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I have a Samsung ASHP linked via Heatmiser controls to a 14 zone, 21 loops UFH system. We commissioned this back in March 2020 just before lock down and after 1 week, it shut down as the property is passive insulated and triple glazed. Airtight too at just 0.17achr. Due to lock down delaying our finishing and selling the old place we finally moved in in September. Since the cold weather in early December I've been trying to get the heating system to work to our requirements. Initially sitting in the lounge and not being warm enough despite the Neostat showing 23deg, I invested in a calibrated thermometer and set about calibrating the Neostats to this. Property has MVHR too, so adds to the overall picture. This has now been nearly two months of adjusting but recently discovered that when the MVHR was switched off, the stats jump up in temperature value and records the house warming up overnight when the heating is off. So further probing found that the stats are recording their own heat. Front panel runs at 27 degs, the thermistor sensor on the bottom edge has no natural vents in the rest of the housing so with no air movement, the central heat of the stat just warms this up. Taking the stat off the wall I found the rear fixed plate running at 29 degrees. Top this all off with the fact that the bathrooms with remote sensors are fine and record the actual temperature and are not affected by MVHR airflow leads me to conclude that the Neostats are not fit for purpose. They generate too much internal heat to read correctly. To prove this I took one apart, drilled four holes in each side of the housing at the top and behold, it reads more accurately, now only 0.6 deg high with still air as opposed to 2 deg plus beforehand. The photos show the temperature with the stat given a gentle hand fan movement of air, still air without fan and the temperature of the face. The final one shows the drop in recorded temperature when. the MVHR cuts in. Note this MVHR is a Brink Flair 400 running at just 50m3/hr in this shot, the dining room being over 120m3, it is not the slightly cooler air hitting the stat directly as the dining room outlet will be serving up only about 6m3/hr but merely the stirring of the air stopping or reducing the stat reading its own temperature. So thanks for reading this far and just wondering if anyone has had head scratching moments with these stats? Going to try buying a decent looking remote thermistor and hooking it up just below the stat to see if this cures the problem and then seek redress through Heatmiser who have already suggested using remote sensing. Their suggestion rather reinforces my view of not fit for purpose.