Ditching gas, going electric immersion only, a wee project

Solarchaser

Hi, you really have been scrutinising, I'm impressed 😁

So the tank, dry stat pockets are basically little inserts in the tank where you can put manual thermoSTAT's if you wanted to regulate temperature that way rather than by heating the immersion elements until they cut off, so they are dry pockets and can't be used for circulation,  I have temperature sensors in there just now so I know what temperatures the tanks are at.

I should probably do a diagram of how the tanks are plumbed for help with understanding,  but for right now I'll just try and explain.
Cold in and dip tubes, on one tank this is the fill spout which I have going back to the kitchen where the combi used to be with a pressure guage so I can monitor and top up the tanks.
On the other tank this is used for expansion vessels connection.
7 bar TandP connection I'm using as exactly that.
Although the tanks will sit between 1 and 1.5bar, there's never any harm in having a safety.
I "tested" they worked on first heating the tanks without an expansion vessel... they do lift at 7bar 🤪
Secondary return is halfway down both tanks, these are teed together (now with check valves to reduce thermosyphon) and then feed the bottom of the central heating pump I'm using for a destrat pump, from the top of the pump the pipe goes across the top between the tanks and goes to both hot draw offs, though the draw off to the smaller tank has a motorised valve blocking it.

So destrat sucks in water at the half way point and pushes it to the other side of the tank at the top.
Really it's just about moving water around at the top so that I don't get a coil drawing all of the heat away from one part of the tank, and so get cold radiators. 
Would the cold draw and dip be better for this? In theory yes, however when I turned the tanks upside down with only a few litres of water in them, the 500L tank leaked water out of here meaning it wasn't sealed as it should have been, and this would potentially cause the pump to pull an air bubble instead of water, which would render it useless.
If the tank was better made, my preference would have been to use the dip tubes.

The 300l tank was an off the shelf tank, and seems well made, the custom 500L one was... shabby. But it holds the pressure, and I needed it in that timeframe.
Emails to telford were not answered!

The 500l tank is the main tank, right now its sitting at 69C, the 300L tank is at 23C.
The only time water will be mixed into the 500L tank from the other one is if the 300L tank is above 40C, this will allow the motorised valve to open when the destrat is running using a 301 type NO switch, probably attached to the hot draw off... not fully decided on that yet.

What's the difference between this and a thermal store, good question,  I initially looked at thermal stores but they were silly money above 250L, I looked at how they did what they did, and essentially decided I'd take a water tank and use it in reverse, so that it works as a thermal store, that's what I'm trying to do here.
The main difference with a thermal store is heat loss from what I can gather,  but really that's just insulation,  so wrapping the tanks in 200mm of loft insulation for £50 when I'm finally happy I have it running should give me similar heat loss to a thermal store, if not better.

Finally, what if it doesn't work, if I can't put enough heat in with my 12Kw of heating elements (4 x 3kw) in 3 hours of go faster... 36kwh of heat, then the heating elements will have to be on for longer.

The house has always used more than 36kwh of heat in winters days, so it's likely I'll need more, but... gas combi was 15 years old, so hopefully inefficient, and we heat the house really for the livingroom, so I'm hoping that the pex loops I've put at the back of the livingroom will help it heat faster, so less overall heat required. 
In the coldest evenings the wife would often put on the fake fire (2.2kw electric fake fire) to heat the livingroom for an hour, so I suppose that's the fallback.

I won't be putting in a system boiler as the gas has been removed, but I'll admit I've been thinking of taking apart one of the showers as a plan D to use the heating element inline on the destrat pump to give 10Kw of heat if the system just won't do it. Problem there being that I'd be at 22kw of a 25kw mains supply without anything else in the house running.

So I guess plan A is 3 hours of go faster, 36kwh of heat and cross fingers.
Plan B is 3 hours of go faster plus leaving just the 500L tank heaters on longer, 6kw can be covered by my batteries for an hour or so... but I'd have to buy some expensive rate leccy.
Plan C is using the fake fire as a livingroom top up.
Plan D is macgiver a shower heating element in as some sort of heat booster.

