DC Spa pre heater

[BenQ]

So I was watching David Poz on YouTube and saw his hot water video and decided to try building a similar system to preheat my spa. So I bought 4 used 305w Sunpower Solar Panels (~60v at ~5.9amps) @$90 each, a Camco 04382/04383 1500W 240V Universal Flange Water Heater Element - High Watt Density @$11, a used Murray ghn321nw 30 amp 240vac/250vdc disconnect switch with fen-r-30 fuses @$10 and some InstallGear 8 Gauge wire @$33. I got it all installed (panels in series for ~240vdc @ ~5.9 amps) At the terminals when the disconnect switch is in the off position, but when I throw the switch to the on position there’s nothing...any ideas why that is? Are the fuses interfering somehow? I checked for continuity before connecting the solar panels to the disconnect switch and there’s no resistance so...?

 

[snoobler]

Did you use his spreadsheet? Whole point of is video was "it's all about the Ohms."

IMHO, you're heating element has a MUCH higher resistance than your panels can handle. He has a calculator right there on his home page.

 

[DThames]

With the switch open(off) you should be able to measure your panel voltage on the panel connection at the switch and see your (60v x 4 series) voltage. This will verify that your panel power is getting to the switch. Next you can disconnect one wire from the heating element and between that wire and the element (where you just disconnected it) you should see that same voltage when the switch is on. This is DC voltage, so be sure your meter will handle 250v DC. If either of those tests show zero volts, you will be able to determine which side of the switch your problem is located. Then work from that point to discover more.

 

[efficientPV]

So you read absolutely nothing? Disconnect one of the wires to the heater and throw the switch to on. Do you then have voltage at lower terminal? Since these are used panels, there may be corrosion on one of the connectors. With a load, that will drop the voltage to zero.

 

[BenQ]

Did you use his spreadsheet? Whole point of is video was "it's all about the Ohms."

IMHO, you're heating element has a MUCH higher resistance than your panels can handle. He has a calculator right there on his home page.

Yeah I used his calculator, but he didn’t have a heating element on it that matched my panel configuration. In the video he says he would have liked to buy an element like mine and connect his panels in series instead of parallel, but he already bought the lower voltage element.

 

[snoobler]

There's a reason your element didn't come up. The resistance is too high.

 

[efficientPV]

I don't put much stock in David's water heating advice. People should learn some math instead of the idiots guide to water heating. That 1500W element is 38.4 ohms and "ideal" should be about 40 ohms. You are probably better off having even higher resistance. Whatever was calculated that those panels would produce, drop it in half. Look at this chart and think of 10A as 100% of the panel current and look at the percentage gained with a properly designed system vs direct connect.

Panels at 60V Direct connect Increase

10A X 60V = 600W 10A X 60V = 600W

9A X 60V = 540W 9A X 54V = 486W 11%

8A X 60V = 480W 8A X 48V = 384W 25%

7A X 60V = 420W 7A X 42V = 294W 50%

6A X 60V = 360W 6A X 36V = 216W 67%

5A X 60V = 300W 5A X 30V = 150W 100%

4A X 60V = 240W 4A X 24V = 96W 250%

3A X 60V = 180W 3A X 18V = 54W 333%

2A X 60V = 120W 2A X 12V = 24W 500%

1A X 60V = 60W 1A X 6V = 6W 1100%

If you have unlimited funds, no clouds, and space for an array this isn't a problem.

 

[BenQ]

So if I added 4 more panels to the mix then I’d be in business?

 

[BenQ]

With the switch open(off) you should be able to measure your panel voltage on the panel connection at the switch and see your (60v x 4 series) voltage. This will verify that your panel power is getting to the switch. Next you can disconnect one wire from the heating element and between that wire and the element (where you just disconnected it) you should see that same voltage when the switch is on. This is DC voltage, so be sure your meter will handle 250v DC. If either of those tests show zero volts, you will be able to determine which side of the switch your problem is located. Then work from that point to discover more.

I’ll try this tomorrow afternoon!

 

[BenQ]

I don't put much stock in David's water heating advice. People should learn some math instead of the idiots guide to water heating. That 1500W element is 38.4 ohms and "ideal" should be about 40 ohms. You are probably better off having even higher resistance. Whatever was calculated that those panels would produce, drop it in half. Look at this chart and think of 10A as 100% of the panel current and look at the percentage gained with a properly designed system vs direct connect.

Panels at 60V Direct connect Increase

10A X 60V = 600W 10A X 60V = 600W

9A X 60V = 540W 9A X 54V = 486W 11%

8A X 60V = 480W 8A X 48V = 384W 25%

7A X 60V = 420W 7A X 42V = 294W 50%

6A X 60V = 360W 6A X 36V = 216W 67%

5A X 60V = 300W 5A X 30V = 150W 100%

4A X 60V = 240W 4A X 24V = 96W 250%

3A X 60V = 180W 3A X 18V = 54W 333%

2A X 60V = 120W 2A X 12V = 24W 500%

1A X 60V = 60W 1A X 6V = 6W 1100%

If you have unlimited funds, no clouds, and space for an array this isn't a problem.

