On 08/12/2011 14:45, Tim Streater wrote:
In article ,
John Rumm wrote:

On 07/12/2011 22:58, Tim Streater wrote:
In article ,
John Rumm wrote:

Modern boilers will load sense and modulate - so as the differential
between flow and return falls (which in a correctly working and
balanced system indicates that the house temperature is rising), the
boiler will reduce its output to better match the rate of heat loss
into the house.

You talking about gas or oil, here, John? Or both?

Mostly gas. Some modern oil boilers may be able to modulate, but with
far less range and flexibility compared to a gas one.

It is worth remembering that condensing model with oil fired boilers is
not so useful as it is with gas fired boilers. The difference between
lower and high CV for gas is just over 10% but for kerosene it is only
just over 6% and for diesel it is marginally less.

Be nice to think an
oil job can vary its output. Otherwise, seems to me, there is a good
risk it just dumps its heat up the flu if the return temp is too high.

Its probably not that bad - as it can't modulate as much, the flow
temp will start to rise and it will then cycle off on its internal
stat. Since the pump will still be running, the hot water in the
boiler ought not be wasted and can still be circulated out to the
rads. Cycling like that is less desirable than a wide modulation
range, but it is not the highly inefficient cycling that one
associates with old cast iron boilers being cycled on the room stat
with no pump overrun.

Ah. In our case we have only one room stat (and its in the hall). Sounds
like I should turn it up high and control matters via the TRVs.

Possibly not. We have had many an argument over the years about where is
the best place to mount the room stat and position the radiator without
a TRV. Having the room stat in a place where you don't much care how
closely the temperature is controlled is not ideal as far as I am
concerned. Not only do you have to ensure that the hall warms up more
slowly than the rooms where you actually spend your time TRVs are also
more susceptible to changes to outside events than a room stat so have
to adjusted a bit more frequently if you happen to be sensitive to cold.
Be that as it may (and many would dispute the living room as the best
place for the room stat) turning the hall stat right up will leave the
boiler firing up just to heat the hall to a temperature you really don't
want to waste gas on. There is no ideal solution but throttling the hall
radiator down rather than turning the hall stat up is the cheaper one
for your circumstances.

For what its worth I have separate upstairs and downstairs heating zones
with programmable stats in the main bedroom and the living room.

--
Roger Chapman