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Caleffi Variable Speed Pumping with Double-Pumped System
This question is for Hot Rod or anyone else with specific experience or knowledge. Thanks in advance.
I'm upgrading an older system's various components including moving to variable speed pumping using the Caleffi iSolar Controller Line (RESOL). The system is going to be double-pumped through an external heat exchanger, meaning a pump on the solar loop and a pump on the potable loop.
What I am curious about and looking for pro-tips on is the variable speed control strategy for the main solar collection loop/s for this. It seems to me there are a few ways to go here:
1) Control both pumps off of a single relay (iSolar 2) and run them through a solid state relay (as their draw exceeds the rating of the relay) when the conditions are correct for solar collection.
2) Separate each pump to run off its own relay (iSolar Plus) w/o the solid state relay when the conditions are correct for solar collection.
3) Control one pump on one variable speed relay and the other on a separate ON/OFF, set speed relay (iSolar Plus) when the conditions are correct for solar collection.
My question is which of these methods is preferred? It seems to me that two variable speed relays (option 2) trying to maintain a constant delta T could end up fighting each other and, ultimately, create less than stellar production, possibly wearing out the pumps and/or relays from overuse/ confusion.
On the other hand, I'm not sure that option 1 is great either, as I believe it would take some tweaking of the internal speed controls on the pumps (they are 3-speed manual Grundfos pumps) in order to dial in on the setting that works best to maintain the appropriate delta T.
Option 3 seems to make sense and I think I'd run the solar side pump on set-speed and the potable side pump on variable, but I could see arguments either way.
Anyway, looking for advice on proceeding from anyone who's got it.