Paranoid and needing comfort - Page 1
Forum section: electronics / projects

author: msknight
author_rank: Regular Contributor
timestamp: February 12, 2025, 12:39:17 pm
content: I know enough to know that I don't know enough. I'm seeking comfort.I'm in the UK.I'm taking the house to smart which includes the heating system... well, we're electric and no mains gas, etc. and the storage radiators need replacing... long, long story.Most of my ducks are lined up in a row with the exception of one thing......the heating element in the hot water tank which is 3kW. - At present, it is on Economy 7 and turns on at start of cycle about 22:30, heats up and then spends the night keeping itself topped up. I believe that if I instead had it come on at 5:30am and heat ready for the day, I can save half the energy (the tank is lagged to hell and back so looses very little heat... the only gaps are holes so that the heating elements can breathe and don't overheat.) - I get that figure from watching the temperature curve at the bottom of the tank, so I know when the element is active.Now... the best I can source on Zigbee2MQTT is this...https://scolmore.com/product/CMA30651... and at 13A at UK 240v the heating element would be right on the margin.I chose it because it is a reputable manufacturer... ie. not One Hung Low... and lists compliant to BS-1363:4 which in theory should be 13A continuous load.The heating element would be on for roughly 1.5 to 2 hours in any 24... so if I understand the standards for the plastics correctly, it should not come anywhere close to melting down.In theory, I've done my home work and I've read the standard but just for belt and braces I called Scolmore group and asked to speak to someone in technical, just to do the belt and braces.The bloke who answered the phone seemed to know his stuff, but sounded American and seemed to be convinced that British homes are still wired from sixty years ago. I did explain that the heating element is not on a spur and goes back to its own 16A fuse at the economy 7 board, and that the whole house was re-wired when we moved in 17 years ago. He said that the fuse might last a couple of years but would eventually go... but 13A fuses don't blow until 20A continuous current.So instead of getting more confident about the situation, I am now closing on being a nervous wreck about the situation.Is the bloke at Scolmore on the money, or wide of the mark? ... there appears to be nothing else trustworthy on the market so I either go with this, or else I wait until the market catches up with smart control of higher power heating elements and continue to waste money.EDIT - He stated that things are generally only reliable for 80% of the load and while I've read that for some other equipment in other countries, I believe that under UK specs, the load as stated should be considered continuous and not peak. The product spec sheets don't actually list the continuous and peak loads, which is something I fed back to him.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 12, 2025, 01:46:20 pm
content: Quote from: msknight on February 12, 2025, 12:39:17 pmWhat exactly led to this belief? I think it's seriously wrong. If the tank was so poorly insulated that it leaked significant amount of heat during night, then it would not last during the day anyway - and you would notice.

author: msknight
author_rank: Regular Contributor
timestamp: February 12, 2025, 01:50:14 pm
content: The monitoring of the tank at the bottom, during the period that the economy 7 is active. Plus, when I'm awake at silly o'clock in the morning, I can hear it. Graph in original post.

author: ch_scr
author_rank: Frequent Contributor
timestamp: February 12, 2025, 02:26:16 pm
content: The way I see it you only measure the "on time" of the heating element, not the current? And "half the energy" is based on the visible "on time" of the graph?First I'd like to see the graph "zoomed in" from 22:00 to 07:00 to check if it's really onthat longand not a display artifact.If there is a current / power measurement, it might give clues as well.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 12, 2025, 02:39:34 pm
content: Quote from: msknight on February 12, 2025, 01:50:14 pmGraph showsstratificationof water; warm rises up, colder falls into bottom. To see amount of energy stored, top sensor gives optimistic, bottom sensor pessimistic idea.Energy loss of the tank is through the insulation from all sides. Sure, by heating closer to use period the tank temperature is allowed to go lower before that, and heat loss is also linearly lower. So maybe you could avoid 30-40% of the loss, or so. But that does not mean saving 30-40% of energy, since loss is only tiny part of total system energy. Easiest way I think to achieve this saving would be a simple clock switch, right? Cutting off like 22-03 or so.Also you can note from the top temperature sensor that the tank really isn't finished heating when the thermostat first clicks off. The next cycle heats it up more.

