I have been following the recent excellent threads and am now coming
out of my confusion, I think.
Our existing solid fuel boiler is not going to cope with the
additional requirement of our extension and will be replaced by oil in
the nearish future. This will allow some degree of control (yippee).
My original plan was simple DHW stat and TRV's on all rads, but this ,
I now see, has difficulties in achieving 'boiler interlock'
From pointers given by various recent posters I believe I want:
1) two CH zones, since I have identified that all our rooms fall into
one of two distinct heating requirement patterns.
2) separate control of hot water.
Now... someone (BillR, I think) mentioned the Honeywell CM67's. Having
investigated, these look excellent, allowing each heating zone to be
controlled to different temperature levels throughout six different
periods of the day. I particularly like the look of the RF versions
and, AIUI, by linking to an additional HC60NG to be used as the
boiler/pump controller - as per honeywell installation guide - I
could then have TRV's on all the radiators except those in the 2 rooms
(1 room in each zone) where the CM67's are installed and this would
provide the required 'boiler interlock'. I think I'm OK with this so
far (?) Someone tell me if I'm not!
Then I come to the DHW....
I cannot see an equivalent beast to the CM67 for DHW purposes....
I guess I can use a simple cylinder thermostat - although I don't
necessarily want to have a tank full of hot water permanently. Even if
I did, how do I get this stat to talk to the HC60NG that is
controlling the boiler?? I am sure I am missing something very simple
here but cannot put my finger on it.
It may become clearer when I have the boiler installation information
to hand but in the meantime, I'd really appreciate if some kind soul
can enlighten me.
Perhaps there is something to be said for the old anthracite boiler...
a) light it once a year - DHW control is sorted
b) switch on the pump when you want the CH
c) twiddle with the TRV's
d) switch the pump off at bedtime
e) heat leak rad keeps the bathroom warm for the overnight tinkles and
f) the heatloss from the boiler keeps it warm for breakfast!
arhhh the simplicity!
Not really. Interlock is easily achieved. Any argument is about whether it
is worth achieving or not. The benefits of implementing the interlock depend
largely on the attitudes of the occupier. If the occupier doesn't mind turn
it off and on manually, then the interlock is not as effective in increasing
efficiency as when they are not. I prefer a system I can just leave well
alone, yet works efficiently.
It is possible to have an all-TRV system, even subzoned for timing. You
simply plumb as S-Plan, with a 3 channel programmer, or with separate
programmers conveniently located. But rather than using the 2 port valve
microswitches, you achieve interlock using a flow switch after the bypass,
but before the heating zone valves. The output from the flow switch can be
combined with the HWC zone valve microswitch to provide a boiler call for
Personally, I prefer to use a programmable room stat, as it allows different
temperatures at different times of day. Having implemented it, it very
effectively maintains temperature in all rooms, including those with TRVs,
provided the system is reasonably balanced. The rooms with the programmable
thermostats are those used most frequently (master bedroom and lounge). This
provides very accurate temperature in those rooms (measured away from the
radiator) and a handy nearby override mechanism.
The only problem is the kitchen, which doesn't get to temperature. However,
this isn't really a control issue, as the room is 5m x 2.5m, with 2 external
solid walls, 3 large windows, 1 door, french doors and a single tiny
radiator at one end suitable only for a little bathroom. I'm planning to put
a fan convector (i.e. Myson Kickspace 800) in to resolve this.
Unfortunately, there's no room for insulation, although the draught proofing
could be seriously improved.
Do you mean an RF cylinder thermostat? I'm sure I've seen one somewhere.
However, I prefer to use proper cable when possible. My experience with
other RF stuff is that it isn't quite as reliable as a bit of twin and
earth. OTOH, if running cable is very difficult, then they are tempting.
When wiring this system up, you'll still need a normal programmer. The hot
water side handles HWC timing. The heating side acts as a master controller
for your heating zones. You normally set this side to either "OFF" or "24H"
and allow the programmable thermostats to handle any timing. There is good
reason for maintaining a separate programmer. Firstly, you can turn off the
entire system simply, without traipsing round the house to all the room
stats. Secondly, wayward teenagers wanting 30C in the middle of the night
(and having a programmable stat in their room) can be defeated. It is best
to leave on 24H, if possible, though, as the programmable thermostat can
keep a reasonable overnight temperature, such as 12C, rather than turning
off completely. However, if you go overboard in balancing up TRV radiators
so as to starve the master room, this may not be as effective. Doing so just
a little, though, helps maintain steady temperatures.
I think that a better and more convenient solution to this would be
the Danfoss Randall TP75-RF (about to be replaced by TP7000-RF) for
the heating zones and a WP75-RF for the hot water. This would give
you completely independent time and temperature operation for each
zone and the hot water.
These have a common receiver type and you can get up to 3 channel
receivers, although probably to order.
www.controlscenter.co.uk should be able to or any heating supplier.
You would sit the receiver at the boiler position and hook all the
demand outputs to the switched input of the boiler and possibly the
pump if the boiler doesn't have an overrun output for the pump.
If the motorised zone valves need to be in a position remote from the
boiler and you can get power there, then you could have additional
single channel receivers if you wanted. The receivers are trained to
the transmitted signal during installation and you can have multiple
receivers listening to one transmitting programmer.
Obviously this would cost a bit more to implement, but the trade off
is against wiring convenience and time .
What you need is, in effect, an S-Plan-Plus [see
http://content.honeywell.com/uk/homes/systems.htm] with 3 zones, 2 for
heating and one for hot water.
