IMM wrote:
"The Natural Philosopher" wrote in message
...
BillP wrote:
"stuart noble" wrote in message
...
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
days.
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.
If the room stat has been removed, the installation does not meet the
minimum requirements of Part L building regs.
Can you show me exactly where this is stated?
It isn't.
My Part L building regulations have no mention of use of thermostats
beyond stating thet either TRV's or other thermostats should be used.
It refers to it as a control interlock. Can be a flow switch. In short, a
device to switch out the burner when the house is up to temp. A boiler is
not allowed to cycle unnecessarily.
What is wrong with letting the CH return stat do this?
That is the point.
I ran a system fuly TRV'ed with the house stat turned flat out for
several years. The boiler neither short cycled nor ran continuously. It
cut off when the TRV's had had enough. There was a single unregulated
radiator in a well insulated airing cupboard.
I have a system now that in part is functinally equivalent, in that I
have room stats on fan blown rads. Plus a few TRV equipped rads. The
pump runs continously, The water circulates continuously, the fans come
and go, and the boiler does the same. It burns a few minutes, and then
shuts down.
Every room is set to the temperature requiertd. I don't have to mess
with heat leaks - which anyway would never work, because there is no
simple place to put a zone stat on this little lot. Dowqnstais hall ae
on a different zone (UF) and upstairs halls/landings are unheated as
they take heat from teh UF system by convection.
Ther is no issue with teh boiler running continuously. Boilers stop when
their own internal stats detect the return flow is up to temperature.
That is cycling - NOT ALLOWED.
I beg your pardon. All boilers cycle under normal usage. They get the
water up to temperature in the primary HW or CH circuits, and then shut
down. That is what they are designed to do for gawds sake!
I seen no reaosn to install ...
That is because you don't understand.
I don't think YOU do then.
There seems to be an urban myth here.
Not so.
With a TRV system, the idea is to pump
more or less continuously, and
let the TRV's decide on demand.
Good idea.
The boiler merely cuts in when the
return flow has dropped to the boiler
stat level.
What if all rads have TRVs? Then the boiler cycles. So a flow switch is
necessary to cut out thre burner.
No, you are confuding 'cycle' with 'short cycle' and also if yopu read
back on all posts, I never said that all rads were TRV'ed without at
least a bypass loop. The point is that short cycling happens when there
is NO flow. With ANY flow the boiler cuts out, and the hystersis of teh
boiler stat means the water cools several degrees before the next burn
comes along to raise it again. Unless you have a stalled pump, that
takes several minutes and more likely up to half an hour.
How quickly it heats up again of course depends on boiler stat
hysteresis and how much water is actually circulating. Obviously if you
stick a bypass loop by the boiler its not a huge amount. However, in my
case a long pipe run to a small radiator in an airing cupboard was more
than enough to mean a 2-3 minute burn was required at the minimum. And
frankly, any house that has its heating on, and requires no heat input
for half an hour, is wrongly timed anyway. I have been sitting here for
the duration of typing this, and my boiler has not fired up, yet the
pump is running all the time in the evening.
Use of TRV's with an overall zone stat
makes them essentially useless.
Not useless, just less effective. If the room stat is in the coolest part
of the house, usually the hall, and the rad is sized and balanced to give
the desired room temp, then if this room is satisfied then all the others
are. As it is the coolest room in the house it will demand heat before the
others, so the other room will not be cool.
Ok, at what point will a system with all TRV'ws and a master stat become
exactly the same as a system with no stat and all TRV's?
Easy. When the stat is turned up beyind the ability of the TRV'd rads to
flip it.
Result, one user adjustable contol that all by itself, renders the house
essentially exactly the way I ran mine. Statless.
Guess why they have stats in the boilers chaps, and make them cycle...
With all due respect, you and Christian are talking what appears to be
twaddle.
'fraid not! It is you who are twaddling along.
No. I still don't see it.
On an ALL TRV'ed system, I'll buy that a poorly designed system equipped
with a very short bypass COULD in theory - but rarely in practice -
short cycle. It will NOT run the boiler continuously tho. If it does the
bloody thing wil explode.! Thats why teh boiler has its stat...
Equipping the house with a stat does NOTHING to remedy that in my
opinion. Either the stat is set too low and the TRV's never do their
work, or the stat is set too high, and might as well not be there at
all. Finding some arbitrary finely tuned position and temperature
setting for the stat is completely beyond the average home owner, and is
far more likely to result in over use of energuy as he rams the stat
open in frustration, and then faffs around with his TRV's.
A non statted sytem that detects a stalled pump is good, except how does
it detect a stalled pump when the pump is off? It can't. So instead of
boiler short cycling we have pump short cycling.
The answer os simple. All TRV's no ghouyse stat and one radiator that
is not TRV'ed that has sufficient thermal inertia and water capacity to
stop short cycling. Or simply put a bypass right across the longest pipe
run. Sure, the pipe run is contributing to heatinbg teh house, but the
situatin where th ehouse is warm enough, all rads are shutdown and teh
bozo still runs his heating every day all day seems rather remote.
Most users can cope with the 'its bloody hot today, sparrows are wilting
in teh park, I'll turn my central heating off'.
The iedal is obviously a separet zone in every room coupled back to the
pump and boiler. TRV's are almost that, the only difference is that you
have to rely on return temperature to tell the boiler its not needed
anymore.
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