Why I Won't Be Buying A Nest Protect

Nest Protect
The Nest Protect was announced in October 2013 and is a new smoke alarm and carbon monoxide sensor from Nest. It includes a photoelectric sensor for smoke detection, a carbon monoxide detector and speaker. It also contains an ultrasonic activity sensor, which allows you to hush it with your arms when the warning noise sounds.

It really is a very clever and good looking piece of kit but, it is clearly not aimed at people like me and there are numerous reasons why I won't be buying one ...

Smart Sensors

I have a problem with 'smart sensors' in general. I simply want my sensors to just sense and report data back as quickly as possible. It is much better to have the decision making process centralised. This way the decisions can be made whilst considering all aspects of the smart home and thus with much better (and ideally 'whole house') context. This would include simple things like is it dark outside, is anyone in the house or are the owners away on holiday.

Once you start adding additional functionality to sensors, then where do your stop? Should all smoke alarms have gesture control and voice recognition?

Also note that the press often refers to simple networked sensors as 'smart sensors' which confuses things.


Adding extra functionality to what should be a simple sensor makes it much more expensive. The Nest Protect costs £109 in the UK, which is quite a lot of money for a smoke alarm but not outrageously so. By comparison, the 12V dc smoke alarm devices we are currently using cost about £15 and the carbon monoxide (CO) devices about £15 also.

Nest recommend that you install one "on every floor, including finished attics and basements". They also add "inside and outside every sleeping area". This could soon mount up to a significant cost, especially when you include the power requirements (assuming no battery only models) and the potential installation costs.


The photoelectric and ionisation sensors used in most smoke alarms have a limited lifetime. Just like any other smoke detector and carbon monoxide alarm, the Nest has a limited lifetime. The specification claims this to be seven years. This is typically about how long it takes for the internal sensors to wear out. This then makes replacing another significant investment. Replacing all the smoke and carbon monoxide sensors in your home could be an expensive exercise.

When the device does finally expire the Nest Protect user manual describes the following process:

About two weeks before it expires, Nest Protect will light up with a yellow light. Wave at Nest Protect and it will say, "Nest Protect has expired. Replace it now.". you are expected to buy a new alarm and recycle the old one. If you try and keep your Nest Protect going belying this expiration period, it will no longer work and you will not be protected. In reality, the technology will have moved on long before this point and you will probably be looking at a totally different replacement device.

Even backup batteries in smoke alarms fail over time. The EI smoke detectors originally installed in our current home have batteries soldered onto the circuit board and when they fail (both failed within 9 years), the whole alarm has to be replaced. There is a lot to be said for making sensors cheap and easily replaceable components in the smart home.

Very few people use still use 802.11b Wi-Fi, even though is was only adopted in 1999 and mainstream use was much later. Building technologies such as Wi-Fi into basic sensors such as smoke alarms may well make them redundant a lot quicker. Wi-Fi technology and versions have progressed many times.

Future Proofing

The way to future proof your smart home is to use a hybrid technology solution and to employ technology abstraction. The use of wires and cabling should ensure that you can connect and upgrade sensors when needed. The underlying principle of providing power and getting a signal back is likely to remain valid for a very long time. Wires are also extremely reliable technology and my preferred approach to any home automation.

For this reason we have used standard alarm cable for basic (digital) sensors and Cat5e cable for anything that might require data networking. 1Gpbs data transfer should support all the sensors we can envisage for many years and even then we have installed cables in ducting so that even they can be upgraded later if need be.

If smoke sensors are simple and dumb, they can be easily and cheaply swapped out and the interface to them is unlikely to change much over long periods of time. They can be treated as simple, powered switches. The internal technology can be improved and upgrade over time but the interface remains unchanged and integration of this and any other sensors into our smart home is a simple process, using common infrastructure and components (e.g. USB or Ethernet IO board).

This model applies to pretty much all sensors in the home and is why we use wired magnetic contact sensors on all the doors in our home. Such technology is likely to have a useful lifetime in excess of 50 years.


The Nest Protect is a thing of beauty and very well engineered too but, my ideal smoke alarm would be invisibly integrated into the structure (ceiling or wall) of our smart home (so long as its air flow and sensitivity is not restricted). Smoke alarms are structural elements of the smart home that don't need to be seen. Just knowing it is there and do its job is enough. I'd rather buy sensors that are white, compact and beautiful, if invisible is not an option. We also consider flashing lights a distraction.

Carbon Monoxide

Whilst carbon monoxide is slightly less dense than air (and so will rise), in the real world there is not enough difference and it will readily mix with the air in your home, due to natural convention and circulation. The reason carbon monoxide kills so many people is that it is odourless and colourless. It destroys the ability of the red blood cells to transport oxygen around your body and can still kill you after it has been removed.

In an ideal world, you would not put a smoke alarm and a carbon monoxide sensor in the same place. I have installed a CO sensor in our kitchen close to the gas boiler. The smoke alarm in our kitchen is mounted at the other end, some way away from our toaster and cookers.

