We have a rinnai convector gas heater and when we turn it on it fills the room with a horrible gassy smell.

Check the room if there are any messy odor

Change the filter

There are a couple of things that can cause this odor. But, being sure it's really a gassy odor is more important. It must smell like "rotten eggs" to be gas. If that is the odor you're smell, turn the heater of and contact an authorized Rinnai dealer for service.

If however the odor does not meet the "rotten egg smell", there are other things that cause the odor, but it's not really from the heater.

If you have painted or stained within the last 2 weeks, the heater has picked up the odor of the fumes (which you probably didn't notice) and has made time 50 times worse. The same holds true for carpet cleaning or new carpeting odors. If this fits the scenario, discontinue using the heater for a week or two until the odors subside. In the meantime, if weather allows, air the house out. Also, remove and clean the heaters filter, before restarting the heater,

I hope this helped you to troubleshoot & solve the problem.