Series Heat Pump Water Heater and Basic Electric Unit

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chill_builds

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I am planning to install a 50 gallon Rheem HPWH(in garage main level) in series with a basic 50 gallon Rheem 4500 watt electric unit(in basement one level down). The reasoning for this is to increase total amount of available hot water for the few times a year we run out of hot water and to save electricity by running the HPWH in heat pump only mode. In this way the HPWH will do most of the work. I plan to set the HPWH to maybe 130 F during the summer to take advantage of the hot garage and maybe set it to 120 F in the winter to reduce the stress on the heat pump when the inlet water and garage get very cold.

Another reason to have this setup is to have a backup water heater if the smart HPWH fails. The Rheem warranty for 10 years is great but they expect you to wait a few days to get parts shipped when you diagnose and fix these units through their support line. I recently had to replace one of their smart units at less than a year due to the control board failing 2x and then a thermistor after replacing the board the second time with no parts available locally. My choices were to remove the smart one and install a dumb one myself that day or plan to receive divorce papers from the wife so the choice was pretty simple. I decided to give Rheem another shot as they provided me with a return auth and I was able to return the unit for full credit that day. I only wish I had installed a marathon lifetime tank now with my new 2 units in series plans.

I figure my savings will be about $350/year even with keeping both tanks up to temp 24/7/365. I have an energy tracker from IoTaWatt and it looks like I spend about $50 per year to keep the water up to the set temp outside of hot water usage.

Although electric cost savings is a goal of this setup, comfort is more important. I may also add a small off-grid solar/battery system to power the water heater in the event of a power outage.

Doing a bit of research I am wondering if I need a check valve or negative relief valve on the cold water inlet (bottom of tank) for the HPWH in the garage or on the hot outlet (side of unit near top of water tank below the heat pump) since the basement unit draining could cause some serious pressure on the main level unit.
 

John Gayewski

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Running heaters in series shouldn't require a check valve or a vacuum breaker.
 

chill_builds

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Also, my current water heater has a 2 gallon thermal expansion tank. Should I add a second 2 gallon on the new added unit or replace the current 2 gallon with a 4 gallon tank?

Seems I prob need a 4 gallon on the cold inlet for the new unit?
 

John Gayewski

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The expansion tank needs to be between the heat source and the check valve that closes your system to the public water supply. Expansion tanks are sized based on volume of water and heat rise. If your adding volume to your closed system then yes you should probably upsize the expansion tank. 5 is the number to remember for heating water.

The expansion rate of water is .00023 (five digits that add up to five) times gallons of water in the system times degrees farenhiet of rise. Or sometimes people just say 5 percent of the volume of water, but that does oversize them.
 

chill_builds

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Got this completed on Sunday. Got the water heater from HD after watching slickdeals for a low price. Got one for $999 plus tax and delivery. About $1500 all in with a diy install in the garage which included running power. Averaging 5 kWh/day so far so it looks like this will save us about $500/year over our standard element electric heater. 3 year break even with a 10 year warranty seems like a no brainer.
 

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