You can also use Air Vent (VENT) found in the Special category. I use it for my Uran reactor. While it only generates 0View Save 0.10pressure, it's still enough for a pretty fast heating of the Uran in the chamber I made.
You can look at my saves called "Basic Uranium Reactor" (ID: 2623543) or "Uranium Reactor V2" (ID:2623896)
P.S: Found out that the more VENT pixels you have next to eachother in a close space, the more pressure you get!