Initially I thought I'd need a valve to shut off flow (to the added heater core) so the van would warm up faster in cold weather. As it turned out the van warms about the same regardless, so I just leave it open all the time. If I did this mod again I'd probable not take the time to install a valve here. Having it was valuable for testing though. With that valve closed the van is essentially the same (regarding coolant flow) as before. With it closed and driving in conditions that previously led to overheating (up a hill on a hot day with AC running) you could see the temp gauge rising, but as soon as I opened it, the temp would start dropping back down.

When selecting a core, I would look for one designed for a vehicle with a large passenger cabin and a single heater core. The Previa fit that description, but there are others out there. The idea is to get one with a large capacity (more BTU's are better). Tim