SolarImmersion + de-strat = do i need it?

[Mark Gee]

4kwpa system and a new solarimmersion...working surprisingly nicely even though it's December.

family of 5 who bath and shower a lot. therefore might benefit from a full tank as it is always used and we work from home and shower in day after sport etc etc

would I benefit from a de-strat pump? What are they £20 + £50 to install? it just mixes up the water right? to average the vertical temp range

does it matter where the thermostat is and the feed/flow pipes - presumably standard (a bit in accessible for me to see)

 

[Worcester]

You probably don't need the destrat, as, unless properly controlled may just give you a lukewarm tank of water.

We had one customer absolutely insisting on it, (top mounted immersion heater) so I offered to install his (immersun) first and if after 3 weeks (of the summer) he wasn't happy with the performance then we'd come back and install the de-strat as he initially requested, with no additional charge for the second visit, just the difference in the two quotes.

He never called back, so I rang him one month later, he was blissfully happy with the performance and was happy not to have spent the extra ...

It's a bit more than £20 as you need a potable water pump - bronze or stainless steel, plus you need to decide how to control it (run for how long?)

 

[hophead]

I have just installed a bronze central heating pump to move water from the bottom to the top of the hot water tank.
My tank is 5 foot tall x 1 foot diameter. I use a Raspberry Pi to control the pump. a 10 second run drops the temperature at the top by 1C at present. I use a SolarImmersion unit to power an immersion at the top of the tank.
I'm not sure how long the payback time will be but its a fun/interesting project. It cut in today for the first time.
Will provide reports as we get more sun...

 

[Worcester]

@ Hophead, we are Pi mad here - well got 7 of them running everything from weather station to environmental monitoring to media server .

What is the control parameters for the de-strat pump? Surely you'd only want it to work if there was solar power available for the immersion, and over the last few weeks I'd be very suprised if you had any spare power over your base load. I'd also query whether 1 degree is worth bothering about. If say the whole top 75 litres of the tank has dropped by an average of 0.5 degree (1 degree at the top) that's only 0.5*75*4187/3600000 = 0.04kWh @15p kWh for leccy = 0.65p

I'd do it based on a ΔT between the top and bottom of the tank, and based on when it had reached the desired top temperature, no point doing it before that, or if the ΔT isn't large enough - try configuring it to work on the solar thermal model which looks for a ΔT between the tank and collectors (or in the PV case have I enough spare power) if there isn't a significant difference / margin of generation, then you are in danger of just ending up with one large tankful of luke warm water - it's the same principles that we use also to manage large buffer tanks on biomass boilers - we DON'T de-stratify, we have target temperatures top and bottom and look for temperature gradients and drops to trigger the boiler - in fact in thermal stores we WANT stratification...

 

[hophead]

Thanks for your interest Worcester. At present I use Gas to heat the top to 50C, 06:00/08:00 and 16:00/1800 . And the top 1/3 is thus at almost the same temperature. Solar heats the water at the top of the tank. If the temperature rises above 55C I switch on the pump for 10 seconds, 3 minute later I will check the temperature again and if necessary run the pump again. The last thing want is a tank of lukewarm water. I want to maximise the availability of water at 50C.

Last Thursday there was enough sun to raise the water temperature the required 5 degrees and the pump kicked in 3 times.

Last summer with no pump the top 1/4 got hot on the top heater by 11:00. Installing a heater in the middle instead meant that the thermostat did not cut in until 12:30. However there was seldom enough hot left to last past 09:00 the next day.

I am using RPi's to log my gas/electric/solar meters, control the boiler for hot water, and have just bought a OS weather station.

 

[Worcester]

@hophead, hop over to openenergymonitor.org and either set up an emontx or jeenode to log the data of a couple of temperature sensors on the cylinder if you then log that to emoncms - that will give you a far better indication of what is actually going on in the cylinder, it's quite interesting to watch the top and bottom temperature movements as baths / showers / the boiler / the immersun (in my case) have their various effects.

6 CT's monitoring 3 inverters, plus total generation, consumption and import/export, inside and outside temperatures, plus real grid and inverter voltages:

 

[hophead]

Thanks. As part of the same RPi project I have 4 temperature probes up the side of the tank which gives me minute by minute graphs.