If you don't have a gas boiler or gas cooker/hob, then you don't really need a carbon monoxide sensor in your home but, it will still offer some protection. Portable gas heaters are responsible for many deaths in the home.


With an off-the-shelf 12V smoke alarm such as we are using now, there are just three simple connections to the alarm sensor itself. These are ground, +12V dc and trigger output. I use standard alarm cable which is cheap and extremely reliable.

Power & Batteries

My approach has been to use a centralised and shared 12V Uninterruptible Power Supply (UPS) for all the essential features in our smart home. This shares the cost of the infrastructure across all smart home functions and provides increased reliability and control.

The Nest Protect comes in two forms:

  1. Battery powered - The battery only powered device does address the real issue with smoke detectors, which is that the batteries run flat. The added technology in the Nest Protect will mean that it uses more batteries and that they will need replacing more often. At least the Nest Protect will warn you when to do this. The battery powered version uses six AA batteries. It will not work with rechargeable batteries.
  2. Mains powered with battery backup - This means you either have to have mains power in place already or install a dedicated mains power circuit to the required locations for your Nest Protect devices. They still require you to purchase and install the three AA backup batteries and these will need replacing at some point too. The Nest Protect will warn you when to do this.

The use of a battery powered Nest Protect is not a step forward in our view and such a device with Wi-Fi and other technology will result in regular battery replacement. I have taken every opportunity to replace battery powered sensors in our home and where this is not possible we have ensured that these devices can report batteries levels back to our Home Control System (HCS), which can then inform us that they need replacing before they run flat. Nest only recommend one type of battery is used in the device, the Energizer Ultimate Lithium batteries (L91).


I have implemented a common alarm system that covers the whole house and can be clearly heard in every room, not just local to the smoke alarm. This is essential in a large house. All of the sensors in our home are effectively linked to it and have the ability to invoke its features.

Our smart home alarm system also has a much wider range of capabilities. This includes an internal alarm, external alarm, external strobe, voice announcements, SMS messaging and IP messaging. This is a common common capability of our Home Control System (HCS) and can be utilised or triggered by any sensor in our home, with the appropriate channels being used based on the sensor triggered and the house status. This also avoids each type or brand of sensors duplicating alarms and notifications.

The smoke alarm modules we use have both audible and visual alarm capabilities on-board but, we can disable these if need be (depending on where the sensor module is located).

Voice Announcements

Our smart home currently has voice announcement capability and it has been installed such that is can be heard in all rooms (if required). The speakers have been installed in optimal locations and are not linked to the smoke detectors or carbon monoxide sensors, which have also been installed in optimal locations. The volume of each speaker has also optimised for each specific location and its surrounding environment.

Our home uses a common language and voice for all announcements and we chose one that we liked. The use of a common and recognisable voice means that it is clearly identifiable as the house communicating with us. If all 'smart' devices had their own voice announcements and speaker, then there would be a wide range of speakers and accents being used within our home. The use of American accents in UK homes and services can often prove frustrating, though I'm sure Nest will have localised voice announcements.

I see voice announcements as being a common capability that should be used and triggered by any sensor in our home. As an example, all the temperature sensors currently in our home each have individual lower and upper alarm points and can generate voice announcements if these are crossed. They also have the ability to detect the rate of temperature rise and raise an alarm if the temperature is rising too quickly. This rate is set on a per zone basis, e.g. parking a car in the garage (with its hot engine) doesn't set off an alarm.

The voice announcements are synchronised across multiple Nest Protect devices (both mains powered and battery powered), so that if one speaks, they all speak. This is achieved using Wi-Fi and assumes all are in range of your Wi-Fi access point. It is interesting as it shows that Nest have thought about the wider problem but, the still haven't fully addressed it. When you give many sensors and devices in your smart home an independent voice, they will at some point start to talk over each other. Our centralised, queue-based approach basically ensures that they all take their turn.


Wi-Fi is a terrible technology for home automation and security and safety systems. It is not reliable and is subject to many internal and external factors. I've run numerous technology trials in my day job and most of the problems I encounter are down to Wi-Fi in people's homes.

The Nest Protect may also be installed in a non-ideal location by some people purely to ensure a good Wi-Fi signal.

This view is further supported by the Nest Protect user manual which states:
WARNING - Nest Protect Smartphone and tablet notifications require a functional Wi-Fi connection. They're only as reliable as your home's Wi-Fi network and aren't a substitute for a third party emergency monitoring service.

The Nest Protect also has 802.15.4 wireless technology, so even if your Wi-Fi is not working the Nest Protect sensors can try to communicate with each other. This has an even shorter range in the real world though and mainly used with mesh networking technology (e.g. ZigBee) to ensure whole home coverage.

