[OpenTRV-interest] Programmable TRV performance

[Tim Small]

I don't know of any standardised controls, but there may well be some.

With a gas boiler you can increase energy efficiency by several means:

1. Reducing burner output - with most boilers the highest efficiency is
reached at the lowest burner rate (since the heat exchanger is a fixed
size, it works most efficiently when handling the smallest input).

2. Reducing the water circuit temperature - the gap between the flow
temperature and the outside temperature always results in wasted energy,
since you can't (without a heat pump) have throw away exhaust gas at
lower than the flow temperature.  This effect is amplified greatly
amplified in a condensing boiler, as the latent heat is recovered much
more effectively as the temperature drops further:

http://www.lemsworld.com/dew.jpg

3. Reducing pump speed (as long as this doesn't mean that other parts of
the system start working less efficiently - the optimum pump speed
varies under different conditions).


So:

1. Max burner output:

This should be as low as possible, but it cannot be less than the
steady-state heat demand, this depends on:

The heat demand of the building (e.g. which rooms are heated at the
moment, and the levels of insulation in the building etc.).

The outside temperature.

How fast you want it to warm up if it's too far from the set-point.


2. Water circuit temperature (flow / return / avg whatever):

This should be as low as possible, but it cannot be lower than necessary
to put enough heat into the building to meet the steady-state heat
demand, this depends on:

Everything which 1. depends on, but also:

The output of the heat emitters at a given circuit temperature  (i.e.
the difference between circuit temperature and room temperature -
radiators with higher output will allow this to be set lower, similarly
lower thermal resistance in underfloor heating installations will also
allow a lower circuit temperature).




An external control system can tell the boiler something like 'heat at
30% of maximum demand' (and it can use some internal algorithms to
decide the values of 1. 2. 3. above, plus maybe other things like the
difference between the flow and return temperatures), and these are
presumably the sort of protocols which your message refers to, or the
control system could just change the variables directly, and some
boilers will allow this e.g. the Vaillant which I have will allow all of
1. 2. 3. to be set via the "eBUS" interface.

For my system, at the moment I have manually limited the max burner
output (for space heating) to it's minimum (6 kW for my boiler I think),
as my max house heat demand in steady state is something like 2 kW
anyway, through the boiler front-panel setup system.

I currently manually control flow temperature (because I have no
controls installed yet, although on the plus side it is giving me me a
greater understanding of how to control this algorithmically).

Tim.