Need Help w/ Desuperheater + Hot Water Tank Configuration

[starman314]

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.

[BaldLoonie]

Seems to me that making 1 tank a preheater from the geo then have the 2nd tank (in series, 1 after another) hooked up to juice to do the final heating would be the best bet.

Having the geo try to heat 120° doesn't do a lot but have it preheat 55° incoming water makes good use of its heat.

Also suggest a timer on the recirc pump.

[TajarTajar]

Hi Starman and BaldLoonie. I'm wondering how this worked out for you. I'm in the process of installing a 4-ton geothermal system with two Carrier 2-ton compressor and fan coil units - one on the second floor and one in the basement. I was planning on doing 2 gas tankless water heaters - one in the basement and one on the second floor - and am now intrigued by the idea of connecting the desuperheater to an indirect storage tank such as a Superstore or Bock Sidekick.

Questions:
1. I've heard that desuperheaters only generate excess heat in the summertime. Is this true? If not, how much can you expect to get in the wintertime?

2. In summer, how much hot water can I realistically expect to produce on a daily basis from each 2-ton system, assuming it only operates intermittently due to good (R-40) insulation?

3. Due to space constraints, I can only put a 30-40- gallon tank on the second floor. Does it make sense to have one bigger tank in the basement and use it as a pre-heat tank for both instant hot water heaters?

4. I'm in Northern Virginia too. Are there plumbers you'd recommend as experienced in connecting desuperheaters?

[superex87]

Desuperheater make heat year round.
From the systems I have worked on each system should be able to bring 40 gallons of water to about 80ish degress.
40 gallon tanks should be more than adaquite.
Any good plumber should be able to do this for you.

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[starman314]

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.