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.

I suggest Fig 12, page 11 here (tanks in series). Then turn off the electricity to the first tank (or at least the first tank lower element). Or consider adding a propane water heater and using the two electric tanks only as buffer tanks (propane is probably much cheaper than electric).

Review this to see if you want to reduce or eliminate the use of the re-circulation pump.

If 2 heat pumps are close they could be plumbed connecting both to un-powered first tank, then to 2nd tank like in Jon's diagram, which will bring up to finish temp. ChrisJ

If running two heat pumps to one tank, I'd run all 4 lines all the way to the tank. The concern being that if one heat pump's water pump is on, you don't want it to cause any flow through the other heat pump. But maybe they have a valve or pump design that prevents such inadvertent flows.

Checkvalve?

Good point - the drawing has check valves and I overlooked them. Hopefully they have very low opening pressure - I suspect that the pump in the HP doesn't produce much pressure.

Just wanted to follow up on this thread. I added circuit level energy monitoring to the house and found that the water heaters are consuming a large amount of energy, particularly the one connected to the 24/7 recirc pump. I put the recirc pump on a timer and aquastat, which helped curb the energy usage considerably. Energy usage is now ~24kWh per day for the heater connected to the recirc pump (down from 42kWh per day before the timer) and ~14kWh for day for the heater not connected to the recirc pump. However, I would still like to get some contribution from the desuperheaters.

I am thinking what we should do is turn both of the existing 55 gallon tanks into buffer tanks (one for each geo unit). We would then take the hot out of those tanks and t it into a new 105 gallon Rheem Marathon electric water heater. I think if we did that we would still need to run the hot water recirc loop off of the Marathon, which means we wouldn't get an assist from the geothermal in brining the water from the recirc loop back up to temperature. However, we should get a decent contribution from the geothermal on the rest of the hot water usage.

Does that sound like a good plan? Are there any further optimizations we should make?

One other thought is that we could use an 80 gallon hybrid heat pump water heater instead of the 105 gallon Marathon. The water heaters are located in the unfinished portion of a walkout basement. Would the hybrid heat pump be better or should we stick with a Marathon?

I would use the hybrid heat pump water heater, even if it is less reliable. Once you have it plumbed, compare the typical return temperature on the recirculation system to the typical temp in the desuperheater tank - might make sense to plumb it to there.

"3 simultaneous baths in a standard size tub at night"

Does that mean 1 filling of the tub?

How does the aquastat work? Shuts off the pump when hot water reaches the far end of the loop?