I haven't heard of heat cracking grout unless maybe you're cleaning in the arctic circle. Too much psi can blow out grout. I regularly clean @ 1800psi and haven't blown out grout in 3.5 years. That doesn't mean I advocate cleaning at that psi, but I know what I'm dealing with so it doesn't bother me a bit. Some commercial kitchens I'll go up to 2500psi, but really, I don't recommend you guys do that.