We are flying from the US to Italy and have a 2:15 layover in Heathrow, will that be enough time to make it from Terminal 3 to 5?
Is this all on one ticket?
Take the train between terminals. Enter your data in heathrowconnect, works well.
Input your flight info to see an answer.
One ticket or two is the key
the key question is if you are on one booking throughout, or if you are separate tickets even on one airline.
Do not automatically take the train between terminals. To do this you have to go through customs and immigration and if you are just changing planes, you shouldn't have to do this. Even if it means changing terminals. If you stay airside, there is a bus that will take you between terminals.
The website Joe32F gave will answer your basic questions and give you good instructions on what to do.
It is all on one ticket.
Should be enough airside providing you aren't late incoming. The minimum time is 90 minutes. You will need to go through security in T5. Do not even consider doing it landside because of the typically long queues at immigration.
Use the flight connections website for detailed information. It's really simple--just follow the flight transfer signs rather than following the crowd to immigration. The flight transfer desk will get you over to Terminal 5.
If you are checking a bag, make sure it has tags for your final destination when checking in.