Sony has been moulding its Xperia Z-series flagship smartphone line into the perfect model. Well, it more-or-less achieved that with the Z3 towards the end of 2014, so for the new Z3+ to arrive mid-2015 led us to believe we'd be getting more hot stuff.

Only this time around it's a case of literally. The Sony Xperia Z3+ – or Z4 as its called in Japan, just to break the naming convention and confuse the bejeezus out of the world – has Qualcomm's new Snapdragon 810 processor on board and, put simply, it gets too hot all too often. It can be felt to the touch, while such overheating causes app-outages and some crashes, so it's not exactly the kind of hot stuff we were hoping for.

After a case of third time's a charm with the Z3, is the Sony Xperia Z3+ an ironic minus and a step too far in the wrong direction for Sony's flagship?

Let's preface this review by stating how hot it is in the UK right now. It's really hot. Like, 35-degrees Celsius hot (somewhere around the 95F mark, American friends). Now that doesn't bode well for any smartphone left to work in the sun. But we've had the Z3+ for a considerable length of time as our personal phone (12 days and counting), where it's seen cooler conditions and even ventured to New York and Marseille's climes for a bit of humidity variety. The end case, whether rain or shine, didn't cease the heat issues.

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Bizarrely it's certain tasks that really push the Z3+ whereas others have little impact. The camera – which we'll get to in more detail later – is the prime culprit for overheating woes. However, other tasks that we expected to put that core into meltdown, such as switching on the wireless hotspot during an hour-long taxi ride, didn't noticeably raise the temperature at all (whereas other phones, such as the Huawei P8 (which looks almost frighteningly similar), would have probably melted their way to the earth's molten core).

Which is a real shame overall, because to look at Sony has got plenty about the Z3+ right. This isn't just a Z3 with a new processor slipped inside, it's actually a slimmer 5.2-inch screen model than last year's flagship. Flip the Z3+ on its side and that 6.9mm thinness can be felt; it's one of the slimmer devices out there and that was particularly noticeable after swapping out from the LG G4 to take on this new Sony for review.

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It's well made too, in terms of materials and aesthetic design anyway. Glass tops the front and rear, sandwiching a smooth metal frame that's chamfered into a curve to the edges, although the protective and partially-rounded corners – presumably there to avoid scuffs and digging into the legs through tight jean pockets – don't seamlessly integrate into the overall design. We said the same thing about the Z3, and the Z3+ follows a similar design language at a marginally adjusted scale.

However, from a more logical design point of view, well, things are amiss. A slimmer design than the Z3, likely adding to heat dissipation issues, coupled with the less space for a battery (it's 2930mAh compared to 3100mAh of the earlier Z3) and hotter-running processor doesn't add up to good battery performance. And that's one of Sony's better-known traits, so to slip in the rankings in this area is a poor show.

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We've rarely succeeded in making it through a normal work day with a single charge of the Xperia Z3+, meaning a spare battery charger has also been a staple in the travel bag. Plenty of other phones suffer the same, but should the Z3+ have an ultra hot moment then you can pretty much watch the battery life cry its life out of that body. Without even using the phone on one day (background processes were running, but the screen was off), we dipped from around 75 per cent charge to under 45 per cent in a few hours, while other days have typically hit under the 40 per cent mark within 11-12-hours.

Fortunately Sony is known for its great battery-handling software, called Stamina Mode. A firm favourite of the earlier Z3, it's back for the Z3+, enabling extra longevity per charge by adjusting what the phone is doing, such as cutting out Wi-Fi. There are Ultra Stamina and Low-battery modes that further limit functionality, the latter a last ditch saver for the basics when you're down to single digits on the battery front.

We've had to manually intervene too, by flicking on Airplane mode when the phone gets too hot, to cease its communications functionality for a short burst (the kind of thing you don't really ever want to be doing with a phone).

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And if it does all get too blisteringly hot – not that we've found it hot enough to be dangerous – then there's no harm throwing this Xperia into an ice bucket. Because, like the rest of the Z-series line, the waterproofing feature is one that sets Sony on a path most of its competitors can't even touch (even Samsung has ditched the waterproofing for its S6 and S6 edge models). It is fun to throw a phone in the bath and not worry about it thanks to IP65 and IP68 compliant ratings – although the most practical use for this will be accidental drops in the sink/toilet/family pond/giant swimming pool (that you obviously have in your five-acre back garden).

One area we heaped praise onto the earlier Z3 was with its 5.2-inch 1920 x 1080 pixel screen. The very same panel appears in the Z3+, making for great results in a variety of lighting conditions thanks to decent auto-brightness.

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It might not be qHD (2560 x 1440 pixels) like some of the competition, but at this scale that really doesn't really matter. More pixels would have only led to poorer battery performance anyway. Sony tries to compensate with a raft of its own technologies – Triluminos, X-Reality, Live Color LED and so on – for ultra punchy visuals, but sometimes they're a bit too punchy. Within the settings we would avoid the hyper blues and reds of Super-Vivid, sticking instead to X-Reality or even selecting Off for a more subtle visual.

