Novel hot water supply???
Just purchased a small home in North Central Florida….approx 800 sq ft.
It has an old electric water heater. What if I replaced it with the following. Take the current 220v 30 amp line supplying it and split to two 15 amp 110v lines. Install an 8 gal small tank 110v water heater and on the outlet side install a 110 volt point of use heater. Both draw 1440 watts. My thinking is that most of the time the 8 gal would be sufficient but the thermostat on the point of use would be set such that when the water from the 8 gal tank dropped below a certain temp it would kick on. It would be adding heat to water coming from the 8 gal tank which would be warmer than water coming into the house. Keeping 8 gallons warm around the clock sounds to me to be much more economical than keeping 30 or 40 gallons warm around the clock. The cost of these two small heaters is a little less than one standard water heater. Thanks and sorry for the long question
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
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
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/
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 .
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
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.
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.
Not to mention that the smaller tanks typically have thinner insulation as well.