I have been thinking about my central heating this morning as it seems to be
getting very toasty at the moment. We have a Combi boiler (Worcester Bosch
Highflow 400) which has a heat setting on it. There are TRV on all the rads
(except the bathroom - I assume to stop the boiler overheating if the TRV's
shut off) , but we dont have a room stat.
Are the TRV's really temperature controlled or do they simply reduce the
amount of water the rad can get? If that is the case, then with no room
stat, the only way I can control the overall temperature is by the boiler,
so I have to go into the garage and turn it down/up. Is this a normal set
Normally I would leave the heating on 24*7 during the winter and control it
via the room stat so it would get to a reasonable temp and then switch off,
but now if I leave the heating on the boiler tuns all the time (this must be
expensive!!) and the bathroom is like a sauna. The missus loves it, but I
guess the heating bills will be huge!
Presumably you have a programmer with a time switch which tells the heating
when to come on? There will be a cable connecting the CH ON terminal on the
programmer to an appropriate point on the boiler. This cable needs to be
diverted to go via the roomstat - so that the live feed only gets to the
boiler when the programmer and roomstat are *both* calling for heating. In
other words, the CH switch in the programmer and the roomstat both need to
be wired in series between the mains and the boiler. You may well have a
junction box somewhere to which the programmer and boiler are both
connected. If so, this probably has a link between the two terminals to
which the roomstat needs to be connected. If this is the case, you simply
need to remove the link and take 2 wires to the stat - probably to the COM
and NC terminals if there are more than 2. I assume that the pump is
integral with the boiler - or at least controlled by the boiler? If so, this
will take care of itself once you have the boiler under control.
I have just pulled out the wiring diagram for the boiler and it looks ok
actually. I simply (!) need top wire in the live, switched live and neutral
from the stat and then remove a link between the live and switched live
A / B-/\//\- C (room stat)
| | |
| | |
1 2 3 (control board)
Where A is live
B is switched live
C is Neutral
and the link beween 1 & 2 is to be removed
Cheers for the advice!
Should be a piece of cake then! As others have suggested, If you use a
wireless programmable stat you will need virtually no wiring and will also
be able to have different temperatures (or switch off completely) at
different times of day - even if you don't currently have a timeswitch in
I spotted a couple of things from your original post.
TRVs are fairly effective in that they do monitor the room temperature
(albeit they are a little influenced by being near the radiator).
There is a wax or equivalent capsule inside which expands with
temperature and reduces and ultimately stops the water flow. In that
sense they are somewhat more "analogue" than "digital" which is not a
bad thing anyway. Of course, you also have the effect in a house
that there is transfer of heat from room to room so the overall
environment is quite complex. However, they are effective for what
Adding a room thermostat is a good way to implement overall control of
the house temperature.
I completely agree with Bill, an RF thermostat would probably be a
good solution for you. Apart from the Honeywell CM67RF there is the
Danfoss TP75-RF (about to be superceded by TP7000-RF)
These consist of a remote unit which you can site anywhere, or even
take from room to room if you wanted. There is a clock and a
temperature setting and measuring function and the whole thing is
One useful capability is of night set back. This means that you can
drop the temperature over night by a few degrees rather than turning
off the heating completely. 6 degrees or so setback is fairly
typical. Apart from the improved comfort, depending on the system
and the house, doing this can also result in less use of energy as
well. One of the principles behind this is that in some systems and
houses, when the heating goes on in a cold house, there is an
overshoot past the set point and more energy is used. If the house
is not as cold then the overshoot is less. However this does all
depend on the complete environment of the system and the house thermal
characteristics. Also, with this type of room thermostat, the
temperature sensing is more sensitive and accurate than an old
fashioned bimetal strip type.
Moreover, there is also what is called a proportional control
facility. This basically turns the boiler on and off in proportion
to the heat required when the temperature is a degree or two either
side of the set point. The effect is that overshoot can be
Added to this, the boiler that you have is a modulating type which
will adjust heat output according to demand.
Overall, this sounds quite complex. In one sense it is - the
control theory for a system like this and a house is involved.
However, you don't need to be overly concerned.
The main points to achieve are that a) you are getting to the right
temperatures when you want them, b) that there is not significant
temperature overshoot when the system begins the start of a heating
period and c) that the boiler doesn't short cycle (= 30-60 second
burns) because it is producing too much heat.
In reality, you have to experiment a little until you are happy with
Regarding installation, at the boiler end you have a receiver which is
connected to the boiler. You can download the installation manual
for the boiler and you will find a wiring diagram. The boiler has a
set of terminals with a permanent live and a switched live for the
heating. At present, these will have a link between them, I would
After removing the link, you would wire out to the receiver (which
should be located outside the case of the boiler) using the permanent
live and neutral and then the switched live, plus an earth if
It's possible that you might already have a timeswitch on these two
terminals, although I note that some versions of this boiler have one
The best thing if there is a time switch already would be to set it to
24x7 as you are now and to use the clock function on the remote room
If you can run the wiring easily, there are non-RF versions of these
two programmers which cost a little less and have the same
I hope that that helps. This is not a particularly difficult task
if you are comfortable with doing wiring. Obviously, sketch down
what you have before you start in case you need to revert back for any
To email, substitute .nospam with .gl
Well, your system certainly wouldn't satisfy the latest building regs on
energy efficiency - because the system has to be capable of turning itself
off when the heating demand is met.
The simplest solution is to install a room stat - which really needs to be
in a room whose radiator *doesn't* have a TRV - otherwise they will fight.
You could probably put it in the hallway, and leave the TRV on that rad
fully open - or, even better, swap it for the conventional valve on the
ISTR that you said something similar in an earlier thread. Whilst true, it
still wouldn't satisfy the new regs because it will come on again when *it*
cools down - even if no heating zones are calling for heat.
No, not when IT cools down, when the water circulating through it cools
down, and if you have insulated pipes and no demand, that will be an
awfully long time. In my case well over ten minutes to overcome the
thermostat hysteresis. Then a quick minute burn up to temp, and a ten
minute gap again.
In fact since I superlagged the CH pipes in the attic, its even longer now.
So your boiler runs 10% of the time during the middle of the day when no
heating is required?
This is simply not allowed for a new system. You might think it acceptable.
HMG thinks not. If no heat is required, the boiler and water must be allowed
to cool down. The requirement is pretty clearly worded (but only applies to
gas/oil, not solid fuel).
Yes, but it doesn't measure the room temp, on nights when it has been cold,
I have to turn the boiler up or it gets cold, and then the next day the
place is boiling so I have to turn it down again. A room stat would give
I actually don't run my central heating when the whole house is warm enough.
And teh boiler cycles in teh house stat when one is used anyway. So waht
is the difference?
Nothing, except the pump runs all the time.
There is no requirement to make sure your pump doesn't run when the
whole house is warm.
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