author: msknight
author_rank: Regular Contributor
timestamp: February 12, 2025, 02:45:43 pm
content: The graph isn't the issue. I've verified the on times by being awake and hearing the burbling, so I'm aware of that. The on times as stated in the graph are correct.My issue is the 3kW consumption of the element against the 13amp rating of the fused switch when regarded UK standards and what the Scolmore engineer was telling me. That's the issue I need comfort on.

author: Zero999
author_rank: Super Contributor
timestamp: February 12, 2025, 02:55:54 pm
content: 3kW is a fairly common power rating for immersion heaters. Heck my parents' have an immersion heater on their hot water tank, which isn't used because they have a gas boiler. I remember on one occasion, when the gas heating stopped working, I wired it up to a 13A plug and plugged it into a socket in their bedroom.No doubt you could safe some power by setting a timer so it turns on a few hours before you get up and need a shower.I would seriously consider a heat pump, if I didn't have gas, but that would require a substantial investment and is off-topic.

author: SteveThackery
author_rank: Super Contributor
timestamp: February 12, 2025, 10:39:16 pm
content: You have sourced a reputable zigbee switch. If it says it's rated for 13A, then of course you can use it at 13A. It will almost certainly have a safety margin built in.Just fit it and move on. It will be fine.By the way, I'm not convinced you'll save much money, but it won't hurt to try.

author: themadhippy
author_rank: Super Contributor
timestamp: February 12, 2025, 10:52:02 pm
content: If yer worried fire a 20A contactor off the zigbee to switch the imersion

author: PCB.Wiz
author_rank: Super Contributor
timestamp: February 13, 2025, 01:12:36 am
content: Quote from: msknight on February 12, 2025, 02:45:43 pmWhich weak link are you worried about?The fuse failing from thermal cycling, or the inbuilt relay having a finite life when switching 13 Amps?Your system does not seem to have too many daily on-off cycles as it is.You could buy one, and dismantle it enough to read the relay rating, and check the fuse rating?

author: Siwastaja
author_rank: Super Contributor
timestamp: February 13, 2025, 09:32:05 am
content: Heaters have no inrush current at all so are quite easy on relays. A legal, standard-conforming product should be able to handle the current without any extra derating. Heaters do have some inductance which causes contact sparking at turn-off, but usually nothing dramatic.

author: CaptDon
author_rank: Super Contributor
timestamp: February 13, 2025, 01:59:55 pm
content: A 13 amp rated element will have a 14 to 15 amp initial draw as the element heats and resistance goes up. The higher cold current does taper off quickly however. The change in cold vs. hot resistance of a heating element is not nearly as drastic as that of an incandescent lamp bulb due to a completely different metallic composition.

author: msknight
author_rank: Regular Contributor
timestamp: February 13, 2025, 03:00:03 pm
content: I'd like to thank everyone for their input.I was concerned about loading with 13A being at the top end of the rating, in line with what the engineer said that the continuous load should only be 80% of the specified rating.I think I'm happy now that the 13A listed is actually the continuous load and that it should be able to handle peak load on start up, a little higher than this. So I'm comforted that I can go ahead and it not overheat.What has been said about the graphs is somewhat correct in that after the first power cycle there is water mixing and then the second cycle finishes the job, but the third and fourth cycles aren't necessary and are about the equivalent time under load, as the first two, so I'm intending on making about a 40% saving. I am aware that the temperature sensors are outside the tank and beyond the layer of insulation that the manufacturer put on the tank, but they are at least indicative, if not precise.I am using that indication to note a sharp rise in temperature at the bottom of the tank to tell when the element is on and, thus, also tell when it is off and hopefully avoid a large spark when turning the relay off... by turning it off when the element has stopped running... although from this I understand that it wouldn't be that much time off the lifespan of the relay anyway.Thank you all so much for your input. I think I'm going to go ahead with this work and see where I go.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 13, 2025, 03:40:59 pm
content: I suspect that even the second on-cycle does not fully do the job; if the tank truly leaked so much heat that those two extra cycles at the end of the night were required, how would it hold the heat for all the day until the next evening?Probably optimal for energy use would be to time the heating close to the morning as you plan on doing and adjust the thermostat a bit higher than it currently is (if possible) so that it heats up to the same amount of stored energy it does now, but in one contiguous cycle.Adding more insulation is also one obvious idea to reduce energy loss.Quote from: CaptDon on February 13, 2025, 01:59:55 pmGood point, this can be thought as one kind of "inrush", so in that sense my comment about "no inrush at all" isn't strictly correct.Then again, the relay itself is also cooler at the start of the cycle.

