My neighbour has just had a new Baxi combi installed. They've put TRVs on
all the rads bar one and have ripped out the old room thermostat. Is this
normal? I find being able to set that to 15 or so useful if you're leaving
the house unoccupied for a few days but maybe that's not how it's done these
The rad without the TRV (in the bathroom) you could fry an egg on. Just as
well there aren't any kids about. It looks as though you can adjust the CH
temperature on the boiler but it's already below 50% on the scale.
Normal for a poorly designed installation, yes. And remember that heating
system electrics are beyond most plumbers.
Most sensible installers would recommend a programmable thermostat which
allows different temperatures to be set automatically throughout the day
and days of the week. You can set them for different temperatures for
night, morning, when you're out at work, and evening. And have designated
'weekend' days where you're likely to be in and want the same temperature
So if the weather gets very cold you alter the thermostat on the boiler?
Very convenient, that. For the installer's profits, certainly.
*Red meat is not bad for you. Fuzzy green meat is bad for you.
Dave Plowman firstname.lastname@example.org London SW 12
True, but they'd have to be brain dead to get rid of the room stat, it's
only 2 wires. At worst they could have just used an old fashioned one and
turned the temp up and down as needed, still easier than going round all the
It sounds more like a rip off than incompetence, I wonder how much each of
those rad stats were charged at?
You switfh the whole heating off really. Or turn the TRV's down to frost
ou need one rad that doesn;t shut down, or some other form of bypass - a
radioator that will take the full flow when all teh others are shut down.
Bathroom is good since its a small room and needs the heat to keep
condesation down. Another option I favoured in the past was a small rad
in an airing cupboard. You can also fit a bypass loop that allopws teh
pump to circulate hot water without doing very much heating at all.
There must be some sort of boiler interlock installed. Otherwise, the system
does not comply with the building regulations. The interlock is normally of
the form of a room thermostat. However, it is permissible to use TRVs and a
flow switch to detect that all the TRVs have closed off. You need to find
out if your system has such a switch. If it does not, get the installer back
and ask how the boiler interlock requirement of Part L1 is met.
Actually, thinking about it, it can't be compliant, as the bathroom radiator
without the TRV would mean that any such flow switch would be ineffective.
But then, I suppose you could put the switch after the bathroom rad. I would
imagine such a system would be compliant.
| | | |
| RAD TRAD TRAD
| | | |
RAD = bathroom radiator/bypass
FS = flow switch interlock
TRAD = TRVed radiator
The bathroom radiator would only be on if another radiator in the system
Lunacy. Who wants to go round the house 2 or 3 times a day turning rad
stats up and down? I'd get the fitter to put in a programmable room stat
and save all the hassle (and a load of money on unnecessary heating).
Can you show me exactly where this is stated?
My Part L building regulations have no mention of use of thermostats
beyond stating thet either TRV's or other thermostats should be used.
Ther is no issue with teh boiler running continuously. Boilers stop when
their own internal stats detect the return flow is up to temperature.
I seen no reaosn to install flow switches unless too simply protect the
pump...but a bypass loop does that anyway. Also how does the system
detect the condition when a TRV has opened up? It can't unless teh pump
is switched on to check flow rate...
There seems to be an urban myth here.
With a TRV system, the idea is to pump more or less continuously, and
let the TRV's decide on demand. The boiler merely cuts in when the
return flow has dropped to the boiler stat level.
Use of TRV's with an overall zone stat makes them essentially useless.
Either they are still demanding heat when the main stat cuts out, or
they have shutdown before it itself does, in which case it never
does.... you cannot have a romm or area controlled by two stats on
essentally the same zone.
With all due respect, you and Christian are talking what appears to be
On Fri, 12 Dec 2003 16:05:30 +0000, The Natural Philosopher wrote:
How about page 16, paragraph 1.41
"b) Where it is proposed to effect control by
thermostatic radiator valves, a room thermostat
(or other device such as a flow switch) should
also be provided to switch off the boiler when
there is no demand for heating or hot water."
Also page 19
"d) Boiler and hot water storage controls.
So that replacement boilers (other than solid
fuel boilers) and hot water vessels can achieve
reasonable seasonal efficiency, the work may
also need to include replacement of the time
switch or programmer, room thermostat, and
hot water vessel thermostat, and provision of a
boiler interlock and fully pumped circulation.
