Water heating without charge controller or batteries

[efficientPV]

Everyone should be aware that when the contact opens, the switch sees the open circuit voltage of the panels. That is why the MW&S thermostats blow up when used on 24V panels. With low voltage elements, you can lose half the power just getting to the heating element. With all this interest in simplicity, I don't see why people don't just use knife switches to charge batteries.

 

[kmin]

Everyone should be aware that when the contact opens, the switch sees the open circuit voltage of the panels.

Important point to remember!
Not just for thermostats, but for everything related to solar setups. Everything has to be rated to open circuit voltage.

 

[DPC]

The thermostats have short throw AC contacts that have no arc quench, on DC when the contacts open, they can continuously arc, and you won't ever know it until too late. You will need something like a Littelfuse DCN high voltage DC contactor. To effectively use an AC water heater on DC may involve a string of 3 or more panels with voltage well over 100. Note that most arc welding is at about 24 volts.

I bet you a pound of bacon your wrong and have never done anything like this....
You might need a DC rated contactor if this was an inductive load but its not.

A 300 watt electric element at 24 volt is far more realistic and practical, will have horrible recovery time but will keep warm up a 45 gallons of water.
Better its there to compliment a solar water heater or alternative heat source

If the sun doesnt shine.
A jacket heater will warm water too.

( not this one, its a rust bucket but you get the idea )

 

[Gold Country Russ]

Years ago, Missouri Wind & Solar really was pushing DC conversions of water heaters. They offered a DC rated contactor specifically for the application and enough data to prove exactly why the AC contacts were unsafe and would fail. They convinced me. That part and its supporting documents are long gone, I suspect over legal issues. They still sell DC heater elements but nothing else.
Later this year I will be converting a slightly used electric water heater to preheat water w/either 116V 1.2kw or 155V 1.6kw PV before feeding into the existing propane water heater. If I choose to use the existing thermostatic contacts it will be with 24vdc control voltage for a DC high voltage contactor similar to a Littelfuse DCH series or equal. This will be the only PV DC I allow in my house, and I will make certain it is 100% safe.

 

[kmin]

Years ago, Missouri Wind & Solar really was pushing DC conversions of water heaters. They offered a DC rated contactor specifically for the application and enough data to prove exactly why the AC contacts were unsafe and would fail. They convinced me. That part and its supporting documents are long gone, I suspect over legal issues. They still sell DC heater elements but nothing else.
Later this year I will be converting a slightly used electric water heater to preheat water w/either 116V 1.2kw or 155V 1.6kw PV before feeding into the existing propane water heater. If I choose to use the existing thermostatic contacts it will be with 24vdc control voltage for a DC high voltage contactor similar to a Littelfuse DCH series or equal. This will be the only PV DC I allow in my house, and I will make certain it is 100% safe.

I agree 100% with you, with those voltages at DC you don't have other options. But in case of 12/24VDC situation is different.
Anyway, all relays and thermostats and contactors, AC and DC can fail. Many different ways and different outputs. In coil failure usually open, in contact failure often closed. AC relay or thermostat can fail closed by welding it's contacts, not so uncommon. So the youtube boiler rocket is ready to launch...
In case of continuous Dc arching inside a relay, I would expect the relay body melting and putting your house in fire before your boiler explodes.
Safety pressure valves are always needed, AC or DC or solar thermal.

 

[DPC]

Years ago, Missouri Wind & Solar really was pushing DC conversions of water heaters. They offered a DC rated contactor specifically for the application and enough data to prove exactly why the AC contacts were unsafe and would fail. They convinced me. That part and its supporting documents are long gone, I suspect over legal issues. They still sell DC heater elements but nothing else.
Later this year I will be converting a slightly used electric water heater to preheat water w/either 116V 1.2kw or 155V 1.6kw PV before feeding into the existing propane water heater. If I choose to use the existing thermostatic contacts it will be with 24vdc control voltage for a DC high voltage contactor similar to a Littelfuse DCH series or equal. This will be the only PV DC I allow in my house, and I will make certain it is 100% safe.

If it bothers you us the thermostat in the tank to run a DC rated relay.
Little fuse bought a Canadian company that used to make special relays I use a lot.
Closed up everything is Saskatoon where the relays were made and moved to the USA where they employ god knows who from god knows where to make an American Analog of the same part.
call them for tech support and they will tell you its a magic relay that does everything.

I don't begrudge Murf for selling out.
But he could tell you what the really really did. ( and I knew him back in the day a very smart engineer )
Little fuse will tell you what they think it could do if you buy it...