Weather Compensating Settings

6 Feb 2015
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United Kingdom

I understand that weather compensating heating controls adjust the boiler flow temperature as a result of the outside temperature (in its simplistic form).

Therefore I would like to know what the set flow temperatures are for various outside temperatures.

My house is well insulated and has a condensation boiler.

Many thanks.
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I have a Veissmann 200 and the control settings are programmable by the user so suit the heat loss of the house

There is no room thermostat as such (though one can be incorporated), rather a "nominal" temperature is set on the boiler control (in our case 17 degrees) and the processor uses that as a reference. 17 is not a true room temperature, rather it is simply a reference point for the boiler control to work around. We find this works very well, with a very even temperature being maintained at all times.

As the outside temperature falls the boiler raises the temperature of the CH water and vice versa. We find that 17 suits "mild" winter temperatures (around the 0C to 10C degree mark) and if the weather is consistently sub-zero we set the reference to about 20

At this moment in time, the outside temperature is 4C and the flow temperature is 50C (within the condensing range)

My slope is set such that with an ambient of 0C the output is 54C and with an ambient of -20C the output is 78C

I'd imagine with a well insulated house a fairly low reference temperature and a virtually flat control curve would be used.
There is no standard figure as it all depends on your house. It needs to be set by guesswork and trial and error.

If your rads were correctly sized for your house then with -1 C outside then the flow temperature would be designed to be 70 C to give a 21 C temp inside.

So as the rad output is roughly linear against temperature then that 70 C would reduce towards 21 C when its 21 C outside.

Perhaps that will help you to understand what is involved.

But there are a couple of difficulties with simple weather compensation ( WC ). If it is used to give a constant temperature continuously then WC works very well indeed.

But if you want a reduced temperature at night then you need a more sophisticated controller.

If you turn it off at night, or to a lower set temperature, then there is usually no facility to give a higher warm up flow temperature when the temperature needs to be higher like in the morning to give a rapid temperature increase.

Some controllers can use a proportion of inside temperature which avoids this problem. I particularly like the Worcester controller which enables you to set the inside/outside reference as you like.

You can actually achieve a pretty good WC effect manually by adjusting the flow temp according to the outside temperature.

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You can actually achieve a pretty good WC effect manually by adjusting the flow temp according to the outside temperature.


Thank you. That's what I have been doing over the last 4 months. So if the outside temp is 0C I would set flow to 54C, for 10C outside I have been setting the flow to 48C. But as temperatures rise I needed some input on the next step, say 15C outside.

There must be a point in setting the flow temperature where it not efficient to set or too low to have an effect on the ambient, say setting a flow of 25C.
The Nest WiFi controller ( and possibly some other similar types ) claims to use weather information obtained from the internet to provide a form of WC.

Anyone looking at that needs to understand that the Nest does not adjust the boiler flow temperature like true WC.

All it does is to use on/off modulation of the boiler leaving the flow temperature as set on the boiler. That is old technology and does not improve boiler efficiency as true WC does by setting a lower flow temperature which allows greater condensing boiler efficiency.

So the Nest is a useful addition to an older heating system but true WC is significantly better as it increases the efficiency. By how much is a more complicated question and there are some wild claims being circulated. Most conservative engineers would probably suggest a figure of 6% as being realistic but it all depends on how it is set up and used.

The Vaillant system uses a built in temperature curves to adjust the flow temperature against outside temperature. You can adjust which curve to use to match the thermal performance of the house but it does take a little while to get it to the best curve.

It relies upon an eBus connection direct into the boiler controls to achieve this so a Vaillant WC programmer needs a modern Vaillant boiler. ;)
The Vaillant system uses a built in temperature curves to adjust the flow temperature against outside temperature.

I have been reading up on curves, how do you determine the best curve for your house. So a modern well insulated house with new condensation boiler?
Trial and error after first digesting the user's manual. The controller will have a default setting to get you started.
You normally have both a curve ( slope ) and an offset ( or level ).

You can adjust the offset as you wish for comfort but the curve is a matter of trial and error.

But the big mistake most people make is to constantly fiddle with it.

Its an automatic system and needs to be left alone to do its job.

Alec Morrow has this link on his website, it shows ones factored take on temps in relation to curve.[/QUOTE]
The Americans call weather compensation "outdoor reset". I can't think of any good reason as "reset" implies starting again.

However, they do have a useful parameter called "reset ratio", which is the difference in water temperature divided by difference in outside temperature.

Example (assuming a required water temp of 70C when it is -1C outside, and a room temp of 20C):

Water temp: Max 70C, Min 20C
Outside: Max 20C, Min -1C

Reset Ratio = (70-20)/(20 - (-1)) = 50/21 = 2.38

This is the same as the curve number in the document mentioned above.

The better insulated the house, the smaller the reset ratio.
That is a Viessmann document and a good one at that too. Not an Alec creation!

Does anyone know if the curves shown by different boiler makers are going to be similar in their effect to each other?

Never said it was Alec's creation, just that the document is on his website, as for the quality, I'm sure most will see the obvious floors in the document without even getting to the claims of WC giving a 20% reduction in your annual fuel bill.

Page 17 of the following link will give you caillants curve(old controller but doesn't matter)

All should be pretty close as they're all calculated as has been detailed above.
I think that was so silly of Veissmann making such an absurd claim.

When preparing the slides for a course I was intrigued to see the curve incorperated in our boiler PCB is not a curve it's linear. When compared against the Open Therm curve, which is a curve, it is very close. I just put it down to an unsophisticated but reasonable 'curve' on the software onboard the appliance until you buy OT which appears to be a little more sophisticated and is 'master' of the boiler software.

I assume, always dangerous, that the OT curves would be the same whereas manufacturers curves may differ. Looking at your link Lee the Vaillant curve differs from the OT. Haven't got the equation we provide with me at home.

All very interesting but fiddle till your're comfortable.

I think it's for comfort not economy but the new ERP is saying raise the system efficiency 2% with WC.

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