I don't know of any sensors that are WiFi enabled, but you could rig up a Raspberry Pi with a DS18B20 or a DHT11 and have the Pi send via WiFi to a web platform.  I have used the DS18B20's before and used a Pi to send an SMS message to a cell phone and it wasn't too difficult to make it work.  It shouldn't be hard to make the Pi send to your web based platform.

Simplelink SensorTag - TI.com is currently Bluetooth enabled and soon WiFi.

Clem

A low-cost solution is to use an ESP8266 with a temperature sensor. There are many examples on the net.

As a practical point, most central heating boilers and controllers are able to fire themselves up when the temperature of the boiler gets too close to freezing. Either one or the other is usually enabled by default, but sometimes they are not.

If you want an almost ready-made Pi solution, the RaspberryPi.org SenseHat device and Node-Red can quickly be set up to send temperature and other data to other devices on your LAN or the Internet. The SenseHat is ridiculously over-featured to make this an economic solution, but Node-Red should be capable of dealing with simpler temperature measurement devices. Node-Red works by setting up a web-server on your Raspberry Pi which you access using a browser and which provides a graphical, drag-and-drop display to connect and process data from the Pi to remote devices.

You can make the temperature measurement easier by buying a low cost Arduino Nano and an Arduino temperature module and programming it via a Pi USB port (the arduino IDE is in the Raspbian repository). Because the Nano is running at 5V rather than the Pi's 3.3v, you have much better sensor choices. Python can then be used to collect the temperature measurements via USB from the Nano and then forward them to the Internet.

I second the ESP8266, I have been using these for a number of projects lately and they are an inexpensive (sub $10) solution to get a wifi connected device on your network for a variety of monitoring tasks. You have the option of programming them in the Arduino IDE, using the Arduino framework, if you would like.

Great suggestion, Doug.

Mike

You could use a ESP8266 with a temperature sensor attached to it. Then use a Raspi with a LCD or a even the 7-inch touch screen and design a GUI for use with it to display the current temperature and any warnings if the temperature reaches a low enough temperature.

The ESP8266 can run off a couple AA batteries (or even a coin cell) for a fair length of time especially if you only send data when you need to. Which would help if the pipes are in a location where you may not be able to easily get a power supply put in. Hope this helps