[OpenTRV-interest] Weather compensation

[John]

On 22/12/16 19:31, Damon Hart-Davis wrote:

> One variant of our hardware (for a district heating project) was
> measuring return temperatures (and flow was measured centrally), but
> in general a modern boiler should manage its flow and return itself,
> eg modulate down then shut down if the return is too high.

Almost all boilers will, as you say, modulate down to maintain their set 
temperature.

>> On 22 Dec 2016, at 17:10, Rob May <rob at themayfamily.me.uk> wrote:
>>
>>> though I think the efficiency and therefore CO2-saving gains are
>>> mainly from reducing flow and return temperatures during milder
>>> weather.
>>
>> Have we considered a way to add sensors to measure the flow and
>> return temperatures? (even if we can't control them).  I may be
>> mis-remembering, and may be showing my lack of knowledge, but I'm
>> sure that I read somewhere that many condensing installations were
>> not efficient as the return flow was always too high for them to
>> actually do the condensing bit.

It's not that they're "not efficient": a standard-efficiency boiler must 
be designed (almost) never to condense, to avoid corrosion, so under 
normal operation its flue gases will be well above boiling point (to the 
extent of being a burn hazard). A "condensing" boiler, even if it's not 
actually condensing (much), can extract more heat from its flue gases.

However with lower flow and return temperatures it can extract quite a 
bit more heat by condensing (more) steam from its flue gases to liquid 
water. The problem is that in practice the user usually sets the 
boiler's flow temperature control high enough to produce enough heat 
during the coldest weather and doesn't turn it down in milder weather: 
this is where modulating controls (either weather compensation or a 
modulating room temperature controller) can extract greater efficiency 
from the boiler.

-- 
John Stumbles                                    http://stumbles.org.uk
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