Hello,

I have an old mechanical central heating control unit made by SMC. It
has stopped working, the dial no longer goes round, so I cannot set the
timer and also I cannot switch between hot water and central heating,
as whichever setting I have the switch on I get both hot water and
central heating.

I am signed up to the British Gas homecare service, and have called
their engineer out, however he just told me that because it is over 7
years old I am not covered.

Knowing how much heating engineers etc cost, I am hoping to replace
this unit myself with a modern, digital type timer/control unit.

Could anyone advise on whether this would be a sensible thing for me to
attempt - I have done work on electrics before and have a sound
understanding of them, however I kwno next to nothing about central
heating systems and I have read that the wiring is different on the old
mechanical control units to the new digital ones.

Also, would I be within the law/building regs to do this job myself, or
do i have to get a qualified person in?

I can post photos of the old unit and wiring if this helps at all....

Any help or advice would be really useful pleaseeeeeeeeeeeeeee

In article . com,
Thomarse wrote:
Could anyone advise on whether this would be a sensible thing for me to
attempt - I have done work on electrics before and have a sound
understanding of them, however I kwno next to nothing about central
heating systems and I have read that the wiring is different on the old
mechanical control units to the new digital ones.

There's not much difference in principle - ignoring any earths needed, you
simply have a live and neutral in, the neutral being for the clock motor
or programmer electronics, and two switched live feeds to control the
heating and hot water. Some types of boiler had low voltage control, so in
which case there would be a low voltage in as well. Nothing terribly
complicated if you check *carefully* how the old one is wired.

--
*There's two theories to arguing with a woman. Neither one works *

Dave Plowman London SW
To e-mail, change noise into sound.

Thanks for the reply Dave! Nice to know somones out there who is able
to advise!

You make it sound very simple, and Im sure on face value when I look at
it, it will be simple, but I do get a little bogged down when it comes
to circuit diagrams and the type of system which I have, I basically
dont know what system I have. My concern is that having read other diy
forums, replies tend to say "if you have this system wire it this way,
and if its this system wire it this way".

I will get the old control box opened up tonight and draw out how it is
wired up, and see if this matches up at all with the new digital
control box I hope to get!

On a side note, do you know if there are any regulations in place that
state that a wualified person must do this work?

Thanks again

Tom
Dave Plowman (News) wrote:
In article . com,
Thomarse wrote:
Could anyone advise on whether this would be a sensible thing for me to
attempt - I have done work on electrics before and have a sound
understanding of them, however I kwno next to nothing about central
heating systems and I have read that the wiring is different on the old
mechanical control units to the new digital ones.

There's not much difference in principle - ignoring any earths needed, you
simply have a live and neutral in, the neutral being for the clock motor
or programmer electronics, and two switched live feeds to control the
heating and hot water. Some types of boiler had low voltage control, so in
which case there would be a low voltage in as well. Nothing terribly
complicated if you check *carefully* how the old one is wired.

--
*There's two theories to arguing with a woman. Neither one works *

Dave Plowman London SW
To e-mail, change noise into sound.

In article . com,
Thomarse wrote:
You make it sound very simple, and Im sure on face value when I look at
it, it will be simple, but I do get a little bogged down when it comes
to circuit diagrams and the type of system which I have, I basically
dont know what system I have. My concern is that having read other diy
forums, replies tend to say "if you have this system wire it this way,
and if its this system wire it this way".

If the old one worked ok then all you have to do is wire the new one the
same. Most basic programmers are simply two changeover relays operated by
the timer mechanism. A changeover switch is the same as a lighting two way
switch - one common terminal which is the input in this case, and two
output terminals - one which is made with the relay off, and the other
with it on. The 'off' one isn't required on some installations. The common
terminals are often both wired to the incoming line so in practice you may
only have two to be confused over. ;-)

Like all electrical circuits, it helps to 'break' them down into the
simplest of blocks no matter how complicated they look at first. In this
case you have three such blocks - the 'motor' which controls two
changeover switches.

--
*What was the best thing before sliced bread? *

Dave Plowman London SW
To e-mail, change noise into sound.

