[OpenTRV-interest] Controls and stuff

[Tim Small]

On 22/12/16 14:30, Simon Hobson wrote:
> For most conventional systems, there will be several benefits. Firstly energy savings from running the boiler cooler and hence losing less heat through the exhaust, elimination of non-condensing operation when combination of low demand and high bypass ratio gets the return temperature above that required to keep the boiler condensing, and a massive reduction (due to effectively maintaining a higher flow rate) in system noise that normally happens when demand is low so there's a high back-pressure making all the valves "hiss"

For my boiler there are a few useful controls which you can set over
eBus (AFAIK), these include:

. Set point
. Set point tracking (either flow or return temp)
. pump speed
. Output %limit (35% is the minimum for my boiler IIRC)
. Cycling back-off time

I think the following are probably useful inputs to try and optimise
efficiency:

. Discomfort (e.g. some measure of how far below the comfortable range
for a human each zone is)
. Number of zones calling for heat
. Wind speed
. Outdoor temperature

Whilst set point is often talked about, what isn't is the % of max
output which the boiler is operating at (modulation) - it's usually just
left up to the boiler to determine based on distance from set point and
flow/return variation over time.  It does have quite a large effect on
efficiency tho' (with some boilers more than others):

e.g.

http://hpac.com/site-files/hpac.com/files/archive/hpac.com/fastrack/f1.gif

http://www.diynot.com/diy/media/boiler-efficiency.27041/full

https://www.automaticheating.com.au/wp-content/uploads/2016/11/Figure-1.jpg

http://www.greenshootscontrols.net/?p=153

... so I think it's probably worth considering too.

My hunch is that for many systems the boiler is operating at high
outputs when it's not really needed, just because the demand for heat
has just come on.

With more information the controller can probably do a better job - if
the total heat input needed to balance losses is quite low, then it may
be worth limiting the output according to some efficiency / discomfort
trade off.  e.g. if it doesn't impact discomfort at all, then limit
output to try to maximise efficiency.

OTOH, if the occupants have suddenly come home unexpectedly, and the
house is freezing, then warming things up quickly should be a priority
so don't limit output - bump up the flow temperature too perhaps.

BTW, with a low set point, and the output limited to 35% (=6kW for my
boiler - the total thermal losses for my house are something like
2.5kW), I've never seen any condensation 'plume' coming from my boiler's
flue, which makes sense if you think that those plumes (and I can often
see 10s of them looking out of my window) are due to condensation in
free air instead of on the heat exchanger.

Tim.