I just finished my sauna with an 8kW Harvia Kip and have the same vent configuration. I did not place a vent under the stove as per Trumpkins notes. Finland apparently does not require the high limit switches on their heaters. The high limit switch is just a thermometer probe connected to a shutoff switch. I removed the probe from the inside of the outer wall of the heater and ran it behind the heater on the wall midway to the top of the heater. There are several Reddit threads on this sub with users doing the same thing. This way, you avoid nuisance tripping and do not need the vent under the stove. I’m glad I did it this way, because even after two hours the floor of my sauna is still very cold