has live exposed parts inside when open, and it is not capable of on-load switching.

You should NOT be attempting the installation of the new circuit or the heater yourself.

 

[Echo the husky]

Something like this is more suitable for the supply. There is no way installation is going to cost less than the £60 product. As above it is not a DIY job.

 

[Lectrician]

A double pole MCB at the origin in an enclosure would be OK. Not ideal.

 

[ericmark]

Where the installation is in charge of an ordinary person type tested distribution units have to be used called consumer units, these are always single phase, and limited to 125 amp, so in a normal home you can't have a three phase or split phase supply.

In blocks of flats where the shared area is under the control of an instructed person or better then you can use split or three phase in the shared area. The instructions state the 7.5 - 11 kW models are non domestic.

I have used under sink heaters, we used 3 KW versions with 7 litre capacity and there was a 1 KW element which could be fitted, I used one in a caravan on a 10 amp supply and insulation was that good the cupboard could be used for all normal items. There is no need for instant water heaters for under sink, 7 litres of hot water was ample, even with just a 1 KW element I never ran out of hot water.

 

[Marty21]

Thanks for the helpful and constructive replies.

Re. the HR17B - those were my initial thoughts, flameport, but as they are being sold in the UK it made me wonder whether they had a legitimate / permitted cable protection role. Rotary isolater was the obvious choice - thanks chivers67 - but there still needs to be cable protection. So possibly a small purpose made enclosure fitted with the isolator and a suitably sized MCB inside after the main fused switch.

To clarify, this is a commercial building and we are looking to replace nine very old individual water heaters in toilets and kitchens with a single hot water source, hence the Dafi. These services are all stacked above each other on the three floors so we're not talking about long pipe runs.

The lower wattage Dafi that we have in another building isn't really up to the job in terms of flow rate and temperature rise. We also have under-sink 10 litre storage units elsewhere, but the heat losses from these really mount up in terms of wasted energy / electricity. We did an energy tracking exercise last year and its quite an eye-opener when you see the consumption from fridges and water heaters kicking in throughout the day and night.

Obviously there are many ways of achieving the objective, including an an indirect or direct hw cylinder, another instantaneous solution or even solar. Just thought it worth trying out the 11kW Dafi as there is 3-phase in the building. The installation will be certified before use by our usual electrical contractors.

 

[flameport]

replace nine very old individual water heaters in toilets and kitchens with a single hot water source, hence the Dafi.

That will not work.
An 11kW heater will supply about 5 litres per minute with a temperature rise of 30C. Barely adequate for a single outlet, and totally useless for two or more.

 

[Simon35]

Thanks for the helpful and constructive replies.

Re. the HR17B - those were my initial thoughts, flameport, but as they are being sold in the UK it made me wonder whether they had a legitimate / permitted cable protection role. Rotary isolater was the obvious choice - thanks chivers67 - but there still needs to be cable protection. So possibly a small purpose made enclosure fitted with the isolator and a suitably sized MCB inside after the main fused switch.

To clarify, this is a commercial building and we are looking to replace nine very old individual water heaters in toilets and kitchens with a single hot water source, hence the Dafi. These services are all stacked above each other on the three floors so we're not talking about long pipe runs.

The lower wattage Dafi that we have in another building isn't really up to the job in terms of flow rate and temperature rise. We also have under-sink 10 litre storage units elsewhere, but the heat losses from these really mount up in terms of wasted energy / electricity. We did an energy tracking exercise last year and its quite an eye-opener when you see the consumption from fridges and water heaters kicking in throughout the day and night.

Obviously there are many ways of achieving the objective, including an an indirect or direct hw cylinder, another instantaneous solution or even solar. Just thought it worth trying out the 11kW Dafi as there is 3-phase in the building. The installation will be certified before use by our usual electrical contractors.

If it's a commercial building, just get your usual electrical contractor to come on site to advise and do it properly.

You shouldn't be using YY as a power cable as per your first post, it's a control cable. 17th Edition advises against it, as do the cable manufacturers.

 

[Marty21]

I think I have figured this out now so will run it past the contractor who will sign it off. We'll put a new 3-phase DB in after the incoming switch.

Thanks Simon35 for flagging up the issue about the YY cable. We've lots of SY in our industrial premises, although mainly used for machine connections where SWA isn't necessary.

Reading around the IET and other forums, there seems to be a lot of discussion and different opinions about xY cables use as power cables:
http://www.basec.org.uk/News/Basec-...ading-Claims-Associated-with-YY-and-SY-cables
http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=60998

 

[flameport]

We'll put a new 3-phase DB in after the incoming switch.

For what purpose?
The heater is totally unsuitable for the application.
Any money spent installing it will be wasted.

 

[Lectrician]

These heaters are designed to be positioned at the closest point of utilisation. They will not work as required with long pipework runs, and will be useless in supplying multiple outlets concurrently.

Your existing setup of multiple heaters is preferred, and most common!

If you want to spend money, invest in a mains pressure cylinder, with dual immersions.

 

[oldbutnotdead]

I've got one of the smaller Dafi's (3.2 kw I think) as a local hot water source for a washbasin. No it isn't brilliant (at anything more than a trickle water temp goes from OK to rubbish) but given the frequency of use of this particular basin (less than once a day on average) it is much more cost-effective than the traditional heater with tank. Plus the design allows you to use whatever tap you fancy rather than having to have a specific tap or an oversink unit so I can understand where the OP is coming from.

 

[Marty21]

Installing a small 3-phase DB like the Nano with RCD (much as I hate to buy Chinese products) isn't going to cost much and it's there if we decide to put a couple of extra 11kW Dafi heaters in and share them between fewer outlets. But until we try it and find out what this unit can produce, we won't know.

Currently we have a 4.5kW Dafi shared between adjacent ladies & gents toilets in other premises and that works fine. My only complaint, as noted earlier, is that the flow rate and temperature rise isn't really adequate even for one drawing. Although I think if we change the taps and install a spray nozzle (free from the water company) the reduced water flow may improve that.

@Lectrician - yes, we have one of those on site - an Ariston that I bought last year. Quite a big install job, though, and requires building regs certification and I think it requires annual check on the expansion vessel (may be wrong on that). Even if it only uses night-time Econ7 energy, the cost in heat losses alone from the cylinder we calculate to be in the order of £200 a year (60p per day). Add in wasted hot water drawn from central point to nine outlets (and at mains pressure too!) and the cost of that particular hot water looks very high. Could put an indirect version in and use cheaper gas energy and heat from the CH boiler(s). That's fall back if the Dafi fails to live up to expectations.

Thanks to all for the comments and advice. Will let you know how we get on.

 

[flameport]

As the number of posts increases, this discussion becomes less and less credible.