Ditching gas, going electric immersion only, a wee project

Reed_Richards

Solarchaser said:
Re the resistance,  I mean, does it really matter?

Well yes, because it's the resistance that determines how much current the heaters actually draw.  You can "call them 13 A" or you can know the actual current, at least for any given supply voltage.  I=V/R

Solarchaser said:

I personally wouldn't be happy with a 32a rcbo on a 13a draw, if that was my house I'd be changing that out for a 16a, I mean worst case scenario it could be wired with single 2.5T&E meaning the if the element went low resistance the supply cable would melt before your rcbo would trip.

My house was rewired in 2019 after we moved in and I trust the electrician who did the work so I trust his decision to use a 32 A fuse and leave a lot of "headroom".  In order to "go low resistance" there would have to be a current-bridge between the live and neutral wires.  Either this will be so good that the current drawn exceeds 32 A or, more likely, there would be some imbalance between live and neutral that would trip the circuit breaker.

Solarchaser

Yeah I can know the actual current for the point in time I measure the resistance,  but it really doesn't matter.
If its 11a or 14a is irrelevant, what's relevant is having the wiring and trips good enough to have headroom to cope.
It's pretty standard to have a 16a trip on a 3kw circuit, as this deals with the manufacturer tolerance aspect and supply variation. 
If it was a 12a trip you would get lots of nuisance trips.

Re the wiring, I'm glad you trust the spark, but thats not my point, if you believe a heating element can only go to earth or go almost short, id suggest you don't have alot of experience with heating elements, the normal fault mode is one part of the element breaks down, so you get a heat spot, this will draw more current,  but not like a short.
So if for instance your element went faulty in a way which grabbed 25a of current,  your 32a trip wouldn't care but your potential 20a wiring will.

But hey, this isn't meant as a dig, you are happy to pay someone to do this stuff as presumably you haven't discovered bad workmanship, I tend to only trust what I'm able to prove based on my experiences. We all have different experiences based on our lives events.

Reed_Richards

Solarchaser said: id suggest you don't have alot of experience with heating elements, the normal fault mode is one part of the element breaks down, so you get a heat spot, this will draw more current,  but not like a short.

So if for instance your element went faulty in a way which grabbed 25a of current,  your 32a trip wouldn't care but your potential 20a wiring will.

Fortunately, I have no experience whatsoever of the failure mode of heating elements.  But hot metal conducts electricity less well than cold metal so I would expect that a "heat spot" would reduce the current rather than increase it.  I found a reference to failure modes here: https://www.marlec.co.uk/wp-content/uploads/2018/08/SiB-Immersion-Common-Faults.pdf .  I'm pleased to see that the RCD component of my RCBO would deal with the common failure modes suggested there as they cause some current leakage to earth.

Anyway, this is off topic.  The issue was that you have parts of the circuit feeding your heaters that trip, presumably after getting too warm.  Could this indicate that one of your heating element is in failure mode and therefore drawing more current than specified?    

Solarchaser

Certainly in any breakdown of the internals which causes the actual heating element to touch the outer shell, will of course cause earth leakage and trip the rcd part of the circuit, but thats not what I was talking about.
Hot metals conduct poorer than cool, yeah, but not that much poorer unless you are talking extremes of temperature.

Through various jobs in my career I have dealt with various heating element failures, I like to be safely dealing with them all, not just ones which leak to earth.
You have a 32a trip on a 13a circuit, id advise you check your wiring to ensure its able to cope with your circuit.... for you, not for me, you don't want to do that, ok 🤷‍♂️

Agreed its off topic, my mcb, not rcd is tripping because the circuit is drawing the max current its rated for over a 3 hour period as I surmised/stated/calculated 3 or 4 pages ago.
This causes the mcb to heat up and eventually causes the bimetallic strip inside to trip.
So nothing is failing, everything is doing as it is supposed to.
Leaving the cupboard door open has stopped it tripping again, so a little air circulating around the hot parts stops them getting hot enough to trip.
The only failure was in my using a quick reference figure of 3kw = 12a and so 12kw = 48a instead of actually doing the calculation to show that with my normal house voltage I'm drawing 50a, not 48a.

The 50a mcb supplies another CU which has an rcd main, which has not tripped (well it did once around the destrat pump wiring, but I'm not sharing that after 4 pages of this🤦‍♂️) and 2 individual 16a and one 32a (for 2 element's) MCB's which have never tripped

I was a little disappointed, but was giving myself a break due to it being a quite involved project and there being multiple things to consider, bearing in mind im not an electrician nor a heating engineer.

The failure is not dangerous in any way, it simply causes nuisance tripping, which I can stave off by opening the CU front flap and the CU cupboard door, so not a massive inconvenience,  I shared it only because I have shared every stupid decision I've made in this project, so that anyone who chooses to do something similar can learn from my mistakes and not their own.

Freepost

Putting the electrics to one side, may I ask a couple of questions, you may have already given the information but what the heck I'll ask anyway.

