Increasing hot water capacity with existing Eddi

[MisterTea]

I’m looking at increasing my hot water capacity that my Eddi can produce.

I have a 3kw solar array, a Zappi charger and an Eddi connected to a hot water cylinder with single top 3kw immersion heater.

The zappi gets first dibs on any surplus solar and during May - August it will run my wife’s BMW i3 purely on free electricity. The Eddi will heat if the car is not plugged in or is fully charged and does a good job of providing a limited amount of hot water, meaning that by careful use we get around 6 months of not heating with our gas boiler.

My issue is that the immersion only heats the top of the tank so once it is hot any surplus solar goes back in the grid, I’m looking at different options to increase the capacity of stored hot water.

I like the look of the Mixergy tanks but they seem to be very expensive, if I had or was planning to install batteries this would be the way forward as I could get a cheap rate tariff and charge the batteries overnight and also the Mixergy when not enough solar. However without batteries I think my peak use would outweigh the savings.

I’ve looked at other cylinders and am wondering about a twin immersion cylinder so as to heat more water with the surplus. Would I be better with a slim cylinder so that I would be heating less in the top before the bottom immersion started?

Thanks.

 

[efficientPV]

There are heating elements which will fit the 3/4 inch drain port of US tanks. There are small heating elements that will fit in almost any tank port. Europe has some strange tanks and don't know what will work. Tall is best for stratification. The lower level of tank acts as preheat. I use a small point of use tank like the BOSCH ES4 as the primary tank because it recovers quickly in the morning in series with the main tank. Big preheat tanks are just good for heat loss if you go above 35C.

 

[SeaGal]

As well as the twin element option, I wondered about whether a little pump that could circulate water from the top to the bottom of the tank could be used to even out the immersion's heating - would be cheaper and easier to install than a new cylinder?

 

[Ampster]

I wondered about whether a little pump that could circulate water from the top to the bottom of the tank could be used to even out the immersion's heating -

If that is not enough, increasing the temperature of the thermostat and then using a tempering valve on the output could give you increased virtual capacity.

 

[SeaGal]

If that is not enough, increasing the temperature of the thermostat and then using a tempering valve on the output could give you increased virtual capacity.

LOL - I do that with my diverter - dynamically use a higher temp via my ESP32 controller for immersion heating than by gas

 

[Steve777]

You might look into a standard AC hot water tank thermostat set. They typically have two elements and two thermostats; the top one heats first (shutting down the lower one) then when the top is hot the bottom element gets power and will come on if its thermostat is calling for heat. Nothing special about it, most standard or larger electric hot water tanks have this setup. It has the advantage of getting hot water at the top quickly so it can be used sooner.

If you aren't in a hurry, heat stratification can help. My solar thermal tank puts the hot water in at the bottom. Eventually that heat rises and the whole tank gets hot, but it can take a while. It's not usually necessary to pump the water around in a hot water tank as convection will eventually do the job for you.

 

[efficientPV]

I have two elements in my garage tank for laundry which are standard 240V elements. The hot water controller is sending a much lower voltage to them from the array. Currently using both elements in parallel with their own thermostat for a closer resistance match to PV voltage. Frankly just lazy and didn't want to put new elements in. I'm actually surprised how well this works keeping the upper 15 gallons hot with a nice preheat in the lower section.

 

[MisterTea]

Thanks for the replies, I currently do heat the water via the immersion as hot as I can get it to without tripping the thermostat on it. But it still only provides for a small volume of hot water.

My thinking is to replace the cylinder with a similar capacity one with twin immersions, the Eddi will use surplus solar to heat the top one until temp reached then switch to heating the lower one.

I’m thinking a slim cylinder may be a better bet as I don’t get sooo much surplus, this way would get the top section hotter quicker and start on the lower. I’m thinking that may be a way to get more hot water.