Backyard Aquaponics
A place to discuss aquaponics

I am having the same problem so I was searching the internet for answere...turns out that not just heat is the culpret with bolting lettuces. length of day is also an issue. will make lettuces bolt and turn bitter.

I had mine in my nice cool basement ( so I know temp was not an issue) but they were bolting anyway. but I was keeping my artificial lighting on too long.

MMM, am thinking heart varieties of lettuce will have to be winter only crops. Days are shorter and cooler.

I grow loose leaf because I can take what I need and leave the plant to continue growing.

No there temperate climate plants,not winter plants,you can grow them successfully in full sun,50% shade cloth will do the trick!leave enough height to loose the heat and make ensure you have airflow this will increase transpiration retarding bolting and increase the vigour of the plant helping draw up more nutrients.
There is a good PDF on the Internet of a hydroponic/Aquaponic lettuce production unit based in Hawaii,they achieved year round growth under shade cloth covers,I don't remember the link but it's the sort of read that helps point you in the right direction,if I can find it I will post it.

Stress will also make them bolt to long a light period later in its growth stage will do this.

http://www.ctahr.hawaii.edu/oc/freepubs/pdf/SA-2.pdf
I'm pretty sure this is the one,have a look see what you think.