Need Help w/ Desuperheater + Hot Water Tank Configuration

Discussion in 'Geothermal Heat Pump Applications' started by Starman, Mar 24, 2016.

  1. Starman

    Starman New Member

    Hi all,

    We recently had a geothermal system installed in a new construction build and I need help figuring out how to optimize the desuperheater + hot water heater setup. Here are the details on what we have:

    2 x Waterfurnace 7-series NVV036 both w/ desuperheaters
    2 x A.O. Smith Promax 55 gallon electric water heaters (we are on propane with no access to natural gas)

    Currently, the upper and lower thermostat settings for both water heaters are set to 140 degrees. We have mixing valves to bring the water temp back down to a reasonable temperature at the tap.

    One desuperheater loop is plumbed to each water heater, with the DSH In coming from the cold water side of the heater, and the DSH out going to the bottom of the water heater.

    We have a hot water recirculation pump attached to one of the water heaters which pulls from the hot water side. The pump runs 24/7 with no timer or aquastat.

    The plumber tells me that the hot water heaters are not zoned and that both heaters are capable of providing water to the whole house, but I can't really tell how the water from the two heaters is mixed.

    We are located in Northern VA and have 2 adults and 4 kids in the house. Our peak water usage is 2 back to back showers in the morning and 3 simultaneous baths in a standard size tub at night.

    What would be the best way to optimize our hot water heater and desuperheater set up? We would love to use the existing 55 gallon electric tanks if possible, but are willing to add something if that is necessary to build the most cost effective solution that meets demand.
  2. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader


    Are the heat pump close or next to each other?

    For a DSH to run efficient, you need a buffer tank, which is non powered. The DSH maxes out at 140F, just where your electric heat elements keep the tank's water temp. Meaning that the DSH hardly distributes anything, if anything at all, with your setup.

    It is wasting a lot of energy to heat the water up, only to mix it down again. Also to have recirculation pump running 24/7 is a waste, since the lines will act like a radiator and loose energy 24/7. Use a timer or a motion sensor to run the recirculation pump.

    You could use each tank as the buffer tank for each heat pump, and have a single larger final conditioning tank, for example a heat pump water heater. Or have a water-water heat pump produce all your hot water needs. Or add 2 electric tanks as final powered tanks.
  3. Starman

    Starman New Member

    Hi Doc,

    Thanks very much for your help. I read a number of your threads on various forums and they were all extremely helpful in figuring out what to do. I wish our original geothermal installers had been as knowledgable and thoughtful about this issue as you.

    Just wanted to follow up on this, since we finally took some action to correct the problem. We added an 80 gallon AO Smith Voltex heat pump water heater. We unplugged the two existing 55 gallon electric water heaters and have each of them plumbed to one of the desuperheaters. The storage tanks T into the cold in of the heat pump water heater. We're in winter right now and the desuperheater loops are heating the water from ~55 degrees to 90-100. The heat pump water heater is heating the water to 140 degrees from there and we are using a tank booster to increase capacity. This configuration is successfully supplying all of our hot water needs at less than half the cost of our existing configuration. We should break even on the cost to purchase and install the heat pump water heater in 2-3 years.
  4. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    It sounds like you have made real improvement and are experiencing good results.

    Why set the heat pump water heater to 140*F? That greatly reduces its efficiency and stresses its compressor. Could you drop that to 120*F or thereabouts?

    I'm unclear on what comprises a "tank booster" as well as its role in increasing capacity - please explain.
  5. Starman

    Starman New Member

    We originally had the heat pump water heater set at 130 degrees, but we found we did not quite have enough hot water capacity at that temperature, so we upped it to 140. TankBooster is a thermostatic mixing valve that takes 140 degree water from the water heater and mixes it with cold water to bring it back down to 120 degrees, effectively increasing hot water capacity. I'm glad you asked about the TankBooster - I just checked and the plumber didn't move the TankBooster over to the new water heater. I don't think the existing configuration of having the water heater set to 140 is safe with kids, so I'm going to turn it back down until we get that sorted out. I imagine we're getting the same effect as the TankBooster right now by mixing in cold water at the point of use, but I don't want the water going out to the point of use at 140 due to scalding risks.

    On another topic, I've noticed in monitoring the setup that the water in the two buffer tanks can sometimes be at significantly different temperatures depending on the runtime of the associated geo units. Currently, the two buffer tanks T into the cold in of the hybrid hot water heater. Is there any solution that will pull water from the hotter side of that T rather than just mixing the two together?
  6. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Check your state laws, but in New York anti scalding laws require between 110F and 120F at the tab, not higher under any circumstances. You putting yourself and others at risk, not just your kids.
  7. Starman

    Starman New Member

    As I mentioned in my previous post, I turned the water heater temperature back down when I noticed that the plumber had not installed the TankBooster on the hybrid water heater. However, I would think that it would be ok to keep the water heater set at 140 degrees when the TankBooster is installed and set at 120 degrees. The TankBooster mixes in cold water right at the point where the hot water line exits the water heater, so the water should not reach the tap at higher than 120 degrees unless the TankBooster is not functioning properly.

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