Adding extra temperature sensor to hot water tank

[Sammy21]

I've just replaced all the old Danfoss controls on my heating system with a 3-channel Wiser system which seems to be working pretty well. However, I'm a bit disappointed that there's no finer control of the water heating circuit. There's a great big old thermostat at the bottom of the tank. the boiler fires up twice a day and heats the whole tank. Pretty coarse-grained, not super efficient. But works.

I'd like to run some experiments with finer control, and to do this, i want to use one of these from Sonoff, that I happen to have lying around. The sensor itself is around 4mm in diameter, and connects to a Wifi Sonoff switch that I can use to control the HW circuit, either based on a simple timer, or using more complicated logic via IFTTT and Sequematic.

The only tricky bit is how to get the tip of the sensor resting agains the metal of the tank. I want this sensor to go nearer the top, halfway between the thermostat and the top of the tank, roughly, the thinking being that if the temp there is around 50C, that'll be enough hot water for a shower.

So how do I get the sensor next to the tank?

The boiler manual says there is 50mm of insulation, "fire retardant polyurethane foam, nominal thickness 50mm".

So, can I carefully drill a 4mm hole through the plastic covering to a depth of say 30mm, then keep going using a wooden skewer or plastic tool that can't possibly damage the metal tank itself? Then I'd push the sensor un until it is touching metal.

Anyone else ever tried this?

I also want to log the temperature every 5 minutes or so and see how it changes throughout the day, see how much heat the boiler loses, and look for ways to optimise the heating cycle so that I'm not heating water unnecessarily. This would be the perfect setup for that as it's easy to create temp spreadsheets with IFTTT and Google Sheets.

 

[stem]

The main issue tends to be with regard to the physical arrangement, and not necessarily the thermostat (unless it is faulty of course) The bottom of the cylinder is where the cold water comes in and is below the heating coil, so tends to stay cold. As heat rises, the temperature will increase gradually as it goes up and the water at the top can be 40 degrees or more warmer than the water at the bottom. To make it worse, the range is not consistent, or linear, it can change according to water usage and the incoming cold water temperature etc.,

With such a varying range, it can be difficult to regulate the temperature precisely when measuring it at just one point. So to get a reasonable average and provide a sufficient supply of hot water, manufacturers tend to recommend that the thermostat be located between one quarter and one third of the way up the cylinder, by which time the water is no longer cold, but not as hot as it is at the top.

If the thermostat was at the top of the cylinder that will heat up quite quickly as hot water rises. Then the thermostat will stop the boiler heating the cylinder anymore. Water to the taps will appear hot, but once a sink full or two has been taken off, it will run out as the water below will still be cold, so not enough for a bath or a shower longer than a couple of minutes.

It's an interesting experiment though, it will be good to hear how you get on.

 

[MJN]

Hi Sammy21. Before you start hacking away at the cylinder might it be worth experimenting with a less-invasive approach? Will your sensor fit alongside the existing thermostat probe in the pocket that houses it?

As an aside, it is not clear to me how you are expecting/intending to improve efficiency. With such a well insulated cylinder heat loss is fairly low, and almost effectively zero during the heating season given the 'waste' heat contributes to space heating. Looking at this from a different perspective, what is your energy consumption outside of the heating season i.e. what is your daily usage when only heating hot water? That puts the absolute maximum limit on potential savings and that's even before accepting that you do use a proportion of the hot water that you heat thus pulling that ceiling even lower.

 

[Sammy21]

If the thermostat was at the top of the cylinder that will heat up quite quickly as hot water rises. Then the thermostat will stop the boiler heating the cylinder anymore. Water to the taps will appear hot, but once a sink full or two has been taken off, it will run out as the water below will still be cold, so not enough for a bath or a shower longer than a couple of minutes.

I have had some experience of water heat stratification before, albeit on larger tanks (1.5kL). My tank is 180L, so I'm estimating that if I put the sensor 1/3 from the top, a temp of 45C will give me 1/3 of a tank of hot water (60L), enough for a quick shower (which is all that is needed during the day, as showers are otherwise in the morning or the evening). I can also set up some logic so that if the sensor drops below 35C during the day, the boiler comes on and heats it back up to 45C, so I'll always have 1/3 of a tank of hot water. In the evening the boiler will execute a full heat cycle and heat the whole tank to 55C for everyone's evening ablutions.

Thoughts?

 

[Sammy21]

Hi Sammy21. Before you start hacking away at the cylinder might it be worth experimenting with a less-invasive approach? Will your sensor fit alongside the existing thermostat probe in the pocket that houses it?

