# Measuring "average" temperature inside Hot Water Cylinder

I guess I am trying to reinvent the wheel here but hear me out. Seems in the year 2022 we still have to second guess how much "hot" water is remaining in a hot water tank. Our family usage varies dramatically from day to day, some days we might as well stick the tank to permanently on (governed by the tank thermostat) on other days we're better of controlling via a timer as it's only required for 30 mins of the day. There are occasions when 3 people will shower one after the other with nothing left for the 4th person and there are occasions when only one person is at home for 2 days. There is no way of telling what our day to day usage is going to be in advance. Always on = wastage if no one is using for 2 days. Timer = Assumes constant routines that don't change much day on day. Neither really cut it IMO.

Clearly, in this modern day and age, it would be beneficial to have a method of determining when the available hot water is getting "low" (I get there is no such thing as low in terms of quantity but I'm talking in terms of temperature). And then use that knowledge (should we wish) to heat the tank some more. We can in theory do that by being able to determine what the average temperature is inside the tank.

I have not come across any tank design where the manufacturer chooses to reveal the internal temperature. The probe sits behind the thermostat, the capability is there in theory to determine this temperature but there's not a single temperature gauge to be seen.

As manufactures of tanks have no interest in fitting smart temperature sensors that can be interrogated by Wi-Fi, RF, BT or Zigbee (\$10 tech), my question is where is the best place to self install a sensor to determine the "internal" tank temperature? That sensor could of course sit near a pipe (that is somehow linked to the internal temprature of the tank). Of course even if I find that spot, it would be a learning/calibration exercise to determine which temperature value means I'm "getting low on hot water" but that's a separate problem.

Thoughts, suggestions, alternatives? Thank you

I've wondered about this myself. In the old days, before cylinders had bonded insulation, you could feel the copper itself, and see how much of the tank was still hot. That experience flags up a problem with measuring the temperature. The cylinder doesn't cool evenly. The new cold water comes in at the bottom, and the remaining hot water stays in a layer on top. It's called stratification. So often it's a case of how much hot water is left, rather than what is its temperature.

I think Mixergy make a cylinder with a sensor which shows how hot the water is at different levels in the cylinder.

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If your cylinder is modern with integral foam insulation etc. the heat loss from it is minimal so you might as well just leave dhw on constantly rather than faffing with a timer.
Other options- you could insert a flow switch on the hot output from the tank, and connect it via a bit of logic to dhw control circuit (so, for example, if dhw runs for more than 5 mins solid then boost dhw).
You could put similar logic on bathroom/shower room light circuits.

Thanks for the suggestion. Mixergy seem to be on top of this. They've written this article which shares my thoughts entirely:

The industry needs to change in light of recent world events. A 20% reduction in energy usage (even 10%) suddenly means something to the majority of us. This kind of tech should be high on the list for government to enforce.

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Thanks for the suggestion. Mixergy seem to be on top of this. They've written this article which shares my thoughts entirely:

The industry needs to change in light of recent world events. A 20% reduction in energy usage (even 10%) suddenly means something to the majority of us. This kind of tech should be high on the list for government to enforce.
Hmm. Not sure how their cylinder is improving energy usage. Reading the article the savings came from monitoring hot water use and varying heating patterns to fit.
Again looking at their numbers, their devices save 1kwh per day on a low occupancy/large cylinder scenario. Worst case loss (heating electric) that at the moment will cost £124. So a worthwhile sum but not really life-changing.
EDIT Savings will be much less on a high occupancy/large cylinder or low occupancy/small cylinder setup.

I think you have an exaggerated view of the energy cost of keeping a cylinder hot. What do you think it costs? Do you seriously thi k you could save 20%? Or even 10%?

What colour is yours, and where is it? In the loft? Or in an occupied part of the house? Do you heat it with gas? Off-peak electricty?

Is the house occupied all day? How many people?

If your cylinder is modern with integral foam insulation etc. the heat loss from it is minimal so you might as well just leave dhw on constantly rather than faffing with a timer.
Exactly. And heat lost goes into the house anyway.

Thanks for the suggestion. Mixergy seem to be on top of this. They've written this article which shares my thoughts entirely:

The industry needs to change in light of recent world events. A 20% reduction in energy usage (even 10%) suddenly means something to the majority of us. This kind of tech should be high on the list for government to enforce.
It says
"Tenants were able to save on average 35% in energy consumption and 16% carbon reduction compared to a traditional cylinder. By enabling the machine learning feature, the tenants were able to save an extra 12% in energy consumption compared to the schedules they had created in the app!"
To misquote the Iron Duke - if you'll believe that you'll believe anything

It says
"Tenants were able to save on average 35% in energy consumption and 16% carbon reduction compared to a traditional cylinder. By enabling the machine learning feature, the tenants were able to save an extra 12% in energy consumption compared to the schedules they had created in the app!"
To misquote the Iron Duke - if you'll believe that you'll believe anything

I have some magic beans for sale....

Good odds the savings came from tenants using less hot water. Bit like 'smart meters are an energy saving device'. No they're not, they just make it easier to see how much energy you are using.

Clearly, in this modern day and age, it would be beneficial to have a method of determining when the available hot water is getting "low" (I get there is no such thing as low in terms of quantity but I'm talking in terms of temperature). And then use that knowledge (should we wish) to heat the tank some more. We can in theory do that by being able to determine what the average temperature is inside the tank.

If you buy a few of these - https://www.ebay.co.uk/itm/225084166392

They run on battery, so no actual wiring involved.

You can make a tiny hole slopping at an angle into the tank insulation, into which you can push the probe tips, at regular places along the height of the cylinder. Mount the displays in a panel, and they will provide a very good idea of exactly how much hot water you have remaining in the cylinder. I would suggest four of the displays might give you the resolution you need, but be aware the water in a cylinder, only gets hot above the heating coil, or the lower end of an immersion heater.

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Goodness, we are a cynical bunch!

This strikes me as a university 'spin out' company, but I could be wrong.
A small operation, designing and researching products, to be manufactured to order elseware.

I have no doubt that the products can save money and CO2 over older, poorly insulated tanks - and just as any company will do - they will naturally take best case scenario data from their products and compare them to worst case data from others.

However, there are some interesting features, such as apparently modulation:

But what fascinates me more, is the 'Smart' nature of the device and the companies work with the national grid to develop a "1MW frequency response service" and "DESIRE - a Distributed Energy Storage Integrating Renewable Energy" - a system utilising smart tanks for load distribution.

So, it might save people energy, but depending on your viewpoint, could also be another big brother intrusion!

Have it timed for 30 minutes a day, tell people to advance it when they go for a shower. We have a regular schedule so ours is timed to our routine - out of routine we "+1hr" on the programmer and give it 10 minutes. We have a well insulated cylinder, heat loss negligible in summer and goes into the house in winter. Otherwise get a combi.

We have a well insulated cylinder, heat loss negligible in summer and goes into the house in winter. Otherwise get a combi.

I'm quite happy with my cylinder's capacity, but it's yellow/orange insulated, 35 or so years old. I'm wondering whether it might be worth attempting to improve the insulation in some way.

You can put a red jacket on it if you want.

But start by insulating any bare pipes.

Rarely seen these days

Maybe you can find an energy saving group.

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