Solar storage tank back up electric element

Dorsey Davis_2

I'm looking at solar thermal systems with the single coil storage tank and back up electric element. So my question is whether the single element can keep up with hot water demand during no solar periods….I’m assuming it simply has a recovery rate typical of a 4500 watt element. On a standard electric water heater it’s usually the top element that fails. Therefore in my experience I know how that effects the hot water but I’m not very familiar with the lower element not functioning (since the solar tank on no sun days basically mimics a lower element failure). Will the single element in the solar tank eventually heat the entire tank? Can I tell my customers that they just have to wait a bit longer during no sun periods? Or will it never quite get the lower portion of the water hot?

Mike Dunn

solar tank

If you have a 80 gallon tank with a coil HX on the bottom for solar and an electric element up top for backup, you probably at best now have a 35-40 gallon electric water heater for those periods of no sun.

Upsize the tank to achieve more back up storage up top or use a two tank system or a solar tank with a tankless backup sized for the domestic load.

Mike

hot_rod

About 1/2

of the tanks capacity, as Mike suggested. But a tad better recovery as the bottom 1/2 does get preheated a bit from ambient temperature.

You could upsize to a 5500, maybe a 6000 watt element for faster recovery. Depending on your incoming voltage, a 5500 or 6000W element would run on a 30 amp breaker.

Another good option is to marry a instantenous HW heater to the solar tank. If you have enough juice (amperage) the solar preheat tank could be backed up by an electric instantanous DHW heater.

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Dorsey Davis_2

Solar storage tank back up electric element

So now I'm seeing that Bradford White has a line of dual element electric tanks with a solar hx coil. Wouldn't that prevent any temp dif between the roof panels and the tank since having both elements would never allow the temp dif to occur?

I like the idea of the solar storage pre-feeding a tankless. I've heard you need a diverting valve. When there's not enough hot out of the solar tank, the diverter switches flow to the tankless. Is this correct? Anybody done this?

Mike Dunn

Correct A mundo

on the first paragraph. I would disable the lower element for normal solar activity then enable it if I was shutting down the solar side of things for either maintenance or what have you.

I have been pondering your second question myself. I think you directly feed the tankless with the preheated solar water. The question is do you put the thermostatic mixing valve downstream of the tankless or between the tankless and the solar tank?

singh

Solar with electric tankless

I have a solar pre heat with a electric tankless unit by seisco.
http://www.seisco.com/Diagrams/tankless-solar-dia.jpg

As long as you have the room in your panel box 200 amp then you should enjoy plenty of hot water.

No diverter valve, just a temper valve to insure really hi temps from the solar does not trip the tankless on hi limit.
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rich pickering

5500 W...

5500W is okay on a 240v 30A circuit, 6000W would be too much (exceeds 80% of circuit rating).

If it's 208V service (e.g. 120/208 3 phase commercial) than a 6000W/240V element will yield 4507W at 208V and a standard 4500W/240V will yield only 3380W at 208V.