help needed with DC thermostat solar water heating





Page 1 of 3 1 2 3 LastLast
Results 1 to 20 of 57
  1. #1
    Join Date
    Oct 2009
    Location
    Mooi River
    Posts
    758
    Thanked: 60

    Default help needed with DC thermostat solar water heating

    i have 2 x 280W solar panels on the roof.

    connected to an AC DC (220v 2kw and 900w DC)element fitted in my kwikhot 150l geyser.

    i have connected it directly for now.

    i am looking for a DC thermostat to switch the dc side off when temp is reached.

    the voltage from the solar panels is around 50V DC and current of 9 Amp

    i have tried a 250v 16A thermostat (30-90 degree)

    it arcs when it clicks off definitely sounds like its burning with a sizzling sound.

    where can i get a simple DC thermostat for this project?

    2000 Ford Courier DC 2.5TD 4x4
    2009 Fiat 500
    2001 Merc C200K
    2007 Disco 3 TDV6 manual

  2. #2
    Join Date
    May 2013
    Location
    Sandton
    Age
    59
    Posts
    256
    Thanked: 14

    Default Re: help needed with DC thermostat solar water heating

    Have you thought of a DC automotive relay they should be able to switch that sort of current?

  3. The Following User Says Thank You to TonySwash For This Useful Post:


  4. #3
    Join Date
    May 2013
    Location
    Sandton
    Age
    59
    Posts
    256
    Thanked: 14

    Default Re: help needed with DC thermostat solar water heating

    Have you thought of a DC automotive relay?

  5. #4
    Join Date
    Jan 2009
    Location
    Durbanville
    Age
    58
    Posts
    35
    Thanked: 0

    Default Re: help needed with DC thermostat solar water heating

    It might help to reduce the arcing if you connect a resistor across the thermostat terminals. Approximately a 4.7KOhm 2watt.
    01 Discovery 2 V8 A/T- Sold
    97 LC80 4500 GX Limited

  6. #5
    Join Date
    Oct 2009
    Location
    Stellenbosch
    Age
    69
    Posts
    3,953
    Thanked: 498

    Default Re: help needed with DC thermostat solar water heating

    Quote Originally Posted by daveburr View Post
    It might help to reduce the arcing if you connect a resistor across the thermostat terminals. Approximately a 4.7KOhm 2watt.
    I'm tempted to say add a 0,1uF 400V capacitor in parallel with your resistor for an even better effect.
    Eggie.

    What this country needs more and more, are more unemployed politicians.
    - apology to Edward Langley.

  7. #6
    Join Date
    May 2009
    Location
    Hillcrest, Malaysia
    Age
    63
    Posts
    18,554
    Thanked: 5975

    Default Re: help needed with DC thermostat solar water heating

    The good old AC vs DC breaker/switch debate.

    Anybody contemplating switch DC should watch this - https://www.youtube.com/watch?v=Zez2r1RPpWY

    At this voltage and current, there is NO easy cheap fixes, no combination of resistors or capacitors that are worth even trying.

    Three solutions, but pricey.

    Use thermostat to switch a HV DC contactor.

    Use thermostat to switch a Silicon Switch. AKA a SSR Solid State Relay.

    Use the thermostat to switch a MPPT controller. Basically the thermostat signal shuts down the DC-DC inverter in the controller. (This is the industry standard for doing what you want to do)
    Cheers

    ZS5KAD
    3 V8's
    NA TwinTurbo SuperCharger
    A V6 and an inline 4

    The frogs are starting to notice that the water is getting warm but it is already too hot to do anything about it.....

  8. The Following User Says Thank You to Fluffy For This Useful Post:


  9. #7
    Join Date
    Oct 2009
    Location
    Stellenbosch
    Age
    69
    Posts
    3,953
    Thanked: 498

    Default Re: help needed with DC thermostat solar water heating

    Quote Originally Posted by Fluffy View Post
    The good old AC vs DC breaker/switch debate.

    Anybody contemplating switch DC should watch this - https://www.youtube.com/watch?v=Zez2r1RPpWY

    At this voltage and current, there is NO easy cheap fixes, no combination of resistors or capacitors that are worth even trying.

    Three solutions, but pricey.

    Use thermostat to switch a HV DC contactor.