author: NE666
author_rank: Frequent Contributor
timestamp: February 14, 2025, 11:32:00 am
content: Seriously, I'd use a decent brand contactor and switch power to its 'coil' with the Zigbee, as suggested earlier by 'themadhippy'.The contactor design will at least have been tested and certified by an insurance underwritten lab, somewhere along the line. Who knows what QA the Zigbee module has been through, or where the components come from. And the contactor would be running underrated, not at the top end.https://www.amazon.co.uk/Schneider-A9C20732-Contactor-230-240-1Piece/dp/B008XJLGKI

author: msknight
author_rank: Regular Contributor
timestamp: February 14, 2025, 12:44:31 pm
content: Quote from: NE666 on February 14, 2025, 11:32:00 amSo does that talk Zigbee2MQTT or would I need something else to do the switching?

author: tooki
author_rank: Super Contributor
timestamp: February 14, 2025, 02:39:57 pm
content: Quote from: msknight on February 14, 2025, 12:44:31 pmIt doesn’t “talk” with anything. Acontractorcontactor is basically just a heavy-duty relay. Energize its coil (for example, with 240V power switches with the Zigbee switch) and thecontractorcontactor closes its load contacts.

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2025, 05:39:40 pm
content: Quote from: tooki on February 14, 2025, 02:39:57 pmYou mean contactor. Autocorrect/dictation tools are crap. lol

author: tooki
author_rank: Super Contributor
timestamp: February 17, 2025, 08:30:41 am
content: Indeed. I didn’t even realize it had “fixed” them.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 18, 2025, 08:32:08 am
content: Give up a bit of comfort. Get off the couch and switch it yourself.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 19, 2025, 08:06:12 am
content: Quote from: Terry Bites on February 18, 2025, 08:32:08 amIn the middle of the night, every night?

author: Zero999
author_rank: Super Contributor
timestamp: February 19, 2025, 09:24:31 am
content: If you're concerned about the inrush current, then just measure the resistance of the heater. I'd be surprises if it's much below 17 Ohms.Anyway, timers capable or switching a 13A load are widely available and not expensive. For peace of mind you could go for something rated to 15A and buy a reputable brand.

author: m k
author_rank: Super Contributor
timestamp: February 19, 2025, 02:43:01 pm
content: Quote from: msknight on February 12, 2025, 02:45:43 pmDoes the burbling means that the tank is emptied when in use?The graph says that bottom water is heated 10 decrees in an hour or so.So is the tank more than 200 liters?(3/11.7kW)Put the heater on for an hour one hour before morning shower or expensive time and forget the rest.If warm water is not enough add a second early night heating period.

author: msknight
author_rank: Regular Contributor
timestamp: February 19, 2025, 07:14:58 pm
content: Quote from: m k on February 19, 2025, 02:43:01 pmThe burbling is just the heating happening and water moving around. Yes, it is a large tank, around 200 litres.I was intending on bringing it on at about 5:30 and off at 7:30 and see where it goes from there.I have started working on a small ESP32 C3 zero board project based on the suggestions before, to hopefully detect the current in the cable non-invasively. Waiting for parts.The problem with the graphs is that there is an initial lagging sprayed onto the tank so I'm not getting a true reading, plus there is convection with the water at the top of the tank so the temperature rise isn't always the direct result of the element.