Use a standard programmer and cyl stat to control the zone valve for the hot
water - with 2 or 3 on/off periods per 24 hours, as desired - but always the
same temperature unless you manually twiddle the cyl stat (which is probably
acceptable). On the same standard programmer, set CH to permanently on and
use its CH ON contact to feed the programmable room stats. The actual on/off
periods (and different temperatures if desired) are then controlled by the
room stats - which in turn control their respective zone valves.
The volt-free contacts on all 3 zone valves are connected in parallel - with
the output side controlling the boiler and pump. That way, the boiler and
pump will run whenever one or more zones requires heat and will shut down
when they are all satisfied - achieving the desired interlock.
This is true. In fact, it's only a small change from the Honeywell diagram.
The basic logic remains the same, but the output from the zone valves'
volt-free contacts drives just the boiler rather than the boiler and pump in
parallel. The pump is then connected to the boiler's pump control terminal -
which is live whenever the boiler is firing plus whenever the boiler's pump
over-run stat calls for circulation to continue.
Ah, I see! You're talking about the by-pass loop rather than the wiring. You
are perfectly correct - the Honeywell schematic doesn't show a by-pass loop
and ought to do so.
My point is also valid. The Honeywell electrical diagram shows the boiler
and pump feeds connected togther - so that the pump stops when the boiler
demand is turned off. For a by-pass to work, the pump needs to be controlled
by the boiler so that it can continue to run after the boiler has finished
Very many thanks to all who responded.
My missing link was the need to still have a normal programmer to
handle the DHW - not realising that the programmable room stats do not
replace the programmer. I now have a fairly large bulge appearing on
I don't really have a problem running cables everywhere since the
whole place will be redecorated following the extension. I
I've had a look at the Danfoss site and can see that that would meet
the bill but have noted the comment from Christian concerning the
potential reliability of the RF stats. Since one may be >20m from its
receiver , I think it makes more sense to go the wire route.
Is there anything to choose in reliability of the electronics or TRV's
between various manufacturers?
Thanks again all
Electronics are normally reliable. I'd buy a room stat mainly on the basis
of appearance, provided it had the required functionality. Brand in itself
wouldn't matter to me, except in that a good brand is more likely to
physically look better.
TRVs should be from a reputable manufacturer and expect to replace every 7
years or so when they start to get a little slow. Cheap ones may fail in a
couple of years.
You don't.. You can have a programmable thermostat for the cylinder
as I mentioned - either wired or wireless.
If you do this plus programmable room controller(s), you don't need to
have an additional programmer at the boiler at all.
On the other hand if you are going for wired connection to the
cylinder, you could cover that using a simple cylinder stat and a
programmer at the boiler instead.
They are pretty solid in fact. There are specific standards that they
are required to meet and incorporate error detection to avoid errors
due to interference problems.
I would regard them as a convenience if wiring is awkward, because
obviously they are more expensive as well. However, if you can wire
easily then that makes more sense.
I've used Danfoss and Honeywell controls and not had a problem with
either. Similarly their TRVs.
Since replacing a TRV typically involves a drain down if the problem
is in the base, then I would avoid the cheap products and again go for
Honeywell, Myson, Drayton, etc. Honeywell seem to be the most
commonly available at the merchants.
Whilst this is true, using a conventional programmer with its CH output
connected to the programmable room stats has the advantage that the heating
can be turned off from a single point (e.g. for the summer) without having
to re-programme the stats.
On Tue, 16 Dec 2003 17:32:02 -0000, "Christian McArdle"
No because they all come together at a common point at the boiler
anyway, to form the switched live to fire it up.
If you have two (or more) room programmers, you could bring their
demand signals and connect them together, then go through the switch,
bring in the HW demand and then into the boiler.
This is an "or" function of the room programmers followed by an "and"
created by the switch followed by an "or" with the HW.
Boiler = ((Prog1 OR Prog2 OR ...... ProgN) AND Switch) OR DHW
To email, substitute .nospam with .gl
You couldn't put the switch on the output of the programmers, as that would
short the outputs of them together, causing the zone valves to open in
unison, rather than being individually controlled. You could put the switch
on the microswitch outputs of the 2 port zone valves. However, this would
not be ideal, as if the boiler fires due to the HWC zone valve demanding
heat, then the heating zone valves may already be open, causing unwanted
heat in the radiators.
You could always add relays. However, the standard (cheaper and simpler)
method is the sequence:
Programmer -> Room Stat (programmable) -> Zone Valve -> Boiler (via
On Wed, 17 Dec 2003 09:56:38 -0000, "Christian McArdle"
Does your last sentence make sense ? If the heating zone valves are already
open, then this can only be because one or both of their timers are on and
one or both of their stats are demanding heat. In what way is heat in the
radiators then "unwanted" ? Apart from that, yes, the outputs can only be
commoned at the boiler after the zone valves, or independent zone control
will not be achieved.
I have a vitally important role serving as a bad example.
It is unwanted because it happens despite you switching the central heating
off. This switch should override the programmable thermostat.
So, the idea is to provide a master switch to turn off the heating. If that
master switch is after the zone valve microswitches, then the programmers
may call for heat and turn on the zone valves. The switch only prevents the
zone valves firing up the boiler. However, if the boiler fires up for DHW,
then the valves are already open and will pass water.
To work properly, you need to put the switch before the zone valves, so they
aren't open unnecessarily. This is easiest done by switching the feed to the
programmers, as doing so after the programmers requires relays, or a multi
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.