Is it reading high or low?

Measurement of the outside temperature will directly affect Weather Compensation where the target flow temps are determined by the outside temperature, but if it's out by a known margin (3-4C), you may be able to offset the WC curve by the same amount.

It may also affect functions such as anti-freezing (if your heat pump has such features), which may be set to trigger at temps below 5C, either meaning the do not trigger when intended or trigger too early depending if it's reading high or low.

Posted by: @cliffhanger

↑

Anyway how important is the accuracy of the external stat?  Mine is out by 3-4 degrees C when compared to two mercury thermometers.

Do you mean sensor rather than stat? If so, and we are talking about the OAT (outside air temperature) as reported by a heat pump, then this something that has cropped up a number of times, particularly when the sensor is inside the heat pump, and so samples air that is affected by the heat pump's own activity. 

More generally, it may not matter too much if it is consistently inaccurate. In effect, the WCC settings will accommodate that consistent inaccuracy. But if it is inconsistent, then things may get more interesting. There's currently a discussion on what the real OAT is on this thread.

Posted by: @cathoderay

↑

Posted by: @cliffhanger

↑

Anyway how important is the accuracy of the external stat?  Mine is out by 3-4 degrees C when compared to two mercury thermometers.

Do you mean sensor rather than stat? If so, and we are talking about the OAT (outside air temperature) as reported by a heat pump, then this something that has cropped up a number of times, particularly when the sensor is inside the heat pump, and so samples air that is affected by the heat pump's own activity. 

More generally, it may not matter too much if it is consistently inaccurate. In effect, the WCC settings will accommodate that consistent inaccuracy. But if it is inconsistent, then things may get more interesting. There's currently a discussion on what the real OAT is on this thread.

I mean OAT.   I have an external stat facing north around 2m up

Reading high incidentally

Posted by: @cliffhanger

↑

I mean OAT.   I have an external stat facing north around 2m up

If it is measuring (and only measuring) the OAT, it is a sensor, if it measures the OAT and then does something like turn the heating on or off, then it is a (thermo)stat. It is difficult to answer your original question (how important is accuracy) if we don't know what you are talking about!

@cathoderay 

Right.  It's the outside weather sensor that's wrong compared to Mercury.

I'm running the pump on external weather compensation.   

@cliffhanger - OK, that's clear. My earlier remark about if it is consistently inaccurate still applies. The weather compensation curve will in effect have the same error, but it will work, although be rather confusing. 

Let's say the OAT sensor is consistently reads 4 degrees higher than the actual OAT, and you want a curve that has 50 @ -2 / 30 @ 15 actual end points, then to get that, you will have to set the curve to 54 @ -2 / 34 @ 15 (because when the OAT sensor says 54 OAT it is actually 50 OAT, if that makes sense. All a bit confusing, but no reason why it should not work.

If it is inconsistently inaccurate, then that won't work.

Are then any reasons why it might be inaccurate, apart from the sensor itself being faulty?    

Posted by: @cathoderay

↑

@cliffhanger - OK, that's clear. My earlier remark about if it is consistently inaccurate still applies. The weather compensation curve will in effect have the same error, but it will work, although be rather confusing. 

Let's say the OAT sensor is consistently reads 4 degrees higher than the actual OAT, and you want a curve that has 50 @ -2 / 30 @ 15 actual end points, then to get that, you will have to set the curve to 54 @ -2 / 34 @ 15 (because when the OAT sensor says 54 OAT it is actually 50 OAT, if that makes sense. All a bit confusing, but no reason why it should not work.

If it is inconsistently inaccurate, then that won't work.

Are then any reasons why it might be inaccurate, apart from the sensor itself being faulty?    

As mentioned earlier it is 2m up sand facing north.  

Looks like I will have to make curve changes as it is consistently inaccurate.   Still under guarantee incidentally.  Are they as accurate normally as Mercury?

Posted by: @cliffhanger

↑

Still under guarantee incidentally.

That's probably the most important point, maybe see if you can get a replacement fitted?

@cathoderay 

Yep. Can look into it.

How common is a inaccurate sensor ant idea?

Thanks

Posted by: @cliffhanger

↑

How common is a inaccurate sensor ant idea?

Dunno, Dec might