Originally posted by SteveP
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It seems that rather than using the temperature forecast earlier in the day for "now", (which would give zero lag but rely on the prediction being accurate) they use the last known temperature measurement which can be a few hours out of date as they don't update in realtime.
So on a day where temperature is rising rapidly you'll see the internet reported figure which is lagging behind being too low, and in the evening during a rapid cool down the internet reported figure will typically be too high.
Another thing to keep in mind is the geographical resolution of different weather services which varies a lot from one service to another. Just because you have your location entered correctly in the TCC portal doesn't mean the weather service Honeywell uses has observation stations near your location. For example I live in Motherwell and while weather.com lets you use Motherwell as a location the data it returns is identical to the data for Glasgow - which is 17 miles away and has quite different temperatures and weather patterns due to the heat island effect. (And probably also affected by the Clyde river)
I don't know which weather service Evohome is using (has anyone figured it out ?) but it may be that you're simply a long way from the nearest weather station that service uses so you're getting weather for a nearby town or an extrapolation from further away weather stations.
In my location the Evohome doesn't do too badly - there is a bit of lag but it's usually within a couple of degrees most of the day so accurate enough for weather compensation. Right now my weather station reports 17.7 degrees - which has risen a degree in the last 30 minutes thanks to clouds finally breaking, and the Evotouch is reporting 15C which it has done for at least the last couple of hours.
No internet weather service will give the same accuracy as an on-site, properly located and installed weather station/sensor, but super high accuracy is not needed for weather compensation.
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