    Use thermostat to switch a Silicon Switch. AKA a SSR Solid State Relay.

    Use the thermostat to switch a MPPT controller. Basically the thermostat signal shuts down the DC-DC inverter in the controller. (This is the industry standard for doing what you want to do)

    This is good debate to remind one & all regularly that there are important differences between AC and DC.

    The U-tube demo is effective in demonstrating what one is dealing with when switching DC currents. Do however remember that the OP is using a 50V circuit. One will get into serious trouble quickly when you substitute 220VDC into a 50VDC circuit.

    Compare this: A very well known automotive circuit breaker, the good old points set in a pre-electronic distributor, switches a 12V / 4A inductive current for several millions of times quite successfully. The "points" is nothing other that a simple and cheap mechanically operated switch.

    One also gets the impression that the demo is cheating a little. Is it just my imagination or is the 220VDC heating up those IR bars rather quicker and brighter than the 220VAC does?? Is the 220V DC really 220V or is it nearer 300V?

    Don't discard the resistor/capacitor method. It does have merit and it is used extensively in practice simply due to the ease of applying it, and its negligible cost.

    BTW: Should Naami (the OP) decide on the SSR solution, it is very important to note that he needs to specify a SSR that is designed for use in DC circuits. The typical (AC) SSR will be of no use.
    Eggie.

    What this country needs more and more, are more unemployed politicians.
    - apology to Edward Langley.

  10. #8
    Join Date
    May 2009
    Location
    Hillcrest, Malaysia
    Age
    63
    Posts
    18,554
    Thanked: 5975

    Default Re: help needed with DC thermostat solar water heating

    Quote Originally Posted by Eggie View Post
    This is good debate to remind one & all regularly that there are important differences between AC and DC.

    The U-tube demo is effective in demonstrating what one is dealing with when switching DC currents. Do however remember that the OP is using a 50V circuit. One will get into serious trouble quickly when you substitute 220VDC into a 50VDC circuit.

    Compare this: A very well known automotive circuit breaker, the good old points set in a pre-electronic distributor, switches a 12V / 4A inductive current for several millions of times quite successfully. The "points" is nothing other that a simple and cheap mechanically operated switch.

    With a VERY VERY fast acting and huge gap compared to an AC bi-metal thermostat. As that tongue slowly seperates it creates a bridge, just like that knife switch, except it never moves far away enough fast enough. Inductive loads are EASY in points systems..The Back EMF generated by the collapsing magnetic field does the same job as an AC circuit, it kills the spark.

    One also gets the impression that the demo is cheating a little. Is it just my imagination or is the 220VDC heating up those IR bars rather quicker and brighter than the 220VAC does?? Is the 220V DC really 220V or is it nearer 300V?

    Doesn't matter. The principle remains, even 380v AC would not sustain an arc like that.

    Don't discard the resistor/capacitor method. It does have merit and it is used extensively in practice simply due to the ease of applying it, and its negligible cost. (Please DISCARD the resistor/capacitor method for this application)

    BTW: Should Naami (the OP) decide on the SSR solution, it is very important to note that he needs to specify a SSR that is designed for use in DC circuits. The typical (AC) SSR will be of no use. (err mmm yep)
    Guys, if you want to try this.

    Please contact Geyserwise in your part of the world.

    They are the world leaders in this application. They have tried everything.

    I promise you, with everything I know, you are NOT quenching the spark on a slow acting bimetal strip contact controlling 50V and 9A with any possible combination of resistors and capacitors. NEVER EVER.

    THIS IS A SAFETY ISSUE

    What Voltage do the telephone and communication networks and exchanges work on. - ANSWER 48V DC

    How many years did I spend designing, installing and maintaining 48V DC system in the telecommunications industry. - ANSWER 17 years.
    Cheers

    ZS5KAD
    3 V8's
    NA TwinTurbo SuperCharger
    A V6 and an inline 4

    The frogs are starting to notice that the water is getting warm but it is already too hot to do anything about it.....

  11. #9
    Join Date
    May 2009
    Location
    Hillcrest, Malaysia
    Age
    63
    Posts
    18,554
    Thanked: 5975

    Default Re: help needed with DC thermostat solar water heating

    Quote Originally Posted by TonySwash View Post
    Have you thought of a DC automotive relay they should be able to switch that sort of current?
    Nope. The current YES, the voltage NO
    Also look at the action of a DC automotive relay.