No idea, I didn't think there would be any room in there but I haven't looked. However, it's not the idea spot (see previous comment), but you are right, it would be less risky!

As an aside, it is not clear to me how you are expecting/intending to improve efficiency. With such a well insulated cylinder heat loss is fairly low, and almost effectively zero during the heating season given the 'waste' heat contributes to space heating. Looking at this from a different perspective, what is your energy consumption outside of the heating season i.e. what is your daily usage when only heating hot water? That puts the absolute maximum limit on potential savings and that's even before accepting that you do use a proportion of the hot water that you heat thus pulling that ceiling even lower.

See my previous reply to stem's comments, but you are right, there may not be much savings. I would like to plot how much heat the tank loses. My own experience with much bigger tanks (higher thermal inertia, so should lose less heat) is that these tanks lose more heat than you might think, as much as 15C in 12 hours. So if I know there will be a maximum of 1 shower in the morning, 1 during the day, and 2 or 3 in the evening, I might be able to regulate my heating cycles to give less hw in the morning and day, and only heat a full tank in the evening. With the current hw control I could guess at this, simply heating less in the morning, more in the evening, but without some accurate regular temp readings, I'll be flying blind.

It could all prove to be a massive waste of time, but it might be fun learning and trying...as long as I don't poke a hole in my tank!

PS There is also the possibility of using the advanced control to program an occasional (say monthly) sterilisation of the HW tank by heating up to 80C.

 

[oldbuffer]

1. That looks like an unvented hot water cylinder. If it is, you must be G3 qualified and accredited to work on it or its controls.
2. If you heat only the top 1/3 of the cylinder to 60, the water below will be at less than 60 degrees. Water should be heated to 60 to remove the risk of legionella.
3. If you drill into the insulation:
3.1 You will invalidate any warranty which may apply.
3.2 You will reduce the effectiveness of the insulation, albeit by very little.
4. As you have the manual for the cylinder, this may contain the hourly heat loss figure. Checking this against the savings you hope to make will enable you to do a cost benefit calculation.

 

[Sammy21]

2. If you heat only the top 1/3 of the cylinder to 60, the water below will be at less than 60 degrees. Water should be heated to 60 to remove the risk of legionella.

the whole tank will get heated to 60C every evening

3. If you drill into the insulation:
3.1 You will invalidate any warranty which may apply.
3.2 You will reduce the effectiveness of the insulation, albeit by very little.

understood

4. As you have the manual for the cylinder, this may contain the hourly heat loss figure. Checking this against the savings you hope to make will enable you to do a cost benefit calculation.

1.32KW/24 hours quoted, but without temp readings I don't really know how this will track in real life, or how much this is costing me with my 10 year old boiler.

 

[MJN]

My own experience with much bigger tanks (higher thermal inertia, so should lose less heat) is that these tanks lose more heat than you might think, as much as 15C in 12 hours.

Be careful extrapolating across systems though - too many variables.

My 150L Range Tribune UVC has a purported standing heat loss of 1.31 kWh/24hrs so converting that into °C heat loss:

1.31 kWh x 3.6 MJ/kWh = 4.716 MJ
4.716 MJ * 1000000 / (150000 mL x 4.2 J/ml) = 7.5 °C

So, at whatever stored and ambient temperatures the UVC standing heat loss is calculated (I'm assuming there's an accepted standard for this calculation otherwise comparisons would be meaningless, and that standard is presumably realistically representative of expected conditions) you'd expect only 7.5 °C loss across the entire 150L tank in 24 hrs which to me seems pretty low, particularly given you are running two heat cycles (I have one which just about covers our capacity needs) so notwithstanding non-linear heat transfer I'd expect you to see only half that in practice.

I suspect you'd see greater loss as a result of the uninsulated pipework so sort that out regardless!

It could all prove to be a massive waste of time, but it might be fun learning and trying...as long as I don't poke a hole in my tank!

And you'd get some graphs out of it... and what's not to like about graphs?!

PS There is also the possibility of using the advanced control to program an occasional (say monthly) sterilisation of the HW tank by heating up to 80C.

I wouldn't be concerned with a UVC given the sealed system and regular throughput. Whilst I don't follow the stats I don't know if there's been a reported case of legionella from a domestic sealed HW system, and unnecessary sterilisation runs counter to your desire for maximising efficiency.

