Initial Design: W-W HX for Domestic Water Production

Discussion in 'General Discussions' started by peecubed, Dec 6, 2018.

  1. peecubed

    peecubed New Member

    Hi all,
    I'm an engineer working through the initial design of a geothermal installation. I've read through the forums a fair amount, and see alot of knowledge on here, and was hoping to get some feedback on my initial thoughts and design. I'm specifically working on the plumbing side, so that's where my questions are coming from.

    Building is a rural farm maintenance building (shop) with attached employee kitchenette, bathrooms, offices, and conference rooms.

    Location is in the Hudson Valley, New York.

    Right now, working with another engineer on the HVAC side, his schematic design is calling for 3 heat pumps: radiant floor shop heating, radiant snow melt, and heated/chilled 'fan coils' for office/commercial spaces.

    The domestic water load is rather small, with only 2 public showers, 2 associated hand sinks, a kitchenette sink, and some small emergency items like eyewash & shower.

    I'm leaning towards linking the domestic water production to the radiant floor heat pump, with a controlled diverting valve. This would use a w-w indirect heat exchanger - I see the Stiebl Elton tanks recommended on here. From experience you guys have, would that be the recommendation since I'm already making hot water?

    Or would you recommend using a desuperheater off the 'fan coil' heat pump since it'll be running year round, and then backing that up with a powered water heater?

    If I use the radiant floor heat pump for the DHW production, the controls would have prioritize calls for heating between the two. Who makes these controls systems? Are these often packaged with the heat pumps?

    On the ground loop side, since there is potential the radiant/DHW heat pump would be in the opposite mode as the fan coil heat pump (during summer), do you guys have a schematic with how this would work? Would it bypass the ground loop and dump building heat into the DHW heat pump?

    I have a bunch more follow ups, but I'll wait to see if I'm even going down the right path.

    Thanks for any help.
  2. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Size of heat pumps? What brand and model?

    Yes, DHW production of the w-w for radiant and an indirect stiebel eltron tank. It has to be the "plus" model with the dual heat exchanger in there. Size of the tank depends on size of heat pumps, since the surface area of the 2 heat exchangers inside the tank are depending on the size of the tank, and the BTUs you can transfer at a given delta T has to go confirm with the size of the heat pump, unless you pick a high temperature heat pump, where you can change the delta T inside the tank better.
    Once you make 100% of the DHW with the geo system, you don't need a DSH anymore.

    Dedicated heat pump for snow melt? How much snow are you trying to melt? Or sqf?

    "Fan coils" is a water to air (forced air) heat pump?

    Controls you can use an outdoor reset and a simple boiler control, Tekmar 256 and 150 will do the job. Diverting via 3 way Belimo valves, the wiring will allow you to prioritizing on DHW. You don't want to prioritize on heating, and then have the shower water go cold.

    I would not work with bypass but use a single variable speed pump to serve all 3 heat pumps, and let the water go through the ground loop permanently.
  3. peecubed

    peecubed New Member

    As a preliminary number, my coworker is looking at 20-24 tons heating. Do you have recommended manufacturers? I've looked at Waterfurnace/Geostar, since that seems to be what most people are referencing on here, but I'm open to other options.

    Preliminary, I'm estimating that a single (of the 3) heat pumps will more than large enough for my purposes in generation of DHW, since the shop area (that will get radiant floor heating and share the heat pump) is ~25,000 sq ft. As a worst case scenario, I know my entering water temp (domestic side, into the tank) is ~35F, and I'd like to have a recovery rate of 80gph, so it looks like I need around 60-70k BTU for my 95F temp rise. From there, how would you recommend I select a tank?

    Again, I'm not sure of the total area that snowmelt is going to be used on, but there is ~16,000 sq ft of equipment pad area directly outside the shop. There's also a parking lot, but I'm not sure if that will be included.

    The "fan coils" I'm referencing are just hot/cold water fan coils in the individual user offices, supplied by the w-w heat pump. We are not doing any forced air.

    Thanks for the recommendation on the controls, and the variable speed pump. I will have to look more into both.
  4. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    You need about 100 BTUs/sqf/hour for snowmelt, so for 16,000 sqf you would need about 1.6 million BTUs/h, or 160 tons alone.
    It takes a lot of energy to melt snow, for a very short period of time. Unless you are trying to get rid of heat in the middle of the winter, that is not a good application for geo.

    What is the heat load of the 25,000 sqf shop?

    Hot water: The Stiebel Eltron tanks come in 85, 108 and 162 gallon size. You usually cover domestic hot water need peak capacity with the storage volume, 70 KBTU/H would be around 7 tons just for hot water, however, it would be rare that due to the small hot water load, you would need 80 gph recovery. The peak use you can cover with the storage volume. As an example, we cover 12 apartments with 12 showers with a single 162 Stiebel tank, and a 5 ton w-w heat pump, with 55 KBTU/H capacity. Your needs seem to be significantly less.

    Fan coils: Do you have console units in the individual offices? How many zones in the office area, and total sqf/ load?
  5. peecubed

    peecubed New Member

    I tried to post this follow up as a new topic, but the forum doesn't seem to be moderated very well right now:

    As stated above, I am looking at a design where I use a single W-W heat pump (say 'HP-1') to supply a radiant floor heating system, as well as the domestic hot water for the building, utilizing a stiebel-eltron dual coil tank for the DHW.

    The geothermal loop will also have 2 other W-W heat pumps ( say 'HP-2' & 'HP-3')utilized for other system in the building.

    My question:
    Looking at WH-1, there will be times in the year when we want 'cooling mode' for the radiant floor system, but we will always want 'heating mode' for the DHW production. How is this accomplished?

    I'm looking at a system like this:, and am unclear how 100% of the domestic hot water load can be met, if all summer the radiant zones are dumping heat to the ground loop. Do controls for the 3-way diverting valve (for when DHW is needed) also switch the reversing valve at the heat pump?

    Additionally, does this get further complicated when I consider I have 2 more heat pumps coming off the ground loop (a scaled down version of this: Do the other heat pumps (presumably in cooling) need to get isolated off once the DHW is needed?

    Any insight would be great. Maybe someone has a controls spec/sequence of operations for these?

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