There may be others, but upon some real fast google research Acu-rite sells a system to monitor temps. According to their website, the "home base" is about $80 and each thermometer is l$13.

That would be good to monitor temps in a particular room (you can do a lot more with the system too)

I don't know what you mean when you say heater, but if you're talking about your whole home heat, you could also get a Nest thermostat (not cheap) which would monitor the temperature at the thermostat (not the individual room unless the thermostat is in that room) BUT would give you the ability to adjust or shut off the heat from your phone.