Overheat protection: hot water cylinder / PV

2 Posts
Apr 26, 2013 05:10 pm
Overheat protection: hot water cylinder / PV

Hi, first time caller Smiley

I have a neat 400l (~100 gal) HW cylinder. It currently has 2 X 3.6kw 240v ACĀ  elements, top and bottom. I have disconnected the bottom one.

I am thinking of trickle heating using a DC element in the bottom and about 400w of PV. (I have grid-tie PV elsewhere, but due to the way my utility buys back power I must always be a net consumer - so it's not worth me going full thermal)

My worry is the thermal cutout - the cylinder has all the usual safety gear, but I would like to be able to shut off the DC if the water hits, say, 70C.

It is fitted with a surface mount 'stat at the bottom, but this is rated at 10A 240vAC...

Any thoughts? Or just don't worry about it? (400w on 400l will take a lot of sunny days... I figure about 20 hours @ 100% efficiency)
97 Posts
Apr 26, 2013 11:53 pm
Re: Overheat protection: hot water cylinder / PV

Hello Adam.  I currently use DC to heat water using 50VDC on a conventional 1440 W, 120VAC element.  I am using the lower "trickle" heater, as you call it.  The element resistance is 10 ohms, which produces a current of 5 amps and a heating power of 250 watts.  Since we use little hot water, it works fine for us.  I use a timer to switch power on for 12 hours during the day, and shut off the power at night, since no hot water is being used during sleep time.  This minimizes any safety problem with overheating due to a stuck thermostat.  I did experience a problem initially, however, and wish to share this with you.  The 240vac/10a stat that you mention is the same as the one that I am using, and the contacts will burn from using DC current.  The life span is then very short.  I modified my electrical configuration to use a DC relay to switch the 50VDC to the heater element.  The thermostat then supplies only a small current to the relay coil and so far has operated without fault for several years time.  I am operating my household off the grid, and find this technique to be very efficient.  I wish you good fortune in your endeavors.  Jon C.
2 Posts
Apr 27, 2013 06:32 pm
Re: Overheat protection: hot water cylinder / PV

Thanks! I was worried about the 'stat, wondered if that may happen (it reminded me of some of the trouble we get on car relays). Sounds like I should give the existing element a try which is great news. I will figure out a relay system. The way you describe sounds pretty straightforward, thanks for that. All the best, Adam.
462 Posts
Apr 30, 2013 12:18 pm
Re: Overheat protection: hot water cylinder / PV

Adam, check out Aquastat controllers. They have a variety of high limit controls for most electrical situations and should be readily available at any plumbing supply.

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