Hi, I have 1 hour and 20 minutes to make a connection at Heathrow from terminal 1 to terminal 3. I'm coming in on Air Canada and the departure airline is Aegean. I will only have carry-on. Will I need to sprint to make m flight. Thanks for your answers.

That is extremely tight IMO. Is this one booking or two separate tickets? If it is one booking, not to worry, they will put you on the next available flight if you miss your connection.

If it is two bookings - you are entirely on your own.

You won't have to clear immigration but you will (as far as I know) have to go through security. And your arrival time is not when you get into T-3 -- it could be 20 minutes or more from the time you touch down til you are inside and heading to your airside transfer.

If you have to collect and recheck luggage then you've no chance of making it - after 1h20m you'll still be waiting in the immigration queue. If your luggage is checked through then you have a good chance of making it - and if on one ticket the airline is responsible for getting you to your destination. If not it's your problem and your bank balance that would get hit heavily

Go to http://www.heathrowairport.com/ click on "airport information" then select either "flight connections" or "travel between terminals" depending on if your luggage is checked through or not

I have a question. If I book a two hour connection at LHR, both BA flights, is it enough? I'll be returning to the U.S., so will I be stuck in the immigration line forever?

My problem at Heathrow is often seems we either circled waiting to get in the landing order, or sat on the tarmac after landing waiting to get a gate to deplane. Maybe I've just been unlucky but this has happened more than once. So I was actually off the plane at least 1/2 hour later than what I expected.

I think I got my question answered on the Heathrow Airport site.