Yes, it's true.

But the thing is that this equipment; an air conditioning equipment with a network interface, I can't configure a gateway.

I only can configure an IP, Mask, and that's all.

So if I want to access through HTTP from the "inside", how I route the answer ? (without a gateway)

Thanks. Regards.

If you want to access this device via HTTP across the inside interface of the ASA, you need to make sure there no are ACLs limiting traffic to / from your inside interface to the DMZ and vice versa.

Two suggestions. Set the mask wide and enable proxy arp on the local routing interface.

Alternatively, configure NAT such that any connections coming in for this device appear to be on the local subnet.

Neither of these are great, but they may just give you connectivity.

No problem - more people suggesting the same thing adds weight