Weather Compensation?

[TheScullster]

Hi all

Can anyone explain the purpose of this? If you set the stat to 19c or whatever, doesn't the heating system simply try to attain this figure regardless of what's going on outside?

Phil

[fred]

In article , TheScullster writes

2 options:

1. Boiler turns the wick up when it's cold outside.

2. Boiler turns on earlier when it's cold outside.

[1] Means that the boiler runs at it's max efficiency point except when full demand is absolutely required.

[Tim W]

fred wibbled on Tuesday 05 January 2010 14:25

1st option elaborated: Warmer outside and the boiler modulates to a lower power and lower output water temperature aiming for the most efficient point WRT condensing mode. You don't need so much power from the rads as it's not so cold outside, so reducing the water temp to a more efficient point is still able to achieve the internal heating required.

I have no idea which one is true though. The 1st is a genuine efficiency gain but requires a modulating boiler that would benefit from reduced output temperature (any condensing would?) and has the appropriat econtrol interface to achieve this.

The 2nd is purely a user convenience feature but still useful...

[TheScullster]

"Tim W" wrote

The condensing angle wrt inside and outside temperature makes sense. The CM67 wireless stats I have include some sort of pre-emptive start-up mode. It was a complete pain and had to be disabled.

Phil

[John]

I thought it tried to predict the start time so that the thermostat setting could be achieved by the set time. eg if set for 20c at 7:00am on a cold day, the boiler may start at 6:15am. On a milder day it may start at 6:45

[js.b1]

That would be the smart way of doing it, Rinnai gas wall heaters do this by learning how long it took to reach a desired temperature and then starting earlier - you say how hot you want it at Time-X rather than start to heat at Time-X.

[Andy Dingley]

2nd can also be implemented by an add-on thermostat, without needing to involve a more complicated boiler.

[Tim W]

TheScullster wibbled on Tuesday 05 January 2010 15:32

OK - the CM67 would satisfy the 2nd mode above.

I would hope a boiler with an expensive control could do 1)...

[Tim W]

Andy Dingley wibbled on Tuesday 05 January 2010 16:13

Exactly (eg teh CM67 just mentioned). I don't discount the possibility that a boiler maker might do the same and claim it's "weather compensation". It is, but not the type that might save you money...

[Andrew Gabriel]

OK, reword 2 as "Boiler turns on later when its warmer outside". if that makes the advantage clearer!

[Martin Pentreath]

But to go back to the original point, why does it matter what the temperature is outside? Surely it should only turn on later if it's warmer INside, as indicated by the interior themostat?

[Tim W]

Martin Pentreath wibbled on Tuesday 05 January 2010 18:47

Both options need knowledge of how cold it is outside as being cold outside will affect heat losses (higher so heating will not achieve the same deltaT for a given power output) and also takes into account the fact that the thermal elements in the building have a much larger temperture gradient across them, so the specific heat capacity effect will require a larger energy input to raise the temperature.

Those two statements are interlinked but it seemed easier to state it both ways.

If your internal walls were lined with 100mm of celotex and you had zero ventilation, then knowledge of the exterior temperature would be by and large irrellevant.