author: Zero999
author_rank: Super Contributor
timestamp: February 19, 2025, 08:03:36 pm
content: What temperature is the mains water?What temperature do you want to heat it to?Water has a heat capacity of 4.184kJ⋅kg−1⋅K−1and 1kg of water is near enough a litre.https://en.wikipedia.org/wiki/Specific_heat_capacity?useskin=vectorAssuming the mains water temperature is 10°C and you want to heat it to 50°C, that's a temperature rise of 40°C or 40K.Your tank is 200lThe heater is 3kW, therefore dissipates 3kJ of heat per second.c = 4.184kJ⋅kg−1⋅K−1P = 3kWm = 200kgΔT = 40Ktime = ΔT*m*c/Ptime = 4.184*200*40/3 = 11157sDivide by 3600 to get hours.11157/3600 = 3.1 hoursTo get the minutes multiply the decimal by 60:0.1*60 = 6 minutes3 hours 6 minutes to heat the full tank up to 50°CThe above of course assumes no heat escapes. I ran through the above calculation with the power set to 2.8kW to account for 200W of leakage and got just under 3 hours 20 minutes.

author: tggzzz
author_rank: Super Contributor
timestamp: February 19, 2025, 08:42:55 pm
content: Quote from: msknight on February 19, 2025, 07:14:58 pmIf it is foam lagging, scrape away a small part of the lagging and thermally attach the sensor. That's what the technician did with my hot water tank.QuoteWhile energy is being input, the top of the tank will be hotter than the lower down. When that stops, the heat will diffuse downwards so that the temperature lower down will continue to slowly rise and the temperature at the top will slowly fall.In my reasonably well insulated hot water tank, the temperature sensor is about 1/3 the way up from the bottom. I set it to heat the water overnight to the set point temperature before I get up, so I can soak in the bath.If I heat the water one day and draw a bath, the next morning the water has been reheated as desired. But if Idon'tdraw hot water, the next day the bath water will be too cool. That's because, although there will be heat loss over the 24 hours, the temperature in the tank has become more uniform and temperature at the sensor is too high for the heating to have come on.TL;DR there is an "interesting" interaction between heat convection, heat diffusion, and the position of the temperature sensor.

author: msknight
author_rank: Regular Contributor
timestamp: February 20, 2025, 06:30:47 am
content: The variable is how hot the water is at the start.That depends on how much we have used the previous day.This morning, the first, "charge," only needed half an hour and didn't kick in a second time within the first phase of the economy 7.

author: msknight
author_rank: Regular Contributor
timestamp: February 20, 2025, 06:32:02 am
content: Quote from: tggzzz on February 19, 2025, 08:42:55 pmYes, that's what I'm finding. Watching the interaction between the top and bottom sensors is proving interesting.

author: tggzzz
author_rank: Super Contributor
timestamp: February 20, 2025, 09:21:42 am
content: Quote from: msknight on February 20, 2025, 06:32:02 amDon't forget to include any hysteresis on sensors.Possible workarounds...Increase either the temperature or the heat loss. Intellectually unappealing.Ensure you always draw off sufficient water the night before. For me that is of the order of a kitchen sink. OK if done occasionally.Modify the algorithm so that heating starts if the top sensor is too low, and stops when the bottom sensor is high enough. Viability depends on detailed inspection of both sensors' temperature vs time.

author: m k
author_rank: Super Contributor
timestamp: February 20, 2025, 10:32:35 am
content: Quote from: msknight on February 20, 2025, 06:30:47 amSo around 6.5C for 200 liters.When was the previous heating period?

