Thanks for the quick reply. I am using the 250 on the intake side but this section is after the heat exchanger which does not have any risk of imploding. The 135 has the same temperature rating (200F) and close to the same pressure rating 200 psi vs 275 psi.
Also, Shields recommends the 135 for “heat exchanger” applications. This is attached to the outlet of the heat exchanger so I am not worried about that end. My concern is the attachment to the mixing elbow
. But if they have the same temp rating, it seems this should also not be a problem.
I am only asking because I have 135 and my local retailers do not carry the 250 in 5/8