DIYbanter - View Single Post

wrote:

Interesting stuff, the installer said it would be fine but I didn't see
him analyse what was already on the ring so not sure how he came to
this conclusion

He was negligent, and bull****ting too.

The room size is 20m2 but the UFH area is probably about 15-16m2

From what I remember and by looking at the wires coming out of the
screed there are 2 heaters and I'm pretty sure they are 2KW each.

(but you go on to say you think (!) they're actually 1kW each)

On the existing ring which serves the dining room and the kitchen are
the following:

Toaster, Dishwasher (spur), Coffee machine, hot tub (connected from a
FCU which was spurred from the ring) and a fridge freezer (Spur) kettle
is also there but very rarely used. The hob and the oven are on their
own circuit of course

Well, it falls pretty squarely into what I called the "DON'T EVEN THINK
ABOUT IT" region. Toaster: 1kW; washdosh - 1kW or 1.5kW (intermittent -
much less when just sloshing already-heated or cold rinse water around,
but up at that figure while heating water and during final 20 mins or so
of air drying); coffee mc - prolly 0.5-1 kW; hot tub - god knows, if
you're heating it electrically then far-too-much not to be on its own
dedicated circuit already (it'll be 3kW), if it's only a pump and heated
off your CH then just a couple of hundred W, maybe, maybe a bit more for
lights too; fridge/freezer - a few hundred W; kettle - 2kW when used.
(Whether they're fed down a spur or from sockets/FCUs directly on the
ring makes not a whit of difference to the load on the ring.)

The hot tub issue alone makes me most uncomfortable (you mention an FCU,
but please God tell us there's a 30mA RCD on that circuit too). Even
without that, you've a peak load of pushing 5kW in the rare cases where
most of these appliances are running - that's 20A. Put even the 2kW
lower guesstimate of your UFH on the same circuit and you've used up the
whole of the circuit capacity, even without worrying about whether most
of that load is at one end of the ring (which means the bulk of the
current is carried by the shorter side of the ring, so making that
shorter cable section run disproportionately hot). Then someone plugs a
vacuum cleaner (1.5kW+) into the ring too... overload city here we come.

You *really* need to get this lot properly sorted: a new separate
circuit with its own RCD/RCBO for that hot tub, and a separate circuit
for the UFH if it's only 2kW (if there's two segments each of 2kW,
you'll want either two separate circuits each in 2.5mmsq, or one in
4mmsq - probably). Running the whole lot off one ring creates the
classic conditions for repeated, sustained, cable-najjering overloads
which are above the cable rating but below the point at which the 32A
MCB on the ring will trip.

Don't go there. Get it sorted.

Oh, and what is the room being UFH'ed? If (shudder) a sauna, does your
installer know about providing additional earthing and the like? (Or
maybe his horse does - y'know, the one he digs his spurs into with a
wild yee-ha! as he rides off into the sunset...)

Stefek