Upgrading an immersion heater from AC to 48V DC

Thread Starter

davelondon

Joined Mar 30, 2021
19
I'd like to change the immersion heater element in a hot water calorifier tank to a 1500W, 48V DC unit, like this.

I've been told the thermostat won't like switching DC and would need to be changed to something DC rated, perhaps with a contactor or relay doing the switching. It seems to me that this should be a relatively simple project, but I'm not an expert...
 

Yaakov

Joined Jan 27, 2019
2,911
A contractor like this one: https://www.ato.com/50a-dc-contactor-single-pole-12v-24v-48v connected to your existing thermostat should work. Nominally you are switching around 31A but using a contractor rated higher is a good idea.

You will have to work out the current voltage being used by the thermostat. If it is directly switching the mains voltage, you have a choice to get an AC mains voltage coil in the contractor or supplying a lower voltage to one like I showed you.
 

Thread Starter

davelondon

Joined Mar 30, 2021
19
Wow so it's as simple as connecting the existing thermostat to the contactor? No extra circuits needed? That would be great...
 

Thread Starter

davelondon

Joined Mar 30, 2021
19
Hopefully this would work without the mains AC running at all (I'm swapping this out so that I can leave my inverter turned off but have the immersion heater running)...
 

Yaakov

Joined Jan 27, 2019
2,911
Wow so it's as simple as connecting the existing thermostat to the contactor? No extra circuits needed? That would be great...
If you can swap out the current heating element for the DC one, it's that simple. It's a basic resistive load, and the thermostat is just a heat operated switch.
 

Thread Starter

davelondon

Joined Mar 30, 2021
19
Over on dirsolarforum I was told that you can't change an AC thermostat to 48V DC: https://diysolarforum.com/threads/upgrading-an-ac-immersion-heater-to-48v-dc.20383/

Most thermostats made for AC will not be reliable with DC, as the points can arc with each opening, causing all sorts of problems, including welding the points together. Generally you can swap out elements if the threads fit. I have considered this but I would get a simple temperature control that could switch a DC relay or build a relay driver circuit for the standard thermostat.
 

Yaakov

Joined Jan 27, 2019
2,911
Hopefully this would work without the mains AC running at all (I'm swapping this out so that I can leave my inverter turned off but have the immersion heater running)...
If the thermostat doesn't require mains to operate (some electronics, etc.) then you could use the same 48V supply to switch the contractor through the thermostat as long as it's contacts are rated for enough DC current to operate the contractor. If not, you'd have to include an intermediate relay that can switch the contractor. I don't think you'd need that, but you have to check. You really don't want the thermostat contacts welding themselves closed.
 

Thread Starter

davelondon

Joined Mar 30, 2021
19
OK so the current design is going to be a custom unpressurised hot water container, with a 1500W immersion heater... So I'll be building the whole circuit including the relay and thermostat....

Yes, but again, check the ratings for the contacts on the thermostat and make sure it can handle the DC load of the contactor coil.
Do you know where I can find a suitable thermostat?
 

Thread Starter

davelondon

Joined Mar 30, 2021
19
One thing to consider is that the water container will now contain heating coolant (water / glycol mixture) at 80C, so higher temperature than your standard fresh water... so the thermostat will need to be able to support 80C.
 
In the old days, large DC currents were switched using mercury relays. The wetted contacts in them and lack of wearing parts means that they last nearly forever. You can still find plenty of old Durakools, etc. for cheap as surplus on E-bay. Nobody wants them aside from hobbyists because they're hazmat waste these days.

Obviously be sure to only buy ones with fully encapsulated vials. The ones with exposed glass should be kept as static displays only.

Example:
https://www.ebay.com/itm/Durakool-C...097073&hash=item442792573f:g:LS0AAOSwybpblrMg
 
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strantor

Joined Oct 3, 2010
5,534
Bear in mind this is a resistive load so the arcing across contacts typical of a DC circuit will be minimal. You could "probably" get away with using just the thermostat as is.
 
Bear in mind this is a resistive load so the arcing across contacts typical of a DC circuit will be minimal. You could "probably" get away with using just the thermostat as is.
I've drawn some pretty impressive DC arcs at low voltages with relatively modest resistive loads. You can arc weld with 30 volts DC. A lot of times if you try to switch DC using an AC-rated contact it will just sit there and hiss until some part of the device overheats and melts down.

[video]
 
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Orson_Cart

Joined Jan 1, 2020
46
You can use a standard mains AC thermostat to switch low power DC to control a relay/contactor to switch the main element - ideally not more than 48VDC and 1A through the normal mains thermostat - so you don't need to find another thermostat - just an intermediary contactor / relay to power the DC load, with its coil energised by the thermostat, ideally 1ADC max.
 

strantor

Joined Oct 3, 2010
5,534
I've drawn some pretty impressive DC arcs at low voltages with relatively modest resistive loads. You can arc weld with 30 volts DC. A lot of times if you try to switch DC using an AC-rated contact it will just sit there and hiss until some part of the device overheats and melts down.

[video]
WOW. Ok, well forget whatever nonsense I said. That's crazy. Thanks for the correction.
 
It all depends on the open ckt driving voltage and the inductance of the line, 30VDC is a whole lot different to 300VDC ...!
Inductance is only relevant for the maintenance of an arc with fluctuating impedance when dealing with DC. It's why old transformer-type DC welders and carbon arc power spplues used big chokes in series.

Beyond that, 30VDC is 30VDC. Two or three car batteries in series will sustain an arc.


Switching DC is no joke. That's why cars and trucks run on 12 or 24VDC, instead of 72 or 90VDC like locomotives - DC equipment gets much more bulky and expensive at higher voltages. E.G. a contactor rated for 40 horsepower at 600VAC will probably only handle 2-5 horsepower at it's maximum DC rating of 90~120VDC... if it even *has* a DC rating at all.
 
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