If those don't work, yeah I dunno, plans to Z I guess. 
What I can say for definite is I've spent too many hours, thinking, planning and changing my mind, so this is the system! I also have no more room for more tanks under the house, so I can't expand the system.... well not easily.
I've also spent way to much energy, and far too many weekends and evenings crawling around under the house, bumping, bruising and cutting myself to give up on it, not to mention more weekends changing it from the original to make it work more efficient. 
Today I spent all day changing the piping for hot water from 22mm to 15mm both to speed up delivery time, reduce wasted heatloss after hot tap closes, and to allow me to double layer the pipe insulation  (first layer black pipe foam 6mm to 28mm, then 28mm pipe insulation at 19mm to 47mm. I also added check valves, 1 just as the cold enters the first coil on the small tank to stop heat wastage into the cold line, and one to the hot line just before the mixer, to let thermosyphon bring the hot water from the tank all the way to the mixer but not be lost to the cold side of the mixer when not in use.

We shall see if it works 🤷‍♂️

Solarchaser

As an aside of sorts, I mentioned before and element thermostats being about 70C, but I notice a large variability in the 4 elements I have.
I've been trying like a bear to find a double isolating high temperature replacement thermostat (x4) to run say 90C with a 95C manual reset, but the various stores I've visited and search terns I use have not came up with any usable solutions. 

It seems the industry standard is a 10% tolerance too, which is mad, so a 70C stat could be somewhere between 63C and 77C, no wonder they are all different. 

I've been carrying about a stat for a couple of weeks to try and get a replacement that will work, and suddenly noticed there was a couple of screws holding it together... I mistakenly assumed it was a sealed unit.

So turns out these ones work by a fine threaded screw pushing against what I assume is a bimetallic strip,  this screw is the pointer you adjust for temperature, and the pointer sits on top of the screw in really hundreds of permutations so no wonder they are all so variable.

So I now need to set up a wee jig to get water in a vessel at a pretty constant 90C and adjust them all to that, then test them all multiple times to ensure the manual reset cuts in at 95C.
So I've saved some money, but gave myself more work.

QrizB

Solarchaser said:

I've been trying like a bear to find a double isolating high temperature replacement thermostat (x4) to run say 90C with a 95C manual reset, but the various stores I've visited and search terns I use have not came up with any usable solutions.

Have you thought of switching from cheap, passive thermostats and moving to a more industrial approach using thermocouples?

This sort of thing (not a specific recommendattion, just the first likely-looking example I could see):
https://www.ebay.co.uk/itm/224040936181

I've run lots of lab- and pilot-scale rigs and controllers like these are pretty foolproof. Depending on how your tank would respond to boiling, if you want belt-and-braces you can run two controllers in series; one set to the working temperature and a second as an overtemperature trip.

Solarchaser

I haven't considered thermocouples as such,  I understand what you are saying about the accuracy,  the problem with thermocouples is their fragility.
In another life I did a fair bit of work with autoclaves using thermocouples and spent alot of time repairing them. 

Actually that has sparked an idea on how to calibrate the thermostats as to calibrate thermocouples you use a calibrated heatbath, so if I can get a hold of one (maybe from my old job) I can calibrate all of the thermostats to a high degree, so thanks 👍

michaels

You do have the pockets for a boiler thermostat so could you put one in there and compare it against the immersion thermal cut out to calibrate?

The more I read this thread the more I think we should get a second hot water tank in the loft in series to our existing 180l so that we could run all our daily hot water off the 5 hour 4.5p window.  Should pay for itself in about 12 months by my calcs assuming we don't overload the max current on the 10A main fuse.

Solarchaser

Unfortunately not due to the distance between the stat pockets and the immersion, there is about 5.5C difference between top and bottom centres but immersion are out at the sides, so can't really be sure what temperature they are seeing. 
For your super high hot water usage, id certainly agree that on the face of it, another tank would be useful for cheap rate and solar consumption in the brighter months

Solarchaser

I crawled back under today to see the difference my modifications of a week or two ago have made, now that the tank is back up to 70C, specifically the destrat pump was always hot, and the hot water from the big tank was to some degree flowing to the smaller tank. Both of these increasing heat loss.
I had added in two single check valves, 1 for the 500l tank to pump and one from 300l tank to pump, so I expected to find the destrat pump stone cold.
I was disappointed. 


This is the secondary returns which I'm using as the feed for the destrat 

This is with 19mm insulation on the 22mm pipe, so there shouldn't be much heat escaping.
The yellow circle is the 1 way valve /check valve and you can see both sides are hot, so it's doing sweet FA.
The one from the 300l tank is doing well though, you can see the far left is all cold.

