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Proposed Water-to-Air+Domestic Hot Water Open System Design Guidance/Critique

RedOakON
RedOakON Member Posts: 1
in THE MAIN WALL

First of all my thanks in advance for the help.

This project is located in near Bancroft ON, Canada and we would be glad to know if there are any hydronic contractors from this community in the area.

The proposed system key points:

  1. Intended for a 1,000 square feet restaurant.
  2. The RTU forced air supply trunk is 24in x 24in down flowing through the roof penetration.
  3. Open system to supply the minimal domestic hot water use of ~15-20 USG per day.
  4. 24in x 24in water-to-air heat exchanger for installation in the forced air supply. (attached)
  5. The hot water tank / boiler unit is a choice between a 34 USG and 50 USG 130,000 BTU with 96% and 95% thermal efficiency respectively. (attached)
  6. The domestic water temperature should not go below 140°F per health requirements.
  7. 100,000 BTUs is allotted for the space heating needs and the balance for the minimal domestic hot water needs.
  8. The piping is planned as 37ft of 1in copper with 3 x 90° elbows on the supply and the return with insulation on the supply.
  9. The intent was to locate a 7 GPM variable circulator on the supply side of the boiler.
  10. From what I've read, the suggested ΔT for a water-to-air heat exchanger is 30°F.

The questions are:

  1. Given the heat exchanger capacity is listed as up to 240,000 BTUs, will there be too much of a ΔT for this design?
  2. The up to 7 GPM circulator was the result of the formula for system delivered BTUs = 500 × GPM × ΔT. Is this the correct approach to sizing the circulator correctly for this application?
  3. What am I missing? Any nuanced details are appreciated.

You may be asking why this design is necessary at all. It has all to do with space constraints, a limited budget, an RTU with a dead heater, but a working AC and the ideal of less gas burners and more consolidation of functions is preferable.

I am not a contractor, I am just the guy who gets brought in to solve the problem when the conventional approach will not work.

P. S. Pumping away is on order.

Thanks again.

Screen Shot 2025-04-01 at 9.16.01 PM.png
aos-commercial-gas-pdv-polaris-pc-spec-sheet-english.pdf

Comments

  • Hot_water_fan
    Hot_water_fan Member Posts: 2,180
    edited April 2

    How did you come up with a 100,000 Btu heat loss?

    That’s the heater almost no one knows exists, so why everyone thinks “endless hot water” is something only a tankless can provide.

    I think this is extremely overcomplicated and expensive. A furnace is the simple solution in my opinion. This building will have multiple gas appliances. One more is okay. If the domestic hot water needs are only 20G a day, an electric tank would work great, keeping the count of gas appliances the same.

    1. Output is dependent on air temp, water temp, air flow rate and water flow rate. So it’s “rated” for 240kbtu. It’s also rated for 24kbtu. Just depends on how you operate it.
    2. that’s the right formula but again, you need to actually determine heat loss. Then you can determine a flow rate and delta T. You can vary these by outdoor temp too.

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