1) With regard to the immersion heaters that came with the telford cylinder/tank are these set at 70°C and have you adjusted them to provide a higher temp. I seem to remember you saying that you wanted to achieve a much higher temp, perhaps 90°C.

2) I think you are using a mixer in order to reduce the temp of water for DHW but also for CH was it the Pegler PEG402 which I think is fine for DHW but will stuggle with higher temps than say 45° for CH.

You have been very up front with your design and the errors/mistakes that have occurred along the way and how you have resolved those errors which I find very uplifting. You may remember that I also wanted to install a similar system albeit with a gas boiler for back up and my system is now in and working. I switched on a couple of days before Christmas and had a leak in the UFH circuit which had to be excuvated ... "a man and his black and decker", actually I found the leak very quickly and it fixed now, so it's testing now.

Thanks for your help and please keep posting.

Happy New Year

F.

Solarchaser

Hi Freepost, yeah I very much remember you were doing similar but with gas back up, and remember your very clear diagrams which were helpful in explaining what you were doing.
I'd suggest you perhaps update your thread you started, id be interested to know what tank you went for in the end, whether you decided on plate or coil heat exchangers and your decision about using the whole tank as part of your central heating from gas or whether you used a coil?

Anyway to answer your questions. 
1. I had to remove the standard thermostats. They were set to somewhere around 70, and I wanted to be at 90c, I did manage with a calibrated heat block to make them trip at 90C, however I was also getting alot of nuisance tripping of the manual reset which made the whole thing quite frustrating. 
QrizB offered a solution of PIDs which I initially dismissing out of hand, but in an embarrassing U turn ended up using them, but with the PID function disabled so that they essentially worked as an electronic thermostat rather than mechanical. 
I use a plug in timer to operate the PIDs on two heaters in my off peak at the moment.
This is the ones I used:-
https://www.ebay.co.uk/itm/275038350833?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=QnDHmConQl6&sssrc=2349624&ssuid=DtbsXk1pRsq&var=&widget_ver=artemis&media=COPY
The temperatures tend to be accurate, and the solid state relays have given me no problems, they are just a bit, blocky and ugly, and the mains connections are exposed, but that can all be resolved by putting them in a wee project box (on my to do list).
The solid state relays do need a decent heat sink to be attached to though, so that could be an additional cost if you don't have any kicking around.

The thermostats on the heaters actually just pull off, they are pushed into spades on the actual element, so when I removed the thermostats I just put spade connectors on the end of my wires and plugged them onto the spades.

You can see one of the spades in this picture. 

2. I used the pegler for hot water, but for the ch I've just used a gate valve to balance the temperature, which is not nearly as good at regulating temperature to be honest.
I would like a thermostatic regulator, however I feel I might give myself a bit of an issue trying to use one, after the ch has been on for say 20 mins, the return temperature isn't that much cooler than the supply temperature, so I wouldn't be able to really regulate down, and with a thermostatic control, that may stop water flow.
It may not, but its a fair amount of hassle to find out, so I think I'm just going to stick with my gate valve for now.

Freepost

Solarchaser said:

So inspired by Freeposts block diagram of the proposed system, I figured I should really do one myself of my system as it sits.
I figure I'll do it over two posts, this one being the plumbing , and the next one the electrical.

Hopefully it's fairly self explanatory, bit i'll prattle on a bit anyway.


Second circuit is the central heating circuit, the pump (P) pulls water through the trap (which should filter out the crud) in through a one way valve, through the internal coil where its heated and out through the next one way valve.
Some of the water will be pulled through the valve before the coil which acts as a mixer valve, this stops the radiators being at 80C, I'm averaging around 56C at the moment, which seems about right,  though even at 40C the radiators appear to be happy keeping the rooms warm.
After the pump the easiest route for the water appears to be through the pex loops (floor coil) I used to make a rudimentary under floor heating at the back of the livingroom, and straight back to the coil, however as its pex, the inner diameter of the tubes is quite a bit smaller than the 22mm copper running to the radiators, and so this seems to work pretty well at distributing heat everywhere.

Okay, I've stopped being lazy and went back to 14 November to the above post, can I ask for more details as to how you are mixing from 80° down to 56°, is it just, say a gate valve?

F.

Freepost

Okay, you seem to have answered me more or less the same time that I posted again.

Thanks

F.

1961Nick

My experience of using MCBs close to their maximum rating for long periods is that over time the tripping point reduces. The solution was either to fit a new one or swap it with one that hadn't been subjected to the same duty - put simply, they wear out!

Solarchaser

Yeah thats expected on some level Nick, anything mechanical that's essentially a safety feature never intended for regular use, will not respond positively to regular use.

I mentioned previously testing pressure vessel safety valves would often mean they would start leaking soon after, the iver temperature valve on my smaller cyclinder after lifting a few times at somewhere around its design temperature,  now lift approximately 10C below its design temperature forcing me to block it up.
So yeah a bimetallic strip that has been exercised a few times, is more likely to trip earlier and earlier the more often it happens.

This is why I need to reduce the current through that mcb as soon as.... well as soon as I can be @rsed really.