Outside sensor slope vs room temperature setpoint

Christoph · Jan 25, 2023

I recently installed weather compensation on my Ideal Vogue Max and I'm very confused by the settings menu item called "outside sensor slope", which lets me select a number between 0 and 40. The manual states that this adjusts boiler performance relative to the building insulation, but doesn't tell me what unit that is measured in. Also, the picture in the menu and manual looks very much like a weather compensation graph (outside vs flow temperature), which I can also adjust by changing the room temperature (nominal, not actual).

When I change the room temperature, the boiler immediately reacts as I would expect it to, but when I change the outside sensor slope, nothing happens at all. Does anybody know what that is supposed to do?

oz-plumber · Jan 26, 2023

Outdoor temp 5C - Indoor Temp requested 21C
Boiler operates to temp set on boiler. ( 70C ) ?

Outdoor temp 15C - Indoor temp requested 21C.
Boiler reduces temp of heating water to 50C.

In a nutshell, as the temperature outside goes up, the temperature the boiler heats the water to is reduced.
I'm not sure of the temperature gradients from indoor to outdoor temperatures, and as for the insulation rating of the home to the reduced temperature that the boiler is working to, would be minimal.

Chuck · Jan 26, 2023

What are the model numbers of the boiler and weather compensation kit?

The Ideal Vogue Max systems I've seen had a setting to choose the weather compensation 'curve' by selecting a curve number, IIRC, between 10 and 30. The curves were plotted in the installation or user guide. (A change of 1 in the curve number, roughly speaking, offset the curve by 1°C.)

I think it's possible that the word 'slope' on your display may be a mistranslation of the word 'curve'. The weather compensation curves are maps between T_flow and T_exterior so the (mathematical) slope is [°C/°C], i.e. dimensionless. The slope (gradient) of these curves is typically around one (i.e. not 40) and varies over the operating range, because the heat output from radiators is not a simple linear function of (T_flow - T_room).

So much for the theory. In practice, if hunting through manufacturer's manuals doesn't provide the answer, try calling the Ideal Vogue helpline. If still no joy, try the middle setting, i.e. 20, and see how the system behaves. In cold weather, if the house is up to temperature and the boiler is cycling on and off several times an hour the compensated flow temperature is too high. On the other hand, if the boiler is running continuously but the house is not getting up to the desired set point temperature the flow temperature is too low. Change the setting up or down by, say, 5 units and see what the effect is. Tweak as necessary.

The aim, as @oz-plumber explained above, is to end up with a system that reduces T_flow to its minimum by the time T_exterior = T_room and increases it at a rate that matches the radiator outputs to the building losses as T_exterior falls. It can be a bit of a fiddle, but it's worth putting a bit of effort in to get the system working correctly. When properly implemented, weather compensation noticeably improves both comfort and fuel consumption.

John.g · Jan 26, 2023

Can you post a link to this Christoph?

As stated above the normal weather compensation curve looks like below so if your rooms require a boiler temp of 72C at -5C outside temperature then you set (this particular curve) the heating curve to 1.4 which means it should reduce the boiler temperature to 58C at a outside temperature of 10C and so on as you probably know anyway.

Christoph · Jan 26, 2023

Thanks all for your answers, I already understand it a bit better than before!

The boiler is an Ideal Vogue Max 32 (manual attached, pages 40 and 45) and the weather compensation kit is from Ideal too (manual attached, page 7).

The compensation kit instructions make sense to me (except that I find it hugely confusing that they use the term "room temperature" for something that selects a curve...). The graphs look like this:

(And yes, set at 22C it can't keep up with the heat loss of my house during coldish periods!)

What doesn't make sense to me is this:

Boiler performance can be adjusted relative to the insulation level of the building. To change the slope press MENU and the following screen will be displayed ...

To me, this feels like it's exactly the same as the "room temperature" setting, except that it seems to have no effect at all. Both settings only are available when the weather compensation kit is fitted, so I suppose it must have some kind of effect. I could imagine that "17" was supposed to read "1.7" though. I just changed it from 17 to 30 and the boiler did not react in any way other than shifting the red line further right on the display (I can see the OpenTherm data ID values; none of the numbers change). But, when I change the room temperature even by one degree, it immediately adjusts the modulation and flow temperature.

Maybe "17" is the recommended thickness in inches of the insulation foam

Thanks for taking the time to look at this everybody!

John.g · Jan 26, 2023

The room compensation bit looks straight forward enough anyway!!
Say you want your rooms at 17C and select "17" this is curve 6th from the left so OT of -5C sets the boiler to 61C, 0C sets it to 54C & 10C sets it to 38C
If you require room temps at 20C...curve 9th from the left so -5 sets 67C, 0c sets 62c & 10C sets 46C

A room temp selection of 22C = curve 11.

If you know the present OT then go vertically upwards until it intercepts whatever curve you have selected and the Y axis will show you the present boiler setpoint temperature.

Will have a look at the other bit in a minute.

John.g · Jan 26, 2023

I think what they mean is that if you have super duper insulation and you find the your selected curve is setting the boiler temperature too high then you simply select a lower room temperature which will then lower the slope and consequently the boiler set point?.

John.g · Jan 26, 2023

Christoph said:
Thanks all for your answers, I already understand it a bit better than before!

The boiler is an Ideal Vogue Max 32 (manual attached, pages 40 and 45) and the weather compensation kit is from Ideal too (manual attached, page 7).

The compensation kit instructions make sense to me (except that I find it hugely confusing that they use the term "room temperature" for something that selects a curve...). The graphs look like this:

View attachment 81143 View attachment 81144

(And yes, set at 22C it can't keep up with the heat loss of my house during coldish periods!)

What doesn't make sense to me is this:

View attachment 81145

To me, this feels like it's exactly the same as the "room temperature" setting, except that it seems to have no effect at all. Both settings only are available when the weather compensation kit is fitted, so I suppose it must have some kind of effect. I could imagine that "17" was supposed to read "1.7" though. I just changed it from 17 to 30 and the boiler did not react in any way other than shifting the red line further right on the display (I can see the OpenTherm data ID values; none of the numbers change). But, when I change the room temperature even by one degree, it immediately adjusts the modulation and flow temperature.

Maybe "17" is the recommended thickness in inches of the insulation foam

Thanks for taking the time to look at this everybody!

I see what you mean now by the outside sensor slope which is selected from the menu.
can you take a few screen shots, say 3, one with the temp setpoint the same as the weather one, one with it say 5C lower and one with it 5C higher.