    It is designed for DC - a fast acting device with a nice wide gap when open.

    Look at that video again.

    What would have happened if the operator pulled the knife switch lever away very fast and very far instead of slowly. No sustained spark.

    The bi-metal thermostat mimics what the operator does. Slowly breaking contact.
    Last edited by Fluffy; 2016/07/25 at 06:58 PM.
    Cheers

    ZS5KAD
    3 V8's
    NA TwinTurbo SuperCharger
    A V6 and an inline 4

    The frogs are starting to notice that the water is getting warm but it is already too hot to do anything about it.....

  12. #10
    Join Date
    Oct 2009
    Location
    Stellenbosch
    Age
    69
    Posts
    3,953
    Thanked: 498

    Default Re: help needed with DC thermostat solar water heating

    Quote Originally Posted by Fluffy View Post
    Nope. The current YES, the voltage NO
    Also look at the action of a DC automotive relay.

    It is designed for DC - a fast acting device with a nice wide gap when open.

    Look at that video again.

    What would have happened if the operator pulled the knife switch lever away very fast and very far instead of slowly. No sustained spark.

    The bi-metal thermostat mimics what the operator does. Slowly breaking contact.

    I wholly agree that a slow bi-metal strip type of thermostat is all but useless in this application, but there are more suitable (spring loaded) techniques. Unfortunately I do not have the inside information on this, but I do expect that a thermostat for DC applications would or should be the type that reacts like a clicksyn. Fast-acting and with integrated hysteresis.

    A semi-conductor approach sounds feasible and not all that complicated. Would Geyserwise not have a solution such as a DC SSR or similar in their bag of tricks?


    BTW: The opening of automotive points is not a "huge gap" at all, it is minute. But the ringing in the circuit has a lot to do in quenching arcing across the contacts.
    Eggie.

    What this country needs more and more, are more unemployed politicians.
    - apology to Edward Langley.

  13. #11
    Join Date
    May 2009
    Location
    Hillcrest, Malaysia
    Age
    63
    Posts
    18,554
    Thanked: 5975

    Default Re: help needed with DC thermostat solar water heating

    Quote Originally Posted by Eggie View Post
    I wholly agree that a slow bi-metal strip type of thermostat is all but useless in this application, but there are more suitable (spring loaded) techniques. Unfortunately I do not have the inside information on this, but I do expect that a thermostat for DC applications would or should be the type that reacts like a clicksyn. Fast-acting and with integrated hysteresis.

    A semi-conductor approach sounds feasible and not all that complicated. Would Geyserwise not have a solution such as a DC SSR or similar in their bag of tricks?


    BTW: The opening of automotive points is not a "huge gap" at all, it is minute. But the ringing in the circuit has a lot to do in quenching arcing across the contacts.
    Eggie, I'm not sucking this info out of my thumb. Honestly.

    I have researched this thoroughly. Even had my own Low voltage heating elements custom made.

    To the best of my knowledge there is no DC type thermostat suitable for this application.

    Up until a while ago GeyserWise's solution was a SSR.

    They proved to be expensive and unreliable, so GeyserWise dropped them and now only supply a MPPT based solution. This has a slight price premium which is recovered fairly fast due to the higher efficiency and better reliability.

    SNAP QUIZ - What is a grasshopper fuse.

    EDIT - There is some interesting work being done on heating elements using Titanium PTC chips, basically these elements go open circuit once the design temperature is reached. Essentially they don't need any thermostat or control. They may even be available already.
    Last edited by Fluffy; 2016/07/26 at 12:15 PM.
    Cheers

    ZS5KAD
    3 V8's
    NA TwinTurbo SuperCharger
    A V6 and an inline 4

    The frogs are starting to notice that the water is getting warm but it is already too hot to do anything about it.....

  14. #12
    Join Date
    Jan 2012
    Location
    durban
    Age
    56
    Posts
    1,113
    Thanked: 823

    Default Re: help needed with DC thermostat solar water heating

    Quote Originally Posted by Fluffy View Post
    Eggie, I'm not sucking this info out of my thumb. Honestly.

    I have researched this thoroughly. Even had my own Low voltage heating elements custom made.

