4300 Gallon Plywood Build (3600+ Take 2)

It would be possible to switch on and off the heater and recirculate the hot water thru the pex until it cools off.

It would require 2 thermostats 1 pump and possibly 1 relay with some minor surgery to the circuitry of an on demand water heater to switch on and off the sensor detecting flow.

That way thermal transfer efficiency is not an issue.
 
The two thermostats two solenoids idea DOESN'T have the THWH, its water is supplied from the hot water line from my household natural gas hot water heater. The same one that supplies water to my washing machine, so the water pressure in the line acts as the pump. I'll draw it up after I eat dinner.

All I need is:

100' pex
solenoid valve normally on
solenoid valve normally off
pex fittings/adapters
3/4" pvc pipe/fittings for frame/water circulation
3/4" pipe holders to secure frame to outside of tank
Y for the hot water faucet
T to plumb return to drain
2 digital thermostats
1 flow through adapter for electrode at end of pex
some way of protecting electrode in tank from fish
 
Ok here we go.

grey square is thermostat and normally open solenoid valve
these are connected to the red pex line
the red pex line connects to household hot water supply

purple square is thermostat and normally closed solenoid valve
these are connected to the end of blue pex line
the blue pex line connects to the drain that washing machine uses

when probe to thermostat 1 determines heat is needed in tank (probe placement to be decided later)

1st thermostat powers up second thermostat (and possibly circulation pump)

second thermostat checks temp of water at end of pex run

the water temp would be cool at this point (less than 100 degrees I'm guessing)

second thermostat opens 2nd solenoid to allow water flow from house hot water supply when pex temp is above base temp valve closes

when thermostat 1 detects tank temp is correct it powers down second thermostat and solenoid which will stay closed because it's normal position is closed

rinse repeat

heat diagram.jpg
 
Have you considered a pool heater?

my dad used to have a fairly large in ground pool in his yard and I once saw it get up over 100 degrees
 
Too expensive to run. They are also designed to be used in sanitized environments and using with a pond voids the warranty. In addition, they aren't cheap. I'd go with titanium pond heaters before considering a swimming pool/spa heater.
 
I figured out how to do it with just one solenoid valve; the second one

first thermostat monitors tank temp then powers up second thermostat when it needs heat

second thermostat checks pex water temp then opens valve if temp isn't hot enough to allow hot water in from the supply.

yay, simplified!

now, if I can find one inexpensive thermostat that can monitor two probes and respond, I can simplify even more...
 
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