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Caleffi Variable Speed Pumping with Double-Pumped System

matt_sunwaysolar Member Posts: 61
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.


  • hot_rod
    hot_rod Member Posts: 22,105
    I would suggest arrangement 2
    Run the solar pump on R1 as a VS function, if it is under 1A, no need to use an isolation trial.
    HX pump on relay 2.
    To be honest I have never compares fixed to VS on the HX pump, in terms of energy efficiency or HX performance measured in BTU transfer.

    I would say the highest fixed speed will give your plate HX the best performance. The VS function would limit on/ off cycling and may extend the pump life. On low harvest conditions the VS in theory tries to match the heat transfer to what is available, just as it does on the solar loop. After all the solar collector on the roof and the plate HX are really the same function. One is in a smaller case:)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • matt_sunwaysolar
    matt_sunwaysolar Member Posts: 61
    Not sure if anyone really cares or not, but I decided on and implemented the control strategy for this system. I went with option #3 from above. The potable pump on variable speed and the solar pump on set speed.

    It turns out, in order to run one pump on variable speed and one pump on set speed, using the same differential ON/OFF with the Caleffi (Resol) controllers, you have to use an iSolar BX (or whatever the Resol crossover # is).

    Two relays can be "paralleled" when programming the controller. Relay one turns on upon proper differential and is variable speed (potable). Relay 4 controls the solar pump and turns on and off when relay one does.

    Visited the system after a week and a half of installation and found good results. With low to moderate sunlight available, this set up had the system running at a 20 deg delta T between collectors and storage at 50% speed on the potable pump. The Caleffi controllers have an hour counter for how long the relays have been operational. Given the short amount of time that the controller has been in place, the counter seemed more than appropriate.
  • hot_rod
    hot_rod Member Posts: 22,105
    thanks for the feedback, good to hear it is harvesting energy.

    it would be nice to have the solar pump on VS also,it limits on off cycling on cloudy days, and consumes less energy. It could also allow you to get a tighter ∆T. It would be nice to see 8-10 between collector and load to get as much energy into the tank. Assuming it doesn't require too much pump energy to accomplish the tighter ∆.

    You should be able to program both outputs on the Plus version?but you are correct that that control cannot share sensors.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • david sweet
    david sweet Member Posts: 7
    Matt there many ways to look at the system you have described in your post…

    I always advise variable speed pumping on all solar thermal systems as they respond much better to changes in solar energy intensity. In the beginning of the day as well as the end, there is not as much solar energy to harvest as there is at midday and likewise on cloudy days…

    I would suggest your application # 2 with one circulator variable speed and the other fixed speed. Variable speed pumping is really variable flow; so slower flow gets a larger temperature rise across solar collector. Better to have 1 GPM of 150 degree water than 5 GPM of 100 degree water and having to use a backup heat source. Informal studies have suggested as much as 25% more energy harvested with variable speed than fixed flow systems.

    When using an external heat exchanger to separate glycol from potable water, either the collector or storage tank circulator could be the variable flow one. So application dependent, either one can control or limit the transfer of energy from collector to storage tank, thus controlling temperature rise. An example could be a drainback system where the installer is worried about collector lousing a syphon if flow becomes insufficient to maintain this syphon. In this case, the collector circulator is fixed speed and the potable circulator is the variable speed one.

    Sometimes you have to think outside box with solar thermal applications and your product mix!