The 'problem' is that Honeywell, like a number of smart heating control manufacturers, uses a single study which allows claims like the following to be made:

Quote Upgrading your basic timer and thermostat control to evohome smart zoning could deliver as much as 40% savings on heating your home.* Unquote

Caveated by:

Quote:* Source Energy Saving Research Unit, Strathclyde University 2013 & TACMA 2013 - Energy savings quoted were calculated using a 3 bedroom, 2 level home, located in the UK, modelling a family of 4 over a typical week’s activity, comparing an evohome smart zoning system, with recommended settings, to a system comprising of a simple timer, single non-programmable room thermostat and no Thermostatic radiator valves. Individual Energy savings gained are dependent upon existing controls, environment and lifestyle. Unquote

Having explored this at length with BRE, as they produce the dataset that is used by all EPC Assessors, they do not accept such claims at face value nor do they accept ErP self certification lodged with The Office of Standards and Safety. The EPC default position for Evohome is 2% as I was told that Honeywell has not submitted Evohome to the BRE for evaluation.

Clearly, Evohome - like a smart TV or a smart phone - is a purchase of personal choice. The perceived benefits of having this technology in the home may well outweigh cost savings. It is though interesting that 6 years on, no other academic institute has AFAIK come up with a study that mirrors the results of the Strathcylde University study.