Section 3 of GPG 302 34 gives more advice on
how this can be done."
The requirement is new from the 2002 building regulations. I suspect you are
simply still thinking in terms of pre-2002 requirements.
No, the thermostatic switch is not enough. It must totally turn off and not
keep the primary circulating water hot. As every single gas/oil boiler in
existence has such a thermostatic device (or similar analogue modulating
control), why do you think they would mention room thermostats or flow
switches as interlock devices?
If the system you propose is installed, the boiler will operate (to heat
primary circulating water) even when all TRVs are closed and the hot water
cylinder is hot. This is not allowed. I'm afraid you appear to be in a
minority of one in insisting that an all TRV system is allowed without room
thermostat or flow switch.
Indeed. There are three methods proposed.
a) Install TRVs and a roomstat. You can balance and design the system such
that the TRV rooms heat up marginally quicker than the room thermostat
rooms. This ensures that the TRVs turn off before the room thermostat
engages the boiler interlock. This provides good temperature control in all
b) Install a flow switch that determines that when no flow occurs through
the TRVs, the boiler is turned off, and the circulating water cools down.
This provides even better temperature control in all rooms.
c) Install a fully controlled zoned system (i.e. room thermostat/no TRV in
Exactly. Because you are a prat was the instant assumption....:-)
Er, but it DOESN'T DO THIS. The primary circulating water, being nice
and lagged, stays hot for a LONG time, and the boiler stat has
hysteresis in its thermostat, so it may take up to an hour before the
little tiddy bit of water looping around gets cool. IF you have the
system on, and it doesn't need ANY heat input in an hour from the rooms,
might I suggest you switch it off anyway?
It makes NO sense to have a house stat.
Nothing in the system I am proposing wastes heat, or short cycles the
boiler. It is merely - and without actually reading the regs in full
detail, I can't even be sure - a TECHNICAL INFRINGEMENT. Not something
that wastes power - which is after all the spirit and purpose of the
Ad the average bozo can just about understand turning up the room TRV's
to get warm, and does so, and then calls in the plumber cos the rads are
cold. Right. Been there, done that.
On Fri, 12 Dec 2003 16:58:21 +0000, Christian McArdle wrote:
AIUI in scenario (a) Only rooms which are
i) Less frequently used
iii) Rooms with additional sources of heat.
Need to have TRVs
It is a very good idea to have the space in which the room stat is located
containing a _correctly_ sized radiator without TRV.
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
FWIW I have a full TRV system (i.e. no rads without TRVs), with an
auto-bypass loop at the boiler, *and* a programmable room stat in the
The TRVs are set so that, in the evening, the rooms are each at the
desired comfort level - thereby fulfilling the aims of a full-TRV
system as you describe it.
However, the hall-stat is set (at 21C in the evening) to shut down the
boiler when all rooms are up to temperature.
In the morning we have the hall stat set to 19C, as we're rushing
around and don't like it too warm in the mornings. There's no way we
could do this without that stat in the hall, unless we went around and
altered the TRVs all the time.
We do get in the situation where some rooms are not up to temperature
and the hall stat shuts the boiler down - this is as you predict, so
against all advice I have fitted a TRV on the radiator in the hall
too. This I set a little cooler than we'd ideally like, so the hall is
the last to warm up - it needs heat "leaking" from other rooms to
finally hit 21.
It's a bit of a fudge, but I can't find a better way of doing it! I
suppose you might argue that that we might as well set the stat to 45C
in the evenings and let the TRVs do their job, which is fair enough,
but we still need the stat to achieve a lower temperature than the
TRVs would allow.
So, with all due respect you are talking what appears to be twaddle
:-) Well, partial twaddle maybe.
Remove HAT before replying
Set the TRV to full (preferably replace, so the radiator is lockshield both
ends). Then balance down the hall radiator using the lockshield so that it
warms up slowly and doesn't get very hot. Then, when the other radiators get
shut down via their TRVs, more water gets to the hall radiator. By the time
all TRVs have shut, the hall radiator gets the full force of the boiler,
heats the hall and the room stat shuts down.
This would defeat the boiler interlock and make your system inefficient, as
the boiler will continue to short cycle, heating your primary water which
may lose heat into areas not required to be heated.
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