I assumed that the H2 error meant that something was dirty, clogged up or that it was overheating because it was dirty or clogged up because the error only showed up when it was in Heater mode.
The heater would run for about 30 seconds, then it would show the error and switch over to the high speed fan , I assume to try to blow out the dust.
I proceeded to open it up to give it a deep clean because the Dyson instructions for cleaning this thing are ridiculous and don't help at all. Gently wiping down the outside and vacuuming the intake holes....really Dyson, really?! I used cotton swabs and 91% alcohol to clean everything I could get to, starting at the base (in hindsight, this part may not be necessary at all). Then I got to the top of the device where the actual air comes out of and noticed that there was a lot of build up on the heater coils. I couldn't get to them without taking apart some vital wiring components, so I didn't want to fool with that.
Then I decided to leave it open and just blow it out with compressed air. Canned air may work (not sure how high the psi is on those) for you, but I used a small air compressor with the limit set to 80psi.
WARNING : This may void your warranty. Do this at your own risk, you assume full responsibility for following the below tips.
Here is what to do:
1. Pry the plastic piece out that is covering up the one screw that holds the base to the top part.
2. Use a T9 bit to take the screw out (point A in the picture below)
After the screw is out, hold onto part B in the picture above with one hand and hold the base with the other hand. Twist until you feel it loosen up. Be very careful opening this part up since there are wires connecting the two pieces. Take your air nozzle and go to town on the inside portion, especially targeting the heater coils. Dust will fly everywhere so I recommend wearing something over your eyes.
Also point your air nozzle into the slits where the air comes out as illustrated by the green lines in the photo below.
Put it back together and test it out.