Mobile Apps

Nest provide both an iOS app and an Android app to work with the Nest Thermostat. Later this month, Nest will update these apps to version 4.0. which will be compatible with the Nest Protect. The Nest Protect will provide mobile notifications when you connect it to a free Nest account using Wi-Fi. It can also deliver low-battery alerts, Heads Up and emergency alarm notifications to your Smartphone or tablet. In an emergency, the 'What to Do' feature will remind you of recommended actions and give you one-button access to an emergency phone number.

Mobile apps can be a really useful way to interact with your smart home but, if I have to install an app for each each brand of device then life is going to get confusing and complicated. A far better approach would to be to have one common mobile app that supports your whole smart home.

Because this is beyond my current coding capabilities I have used SMS/iMessage for notifications and also have a natural language interface and web app for remote access.

Safety Lighting

I have installed Safety lighting at optimal locations in our home and it has also been optimised in terms of lighting colour and brightness. As an example the safety lighting at the top of our landing is just one 5mm warm-white LED consuming 4mA at 12V dc. It was specifically configured to not dazzle and provide enough light to see, but not keep people wake when a bedroom door has been left open.

This is controlled by a single twilight sensor in our smart home, which is also used as an input to a whole set of features and functions. This is also installed in an optimal location and has intelligence to avoid false readings and hysteresis.

Emergency Lighting

Nest Protect features a 'path light' features where it briefly lights up if you walk underneath it at night. Similarly, we have emergency lighting installed in our home.


If you already own a Nest Thermostat, then the Nest Protects can act as secondary occupancy sensors around your house to tell the thermostat that you are still at home, even if you haven't passed by a thermostat for a while. This would keep your thermostat from putting itself in Away mode because it thinks you have left the house.

This is no substitute for proper smart home presence and occupancy detection as we have implemented in our smart home. Occupancy detection needs a much greater number of sensors to work well and different types of sensors to work accurately. Such a hybrid technology approach is beyond the scope of the Nest Protect.

Software updates

The Nest Protect now introduces the concept of software updates to smoke detectors!


Nest state: "We all know why smoke alarms are torn off the ceiling or missing batteries: because every time you make stir-fry, the smoke alarm cries wolf. Or just as you're falling asleep, you hear a low-battery chirp. They've become annoying. And that's a safety issue.".

In the 18 years we have spent in our current home, the smoke alarm has gone off just 3 or 4 times. In two cases it was due to burnt toast. In the other case, it was something left under the grill by accident and in this instance there was actually a small fire. This isn't really much of an annoyance in my view.

This video shows the mains sales features of the Nest Protect:

Heads Up

The Nest Protect features a 'Heads-Up' capability. If it sees that smoke or CO levels are rising but have not yet reached emergency alarm levels, it will give you an early warning, without actually sounding the alarm. Personally, I rather just get an alarm and deal with it. It happens so infrequently and I'd rather it was me making the judgement call as to what is excessive smoke or carbon monoxide.

Nest Wave

Nest Wave lets you silence nuisance alarms with a wave of your hand and is slightly less of a gimmick but, it will be used very infrequently. It involves waving your arms at the smoke detector, which is kind of what most people do with their current smoke detector.

Nightly Promise

Nightly Promise is a feature that shows you its sensors and batteries are working with a quick green glow after you turn off the lights. Instead of lighting up green during Nightly Promise, it will glow yellow.

What Is Missing?

It would have made a lot of sense to add a temperature sensor to the Nest Protect, so that it could be linked to the Nest Thermostat. One of the biggest flaws with the Nest Thermostat is that it only monitors your house temperature at one point. We have a temperature sensor in every room in our smart home, so that we can maintain accurate control down to zone level. The downside of this is that the Nest Protects would be typically placed on the ceiling, which is a bad place to measure actual room temperature. Although the Nest Protect has a 'heat sensor', it is not currently used for this purpose as far as I am aware.

What Is Good About Nest Protect?

Firstly, the Nest Protect is a real, retail product you can buy now. Despite this article describing why I wouldn't buy one, much of what I have built and implemented is not available in any store.

The Nest Protect is very good looking and if it is anything like the Nest Thermostat, it will be very well made indeed.

All of the the publicity and discussion around this device will get people thinking about the safety systems in their home.

Should You Buy A Nest Protect?

If you don't have a smoke alarm in your house then yes, buy one now. Tell all your friends about it and your bit of the world will be a safer place. Any smoke alarm is better than none.

If you are the kind of person that buys the latest Smartphone and you are thinking about upgrading, I would say do it. You can obviously afford it and your will think more about safety in your home. You will very likely also become an evangelist for these smoke alarms and spread the word. This can only be good.

If you are about to buy a new home or embark on a self build then my answer would be no. Now is the best time to re-assess your understanding of what a smart home is and could be. Your money could be better spent by taking a more holistic approach as described in this article.

In Summary

I like the Nest Protect but it is not aimed at people like me. I know that a smart home is greater than the sum of its parts and dumb sensors linked into the larger smart home infrastructure is the future.

This is how we have taken a commodity, low-cost smoke sensor and made it smart. We have also done this with a carbon monoxide alarm.

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