 

[Sammy21]

you'd expect only 7.5 °C loss across the entire 150L tank in 24 hrs which to me seems pretty low, particularly given you are running two heat cycles (I have one which just about covers our capacity needs) so notwithstanding non-linear heat transfer I'd expect you to see only half that in practice.

hmm...that's not a lot is it?

I suspect you'd see greater loss as a result of the uninsulated pipework so sort that out regardless!

yup, just insulated the pipework coming out of the boiler which was naked in our freezing garage. pipes in the tank cupboard are all uninsulated.

And you'd get some graphs out of it... and what's not to like about graphs?!

graphs are BRILLIANT!

I feel like I'm being talked out of drilling a hole in my tank, which is a shame, though possibly wise.

 

[MJN]

I feel like I'm being talked out of drilling a hole in my tank, which is a shame, though possibly wise.

On the contrary! I'm just steering you towards a more incremental approach...

The end state won't be a hole in the tank - it'll be several holes at various points up the side so you can actually measure the stratification in real time!

 

[Sammy21]

The end state won't be a hole in the tank - it'll be several holes at various points up the side so you can actually measure the stratification in real time!

fun! but perhaps a tad over-ambitious?

 

[Sammy21]

Well, I went ahead and did it, and everything seems to be working fine. My only issue now is that I can't work out how to get the temp data into a spreadsheet! I did this years ago but I'll be buggered if I can remember how I did it...

 

[SpecialK]

Well, I went ahead and did it, and everything seems to be working fine. My only issue now is that I can't work out how to get the temp data into a spreadsheet! I did this years ago but I'll be buggered if I can remember how I did it...

How did you do it? I'm thinking of fitting 3 temp sensors to my tank to monitor the temp top middle and bottom.....

 

[Harry Bloomfield]

It could all prove to be a massive waste of time, but it might be fun learning and trying...as long as I don't poke a hole in my tank!

You could drill the outer metal jacket, then poke through the insulation with the likes of a blunt screwdriver.

As you probably know, some cylinders use electric immersion heaters. Sometimes these use an E7 tariff, sometimes also using two immersions each with it's own stat, one short reach, the other longer to heat the entire cylinder. The longer element used during the cheaper off-peak tariff, the shorter one for a quick boost at peak times. The difference, of course, is that with a gas heated system, there is only the one source of heat, at the bottom.

 

[Pete the Feet]

I have a similar sensor to the one that is illustrated on the opening post. I too was concerned about stratification and I did two things. Firstly I bought a NTC sensor to link into my boiler controls. The boiler is a Vaillant System boiler with a VR66 control box and VSmart controls. This enables a digital readout on the boiler control panel as opposed to using the dual thermostat on the Ariston Primo 300L cylinder. The NTC negates the use of the cylinder stat, that is unless you are using the backup immersion heater which utilises the stat for the immersion and boiler control if required.
As Sammy21 and others have stated there is a huge temperature difference between the top and bottom of the cylinder and in order to prevent the boiler cycling needlessly every time we draw off hot water I have limited the heat up time to 1 hour in the morning, with an occasional daytime boost if required. The disappointing thing about the VSmart is that it doesn't give you a DHW readout, you have to access the boiler control panel for this.
However being a bit of a nerd like Sammy21 is, I installed an additional sensor similar to the one in the opening post. It is a Sonoff TH10 and is actually a switch and the data is accessible via a Wi-Fi connection. Sadly it does not quite satisfy my nerdery as I would have liked one which logs the data and produces graphs, but it does give me the info that I need and I now know when to give the hot water a boost should it need it. The sensor for this is installed without gouging out holes in the lagging and gives a reasonably accurate temperature reading. Firstly I tried it in the sensor pocket for purposes of comparison with the other sensor that controls the boiler. Then alongside the connection boss for the thermostat. Satisfied that they both gave similar readings I then placed the sensor where I needed it and that is alongside the flow outlet boss behind the lagging. You should be able to squeeze the sensor in there and get a reasonably accurate reading. To verify this try it on the thermostat pocket as I did.
As a consequence of this I now know to give the hot water a boost when it drops to around 44C if we want one decent shower. At 48C there is enough for 2 showers, just. It is reheated to about 60C but seldom attains this unless I give it a bit extra, as I do for legionella purposes.

My next project on the cylinder is a destratification pump installed between the outlet and the cold water input. This might yield interesting results. I must state at this point that being a complete nerd, I have previously installed timer counters for the both heating and hot water zones. The only cure is the bullet I'm afraid. The one thing I have learned is that running a DHW cylinder is far far less efficient than drawing hot water from a combi but at least it meets our needs.