# Weather Compensating Settings

On this occasion I am largely inclined to agree with David above to a degree.

However radiator heat output is not so linear with low room/rad differentials because convection air flows need a larger temperature to become significant. So that could be a factor in producing curves with a finite radius.

But that's not usually explored because radiator makers are lagging behind the trends towards WC where low rad temperatures are used significantly. Instead they concentrate on maximum heat outputs as that's what the cost conscious are looking for to enable the smallest cost rad.

Tony

Heat transfer is faster (or slower) the greater or lesser the temperature difference. Therefore as the delta T between inside and out narrows the line is not linear. Well, that's how it was taught to me.
I have to disagree with that.

Heat loss is directly proportional to the difference in temperature between inside and outside. If you double (or halve) the delta, you double (or halve) the heat loss (transfer). So the radiator output will also double or halve. It is therefore a linear relationship.

Stelrad Stars will confirm what I am saying.

Stelrad Stars confirms the opposite actually as rad output is determined in a room at a constant 20 degrees C.

Who cares, you get the drift.

Stelrad Stars confirms the opposite actually as rad output is determined in a room at a constant 20 degrees C.
I don't see how Stars does confirm the opposite; in fact I would be amazed if it did.

We are not talking about changes in the inside temperature but the outside.

Let's assume you want a constant 20°C inside, and you need 10kW when it is 0°C outside. Then if it is 10°C outside you will only need 5kW to maintain 20°C inside. For a constant internal temperature, the heat required is proportional to the difference in temperature between inside and outside.

Agile said:
However radiator heat output is not so linear with low room/rad differentials because convection air flows need a larger temperature to become significant. So that could be a factor in producing curves with a finite radius.
I think you may have hit the nail on the head.

All rad manufacturers publish a conversion table showing how the output varies with Delta T ( difference between mean rad temperature and room temperature). I looked up the Stelrad table and then plotted it on a graph. It was virtually linear! I am beginning to think that the Conversion Table is based on theory and does not take into account the effect of temperature on the convection output. Maybe the WC gurus are aware of this, hence the non-linearity in the curves.

Because it is of no practical relevance I would not be surprised if many makers just take the easy solution and show things as linear even if they are not at low diffs.

As an amusing aside, if you look up one of the Remeha boiler instructions, they publish a graph labelled "heating curve" which is actually shown as a straight line!

Tony

Has anyone here actually looked at a chart plotting the heating up of something to a target temperature?

Linear it is not.

Has anyone here actually looked at a chart plotting the heating up of something to a target temperature?

Linear it is not.
I think Newton's Law of Cooling may have something to do with that.

FYI. There is nothing in the protocol that accepts input from an external sensor, that would therefore have to be included in the control so the curve will differ between manufacturers and manufacturer's own software, or so I'm told by the Open Therm Association. Another reason to keep controls and boiler manufacturer products paired.

Just reading a EU guide to the ErP coming in Sept. You can see Honeywell have two guys on the advisory panel. All other options for controls classification are simply described and given a percentage package efficiency uplift. Describing TPI thermostats there's an added paragragh of what it does and how good it is.

MJGas was asking the other day if Evo contributed to SAP.... forgot to ask Honeywell - will have to drop them a line later.

As I understood it, if the boiler had a weather sensor, it passed the info onto the OT controller and the end result was down to that controller.

Is certainly what happens with my boiler at home.

MJGas was asking the other day if Evo contributed to SAP.... forgot to ask Honeywell - will have to drop them a line later.

As I understood it, if the boiler had a weather sensor, it passed the info onto the OT controller and the end result was down to that controller.

Is certainly what happens with my boiler at home.

Don't know the answer to the SAP question but in my experience the control has to modulate the boiler flow temp for the improvement in SAP.

Yes to the OT controller doing the calculation but some have the WC connected directly to the controller others through the boiler.

FYI. There is nothing in the protocol that accepts input from an external sensor
But the OT Protocol has parameters for OTC (Outside Temperature Compensation). In particular: ID0, ID50 and ID58.

EU guide to the ErP coming in Sept.

FYI. There is nothing in the protocol that accepts input from an external sensor
But the OT Protocol has parameters for OTC (Outside Temperature Compensation). In particular: ID0, ID50 and ID58.

EU guide to the ErP coming in Sept.

Best place for ErP at the moment is the HHIC site.

I am not offay with protocols, I can't even figure out COM ports, but I wrote to the OT Association and got a mail back saying no it doesn't.

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