"Thomarse" wrote in message
ups.com...
Hello,

I have an old mechanical central heating control unit made by SMC. It
has stopped working, the dial no longer goes round, so I cannot set the
timer and also I cannot switch between hot water and central heating,
as whichever setting I have the switch on I get both hot water and
central heating.

I haven't the same problem as you but I've just replaced a digital timer
that required a degree in electronics, the instruction manual and at least
two new swear words everytime you wanted to change the programme. I replaced
it with a nice new analogue mechanical timer made by Horstman and am very
happy indeed.

I discovered that the old timer had the wiring identification on its rear
(hot water demand. hot water satisfied etc etc) and it was a very simple job
to swap wires over like for like, especially as the new timer came with
standard wiring diagrams for umpteen other controllers.

Be brave, give it a go!

Julian.

Thanks Julian!

I think I will be brave and go for it! Will amke sure I know exactly
what Im doing before I rewire anything in though!

Im hoping just to geta basic digital controller that doesnt demand a
compter science degree to operate! I am easily pleased at teh moment as
I have no timer control at all at present, so anything is better than
that!

Thanks for your advice!
Tom
Julian wrote:
"Thomarse" wrote in message
ups.com...
Hello,

I have an old mechanical central heating control unit made by SMC. It
has stopped working, the dial no longer goes round, so I cannot set the
timer and also I cannot switch between hot water and central heating,
as whichever setting I have the switch on I get both hot water and
central heating.

I haven't the same problem as you but I've just replaced a digital timer
that required a degree in electronics, the instruction manual and at least
two new swear words everytime you wanted to change the programme. I replaced
it with a nice new analogue mechanical timer made by Horstman and am very
happy indeed.

I discovered that the old timer had the wiring identification on its rear
(hot water demand. hot water satisfied etc etc) and it was a very simple job
to swap wires over like for like, especially as the new timer came with
standard wiring diagrams for umpteen other controllers.

Be brave, give it a go!

Julian.

"Thomarse" wrote in message
ups.com...
!

Im hoping just to geta basic digital controller that doesnt demand a
compter science degree to operate!

No such animal in my book. They are all a nightmare to change!

I got the Horstmann one from the Screwfix book, cheap, reliable and soooo
simple and quick to change!

Anyhow, good luck....

On Wed, 01 Nov 2006 04:30:43 -0800, Thomarse wrote:

Hello,

I have an old mechanical central heating control unit made by SMC. It
has stopped working, the dial no longer goes round, so I cannot set the
timer and also I cannot switch between hot water and central heating,
as whichever setting I have the switch on I get both hot water and
central heating.

I am signed up to the British Gas homecare service, and have called
their engineer out, however he just told me that because it is over 7
years old I am not covered.

Knowing how much heating engineers etc cost, I am hoping to replace
this unit myself with a modern, digital type timer/control unit.

Could anyone advise on whether this would be a sensible thing for me to
attempt - I have done work on electrics before and have a sound
understanding of them, however I kwno next to nothing about central
heating systems and I have read that the wiring is different on the old
mechanical control units to the new digital ones.

Also, would I be within the law/building regs to do this job myself, or
do i have to get a qualified person in?

I can post photos of the old unit and wiring if this helps at all....

Any help or advice would be really useful pleaseeeeeeeeeeeeeee

The main FAQ covers control systems. There is easily enough out there
about the subject. If you come across any misunderstandings
then ask here. If this is all too much for you then you will have to get
in someone who's done it before.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html
Gas Fitting Standards Docs he http://www.makewrite.demon.co.uk/GasFittingStandards

Hi Ed
I hope you can help me with my problem i haved moved the manifold from
one room to other
and put all pipes back as thay off but all rads are cold all up rads
are normal hot the system is 8mm microbore the check valves are the
ones with both pipes going to one valve only one valve on each rad
please email direct if you would i will give my phone number and would
be better to explane things thanks steve

replying to Thomarse, Janet wrote:
Hi Thomarse Did you manage to find a solution for your SS heating control
unit as I am in the same situation?

--
for full context, visit https://www.homeownershub.com/uk-diy...it-299250-.htm