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Hot water heater strategy

Been thinking about the best way to set up my hot water heater system and wanted to bounce my idea past the readers of GBA.

Zone 5A - Central Illinois. Water heaters will be located in basement. Family of 5 (3 young kids) with plans to accommodate elderly parents if necessary in the near future.

HVAC system is a 3-ton GSHP with desuperheater that can provide some water pre-heating but not enough to meet DHW demand by any means.

My original plan was to buy one HPWH and one electric resistance water heater. I would plumb the HPWH to the geothermal unit and HPWH would preheat the water to set temperature. This pre-heated water would then draw into the electric resistance storage/"boost" tank which then would have the hot water lines running out to the house.

My theory is that the "boost" tank would rarely operate...only to maintain set temperature due to stand-by losses. Otherwise majority of hot water would be generated by HPWH and "free" heat from geothermal. Plan would also include to wire and plumb such that the "boost" tank could be bypassed/turned off down the road as occupancy fluctuated.

Thoughts on this plan?

The monkey wrench that just got thrown into it is big box store is offering 50 gallon GE Geospring for $999 (regular $1199) and my local utility is offering $300 rebate on purchase as well (only on one though). So that gets down to $699 which is cost competitive with a lot of straight electric resistance tanks. So now I'm thinking, "Maybe I should just buy two Geosprings for the $1698." That's still $700 less than the Stiebel Eltron 300 80 gallon.

I'm thinking this might be the way to go. Any input/advice is appreciated!

Asked by Robert Kohaus
Posted May 22, 2014 1:02 AM ET
Edited May 22, 2014 4:27 AM ET


6 Answers

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First of all, the Geospring water heater includes electric resistance elements in addition to a heat pump. That means that the Geospring can operate as an electric resistance water heater when necessary. Since you have told us that the cost of the Geospring is about the same as an electric-resistance water heater, I'm not sure why your plan includes an electric-resistance water heater in addition to a Geospring.

Hopefully, you are working with an engineer; at the very least, someone must have designed your ground-source heat pump system. The designer of the system should be able to estimate the contribution of your GSHP to your domestic hot water needs -- a contribution which will be different in winter than in summer. Whether you need one or two water heaters to supplement the output of your GSHP depends on calculations that need to be made by the designer of your GSHP system.

Answered by Martin Holladay
Posted May 22, 2014 4:23 AM ET


Martin's right in that the Geospring unit has electric resistance units for the tank as well, so it can get as much boost as is necessary in order to meet any demand that its internal heat pump can't handle.

Generally, directly plumbing a desuperheater to a heated tank is bad. You'll get more superheat extraction the lower the temperature of the buffer tank. Leaving it unheated, at the whim of the output of the desuperheater and cold water input is pretty much the optimal setup. Let the buffer tank (whose temperature varies based upon usage of water and GSHP) feed the HPWH, and let the HPWH's own heat pump make up the balance (and handle standby losses) Electric resistance heaters in the HPWH will only kick in as necessary during high demand periods.

I believe an unheated buffer tank feeding the HPWH would have lower operating costs than a HPWH buffer tank feeding an electric or HPWH finishing tank.

Answered by Aaron Birkland
Posted May 22, 2014 12:26 PM ET


Aaron is absolutely correct . A wide Delta T in the preliminary tank is most beneficial and will allow the highest rate of HX between the source and load as it were . Unheated buffer is the way to go , a simple indirect water heater would suffice .

Answered by Richard McGrath
Posted May 23, 2014 7:37 AM ET


Any suggestions or strategies on tank sizing? I am looking at a similar setup, with a buffering and finishing tank. Would there be any advantage to having either tank larger than the other? I was considering an 85 gallon finishing tank plumbed downstream of a 50 gallon buffering tank.

Answered by Jonathan Rich
Posted May 24, 2014 8:49 AM ET


This would be use specific . How much energy would the equipment be supplying to the buffer tank and what are your use patterns . It takes 1 BTU to heat 1 pound of water 1 degree in an hour . 50 gallons of water would require 8,330 BTUh to raise 20 degrees . You are more than likely capable of the math required to size the tank for optimum benefit year round . Remember the warmer you get the buffer the less energy the finishing tank will require . Also please remember to set the finishing tank temp at 140* minimum and use a mixing valve to temper down . Legionella is a real threat , see recent tests where 50%+ of municipal water contained the bacteria .

Answered by Richard McGrath
Posted May 24, 2014 9:11 AM ET


Just read the original post again . Never bypass the boost tank , this is your free heat and what will keep your finishing tank using the last energy . Storing the rejected energy from the HP is your best friend and will increase the production of your finishing heater . Setting the temp to 140 in the finishing tank will also provide you the luxury mixing about a bit more than a half gallon of tank water with the remainder from the cold water supply to deliver the 115* DHW . I would probably use a 30 gallon buffer and a 50 gallon finishing tank using this proven strategy . Make sure your buffer has a generous , quality insulation to keep standby losses to less than 1* per hour and your finisher should also be very efficient . Her is what I would put in my home , I would not recommend anything I would not use myself . I would also get the temp in the buffer as high as the manufacturer would allow w/o any warranty issues , this provides the least energy used by your HPWH .
Finishing : http://www.htproducts.com/literature/HTP-Hybrid-Water-Heater.pdf

Buffer : http://www.htproducts.com/superstor-ultra-waterheater.html

You choose the size depending on the amount of heat you expect from DeSuperheater. Whomever designed your system should be able to supply this information .

Answered by Richard McGrath
Posted May 24, 2014 9:12 PM ET
Edited May 24, 2014 9:17 PM ET.

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