I'm hoping someone can explain how it works. According to one source you
can use it with no form of heat sensing from inside the house at all.
Seems weird to me.

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Dave Plowman London SW
To e-mail, change noise into sound.

In an earlier contribution to this discussion,
Dave Plowman (News) wrote:

I'm hoping someone can explain how it works. According to one source
you can use it with no form of heat sensing from inside the house at
all. Seems weird to me.


AIUI, it only works in an open-loop sort of fashion. A friend of mine has
still got a system - installed many years ago - using a Drayton Theta
controller. This controls the temperature of the water going to the
radiators by mixing boiler output water with return water - and creates a
flow temperature which is a function of outside temperature. The aim is
that, by so doing, the radiators will emit the right amount of heat to
balance the heat losses under all ambient conditions. But it's rather crude,
and takes no account of *actual* room temperatures - and so relies on the
radiator sizing, and the gearing between external and flow temperature being
correct.

There may well be modern systems which do something similar by modulating
the boiler without any need of a mixing valve.
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Cheers,
Roger
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On Thu, 14 Aug 2008 00:08:35 +0100, Dave Plowman (News) wrote:

I'm hoping someone can explain how it works. According to one source you
can use it with no form of heat sensing from inside the house at all.
Seems weird to me.

We had an expensive new building at work with 'weather compensation'.

Apparently it means that when it's cold outside, you get really cold
inside as well. If it's hot outside, it's sweltering inside.

That's how ours worked, anyway :-(

In article ,
"Dave Plowman (News)" writes:
I'm hoping someone can explain how it works. According to one source you

I'll have a go. I'm not familiar with current commercial systems,
but I know well the system my dad installed around 1959, and the
system I designed myself and benchmarked about 4 years ago,
but haven't got round to installing properly.

can use it with no form of heat sensing from inside the house at all.

That's how my dad's system worked. The controller was called
BMT (don't know what that stands for). They were quite common
in very large commercial heating installations, but very new
to domestic heating systems at the time. It was promoted by
the Coal Board, and dad had a coal-fired boiler at the time,
although it was retained for the heating control with the two
subsequent gas boilers dad installed. It was retired in 1999,
mainly because dad had a CORGI to install the 3rd gas boiler
and the bloke didn't have a clue what the BMT was or how it
worked.

Anyway, on to how it worked... You had to do heat loss
calculations very much more accurately than is done today.
Maintenance of room temperatures relied on knowing exactly
what the heat loss would be at a given outside temperature,
and sizing the radiator to match that exactly. Then the BMT
monitored the outside temperature and adjusted the radiator
flow temperature based on how much higher or lower the
outdoor temperature was than the temperature on which the
calculations were based. This was done by mixing water from
the boiler with return water in varying proportions inside
the unit. It had an outdoor thermostat phile and sensed the
flow temperature (so it didn't care what the boiler temperature
was, providing it was higher than needed). It was powered by
the water pressure across the pump (a thin pipe was required
from it to the other side of the pump for the "power").
The pump therefore ran continuously, and the radiator water
gradually changed temperature as the outside temperature
changed. It couldn't respond to indoor events, such as
someone leaving a window open, but that was generally a good
thing. Some manual controls on the BMT allowed you to shift
the temperature scale, either to change the indoor temperature,
or to boost the system whilst heating the house from cold.

My much more recent system worked along the same lines, but
uses computer control. The radiator flow temperature is set
by dynamically adjusting the set temperature of the boiler
and having it modulate to meet this. The ideal is that you
reduce the radiator temperature such that the room stat (if
you have one) almost, but doesn't quite switch off. This is
the coolest you can have the radiator water circulate and
get sufficient power from the radiators, whilst having the
condensing boiler operate most efficiently. This temperature
again depends on the outside temperature, and therefore on
how much power you need the radiators to give off. A modern
system will also monitor the indoor temperature, and will
boost the system when the indoor temperature is far short
of the set temperature, which will speed up heating up from
cold, and compensate for things like having the front door
open.

Once you have that level of computer control, it's easy to
also have automatic adjustment of the set room temperature,
e.g. setback at night or when the house is not occupied.
This is strictly outside the scope of weather compensation
though -- it's really occupancy prediction or occupancy
sensing, depending on how you do it.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

In article ,
Andrew Gabriel wrote:
I'm hoping someone can explain how it works. According to one source you

I'll have a go. I'm not familiar with current commercial systems,
but I know well the system my dad installed around 1959, and the
system I designed myself and benchmarked about 4 years ago,
but haven't got round to installing properly.

can use it with no form of heat sensing from inside the house at all.

[snip]

Thanks for the explanation, Andrew.

To explain I've just fitted a Viessmann boiler and included this option.
Which is reversible. I also fitted a remote extension programmer which
includes a room temperature sensor. With the lack of any real information
on their site I sort of assumed they worked in conjunction - but their
technical guy says it's one or the other.
Of course until the cold weather arrives I can't make any decision - and
the options are all in software - but I've a feeling my lifestyle means I
won't want a system that has to be on 24/7.

--
*I got a sweater for Christmas. I really wanted a screamer or a moaner*

Dave Plowman London SW
To e-mail, change noise into sound.