How to measure the remaining HW in an unvented cylinder

[Adsibob]

We have an unvented 300L HW cylinder, heated by a system boiler. We are usually quite good at working out how much we use, so that we can plan the timer accordingly, but occasionally we miscalculate, or somebody will have an unforeseen need and not think to run the boiler for an extra 20 min to compensate, and then we run out of HW.
 

I recall coveting a mixergy cylinder because of the smart tech that came with it which monitored usage and had clever algorithms that could help with topping up the HW automatically. But it was expensive and we couldn’t really make it fit into our pump room, so just went with a traditional copper cylinder. 
 

I’m aware plenty of companies make clever temperature sensors and I wonder how I might be able to rig something up that might alert me when, say, I only have 45L of HW left. 
 

At the moment, the only thing I can think of is to put a pipe mounted sensor on our HW return loop, capture a week’s worth of data from that and analyse it to see what patterns we can spot as it gets closer to running cold. Our HW return loop provides virtually nstantaneous HW to three of our most used taps in the house. The bloody installer did not insulate it very well (it is insulated, but the insulation is cheap and thin) and so we were losing quite a lot of heat from the cylinder via this pipe. We fixed that by rigging up a Shelley PIR system above the relevant taps so that the pump only comes on when you are near the tap. But the heat loss still happens, just at a fraction of the amount of happened before. So maybe a sensor on the pipe could produce some useful info.

 

Any nerds see something up like this and then managed to to use something like IFTTT to solve it?

 

 

Edited January 26 by Adsibob

[ProDave]

11 minutes ago, Adsibob said:

or somebody will have an unforeseen need and not think to run the boiler for an extra 20 min to compensate, and then we run out of HW.

The rest of us have a cylinder thermostat, and when that senses the water going cold the boiler ASHP fires up and re heats the cylinder. No need to put the boiler on for a timed period.

 

The thermostat in in a pocket half way up the cylinder in our case.  So when that senses the cylinder is "cold" you still have the best part of half a cylinder of hot water left.

[joth]

A cylinder stat was popular energy saving measure in the 90s, surprised anyone can  install a new build without one. 

Does your UVC have temperature probe pockets?

 

[SimonD]

I don't think you need anything nerdy. I reckon you're best off simply setting up your system to be priority domestic hot water and keep the cylinder topped up. By modern accounts that's more efficient than letting the cylinder drop to cold and doing a full re-heat cycle, especially if there are times the timer isn't set long enough. Just make sure the cylinder temp sensors are installed on the cylinder correctly and adjust to the right temp.

 

I can't remember exactly which Viessmann boiler you have but it will have a 4 pipe layout capability for PDHW using the Viessmann low voltage cylinder stat. I'm surprised this wasn't the installed setup, or is it?

 

 

[JohnMo]

My attempt. Cheap eBay sensor with a 6mm probe. Drilled 4x 6mm holes in casing of the UVC down to 50% height.

 

The 3rd one down coincides with the height of the secondary returns. There is about 4 deg under read compared to the thermostat pocket. But all good enough to get an idea of the hot water left in the cylinder. Especially good for a slim line cylinder.

 

 

Night before temporary mounts with tape. Showing level of hot water (bottom gauge reads 20 degs)

[Adsibob]

1 hour ago, SimonD said:

^{I don't think you need anything nerdy. I reckon you're best off simply setting up your system to be priority domestic hot water and keep the cylinder topped up.}
 

 

 

we do this already.

 

1 hour ago, SimonD said:

By modern accounts that's more efficient than letting the cylinder drop to cold and doing a full re-heat cycle, especially if there are times the timer isn't set long enough.
 

 

It is set up for priority domestic HW and when the boiler fires, it heats the cylinder pretty quickly. But I would like to automate the topping up when there is a surprise exhaustion of the HW, or if that’s not possible, an early warning system that warns me we’re about to run out. But I don’t see how your suggestion fixes my issue. We already have the system boiler cylinder fire up for about 45 min in the morning and 40 min in the evening. At weekends we also have it come on for 20min in the middle of the day, as we tend to use more HW on weekends.

1 hour ago, SimonD said:

 

Just make sure the cylinder temp sensors are installed on the cylinder correctly and adjust to the right temp.

 

There of course is a thermostat on the cylinder set to a specific temperature, I can’t remember which one, something like 55C. Unfortunately I don’t think it’s a fancy Veissman one, but it does the job in that it tells the boiler to stop firing when it reaches temperature. Is that what you mean by sensor? Do I need more than one thermostat? Where is the right place for it?