author: msknight
author_rank: Regular Contributor
timestamp: February 20, 2025, 03:58:46 pm
content: For those with no UK familiarity... We are using a system called Economy 7.Decades ago, the UK had a surplus of overnight energy going to waste and high demand in the day. Economy 7 was a way of charging people less if they moved their load to the night.This was so successful that they had to produce split periods to different households to even up some of the load.Mine comes on at 22:30 and goes off at 00:30 then on again at 2:30 and off at 7:30. So I get 7 hours of cheap rate.At the meter, there is a mechanical switch that turns on supply to the E7 circuit which goes to a separate fuse board. This has individual fuses and cable runs to each of the storage heaters and also the hot water tank. Thus, I have little control over the timing.The element comes on at 22:30 and then runs on its own internal thermostat. There is a chance for me to alter it by removing the housing and altering a screw, but there's no gauge as to what temperature that would be. It's purely a suck it and see approach. After a little while of the water mixing, the element comes on again for a short period, before E7 shuts the circuit off.Then there's a bit more mixing and a little cooling before E7 switches it back on again and it spends the next five hours mixing and topping itself up... a period which looks to use about the same amount of energy as it took to bring the tank up to temperature in the first place.The tank itself not only has the lagging it came with, but I've put more lagging around it as much as I can to retain as much heat as possible.I will be fitting a 13A zigbee switch. That switch will "wake up" and go "unavailable" as E7 flips the circuit, but I'll have the automation talk with it at 5:30am and thus turn on the element which should then have a clear 2 hours before 7:30 to bring the tank up to a reasonable temperature ready for the day ahead.The period between 22:30 and 5:30 will see the temperature drop a bit more than it would otherwise, but the two hours of heating window should make up for that. It will need a little tweaking.I have storage heaters and they will be changed for wall panels which will also be controlled by zigbee, and at that point I'll take the house off E7 all together... although the meter will still have the E7 timings, so if I time things correctly I'll still get cheap rate ;-)

author: tggzzz
author_rank: Super Contributor
timestamp: February 20, 2025, 04:52:40 pm
content: Quote from: msknight on February 20, 2025, 03:58:46 pmMake sure it is rated for your load, e.g. inrush current when cold and inductive spikes when switching off, if any.A related example is the power required for domestic freezers, typically specified to be ~100W. I've measured startup currents equivalent to >1kW for >1s, so don't expect a 300W inverter to work as expected!

author: themadhippy
author_rank: Super Contributor
timestamp: February 20, 2025, 05:16:22 pm
content: Quoteslightly different bucket of fish,freezers tend to have a motor hidden inside em so aint purely resistive and with the added fun that the load goes from a static lump of sludge to a moving liquid.

author: tggzzz
author_rank: Super Contributor
timestamp: February 20, 2025, 07:15:27 pm
content: Quote from: themadhippy on February 20, 2025, 05:16:22 pmHence the word "related".Heaters aren't simple resistors; they take more current when they are cold. How much more is, as they say, left as an exercise for the student.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 20, 2025, 07:24:46 pm
content: Quote from: tggzzz on February 20, 2025, 07:15:27 pmJust some 10%-ish for a heater submerged in water, based on observations on my hot water tank. And remember the relay is also cold when the heating element is cold. I would not call that "inrush" at all, the effect is so small, it goes within tolerances.--Really, this whole discussion is about reducing losses (through insulation) by moving the heating period closer to the use period. Which of course makes sense. Then again, the heat would still be escaping throughout the day. Therefore adding more insulation would probably be a better investment. One can of course do both to maximize both the savings and the fun of having a hobby.

author: m k
author_rank: Super Contributor
timestamp: February 21, 2025, 10:15:22 am
content: DIY insulating is fine, but external job, does it pay back before the tank is gone.I thought once that putting a black tank out to the sun and preheat warm water is the thing.After some calculations it wasn't.Annual water heating here is around 3000 kWh.Sun is "away" half a year and permanent liquid needs antifreeze.Night time tank temp goes down, so other tank is needed indoors, heat exchanger is also needed, with or without the indoor tank.Better pay that few hundred kW hours to electricity company.Quote from: msknight on February 19, 2025, 07:14:58 pmSince that one morning heating time was so short you can do some tests.Put the heating available around half an hour before your morning stuff and try to be ready a bit earlier than estimated reheating need.The goal is to get the tank full before you start using it and again before Economy 7 ends.If everything goes right you can estimate how many morning routines the tank can handle.If the tank can handle many mornings, cut the heating off, be early and do the next morning without any extra heating.When warm water start finishing you can still do fine for some time, but don't turn the water off before you're done.For relays,if DIN track is available it's obviously the best base.If not then maybe solid state relay is easier.