Central heating thermostat
 

I have a gravity fed HW and pumped CH system with a programable
thermostat. If I have the thermostat programmed to come on all day
inc. overnight then the boiler keeps tripping in to heat the water
despite heat not being required for the CH.
Can I fit a thermostat to the airing cupboard tank so that when the
room temp is OK the boiler will not light up or is there some other
easy solution?

Central heating thermostat
 

stevep wrote:
I have a gravity fed HW and pumped CH system with a programable
thermostat. If I have the thermostat programmed to come on all day
inc. overnight then the boiler keeps tripping in to heat the water
despite heat not being required for the CH.
Can I fit a thermostat to the airing cupboard tank so that when the
room temp is OK the boiler will not light up or is there some other
easy solution?


Yes - I did that in two previous houses and I can recommend doing it.
The problem is that the system tries to keep the water inside the boiler
jacket hot all the time that the heating is on and it just pumps water
to feed the radiators when the room thermostat is cold. You don't need
to keep that boiler hot and a cylinder thermostat is the answer.
Previously the boiler rarely stopped, afterwards it rarely came on - I
think it made a big difference to bills but didn't measure the savings.
Let's have a go at explaining it:

I had a clock timer with two outputs - 'need room heating' and 'need hot
water'. As it was a gravity fed system you have to have hot water when
you have room heating. There was a little cam or peg I recall on the
one which was a mechanical clock, a switch or wiring setting on the
electronic one, to set that rather than having them independent as you
can have with solenoid valve systems. Your timer will be set like that
already I expect.

I fed the circulating pump from the output of the room thermostat.

The hot water cylinder thermostat was a two way changeover switch - the
common went to the gas solenoid (via the overheat thermostat inside the
boiler). The contact which is made when the tank is cold went to the
timer contact 'need hot water'. The other one to the output of the
heating thermostat.


An ascii sketch:

BEFORE
-------
+ HW-timed--------------*-------------------- to gas solenoid
!
Clock !
! room stat
+ heating-timed----/ ---*--------------------- to pump
-------


AFTER
-------
+ HW-timed------------*---! cylinder stat
! -------------------- to gas solenoid
Clock ! !
! room stat !
+ heating-timed----/ ---*-+--------------------- to pump
-------

The room stat is 'down' or closed in the sketch when the room is cold.
The cylinder stat is 'down' in the sketch when the cylinder is hot so
gas is only supplied if
- timer set for hot water AND the tank is cold OR
- timer set for (heating plus hot water) AND the room is cold.

Neutral connections are not shown, but it's worth checking whether your
room thermostat needs and has one - cowboys often omit it as it needs
another cable but omitting it causes sluggish switching with the rooms
hot for a bit then freezing cold an hour or so later.

If you have a combined thermostat and clock then you might have
commections at the points marked --*-- . You'll certainly have two outputs.

Hope this helps, but email if you have any questions.
good luck!

Phil

Central heating thermostat
 

On 22 Mar 2004 05:32:21 -0800, stevep wrote:

I have a gravity fed HW and pumped CH system with a programable
thermostat. If I have the thermostat programmed to come on all day
inc. overnight then the boiler keeps tripping in to heat the water
despite heat not being required for the CH.
Can I fit a thermostat to the airing cupboard tank so that when the
room temp is OK the boiler will not light up or is there some other
easy solution?


Honeywell 'C' plan (or equivalent) is probably the classic answer
(See Sundial plans on Honeywell web site.) In this you use a zone valve to
stop the gravity circulation when the H/W tank is hot.

Alternatively (or in combination) a Danfoss TP9 programmmer/thermostat
would supply power to the boiler when either:

a) There is a need to heat house (i.e. room thermostat calling or more
precisely the programmer uses its sensor and determines that heat should be
supplied by powering the pump and the boiler.)

b) When you want to heat H/W e.g. 7-9am and 5-9pm

You can download the TP9 installation instructions from
http://www.danfoss-randall.co.uk/default.asp

Michael Chare

Central heating thermostat
 

On 22 Mar 2004 05:32:21 -0800, in uk.d-i-y
(stevep) strung together this:

I have a gravity fed HW and pumped CH system with a programable
thermostat. If I have the thermostat programmed to come on all day
inc. overnight then the boiler keeps tripping in to heat the water
despite heat not being required for the CH.
Can I fit a thermostat to the airing cupboard tank so that when the
room temp is OK the boiler will not light up or is there some other
easy solution?