1 hour ago, SimonD said:

I can't remember exactly which Viessmann boiler you have but it will have a 4 pipe layout capability for PDHW using the Viessmann low voltage cylinder stat. I'm surprised this wasn't the installed setup, or is it?

 

 

No idea which one was installed. I can take a picture, but I didn’t specify or discuss thermostats with the installer, I just assumed a thermostat for a cylinder was a standard piece of kit.

 

 What’s special about something like this?

https://viessmanndirect.co.uk/Catalogue/Accessories/Vitodens/Vitodens-100-W-System-Boiler-Up-o-35kW/DHW-Cylinder-Sensor-7179114

[Adsibob]

43 minutes ago, JohnMo said:

Drilled 4x 6mm holes in casing of the UVC down to 50% height.

How do you do this without causing leaks in the UVC?

 

 

[Adsibob]

2 hours ago, joth said:

A cylinder stat was popular energy saving measure in the 90s, surprised anyone can  install a new build without one. 

My house isn’t a new build. It’s almost 100 years old! But I’ve modernised it by gutting it, adding EWI, making it airtight and then adding MVHR (as well as a couple of extensions).

 

2 hours ago, ProDave said:

The rest of us have a cylinder thermostat, and when that senses the water going cold the boiler ASHP fires up and re heats the cylinder.

For various reasons including that there is a limit to how much insulation you can add to a 1930s semi before it looks silly and it starts to develop interstitial condensation issues), ASHP wasn’t going to give us a great outcome. I have however laid oversized UFH pipes so that we can reconsider this in 12 - 15 years time in case the technology improves.

[Adsibob]

2 hours ago, joth said:

Does your UVC have temperature probe pockets?

Not sure what I’m looking for, but the technical drawing does suggest it has one (but only one, as far as I can see): https://www.cylinders2go.co.uk/shop/stainless-steel-unvented-cylinders/indirect-unvented-stainless-steel-cylinders/telford-tempest-300-litre-stainless-steel-indirect-unvented-cylinder/?gad_source=1&gclid=CjwKCAiAzc2tBhA6EiwArv-i6ehM54AtIU2QajESJZ6Ts6pHqEvo7VlvFlGqaVifpNb62SsLV2rqFhoCtD0QAvD_BwE

[marshian]

1 hour ago, JohnMo said:

My attempt. Cheap eBay sensor with a 6mm probe. Drilled 4x 6mm holes in casing of the UVC down to 50% height.

 

The 3rd one down coincides with the height of the secondary returns. There is about 4 deg under read compared to the thermostat pocket. But all good enough to get an idea of the hot water left in the cylinder. Especially good for a slim line cylinder.

 

 

Night before temporary mounts with tape. Showing level of hot water (bottom gauge reads 20 degs)

 

I did similar but only top and bottom

 

 

[ProDave]

7 hours ago, Adsibob said:

t is set up for priority domestic HW and when the boiler fires, it heats the cylinder pretty quickly. But I would like to automate the topping up when there is a surprise exhaustion of the HW, or if that’s not possible, an early warning system that warns me we’re about to run out. But I don’t see how your suggestion fixes my issue. We already have the system boiler cylinder fire up for about 45 min in the morning and 40 min in the evening. At weekends we also have it come on for 20min in the middle of the day, as we tend to use more HW on weekends.

You missed my point.  Ignore the fact I have an ASHP, that distracted you.

 

Your boiler should be "on" all day in DHW mode and a cylinder thermostat will fire the boiler whenever the tank needs more heat.  

[JohnMo]

8 hours ago, Adsibob said:

How do you do this without causing leaks in the UVC?

 

 

The cylinder consists of

1. An outer metal jacket/casing - decorative cover.

2. Then insulation

3. Then the cylinder.

 

I drilled in item 1. Then with a drill bit in fingers removed the insulation until I touched the cylinder 3. You don't drill into item 3. The probe butts but against the cylinder wall and you get a slight under read.

 

[SteamyTea]

Not read all the replies properly, but what you are trying/hoping to achieve is not simple, prediction of known usage at unknown times.

This is usually done statistically with the Poisson Distribution.

To quote Wikipedia.

 

Poisson Distribution

"In probability theory and statistics, the Poisson distribution is a discrete probability distribution that expresses the probability of a given number of events occurring in a fixed interval of time if these events occur with a known constant mean rate and independently of the time since the last event. It can also be used for the number of events in other types of intervals than time, and in dimension greater than 1 (e.g, number of events in a given area or volume)."

 

With enough collected data, several years in this case, you may spot a pattern, but I doubt it.  It is like stotting a pattern is cloud coverage, some time in the future, based on only the cloud coverage from the past i.e. ignoring temperature, wind speed and direction, air pressure, time of year, latitude and longitude etc.

