I fitted the Eberspatcher on our B34 back in 2002 and used a D4 but with 5 outlets. Forward cabin, aft cabin, heads and two in the main saloon. may seem like overkill but at the time we did a lot of winter sailing. The guy I bought from was a fellow sailer with a Sigma 38 and the best bit of advice he gave me was to set the air intake in a position so that it pulls its air from inside of the boat. We have our inlet pipe terminating behind the electrics cupboard by the chart table with the heater sited at the aft end of the starboard cockpit locker. The effect is that instead of trying to force air into the boat from an outside source you end up sucking out the same amount as you are blowing in and the incoming air is warmer than that from outside. it appears to make our unit perform very well and also creates airflow throughout the areas of the boat below the saloon berths on the starboard side and the electrics cupboard, which as a result has always remained dry and never suffered any corrosion problems.