Washington Heat Exchanger for Spa

Discussion in 'General Discussions' started by Air Locked, Oct 2, 2020.

  1. Air Locked

    Air Locked Member

    So an update...

    I asked the engineering dean of my alma mater for a referral. She sent me to the retired Chair of the ME department, who turns out to be a published expert on Heat Exchangers. He even worked on some in deep space probes.

    He looked at the specs I listed above, and confirmed they made sense. Box Checked.

    When I also asked why a larger HX would need more flow, he paused and said "that depends" on which way it was larger: diameter or length. Seems that a longer HX might need more flow but it got complicated why that is.

    So that part is resolved. And I've learned something new.

    Finding the pool fittings has been much less fun, alas. The dealers want to tell me the colors and such but asking about flow rate vs. head gets me "It's Blue" responses.....
    Last edited: Nov 24, 2020
  2. gsmith22

    gsmith22 Active Member Forum Leader

    Maybe just ask for the Cv or "Flow coefficient" if its on a box or a spec sheet. Maybe its next to the color :)
  3. Air Locked

    Air Locked Member

    Here's an update. We have the Bowman HX on hand. It took 3X as long to get the check to Toronto as it took to get the HX to Seattle.
    We'll use a pair of the 2HP Pentair pumps; the source side is low flow but has a 160 foot run; the spa side is ~10 ft but 50 gpm.

    We found a spa/pool contractor experienced at adding fittings to the spa; one issue outsourced.

    The Pentair's have a RS-485 control system but they consider the protocol secret; "Buy our automation system...." That's a clear challenge.

    A friend's day job is reverse engineering embedded systems for Uncle Sam. I'd encouraged him to consider a GSHP for his house before the 30% window closed. He did so, and promptly started looking at the Water Furnace thermostat RS-485 system.

    We had a pump shipped to him, and within a few days of searching the existing work out there, and sniffing the RS-485 bus, he had the protocol mapped out.

    We are now building a microcontroller (It's a om13088 dev board from NXP...) & an added custom PCB with the RS-485 adapters that, when there is a demand for heat, shall:
    1. Start the source pump
    2. Monitor the HX temp. (There's a pocket for a 0.25" dia probe.)
    3. When the HX is warm, start the spa-side pump.
    4. When spa is happy, stop the source pump.
    5. When the HX is cool, stop the spa side pump.

    We sense the heat demand by sending the switched 240V not to the 4KW heater but a 240VAC-5vdc wall-wart; that 5v goes to the NXP.
    We'll fall back to timing parameters iffen we lose the temperature data.

    We are using an EDS 1-Wire OWServer system with stainless probes for the temperature sensing.

    We'll look at tweaking the flow rates for best efficiency once it is installed and we have enough data logged.

    I'll provide imagery when we are further along.

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