I run around 40-42 psi on my 18s depending on how the tyre is wearing. I would class 34 as under inflated on a tyre with such a small sidewall. The smaller the sidewall the higher the pressure you need. Roughly speaking, if you have half the sidewall with the same amount of flex it will get twice as hot.
It is also a good idea to check your pressures regularly, particularly after you have new tyres fitted. You will find that in the first 500km or so they will drop pressure as they bed in. I have had tyres drop 10 psi in the first week and then not drop again for 10,000k