To interact with the screen a double-tap will wake it, just as it does with the LG G4. Only with the Sony it's overly sensitive: the number of times we've had the phone in a pocket and it's switched on, assuming it's being tapped, we've lost count. Too many times we've had temporary 30-second blocks due to PIN mis-entry (by our thighs, no less) or pick up the phone with 75 numbers typed into the dialler. Whoops.

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With good visuals you'll want good sound to match, and the Xperia Z3+ doesn't fare too badly in this department. Although the two small speaker slits top and bottom at the front look insignificant, they push out sound to a reasonable level without distorting to the max. However, given the available space above and below the screen, we're not sure why they aren't larger still.

Using the Xperia Z3+ will be a familiar experience for Sony fans, with its Android 5.0-based reskin taking the base Google operating system and jazzing things up a bit in visual terms. We rather like the look of it; it's smart, typically-Sony-like, complete with suave colour palette. It's a good balance between tweaked user interface and Android feel.

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It's all setup in a very much pro-Sony format, though, just like the Z3 before it (no surprises and, to be honest, fair enough). That means Sony services galore: from Remote Play with your PlayStation 4, through to its movie and Music Unlimited entertainment services. You can move these Sony icon staples around on the home screen should you choose, while the usual Android options to create your own folders, arrangements and widgets will keep the phone feeling just as personal you want.

One other interesting addition is the Movie Creator app, which takes your photos and video to create highlight reels from days out. It feels a little like a work in progress, as the results are rather staid, but it's a sign that Sony wants to get some of the love that the HTC M9's video highlights attract.

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However, there are some oddities. No voice recorder is installed, for example, while the swipe-down shortcuts control panel ditches the press-and-hold approach, so if you need to make deeper changes than on/off you'll need to dig into the settings and search out what you're looking for. That sounds like a small thing, but it's really annoying: we like to press-and-hold on Wi-Fi, for example, should we need to force change a network (you know, opt out of one of those annoying BT networks scattered throughout towns); but that's not possible, so you'll need to hit the settings button, seek out the Wi-Fi section and make adjustments the long-winded way.

If there's ever a feature that Sony has sold itself on, it has to be the camera. The Xperia Z3+ features the same 1/2.3in-size, 20-megapixel sensor as the earlier Z3. That means it's the same physical size as you'll find in most current dedicated compact cameras, spreading that significant resolution over a larger physical surface than many smartphone competitors. The image quality, therefore, is better than many.

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And while that might remain strong when the Z3+ is capturing delightful snaps, the absence of optical image stabilisation is a letdown. That and the overheating issue is at its very worst in this department.

Open the camera, use it normally and it might shut down after snapping as few as 10 frames, warning of overheating. You can load it again, but the same thing will happen. It's a catastrophe for a smartphone maker that prides itself on offering among the best camera experience.

Even the video capture – which at 4K resolution ought to be impressive – is slow and clunky in real-time capture (not playback). Record a clip and there's no real-time feedback on screen for a couple of seconds, just the occasional frame that jitters its way into view. With the flagship of flagships that just shouldn't happen.

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There are heaps of additional fun shooting options on board too, such as augmented reality features where you can throw a cat/gorilla/different colour skin over your face. Which is, um, insane and utterly pointless – and will see the camera shut down within 60-seconds due to overheating.

Again, this is all a massive shame, because the combination of camera smarts at the phone's heart is great. Seemingly it's just incompatible with the Snapdragon 810 processor or, perhaps, Sony has been lazy with software optimisation on its side. Maybe it's a bit of both. Whatever the cause, it doesn't matter: because the Z3+ camera experience is, quite simply, broken. It shouldn't have made it to market like this.

Verdict

From the lofty highs of the Z3, the Xperia Z3+ is, ironically, a big minus against Sony's Z-series. It's hotted things up in terms of design, but also all too literally: that Qualcomm Snapdragon 810 processor gets excessively hot, causing issues with battery life and making some features, such as the camera, approximately useless.

Which is all a massive and unexpected shame. Given the frequency of how often Sony outs flagship phones, we're not entirely sure why this particular one made it onto the production run or, for that matter, was even a necessity. That and the confusing name – Z3+ in Europe, Z4 in Japan – makes us think that, come September this year, we'll see Sony introduce a totally fresh and re-worked flagship, perhaps complete with a new name (if not then keep your eyes peeled for the Z5).

The Z3+ isn't all bad, mind, with decent waterproof design, solid build quality, a snappy user interface and punchy screen. Use it casually and you won't encounter too many overheating issues, but then that's hardly the point of a phone like this.

Yep, the Sony Xperia Z3+ is hot stuff, just a shame that's all too literal. We're going to look at the Z3 and Z3 Compact models fondly, as those are the ones to plump for right now if Sony is your Android phone maker of choice (something the arrival of the Z3+ might bring into question). Hold tight for the Z5, then.