I have two pumps going through the heater just make sure you fit in this order
Tank
Pumps
One way valve of each pump into t piece
Then through heater
Then out heater into t piece
Then to both deals with a tap before one real to even flow on both deals
One way valves are so the pumps don't fight against each over and pump back on eachover which they do lol one way valve is esential
If you require pics please from me an email
deanlawrence89@hotmail.co.uk