Discussion: The above pictures are for illustrative purposes and shows what two inch flex pvc pipe looks like. It typically comes in a roll as shown in the first picture. The next two pictures illustrate that these pipes have a thick wall similar to schedule 40 rigid pvc pipe. DO NOT USE any flex pipe that is not thick walled like this. Use two inch pipe to reduce friction loss. When gluing the flex pipe to the LDR Unions, use the best pvc primer and cement you can locate.
Both the PCS3 attic solar pool heater and SolarAttic's prior model PCS2 were designed for use with LDR Industries PVC Unions fitted on both the intake and outflow manifolds for easy connection and disconnection. This would be important in Northern climates for winterizing the heat exchanger coil.
In practice, if two (2) inch flex pipe was used in the attic, only a single glued joint would be required within the attic. That joint would be in the form of a quick connect/disconnect LDR Union.
This type of installation eliminates most pvc joints and cementing in the attic, which can result in toxic cement fumes in a very enclosed attic area with little ventilation.
If rigid pvc pipe is used within the attic, you need to ensure that proper ventilation is present and limit the exposure to the pvc cement fumes.
Caution: One customer using flexible PVC reported in 2015 that the flex pipe was flattened near the pipe support areas and had to be replaced and, in his attic, needs to be insulated. The suspected cause is excessive heat on the flex pvc pipe, which may have exceeded the pipe's rated maximum of 158 degrees F operating temperatures when he was on vacation and the system was turned off. Under normal operating conditions, the attic does not experience sustained high temperatures that can damage the pipe. This is because the system removes internal heat within 20-30 minutes and then draws heat from the roof deck. If your attic is capable of exceeding 158 F temperatures while on vacation, you should make arrangements to either allow the solar system to operate automatically at maximum [which cools the attic] or to vent the attic. Fully supporting the flexible PVC horizontal pipe runs by allowing the pipe to fully rest along its length on a horizontal board may alleviate this problem. If you have an attic that needs to exceed 158 degrees F for prolonged periods of times, use of rigid PVC pipe or other pipe may be a better pipe solution for the plumbing.