Ditching gas, going electric immersion only, a wee project

Heedtheadvice

That really is a shame that you will need to do more work. I admire your tenacity.

How do you intend to cope with the really cold days? We have not had any so far. Although we have been having much warmer winters of late ( I am not too far from you ) we could still get several days together  below freezing during daytime, and I can well remember very hard frost overnight for over a week at a time - without going back to the 80s or earlier! Of course we often get sunny weather associated with those very cold days which may help generation.

JKenH

I have been following this thread but admit having skipped through the technical bits so the answer to the question I am about to ask may be there but I have just missed it. My question is about heat loss from such large tanks. Have you calculated the heat loss (in terms of say kWh needed to maintain the temperature in the tanks over a 24 hour period)? I ask this because I am aware of the heat loss from my DHW storage between heating it during the day from solar with my IBoost and using it the next morning for showers. I don’t know just when your Go Faster period is but, if like some you have chosen late evening, there could be 8-20 hours of cooling before most of the stored heat is used.

The higher the temperature differential between the contents of the tank and the air outside then the higher the heat loss. You do seem to be operating parts of the system at quite high temperatures so presumably you have worked out that your design is more cost effective than operating a larger tank at a lower temperature.

If I have understood correctly the tanks are located under the house. Now I am not familiar with Scottish properties but is this area within the insulated envelope of the house or is it exposed to external temperatures and hence higher temperature differentials. So far this autumn it has been quite mild (at least where I live with only 2 air frosts and then only just below freezing) so I would also be concerned (like @Heedtheadvice) if we get a really cold spell.

I apologise if this has already been covered (or I have missed a bit) but is heating large volumes of water in the tanks just for DHW in the summer efficient? Presumably you operate at a different temperature? I always used to be a tad envious of those with Combi boilers, (having never had one and only lived in one house (out of 9) with gas CH/DHW).

Finally, did you factor in the rise in off peak tariffs from Octopus? I know there are still other cheap alternatives out there but given the Octopus move how long will they be available? 

Solarchaser

I didn't know octopus had removed the 3 hour tarrif, but yeah I think 5 hours makes more sense for heating, the other part of that being I'm going to have to start charging the Tesla overnight soon as free charging will have to come to an end ... I assume, so it makes sense to have a bit more headroom on the 100a fuse.

I did chuckle at the average comment, it's something my boss says all the time, though more related to intelligence of people you speak to 🤣

Heedtheadvice, my plan is simply to put the heaters on for longer, or I should say, activate the heating boost on the eddi, that's what I've been doing manually when I've noticed the temperature in the tanks is below 50C.
With the lack of electric showers thrashing my batteries I'm tending to have around 3-6 extra kwh left in a day (though not every day) and so have been using that when required.
I'd imagine that when it's much colder.... assuming in a couple of weeks time until maybe mid Feb I will have the two elements controlled by PID's on for an extra hour, or perhaps 2 if required.
The house cannot be cold, that's just not acceptable. So if it costs more to heat it, then it costs more.

There is no problem in supplying enough heat when the heaters are on, it's more the lack of storage, which with hindsight seems pretty obvious. (Wishing I'd stuck with the original idea of two 500L tanks instead of going for the cheaper 300L tank as the secondary)
I am still finding that 3kw of heat in either tank is enough to maintain the temperature of the tanks (whichever tank is heating goes up by a little more than the other goes down) while the central heating is on, though I do expect that to change with temperatures closer to or under zero, currently showing 4C outside.

I was reminded by my good lady yesterday that we would often use the electric fire in the livingroom to supplement the gas central heating on colder days due to the combi being unable to maintain a comfortable heat, she mentioned this to highlight her barefooted pleasure of the (rudimentary pex loops)heated floor, and so I can at least feel that has been an improvement. 

I'm reminded that my current peak rate is only 14.5p, and when I renew in Feb, its likely my off peak will be around 12-13p and the peak 40-45p

michaels

I was about to ask re the 12kw limit as 100A main fuse should give about 23kw max so if you were adding tanks you could also add elements?

Solarchaser

100a fuse so most folk would say 25kw fuse,  but equally can say 23 or 24kw

12kw of elements and 7kw of battery takes me to 19kw.
Wife puts her leaf on charge and the background house is at 23kw.

My plan would be only activate 1 element in each of the 300l tanks, so sticking with 12kw total.

JKenH

Perhaps @Solarchaser you missed my earlier post today. I would be interested in your observations on heat loss.

Solarchaser

Apologies Ken,  after having a look round the board there I came back and realised it was on this thread,  yeah I totally missed your post.

The tanks I have are the largest I could fit through the opening I carved out under my patio doors, which was 500L horizontal. 
They are both outside the envelope of the house, and have 200mm of insulation above them, so any heat leaked will not be to the benefit of the house
These are the standard standing losses for the 300l and 500l tanks


So I should be losing 5.5kwh every 24 hours roughly.
From the end of my heating cycle to the start of the central heating in the morning is 8 hours, so I should lose roughly 1.8kwh or 2.2C and I'd say that's probably about right.
I tend to finish heating around 90c and see 88C in the morning roughly, and its only as I type this I understand what you are saying, and yeah you are correct, since the tanks are my major electric draw now I will move the go period to later morning when renewing, that makes sense to reduce standing losses effects on the useable tank heat.

I based the move to electric only on off peak tarrifs, my plan is to stay with octopus, but really cash is king so it depends what deals they are offering.

In regards to dhw in summer, I only plan to heat the main tank in summer, but this is also related to the fact I'm on FIT and have a second off fit array, so I plan to self consume as much as possible,  and so if I have no hot water bills then its 100% efficient for my needs if you see what I mean?
Heating such a large volume is obviously not necessarily efficient vs small volume, but it will definitely give me a buffer for the not so sunny days where I won't have to pay to heat, as well as a large volume if the wife decides she wants to fill the paddling pool with hot water.

chris_n

You will lose a lot more than that when it's cold. Those heat loss figures will be worked on 20 degrees ambient as a minimum possibly more if they are expecting it to be in a cupboard. With an outside temp of 0 you can probably add 50% to those figures. 
Why insulate the tanks from the house? I can understand keeping the heat from DHW separate in summer but surely allowing any heat that escapes from the heating tanks into the house is beneficial. 

Solarchaser

Those figures are from my actual temperature probes in the tanks over the last week where the highest external temperature has been 9c and the lowest -2C.

The figures from the manufacturer were apparently based on their recommended tank temperature of 60C.

Under the house was already insulated before installing the tanks.
In actual fact I said there was 200mm of insulation,  that's not actually true, there is 200mm between the floor joists, but there is another 200mm on top and around the tanks as well, which is probably why my loss is working out similar even with a greater heat differential between inside and out.
External temps being single figures, internal being high 80's to low 90's

There is a possibility that when I do add another tank in the spring that I will build a PIR box around all 3 tanks, and in that eventuality I may remove insulation from above to let the heat escape into the home envelope, but tbh my laziness is more likely to just add more loft type insulation around the tanks instead.

Heedtheadvice

I would also opt (if I were not more lazy than you!) to extra insulate the tanks rather than remove underfloor insulation.

That allows better control with tank heat being  well preserved and used under control when required and retained Ed when not.

There would be little point allowing heat from tanks to rise into the house when not required such as (the) good summer day(s) when not required. If you needed to cool/ventilate your rooms cos they were too warm that would just be a waste.

I had forgotten you planned to top up via Eddi when required!