So how much heat is going through the pump?

Not sure how clear that is, but its 59.9C on the pump body, which is pretty disappointing. 
So I'll need to empty the tank again and fit a double check valve and hope it can stand up to the thermosyphon.

If you are thinking that tank seems to be leeching quite a lot of heat, I agree. Again 1 way valve circled yellow, just for perspective, this is the immersion heater which I've been using most, leeching quite a bit of heat too.


What about the other end of the tank?


So you can see where the tank end caps dome out, the manufacturers insulation is bloody poor.

Just thinking it might help understanding of how the system works if i drew some arrows on flir pics I took today

Red line is coming up from top of destrat pump and running along from right to left between the tanks to go to what would normally be the hot outlet. This water never leaves the tanks.

The other arrows are tap water, first blue is cold water coming into the 300l tank, this preheats the water in its solar coil (if the tank is warm) and so leaves the solar coil as the light green line and goes into the 500l tank, then comes out as thr orange line, which is hot water after going through that tanks interal coil and comes into the house where it meets a mixer valve, the cold patch on the orange line is just a bit of undrr floor insulation that's hanging down.
I was surprised to see that even as 57mm pipe where only the middle 15mm is hot, still I'm getting some heat coming off the hot water pipe, but what more can I do?
I'll probably wrap some loft insulation round it, or some pipe wrap, a little of which you see after the motorised valve in the next pic.

This picture is just to the left of the previous one, so orange line still represents the pipe coming from the destrat to the top of the hot inlet on both 300 and 500l tanks.
Black circle is the motorised zone valve on the top of the 300L tank which, although obviously getting hot itself, seems to stop most of the heat from the 500l entering the 300l when I don't want it to.

The background pieces are the central heating pump and the magnetic filter return into the central heating coil, and although the circuit has been drained of fluid, ready for me adding in check valves and loops, you can still see the effect of being bolted onto the top of the coil with the magnetic filter at 47C.

This pic also shows both how little heat is reaching the 300L tank, and how much is pouring out of the top of the 500L .. disappointing!!

Hope that helps with understanding it, and apologies for the pic heavy post.

michaels

Interesting seeing the heat l0ss at different point, I wonder if we have similar around our immersions, were yours on when you saw this loss or was it loss from the hot water in the tank only?

Interesting that you actively destrat.  In our last property where we ran a tank on its side as a heat store (direct to gas boiler with pumped circuit to plate heat exchanger for hot water we wanted to keep the top layer (where the draw off for the hot water was as hot as possible even when the rest of the tank cooled so the circuits were deliberately set up to minimise mixing and I seem to remember we had two themostatic valves set up so the boiler and rad circuits could draw a mix from the top and bottom of the tank to max efficiency.

Wish I could remember the exact configuration.

Solarchaser

The immersion had not long switched off on the side with the pump, the other side hasn't been on for weeks, it's tripped the manual reset cutout, I'll reset it at some point.

The destrat, I really don't know if it will be needed, but figured it was better to design one in and find it's not needed than not have one and find I needed it.... though right now, I wish I hadn't as its that pump that's wasting lots of heat just now.

I have a double delay circuit linked to the destrat (or will do when the myenergi Eddi finally finally gets here) so that when the central heating is run for more than 10 mins, the detrat kicks in, just grabbing some water from the middle of the tank and pushing it to the top, but more importantly creating a flow from right to left outside the tank to create left to right inside, at 10 mins the pump kicks in, runs for 5 then kicks off again, resetting.

However as normal solar heating goes it could heat only one side of the tank, and so the Eddi will have a direct line to the destrat to run as a destrat to allow it to heat the whole tank.

That's the theory anyway.

Heedtheadvice

Mainly been lurking on your thread but impressed with your work.

On the question of temp control I am surprised that you are not keen on thermocouples from your experience. Certainly not mine and they are also used in many an application from monitoring to control systems requiring robustness. Take gas fired boilers as an example.

I'm sure you appreciate the advantages of response speed, accuracy, repeatability etc. Those along with associated electronics make for better control without the huge variation in switching temp and the wide hysteresis. Obviously some of the latter is essential to prevent 'chatter' but the amount is far more controllable.

Could be advantageous for you with the tight control spec you desire?