HW Cylinder - 2 x immersions

[brizospark]

I am working on a property refurb where there is a new hot water system to be put in place. The hot water is to be stored in a cylinder located in the hallway. The hot water is to be heated by a combi boiler but there are also 2 x immersions in this large tank, one at the bottom and 1 x halfway up in case the combi fails.

These immersions are 3 kW each. I was just wondering what you guys reckon is the best method of control for these? Cable size wise I was thinking of running a 6mm to a control switch where I would then branch off to each immersion with a 2.5

Do you think I should just use a standard on/off switch (rated 25A plus) or have two seperate switches or even a programmer type control with one immersion being a boost

Cheers Guys

 

[HandySparks]

Are you sure it's a combi boiler? Seems unlikely if there's a hot water cylinder.

As it's just for backup, I'd just run two 16A radials, one for each immersion heater, with a 20A local switch for control.

Is the supply Economy 7 or similar?

 

[brizospark]

HandySparks it is definitely a combi boiler, I asked the same question to the plumber.

he said because it was a large property with 5 x bathrooms there was a need for a storage tank for the hot water.

i'm still dubious to if the immersions are purely backup. Can anyone shed some light on this?

 

[tigi]

would run 2 16A circuits in 2.5 mm (seems overkill as they will possibly never be used)
the highest placed i/htr in the cylinder would be used as a boost
the lowest placed i/htr would be used for for a full tank of hot water
these would only be used in emergency if combi fails so possibly only wire in 1 i/htr
on one circuit whichever one suits best

 

[tigi]

seems like they need more btu's in the boiler !!!!!
on that basis perhaps wire them both in to cover all angles

 

[Marvo]

Maybe a visit to the plumbersforum might shed some light on it....but don't go alone, take a few of the more burly sparks with you to watch your back especially after dark .

 

[sima24]

I would run 2 radials and fit a standard 7 day timer to both, or if its just for backup forget the timers.

 

[HandySparks]

HandySparks it is definitely a combi boiler, I asked the same question to the plumber.

he said because it was a large property with 5 x bathrooms there was a need for a storage tank for the hot water.

i'm still dubious to if the immersions are purely backup. Can anyone shed some light on this?

Hmm. The whole point about a combi is that the domestic hot water heating is instantaneous, so no need for a cylinder.

I guess, in the end, it doesn't really matter about the detail if the immersion heaters are just for backup in the event of boiler failure; if it doesn't work, it's down to the designer of the heating system, who should tell you what controls are required, if any.

 

[daniel hill]

Would guess that the boiler is a system boiler (wired as a s plan or y plan) and not a combi, would not be able to use a combination boiler with a water storage tank.

best to wire 2 x 16A radials in suitably sized cable.

 

[chris murphy]

"Hello All",

Regarding the 2 Immersion Heaters in the Cylinder that brizospark / the OP asked about:

When a Combi Boiler is installed in a Property that has more than one Bathroom or has a high Hot Water demand - perhaps after a refurbishment or an Extension is built - quite often a Cylinder is installed which is Heated by the Combi via Flow & Return Pipework to the Cylinder Coil and a Zone Valve & Cylinder Thermostat and Timeswitch - the Cylinder would usually supply the Hot Water for a Second Bathroom for example.

Unless the Cylinder in this case is a Large Capacity Cylinder I am surprised to read that it is in a Property that has 5 Bathrooms !

Theoretically even with 28mm Plumbing Pipework a Hot Water Cylinder would only be able to supply 2 X Bathrooms simultaneously - with perhaps the Combi supplying another - only ONE Hot Water draw off is possible from MOST `Average Capacity` Combi`s.

Although with a High Capacity Combi the Manufacturers state that 2 Hot Water draw offs are attainable - dependant on the correct Volume of Water Mains being available and other factors relating to the Hot Water system pipework resistance factors - and the correct Volume and pressure of Gas available at the Combi.

I realise that in this case the Family members might all have their own Bathrooms which are all normally used at different times [?] - BUT I am fairly sure that there would not be more than 2 Bathrooms connected to the Combi even if it is a High Capacity model - ?

It IS obviously possible depending upon the Lifestyle of the Family that the Combi could heat the Cylinder to meet the Hot Water requirements of a 5 Bathroom Home in conjunction with the Instant Hot Water from the Combi - BUT - Personally I am very sceptical of that.


Obviously I have written these details knowing nothing more about the Property or the People who live there than I have read here.


I would definitely suggest that the OP wires these 2 Immersion Heaters as if they will BOTH be being used DAILY - and possibly for long periods of time - depending on the Hot Water usage in this 5 Bathroom Home.

Even IF the Immersion Heaters are NOT going to be used as regularly as I am GUESSING here - what about if another Family purchase the House in the future and their Hot Water requirements are totally different from the existing owners - meaning that they do have the Immersion Heaters On Daily for Hours at a time - ?

I realise that there is no suggestion of the Immersion Heaters being wired in an unsafe manner - but would there be different requirements [apart from Timed operation] for the Electrical Installation for 2 Immersion Heaters that are used on a Daily basis for Hours at a time - ?


I am NOT trying to complicate this Electrical Installation for brizospark / the OP - but I would be guessing that these 2 Immersion Heaters will be getting far more use that the Plumber suggested to Him - ?


Regards,

Chris