Element: 1500 watts / 240 volts = 6.25 amps
240 volts / 6.25 amps = 38.4 ohms

Solar panels: 1220 watts / 240 volts = 5.08 amps & 240 volts / 5.08 amps = 47.24 ohms (that’s if they’re cranking at 100% which they probably not. Probably more like 80% so 976 watts / 240 volts = 4.07 amps then 240 volts / 4.07 amps = 58.97 ohms

So we have a potential difference of 20 ohms I can’t imagine that’s helping at all

If I connected 4 more panels in series and added it to this system in parallel I’d have 240 volts / 8.14 amps = 29.48 ohms. Would this im

With the switch open(off) you should be able to measure your panel voltage on the panel connection at the switch and see your (60v x 4 series) voltage. This will verify that your panel power is getting to the switch. Next you can disconnect one wire from the heating element and between that wire and the element (where you just disconnected it) you should see that same voltage when the switch is on. This is DC voltage, so be sure your meter will handle 250v DC. If either of those tests show zero volts, you will be able to determine which side of the switch your problem is located. Then work from that point to discover more.

Okay so I did this test and good news! It’s not the switch! Bad news, it’s definitely the element. So...now I need to find a better element for my panels...if my calculations from last night are right I’ll need something that takes about 1000 watts at 240 volts and 4 amps? Time to go shopping!

 

[BenQ]

If I put two of these in parallel, would they work better? DERNORD Immersion Cartridge Heater 240V 500W Hot Rod Heating Element Replacement 1/2 Inch Thread (240V 500W)

 

[efficientPV]

I can only go on what you told me since you didn't post the label of the panel, 60V @ 5.9A. The odds of you having a shorted element are astronomical. Even a mismatched element would show some voltage. Only a short in the wiring or switch. 40ish ohms should be easy to measure with even the worst ohm meter. Just how did you determine the element was bad? With extra panels the voltage on the element will go above power point and closer to 280V.

If you has those two 500W elements and connected one all the time and switched the other one in parallel when the current rose, that would be better.

 

[BenQ]

Or this! I think this will work great right?

Zoro Select #SG1107 430414 Specifications

Zoro #: G1739062 Mfr #: SG1107 430414
Fits: -	Sheath Material: Copper
Insert Length (In.): 7-3/4	For Use With: Moderately Hard Water
Plating: Zinc	Type: Low-Watt Density
Hz: 60	Watts: 1000
Item: Water Heater Element	Includes: 1" NPSM Screw Plug And Type AG-1 Rubber Gasket
Style: Screw In	Voltage: 240

 

[efficientPV]

Why are you even thinking of running 240V DC anywhere near a hot tub. At least go to 120, preferably 60V. At 120V the heater element will be one quarter of the rated valure. Low density elements, longer or at reduced voltage, are much better to use. Don't use ahy heating element which is grey or black. These have an insulated sheath from ground. You haven't answered my question of why you think the heater element is bad.

 

[BenQ]

Why are you even thinking of running 240V DC anywhere near a hot tub. At least go to 120, preferably 60V. At 120V the heater element will be one quarter of the rated valure. Low density elements, longer or at reduced voltage, are much better to use. Don't use ahy heating element which is grey or black. These have an insulated sheath from ground. You haven't answered my question of why you think the heater element is bad.

I think the heater element is bad because I tested the voltage through the wires when it’s disconnected and I get 240v. I suppose I could have a short, but I’d have to unseal the element and that’s more work than testing the voltage at both ends of the disconnect switch. No grey or black, got it! I figured high volt/low current amps would be better

 

[DThames]

I think the heater element is bad because I tested the voltage through the wires when it’s disconnected and I get 240v. I suppose I could have a short, but I’d have to unseal the element and that’s more work than testing the voltage at both ends of the disconnect switch. No grey or black, got it! I figured high volt/low current amps would be better

So you know you have the voltage at the element, from your test. If you left one of the leads connected and measured from the disconnected side of the element to the disconnected wire, you measured the voltage that was from the panels AND through the element. This proves the element is not broken as an open circuit. If you reconnect it and turn it on, you should see the element lower the open circuit voltage of the 240v that you measured. If it lowers to zero volts (measured across the connections of the element, then the element is shorted. All of your 1200 watts should be turning into heat in the wiring if the sun is bright on the panels with NO shade. In series your panels can't produce anything to speak of if any part of any panel is in shade.....assuming they are typical panels. If your voltage drops to some low value like 50volts, then the element is just too large for the power of the panels. In that case more panels would help bring the power level up. So, with full sun on all panels and the element connected and turned on, what is the DC voltage measured on the two connections of the element?