Is the sensor exposed to direct sunlight in the morning?

Posted by: @bobflux

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Is the sensor exposed to direct sunlight in the morning?

I think not but have now added the cloud coverage weather graph, just in case. 

Potentially the temperature sensor could be shielded temporarily, to see if it makes a difference… 

North facing wall out of direct sunlight atcall times of day and yes not exposed to next doors flue.

I suppose you could put it in a Stevenson screen!

Posted by: @batpred

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All being well, we would expect the two external temperature lines to move along roughly in sync. But they do not.

This is a Midea problem as well. The outside air temperature sensor, which as you say is used for weather compensation, is actually inside the the heat pump, near the air intake, so in reality it measures the air intake temperature, which is not the same as the wider OAT (outside air temperature). My heat pump has adequate ventilation, but when running it, as it should, pushes out colder air, some of which finds its way round to the back of the unit, and so gets sucked into the air intake. This then appears as small changes in the AIT (air intake temperature) when can be seen on my monitoring charts. These become more noticeable when the heat pump starts using defrosts, when it starts pumping out warmer air.

Does this matter? Perhaps not. As long as the AIT is in proportion to the OAT, then the AIT can act as a proxy for the OAT. But the real question is does my heat pump's weather compensation work as it should and the answer is yes, so the AIT is an adequate proxy for the OAT. I just have to remember when looking at the line labelled OAT on my charts that it isn't the true OAT.

By the way, all charts should have the Y axis start at zero (except on very rare occasion, and when that happens it should be made very explicit that the axis does not start at zero). Not starting at zero produces what are known as gee-whizz charts, beloved of marketing types and other racketeers, because they visually exaggerate the apparent effect, gradient, difference, growth in the value of your investment, number of extra tractors produced etc. The real world starts at zero, the visual human brain starts from zero, and charts should do the same thing, start at zero. 

Example: growth of 1%, from £100 to £101 over a decade:

Both charts show exactly the same data, only the Y axis has been changed. The gee whizz chart on the left looks most impressive, but it is a distortion of reality. The one on the right, far less impressive, is what actually happened.  

Posted by: @jamespa

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I suppose you could put it in a Stevenson screen!

That’s what we will potentially do. But i did have a close look this morning and @bobflux is right, the wall is not completely north facing and the morning sun does reach it..

I am now less concerned as it seems the boiler flue is unlikely to interfere. I will ideally find a uv safe shield matching the wall paint and leave the sensor where it is… 

From a quick glance at the manuals, I think the arotherm also has an air intake sensor, which would work better in our case, but I doubt it is configurable.. 

Posted by: @batpred

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From a quick glance at the manuals, I think the arotherm also has an air intake sensor, which would work better in our case, but I doubt it is configurable.. 

 

Mine certainly does.

My oat sensor is tucked under the eves of a north facing garage which ensures sun never gets to it.

Posted by: @batpred

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From a quick glance at the manuals, I think the arotherm also has an air intake sensor, which would work better in our case, but I doubt it is configurable.. 

 

Mine certainly does.

My oat sensor is tucked under the eves of a north facing garage which ensures sun never gets to it.

It wouldn't be my first go-to suggestion for most people, @batpred, but given your history with Home Assistant I'd say you might want to consider the eBUS Adapter Shield C6 Stick configured to talk MQTT to Mosquitto on HA. It would take a bit of research to see if that setup would allow you to override OAT from the Vaillant sensor with a temperature from your own third party sensor, but if you could then you could site your own sensor wherever you want.

Hardware cost is abou5 EUR30, so not a big risk but definitely something to research before buying. If you do get it, you'll obviously then end up with local control of your Arotherm.

Posted by: @cathoderay

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The outside air temperature sensor, which as you say is used for weather compensation, is actually inside the the heat pump, near the air intake, so in reality it measures the air intake temperature, which is not the same as the wider OAT

I would expect the air being pulled past the temp sensor to have a cooling effect.

Don't any ASHPs have a connector on an internal control board to which a completely separate sensor can be connected?

Surely manufacturers must've contemplated a situation where sunlight will affect the readings from a sensor which is physically attached to their own product?!

Posted by: @transparent

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Don't any ASHPs have a connector on an internal control board to which a completely separate sensor can be connected?

Yes.  Vaillant for example has a wireless oat sensor (not sure if there is also a wired version) which can be suitably located.  I would imagine that they are not unique.

Posted by: @batpred

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the wall is not completely north facing and the morning sun does reach it..

Had the same problem. I ended up with a Zigbee sensor stuck to the trunk of a tree 🤣 

What the sensor is mounted on matters: if it has high thermal mass, for example a wall, then its temperature will change much slower than air temperature and that will influence the sensor. Ideally it should be in free air and not exposed to sunlight.