Replies

1. Expert Member
   BILL WICHERS | Feb 17, 2020 11:43am | #1

   Usually you’d have the tankless water heater ahead of the tank heater, but it can work big ways IF your tankless is ok with hot supply water. A 120v tankless is going to be pretty limited in capacity, so my guess is you’ll find you’re pretty limited in terms of hot water supply even with the help of the 8 gallon tank.

   You probably won’t be able to split the 240v circuit the way you describe. Every electric hot water heater I’ve seen uses two hits and a ground, no neutral, so you won’t be able to split the circuit because of the lack of a neutral. What you could do is repurpose the existing 30A 240v circuit as a single 120v 20A circuit (I wouldn’t put in a 15A circuit for this application), and run some 12-2 NM for a second 120v 20A circuit.

   Bill

2. 
   jameshowison | Feb 17, 2020 12:43pm | #2

   So this makes me think:

   Where is the system that, like the Nest thermostat, watches your hot water consumption patterns and prepares hot water as needed? Unexpected demand handled by electric tankless after the tank. Could even recommend a tank size (which is just a battery and a surge in demand buffer) when it's time to replace.

   I expect lots of people have thought of this, but the issue is the first cost of both a heat pump water heater and the tankless electric.

   Sort of reminds me of the Mixergy idea of using everyone's hot water tanks as a over-generation dump for grid management: https://fullycharged.show/episodes/mixergy-hot-water-tank/

1. 
   DCContrarian | Feb 17, 2020 02:22pm | #3

   I have a house in RI that has the vestiges of a similar system, installed years ago. There is some sort of communications wire attached to the electric water heater. When the electric company needs to manage demand they can turn off the power to the water heater. The homeowner got a discount in the rate for having this setup. It no longer works and I don't know how long ago they were doing this .

1. Expert Member
   BILL WICHERS | Feb 17, 2020 03:17pm | #5

   That is what is known as “interruptible service”. Utilities commonly offer this service for large electric loads like air conditioners and water heaters. You get a lower per-kWh electric rate with these services. The utility gets the ability to remotely shut off the service to these devices to help them manage loads. The contract you have with the utility will usually specify how often and for long they’re permitted to interrupt service. Service interruptions are usually during times of extremely high loads (typical summer during the mid-late afternoon).

   Bill

3. 
   DCContrarian | Feb 17, 2020 02:27pm | #4

   Figure out your maximum gallons-per-minute needed, and what temperature you want water at and what temperature it comes into your house at. Multiply gallons per minute by 8.3 to get pounds per minute, and multiply by the degree increase to get BTU/minute. Multiply by 60 to get BTU/hr. Multiply by 0.293 to get watts. That's how many watts your tankless heater needs to be to deliver the hot water you want. My bet is it's going to be a really big number, like 10,000 watts or more. That's the basic reason on-demand electric heaters aren't popular.

4. 
   gary__b | Feb 17, 2020 04:22pm | #6

   Even if it functioned (as noted above, it probably wouldn't), the theoretical energy/money savings is the difference between keeping 8 gallons at temperature and keeping a full size tank (probably 30 gallons in ample in a small home) at temperature. The surface area difference between the two isn't that much, and thus the extra heat loss isn't that much. I suspect the payback would be much longer than the life of the appliances.

1. 
   Trevor_Lambert | Feb 18, 2020 08:10am | #7

   Not to mention that the smaller tanks typically have thinner insulation as well.