I have a pair of Vega Touring boots from CompAcc.com. I find them extremely warm. I am looking for a riding boot that has good ventilation and good support in ankle.
What do you ride with and how do they perform in the summer heat?
Tourmaster WP Solution boots... not very warm during the summer... are waterproof... decent comfort... and can find them under $125 shipped pretty much anywhere.
All day comfortable even if you find yourself doing some walking, like in a National park or something. They're called something else now. My pair are about six years old!
The best boots that I've ever used for riding in warm weather were Icon Tarmac boots. These flow a ton of air! Plus you can't beat the walking around comfort. I just looked online and it seems that they've updated this boot to version 2. I can't vouch for that version but the 1's were great.
I have an older pair of BMW boots when cold but need a larger size so may go with the Tourmaster WP to replace them. My go to is a pair of these for hot weather http://www.originalswat.com/products/115201/
I would check the swat website because they have added a number of other side zips since the originals. They are very comfortable and come in wide sizes.
I also use the TourMaster Solution waterproof boots. Mine are many years old (prior to the version 2.0) and still in good shape. Very comfortable for walking and riding and waterproof in a downpour. When I finally wear them out, I'll buy another pair.
I always mention the Vendramini Marathon Steel. I've had mine 6 years and really like the comfort (and you can walk in them too!). I hardly ever see reference to them....maybe price or # of dealers?