There is nothing on the HW side of a gravity system that would bring
the boiler on, so it must be the room stat calling for heat.
--

SJW
A.C.S. Ltd.

Central heating thermostat
 

On Mon, 22 Mar 2004 19:58:14 GMT, Lurch wrote:

On 22 Mar 2004 05:32:21 -0800, in uk.d-i-y
(stevep) strung together this:

I have a gravity fed HW and pumped CH system with a programable
thermostat. If I have the thermostat programmed to come on all day
inc. overnight then the boiler keeps tripping in to heat the water
despite heat not being required for the CH.
Can I fit a thermostat to the airing cupboard tank so that when the
room temp is OK the boiler will not light up or is there some other
easy solution?

There is nothing on the HW side of a gravity system that would bring
the boiler on, so it must be the room stat calling for heat.


In a simple gravity H/W and pumped C/H system:

a) The programmer will only allow the C/H to be on if the H/W is on.

b) The H/W terminal on the programmer powers the boiler.

c) The boiler uses its own internal thermostat to determine when to run.
(which it can only do when powered)

d) The C/H terminal on the programmer supplies power to the pump via a room
thermostat.

If you swop the room thermostat for a programmable room thermostat you end
up having to leave the boiler and H/W on all the time.

This can be avoided by either converting to Honeywell C or using a
Danfoss-Randall TP9.

Michael Chare

Central heating thermostat
 

On Mon, 22 Mar 2004 19:58:14 GMT, in uk.d-i-y
(Lurch) strung together this:

On 22 Mar 2004 05:32:21 -0800, in uk.d-i-y
(stevep) strung together this:

I have a gravity fed HW and pumped CH system with a programable
thermostat. If I have the thermostat programmed to come on all day
inc. overnight then the boiler keeps tripping in to heat the water
despite heat not being required for the CH.
Can I fit a thermostat to the airing cupboard tank so that when the
room temp is OK the boiler will not light up or is there some other
easy solution?

There is nothing on the HW side of a gravity system that would bring
the boiler on, so it must be the room stat calling for heat.

Now I've re-read the OP a couple of times I've decided that the above
statement is utter nonsense. Sorry for having to make everyone read
it.
On a more sane note, if the original two channel programmer is still
wired in then the system could be altered so that a permanent feed is
supplied to the programmable room stat, which then switches the boiler
and pump on. And the HW channel of the programmer switches on the
boiler only. The heating channel on the existing two channel
programmer would be defunct.
--

SJW
A.C.S. Ltd.

Central heating thermostat
 

Central heating thermostat
 

On 5 Apr 2004 02:57:28 -0700, in uk.d-i-y
(stevep) strung together this:

(Lurch) wrote in message ...
On Mon, 22 Mar 2004 19:58:14 GMT, in uk.d-i-y
(Lurch) strung together this:

On 22 Mar 2004 05:32:21 -0800, in uk.d-i-y
(stevep) strung together this:

I have a gravity fed HW and pumped CH system with a programable
thermostat. If I have the thermostat programmed to come on all day
inc. overnight then the boiler keeps tripping in to heat the water
despite heat not being required for the CH.
Can I fit a thermostat to the airing cupboard tank so that when the
room temp is OK the boiler will not light up or is there some other
easy solution?

There is nothing on the HW side of a gravity system that would bring
the boiler on, so it must be the room stat calling for heat.

Now I've re-read the OP a couple of times I've decided that the above
statement is utter nonsense. Sorry for having to make everyone read
it.
On a more sane note, if the original two channel programmer is still
wired in then the system could be altered so that a permanent feed is
supplied to the programmable room stat, which then switches the boiler
and pump on. And the HW channel of the programmer switches on the
boiler only. The heating channel on the existing two channel
programmer would be defunct.

Have been on holiday but thanks for this reply-it looks like this is
the answer but how do I do it? I have a programable thermostat
connected to a landis and gyr rwb2.9 mechanical programmer.

Easiest solution is to change the existing 2 channel programmer for a
single channel to power the HW only. You will need to move the current
CH switch wire to the permanent feed in order to send a permanent feed
to the room stat. (You could just use the one channel on the current
programmer instead of buying a single channel.) That should do the
trick.
--

SJW
A.C.S. Ltd.