I picked up a wifi data logger off eBay for a steal, $50 best offer accepted price straight from lascar the manufacturer. It is a lascar wifi tp single sensor. It is cloud based or computer based. I set mine up to be cloud based. They have an app too for the phone that I am using. It shows current temp of sensor and a historical graph. I zip tied mine to the outgoing line back to the boiler right after the flat plate. My temp readings are 4 degrees lower than actual temp. No big deal, I just add 4 degrees for the actual temps. The historical graph is great. Since I strapped it to the return line after the flat plate I can see when the house calls for heat because I get a drop in temp on the graph. I can see how long the house runs for and how often. I can also see when the boiler does a cycle and how long it takes. Here's a screen shot of the app from overnight last night.