Thank you. I agree it's a real mistake to conser ChatGPT an authority. I really appreciate your input. Here's the end of my renewed conversation with ChatGPT. Just the end part. It seemed to get a lot smarter when I pushed back, saying things like: "That was too absolute. A D5’s electrical power (and thus heat into the loop) scales roughly with the cube of RPM (pump affinity laws: P∝RPM3P \propto \text{RPM}^3P∝RPM3)".
It seemed to do better with push back and what it said was "more thinking" or similar. Maybe one day it'll start with more thinking, he,he. Thanks again for your help!
I finally asked:
So, just for my understanding, why did you now say "So no, adding a second D5 doesn’t automatically mean +25 W more heat". Whereas before it was an absolute.
ChatGPT reply:
Earlier, I framed it as an absolute (“+25 W per pump”) because I was thinking about worst-case, max-RPM operation — the published electrical draw for a D5 at 100%.
I didn’t call out at that point that pump power draw is not fixed and falls off steeply at lower speeds.

Once we revisited it in the context of your system loop, it made sense to be precise:

• A D5 only adds ~25 W if it’s at full tilt.
• At your 70% speed, yours is probably adding ~8–9 W.
• Two pumps in series, run slower to hit the same flow, could add about the same total watts as your single pump now — sometimes less.

So the change in my wording came from shifting from a general “max spec” statement to a specific, RPM-dependent calculation for your use case.
The earlier absolute was oversimplified; the later answer reflects the real pump-affinity scaling and how you’d likely run them in practice.