    To the best of my knowledge there is no DC type thermostat suitable for this application.

    Up until a while ago GeyserWise's solution was a SSR.

    They proved to be expensive and unreliable, so GeyserWise dropped them and now only supply a MPPT based solution. This has a slight price premium which is recovered fairly fast due to the higher efficiency and better reliability.

    SNAP QUIZ - What is a grasshopper fuse.

    EDIT - There is some interesting work being done on heating elements using Titanium PTC chips, basically these elements go open circuit once the design temperature is reached. Essentially they don't need any thermostat or control. They may even be available already.

    I was under the impression that the dual elements were already working on this technology?
    Last edited by plunger; 2016/07/26 at 07:54 PM.

  15. #13
    Join Date
    May 2009
    Location
    Hillcrest, Malaysia
    Age
    63
    Posts
    18,554
    Thanked: 5975

    Default Re: help needed with DC thermostat solar water heating

    Quote Originally Posted by plunger View Post
    I was under the impression that the dual elements were already working on this technology?
    The standard duel elements that I am aware of are just that.

    Standard.

    I think it's either or. A standard wire dual or a single PTC Chip element.

    But like I said in my post. The new Titanium chip ones may be available, haven't looked at this for about a year.

    Google nuThermo? Or something similar.
    Cheers

    ZS5KAD
    3 V8's
    NA TwinTurbo SuperCharger
    A V6 and an inline 4

    The frogs are starting to notice that the water is getting warm but it is already too hot to do anything about it.....

  16. #14
    Join Date
    Sep 2006
    Location
    Oppierotse
    Age
    55
    Posts
    1,714
    Thanked: 222

    Default Re: help needed with DC thermostat solar water heating

    Why cant the OP use a 220v/16a thermostat connected to a 220v relay that switches the 50v DC on and off?IOW the thermostat is connected to the relay coil to energise or de energise it with very little current draw and the switching side of the relay controls the DC side.
    98 Hilux 2.8D D/C 4x4

  17. #15
    Join Date
    Oct 2009
    Location
    Stellenbosch
    Age
    69
    Posts
    3,953
    Thanked: 498

    Default Re: help needed with DC thermostat solar water heating

    Quote Originally Posted by wiel View Post
    Why cant the OP use a 220v/16a thermostat connected to a 220v relay that switches the 50v DC on and off?IOW the thermostat is connected to the relay coil to energise or de energise it with very little current draw and the switching side of the relay controls the DC side.

    The OP has been absent since his 1st post, but we take it that he wants to switch a 50VDC current of approx 10A.

    Here lies the problem:
    He will need a relay that is capable of handling this 50VDC/10A switching job repeatedly over a long period of time. So your approach can switch the realy coil on/off as you describe, but the problem then shifts to the relay itself. Can the relay contacts handle the job or will they also burn up?
    Eggie.

    What this country needs more and more, are more unemployed politicians.
    - apology to Edward Langley.

  18. #16
    Join Date
    Oct 2009
    Location
    Stellenbosch
    Age
    69
    Posts
    3,953
    Thanked: 498

    Default Re: help needed with DC thermostat solar water heating

    Fluffy, I'm curious as the PV way of achieving solar water heating is a rather topical subject nowadays. So, I do not wish to challenge you in any way, but just asking if you have some input here, as it sounded like you have already given the subject some thought.

    The company SES (based in United Arab Emirates- see below) offers not only DC heating elements, but also an array of DC heating elements with integrated adjustable thermostats. Do you have any pointers or advice for potential buyers of such equipment?

    Another thing, is there really a difference between DC and AC heating elements?
    For instance, can a 110VAC element of say 5kW be used as a 1kW 50VDC heater (or a 240W @ 24VDC)? I doubt there would be insulation issues.


    12-48 Volt Heating Elements with Adjustable Thermostat
    http://www.sespvt.com/12-48-Volt-Hea...ble-Thermostat


    The SES returns policy (given verbatim below) is worrying:
    "You may return most new, unopened items within 30 days of delivery for a full refund. We'll also pay the return shipping costs if the return is a result of our error (you received an incorrect item, etc.). All the Electronics are checked before shipping them. However there is no guarranttee of electrnic devices. If the Package Opened it return can not accept."
    Eggie.

    What this country needs more and more, are more unemployed politicians.
    - apology to Edward Langley.

