AC Relay with DC contacts

[Jet]

I want to run a hot water heater off 24V DC battery power (24V 715 AH lithium). My idea was to connect the water heater to AC power and use the thermostat wires that would go to the heating element to activate a relay that would close a DC circuit to power a DC heating element. Are there low voltage high amperage, DC relays that are controlled by AC. What are they called and what brands would you recommend? Thanks.

Edit: also, does anyone know a website that sells a 24V 900W water heater element. All the ones I have found with the exception of one questionable looking solar website has been eBay, Alibaba, amazon or similar websites. With something like a heating element, I want to make sure it's a quality product for safety reasons.

 

[Samsonite801]

I might consider to just take out the AC altogether (the factory thermostat control) and install my own electronic thermostat and thermistor probe. Or use their thermistor (if electronic), and the resistance range can be adapted to the board you would use.

But if you really wanted to do this the way you describe, you could just use an AC to DC transformer (w/ rectifier, like a wall-wort) to drop down to 12v or 24v DC relay trigger voltage and trigger a standard relay (or a solid-state relay)... It's common to see normal wall-wort AC to DC adapters that can accept 90-250v AC (50 or 60 Hz) since many electronics are made for worldwide power use...

I have seen industrial DC contactor relays with AC trigger, but they are more spendy since they are specialty and usually built more for industrial applications.

 

[Jet]

I might consider to just take out the AC altogether (factory thermostat control) and install my own electronic thermostat and thermistor probe. Or use their thermistor (if electronic), and the resistance range can be adapted to the board you would use.

But if you really wanted to do this the way you describe, you could just use an AC to DC transformer (w/ rectifier, like a wall-wort) to drop down to 12v or 24v DC relay trigger voltage and trigger a standard relay (or a solid-state relay)... It's common to see normal wall-worts AC to DC adapters that can accept 90-250v AC (50 or 60 Hz) since many electronics are made for worldwide power use...

I have seen industrial DC contactor relays with AC trigger, but they are more spendy since they are specialty and usually built more for industrial applications.

Thanks for the idea. I guess the AC to DC transformer wouldn't be that inefficient because it would only be running a little power to the relay. I don't know why I didn't think about that. Thanks.

 

[MichaelK]

Draining a Li battery just to make hot water doesn't really seem like the best application for such an expensive battery. A much simpler system might be put together with with some panels in series to put out ~48V and just power the heater through straight DC. Maybe three or four parallel strings of grid-tie panels wired 2S3P or 2S4P. This has been discussed in detail before. Search for previous threads on DC hot water heating.

https://www.amazon.com/Dernord-Submersible-Element-Immersion-Heating/dp/B01GPSL6IU/ref=asc_df_B01GPSL6IU?tag=bingshoppinga-20&linkCode=df0&hvadid=80264400674375&hvnetw=o&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4583863979620845&psc=1

You need to be careful though to select the proper thermostat that is DC rated. Missouri Wind & Solar offers a temperature range of different DC-rated thermostats.

DC Water Heating Element Thermostat

Missouri Wind and Solar carries a range of DC or AC water heating thermostats including a thermostat that just keeps liquids from freezing. Use these with our DC water heating elements.

windandsolar.com

 

[Jet]

Draining a Li battery just to make hot water doesn't really seem like the best application for such an expensive battery. A much simpler system might be put together with with some panels in series to put out ~48V and just power the heater through straight DC. Maybe three or four parallel strings of grid-tie panels wired 2S3P or 2S4P. This has been discussed in detail before. Search for previous threads on DC hot water heating.

https://www.amazon.com/Dernord-Submersible-Element-Immersion-Heating/dp/B01GPSL6IU/ref=asc_df_B01GPSL6IU?tag=bingshoppinga-20&linkCode=df0&hvadid=80264400674375&hvnetw=o&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4583863979620845&psc=1

View attachment 142557

You need to be careful though to select the proper thermostat that is DC rated. Missouri Wind & Solar offers a temperature range of different DC-rated thermostats.

DC Water Heating Element Thermostat

Missouri Wind and Solar carries a range of DC or AC water heating thermostats including a thermostat that just keeps liquids from freezing. Use these with our DC water heating elements.

windandsolar.com

View attachment 142558

I'll Read more what people say on the forms about diversion. I was really thinking about heating water directly with panels but with diversion. I am using Diacan's Electrodacus Solar BMS and a 24V battery bank. This is a completely off grid installation. This was originally for a van but I'm putting it in a cabin. Had I known that when I purchased the stuff I would have went with a 48 or 60 volt system. With 24 volt panels people seem to use 36 volt water heater elements. I think this 48 volt heater element would would work good with three strings of 12 volt panels as that would produce 51-54 volts.

The reason I was thinking of powering the water heater from the battery was to be more efficient than AC. With a solar panel driven water heater, it only works when the sun is out. It was reassuring with people saying that built such the water was still hot in the morning from the day before. However, if you take showers the night before you probably won't have any hot water in the morning. I kind of forgot. I have this electric miniature on demand water heater, shower head thing that could heat the water in the morning in such a situation.

"Diacan" the owner of Electrodocus won't have the DSSR50s ready and tell June and there is a new way you have to hook two up together to have diversion. His previous solar controller only switched 20 amps max. If I go that route I'll have three 450W 24V panels running a 1200W 36V element. You can set up the controller where it "diverts" the energy to the water heater wants to batteries are charged. I believe you can also set it to prioritize the water heater and recharge the batteries once the water is hot. I will have 7 other panels dedicated to just charging the battery and three 24 volt 330W panels will be in series connected to a hybrid solar air conditioner.

 

[TorC]

I've lived with solar domestic hot water for years. Hot water in the morning after showers the night before is a function of how big the tank is, how hot the water was, how long the showers were, and such. That said, a well sized system will do just fine. You can also significantly increase the performance per amount of water stored with a dual tank system. Cold water comes into the lower tank, thermosiphon hot water to top of hot tank. Then when you draw in the evening, the water in your top tank gets replaced with partially to mostly pre-heated water. A single tank will stratify to a degree, but there's more mixing than a two tank system.

 

[efficientPV]

Electrodacus uses multiple resistance heater elements to match the load to the solar panel. If you don't have multiple elements there is no magic. I have a 40 gallon tank for laundry which easily stratifies and heats quickly the upper 10-15 gallons which is enough for many uses. To heat 6 gallons is only about 800WH to heat. The lower section of the tank is preheat and losses are minimal until temp reaches over 80F. Tank losses can easily be 1500WH a day or 70W needed just to maintain temperature. I use two tanks for domestic hot water with the primary being only 6 gallons. Most tanks require extra insulation. I built my secondary tank into an insulated box. Well, only a half box at this time and the difference in heat loss was dramatic. While not available to most here, Operating as diversion from raw array power is an amazing way to heat water. I'm surprised that there isn't a community here doing this. China should be selling thousands of these units because they are cheap to build. But, a lack of understanding of the principals of operation has resulted in little demand for these products.

Pseudo MPPT can be made by using elements with multiple elements. They do make three phase elements which fit standard water heater threads. Having multiple panel voltage sensors turn on elements in sequence is one possibility. though this is far more expensive than a PWM control and less efficient. At only 70% of panel current in direct connect, you can get 50% more heating with a power point control. When panel current drops to 50%, a power point control will produce 100% more heating. Most of the day or with clouds the panel current will be less than 50% of label. If considering direct connect, one university study indicated that almost doubling "ideal resistance" produced far more daily heating.

Many common small wall electronic wall warts operate over a wide range of voltages and produce enough current to operate DC relays on AC up to 240V. I even use these on array DC when over 60V.