Geysers in Series puzzle

[Clivevan]

My problem has too many variables for me to solve. Please help it is driving me bats.

So how do I build this puzzle?

Brand new solar geyser 150 litre on the roof (with element if necessary currently switched off);

Existing electrical bottom geyser on the ceiling below (witched on) also 150 litres;

Collector in between ie above the existing but below the solar.

Currently a normal retrofit pre-feed type installation with cold supply into cold inlet on the roof (into solar); the solar hot outlet then serves as prefeed and drops down into the cold inlet of existing bottom geyser. The bottom hot exits as supply to the house.

Problem: for 9 months I run the bottom electrical geyser inclusive of standing losses completely unnecessarily for two people. It should be switched off. BUT then the bottom geyser goes cold and what is received from the solar pre-feed mixes into a lukewarm mix before it exits into the house - this is no good at all. I have been told in no uncertain terms . . . I also received a warning that I am on a yellow card for spending much on a useless system.

During 3 months 12 paying guests and the electricity is on but on cloudy cool days the guests will exhaust the bottom electrical supply of water and the solar pre-feed will be too cold to catch up with demand. On those days the solar element should also kick in!

Remember - no ongoing supervision here - I have to set it up with minimal ongoing tinkering.

How do I set up this system??

Complicating factors but also potential solutions:

The house has 3000 watts in PV panels on the roof which during the day is usually overkill and on average (on sunny days) I have 10kwh available to mix in with the mains (a Victron Invertor supplies about 500 watts essential circuit via batteries with the rest available to power non-essentials) PROVIDED THERE IS MAINS. This is currently lost to me as there are no heavy using  essentials and could be connected (presumably) to an element here or there.

Holiday house in Mozambique so not easily reached/accessed/maintained/monitored/repaired/adjusted etc. Tropical climate.

9 months of the year there are only 2 or 3 people using hot water - so the brand new solar should have been sufficient without having to switch on the electricity at all - 150 litres solar in the tropics are just fine.

3 months of the year there are paying guests who demand hot water and there may be as many as 10 or 12 people and 150 litres will not do at all.

Mains is more reliable than in RSA (no loadshedding) BUT sometimes unexpectedly there is a power interruption for a few days.  And of course sometimes cloudy and drizzle for a few days so my 9 month hiatus is punctuated with a few crisis  

 

[TimCam]

Do you have a controller that pumps the hot upper geyser water through the lower geyser, when the temperatures are different? This should happen throughout the day when the upper geyser reaches a pre-defined temperature above the lower (colder) geyser.  Hot water rises, so will never flow into the lower geyser unless there is water flow due to usage, or a pump.

[Clivevan]

No I do not believe so (no controller). So the top tank heats up. Only serves as a pre-feed for the lower tank and feeds it when there is usage as you pointed out. My installer sent the attached picture of the current state of affairs.

This will save on electricity but really only if the bottom tank is fully drained sucking in all the solar heated water. So for 3 months of the year I will use the solar probably to full capacity. Then for 9 months that bottom geyser must be kept on 24/7 to serve 1 or 2 people. If I switch it off it cools down, sucks in pre-heated water but mixes it with its cold water and no good. 

[Clivevan]

The collector is only sufficient for the top 150 litres. If I mix both tanks I end up with 300 litres lukewarm water and an element going like crazy to get it to 55*

[Clivevan]

So R30k down the tube so to speak.

Had no better luck at home where a R30k system first sprung a leak and then burst a few months out the 2 yr warranty.

So a yellow card looms seriously. If I don't run an entire boarding house of solar for the next 10 years I can NEVER make up those capital losses. So I have seriously been recommending that people by very very careful to install solar heated water - the risk on the capital outlay results in a debatable benefit.

[Clivevan]

It suddenly occurs that if I have 10 people the TOP tank will frequently be cold when the bottom has been drained and I will have to leave the top element on as well. (during the 3 month busy period that is). So TWO tanks loosing standing heat and two elements on the whole time. The benefit will be diluted so to speak by having a duplicated system. 

[root]

Get a geyserwise system installed with remote access via their own tuya implementation or get the even better geyserwala unit. Then you can remotely or via timers switch your geysers as needed.

[Clivevan]

I am starting to think the same. Also if I route my unused PV power to the geyser element and spread the element running over say 4 hours ie a 2kw element over four hours rather than a 4 kw over 2 hours, then I will suck in all the pv power (about 2kw available for 4 or 5 hours) into the element and use no mains.

Tx I will google those two - do you know anything re their reliability?

[TimCam]

3 hours ago, Clivevan said:

I am starting to think the same. Also if I route my unused PV power to the geyser element and spread the element running over say 4 hours ie a 2kw element over four hours rather than a 4 kw over 2 hours, then I will suck in all the pv power (about 2kw available for 4 or 5 hours) into the element and use no mains.

Tx I will google those two - do you know anything re their reliability?

Using "free" PV is always a good idea, and a 2 kW geyserwise element will be gentile on your solar system. I don't personally have experience with the geyserwise system, as I run a custom controller. Most electronic equipment should be reliable, but lightning and power spikes can destroy any equipment. Others can comment on the geyserwise performance.

Hope you can get good results with a little more outlay on the geyserwise controller.

[Clivevan]

Tx all for all the advice.

[Unity]

On 2023/08/21 at 7:35 PM, Clivevan said:

I am starting to think the same. Also if I route my unused PV power to the geyser element and spread the element running over say 4 hours ie a 2kw element over four hours rather than a 4 kw over 2 hours, then I will suck in all the pv power (about 2kw available for 4 or 5 hours) into the element and use no mains.

Tx I will google those two - do you know anything re their reliability?

I did this change over the weekend. Changed the 4kw element to 3kw. Installed a timer and change over switch to toggle the geyser in and out of eskom. My 5kva Luxpower doesnt have the Aux so I had to improvise. A 2nd CBI astute timer is at the geyser to allow for morning eskom water temp topping up. Nice thing about the smart timer is that it allows you to set rules for days when there is cloud cover and not draw from batteries. Will probably also change the other timer to a smart for more remote control. Laundry days are Monday, Wednesday and Friday until 3pm. I'm not home at those times so to void tripping the inverter for those days I switch the geyser on from 15h00 to 16h00 to allow battery to be topped up before sunset. On the no Laundry days, switch on is programmed from 11h00 as battery is fully charged by 10h30 from 34%.  Still early days and I will tweak the eskom usage after making sure that there is enough hot water in the morning. Three people on a 200L should be okay in the warm months ahead. This allows me to make better use of the panels that just decorate the roof during the day as the base load from the fridges is very low when no one is home or battery is fully charged. Will buy a 2nd inverter with the saving and be less restricted on the other days.

See the difference in solar usage before the geyser change, almost double solar and will get better as I gain confidence. 

Edited September 6, 2023 by Unity
Graph upload