  19. #17
    Join Date
    May 2009
    Location
    Hillcrest, Malaysia
    Age
    63
    Posts
    18,554
    Thanked: 5975

    Default Re: help needed with DC thermostat solar water heating

    Quote Originally Posted by Eggie View Post
    Fluffy, I'm curious as the PV way of achieving solar water heating is a rather topical subject nowadays. So, I do not wish to challenge you in any way, but just asking if you have some input here, as it sounded like you have already given the subject some thought.

    The company SES (based in United Arab Emirates- see below) offers not only DC heating elements, but also an array of DC heating elements with integrated adjustable thermostats. Do you have any pointers or advice for potential buyers of such equipment?

    Another thing, is there really a difference between DC and AC heating elements?
    For instance, can a 110VAC element of say 5kW be used as a 1kW 50VDC heater (or a 240W @ 24VDC)? I doubt there would be insulation issues.


    12-48 Volt Heating Elements with Adjustable Thermostato
    http://www.sespvt.com/12-48-Volt-Hea...ble-Thermostat


    The SES returns policy (given verbatim below) is worrying:
    "You may return most new, unopened items within 30 days of delivery for a full refund. We'll also pay the return shipping costs if the return is a result of our error (you received an incorrect item, etc.). All the Electronics are checked before shipping them. However there is no guarranttee of electrnic devices. If the Package Opened it return can not accept."
    Sorry for the belated reply.

    That website and spec looks nearly as contrived and smoke and mirrors as the Betta battery LC DATA Sheets.

    The interesting part is the last sentence about "electrnic (sic) devices". Go figure, it's a cheap SSR based thermostat at best.

    The price is excellent at $69 and worth a punt for an element with an electronic thermostat.

    Or is it? Wonder if my insurance will pay if my house burns down.
    Last edited by Fluffy; 2016/08/01 at 10:25 PM.
    Cheers

    ZS5KAD
    3 V8's
    NA TwinTurbo SuperCharger
    A V6 and an inline 4

    The frogs are starting to notice that the water is getting warm but it is already too hot to do anything about it.....

  20. #18
    Join Date
    May 2009
    Location
    Hillcrest, Malaysia
    Age
    63
    Posts
    18,554
    Thanked: 5975

    Default Re: help needed with DC thermostat solar water heating

    Quote Originally Posted by Eggie View Post

    Another thing, is there really a difference between DC and AC heating elements?
    For instance, can a 110VAC element of say 5kW be used as a 1kW 50VDC heater (or a 240W @ 24VDC)? I doubt there would be insulation issues.

    [/COLOR]
    I answered this one separately on purpose.

    The answer, I dunno, yes, and no.

    Depends how you spec it. How and when do you consider power spec vs AC RMS vs DC equivalent power issues.

    But based on a derived Element Resistance spec, once normalized there is no difference between an element used on AC or DC in my opinion.

    A real problem with these things is the difference/tracking of element resistance and tracking between temperature and resistance (power dissipation)

    In a nutshell, as per your observation. Any element will have a resistance (let's assume on resistance), so it will dissipate the power that Ohms law dictates for the voltage applied against the element resistance.

    That is actually simple. But I (anybody) can make an element that dissipates 1000W at one voltage, and the Maths says it will generate 3000W at another voltage. But at 3000W it will self district and burn out. So there is more to it than just resistance.

    And this is exactly the issue I have faced. You can't just blindly take an element from one application and apply it elsewhere using resistance as the design driver. I dunno all the facts. And neither do the vendors, (well they probably do, but they ain't telling)
    Last edited by Fluffy; 2016/08/02 at 01:37 PM.
    Cheers

    ZS5KAD
    3 V8's
    NA TwinTurbo SuperCharger
    A V6 and an inline 4

    The frogs are starting to notice that the water is getting warm but it is already too hot to do anything about it.....

  21. #19
    Join Date
    May 2013
    Location
    Sandton
    Age
    59
    Posts
    256
    Thanked: 14

    Default Re: help needed with DC thermostat solar water heating


  22. #20
    Join Date
    May 2013
    Location
    Sandton
    Age
    59
    Posts
    256
    Thanked: 14

    Default Re: help needed with DC thermostat solar water heating


Page 1 of 3 1 2 3 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •