RE: 24/7 versus limited on and off time for water heating

["Dave McLaughlin" <dave_mclaughlin@xxxxxxxxxxxxx>]

Thanks Simon, Keith,
=20
I am quite lucky as I live in the tropics and the water tank is located an
=
open utility room area where the temp various from 25 at night to 34
during=
the day all year round so based on some basic calcs it should work out che=
aper to keep it off during the day and only switch on for 2 hours in the
mo=
rning before it is needed for showers. If I do have a shower in the
evening=
, it should still be hot enough to have a cooler shower which is what I
pre=
fer. Even though I live in a hot climate, I like the morning shower to be
h=
ot.
=20
I am going to build my own interface to switch the heating on and off via
a=
n industrial relay (2 pole) that I have. Price is not a major issue. They
r=
ecently increased the cost of electricity here by 130% so even if I spend
$=
50 on something, it will pay for itself in a short time if there is a
savin=
g to be had with switching it off daytime.
=20
In the mean time, until I can get the complete circuit designed and build,
=
I will switch off the water heater in the morning and see what the
temperat=
ure drop is and as have a way to measure the power usage, I can see if
ther=
e is any real saving. I have a temperature data logger so I will see what
t=
he temperature drop is at the tank. Don=E2=80=99t need the real temp
inside=
, just some way to monitor the drop. I=E2=80=99ll report back what I find.
=20
Cheers
Dave=E2=80=A6
=20
From: ukha_d@xxxxxxx [mailto:ukha_d@xxxxxxx] On Behalf Of S=
imon Haslam
Sent: 11 September 2010 20:53
To: ukha_d@xxxxxxx
Subject: Re: [ukha_d] 24/7 versus limited on and off time for water heating
=20
=20=20
You can work it out. The heat loss of the cylinder is proportional to the
t=
emperature difference between the water and the room, and the amount of
ins=
ulation. Therefore the higher the average temperature of the water the
more=
heat you will lose so keeping it at 60C for 24/7 will always lose more hea=
t than heating it on demand (when the average temp may be 30 or less).

Most larger new cylinders lose around 2kWh per day when hot (depending on
s=
ize and insulation), and take a few kWh (e.g. 6.1kWh for 210l from 10 to
60=
C) to heat up the water from cold (I haven't investigated smaller
cylinders=
). So the heat loss may be a significant part of the energy used, though
of=
course in the winter the heat will go into your apartment so isn't really =
"lost". To do more calculations you need to know the size of your
cylinder,=
level of insulation (preferably manufacturer's data) and whether there's a=
ny extra insulation (doesn't do any harm!).

That said buying something to control the load may not be cheap - you need
=
something that will safely dual-pole switch >13A. Even a Smiths digital
imm=
ersion timer is about =C2=A325, and I don't know what HA compatible
devices=
are available. (Please post what you find as others on the list may be int=
erested)

HTH

Simon




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