I have tried to do that, and only got a null result.

 

You may be able to set up a warning but looking at pipe flow temperatures and how they change i.e. anomalies between the last half hour ∆T of the hot and cold pipes.

But I am not sure what you would really gain.

 

Why do you only heat water on a schedule?

Why not just keep it topped up when it goes below a threshold i.e. DHW priority. Not as if gas is on a ToU tariff.  You could lock it out during certain times i.e. when sleeping if it is noisy.

 

 

[ProDave]

@Adsibob can you post a picture of your cylinder please.

 

It SHOULD already have a thermostat, that is a safety requirement and part of G3 for an unvented cylinder to ensure it cannot overheat, so I don't see why you are only timing the boiler to come on for set periods.  Have you actually tried leaving the hot water on all day and just letting the thermostat do it's job?

[Adsibob]

1 hour ago, ProDave said:

Your boiler should be "on" all day in DHW mode and a cylinder thermostat will fire the boiler whenever the tank needs more heat. 

I see. In fact I think this is something the plumber recommended, but I overruled that suggestion on the basis that I didn’t think it would be an efficient use of gas. For example, it’s virtually certain that we don’t need hot water between 11:30am and 7am, so heat loss during those hours is not a  problem. 

[dpmiller]

4 minutes ago, Adsibob said:

I see. In fact I think this is something the plumber recommended, but I overruled that suggestion on the basis that I didn’t think it would be an efficient use of gas. For example, it’s virtually certain that we don’t need hot water between 11:30am and 7am, so heat loss during those hours is not a  problem. 

so does this mean that the short heating periods youve set might not even be fully satisfying the tank currently?...

[Adsibob]

12 minutes ago, ProDave said:

@Adsibob can you post a picture of your cylinder please.

 

yes, as mentioned above it does has a thermostat. I use this to choose the water temp. The thermostat is a bit difficult to read, but it’s about 55C.

 

12 minutes ago, ProDave said:

 

It SHOULD already have a thermostat, that is a safety requirement and part of G3 for an unvented cylinder to ensure it cannot overheat, so I don't see why you are only timing the boiler to come on for set periods.  Have you actually tried leaving the hot water on all day and just letting the thermostat do it's job?

I time it for two reasons:

1) I can control the periods of heat loss to when it doesn’t matter (eg when we are asleep)

2) in winter I can heat the cylinder at the same time as running the central heating, to get better efficiency from the boiler. Questionable whether this makes a difference though, given the Veissman boiler is very efficient and modulates to 1/17th of its max output.

[Adsibob]

6 minutes ago, dpmiller said:

so does this mean that the short heating periods youve set might not even be fully satisfying the tank currently?...

Yes, it’s possible. Though one would have thought that a 35kw boiler can heat 300L of water pretty quickly. The Telford Tempest technical details says:

1) it takes 40min to heat all 300L (actually 294L) from 10C to 60C. My thermostat is set around 5C cooler than that, and I run it for 45min in the morning and again for 40min in the early evening. Plus weekends we run a bit extra midday.

2) standing loss is 92 watts, though this doesn’t factor in my poorly insulated recirculating pipe for instant HW.

 

[ProDave]

I would at least start by making your present timed periods much longer, so you are sure the cylinder has reached set temperature.

 

Personally I would leave the HW on all the time people are in the house.  If you really really don't want to do that, then whenever someone takes a shower get into the habit og pressing the +1hour boost button on your programmer.

 

Your recirculating water pipes will be losing far far more heat than the tank, and that is where your attention should be, e.g make the circulating pump only come on when hot water usage is likely, e.g by motion detectors in the bathrooms.

[SteamyTea]

As I was laying in the bath just now, I wondered it you could use the bathroom humidity changes as a trigger to check and control the DHW heating.

I think there was someone that did this for the boost on their MVHR, if I remember, if the RH increased by 5% over a set time period (say 5 minutes), then the MVHR went onto boost.  This got around the problem of it boosting (the whole house) unnecessarily.

[TonyT]

1 hour ago, Adsibob said:

I see. In fact I think this is something the plumber recommended, but I overruled that suggestion on the basis that I didn’t think it would be an efficient use of gas. For example, it’s virtually certain that we don’t need hot water between 11:30am and 7am, so heat loss during those hours is not a  problem. 

So add a programmer in series with the thermostat so that you can over rule it if you want.

 

 

[marshian]

10 hours ago, marshian said:

 

I did similar but only top and bottom

 

 

 

Forgot to add a Picture

 

 

[marshian]

You could use something like a Sonoff Temp Sensor 3/4 of the way up the tank - set the temp set point to just below your target tank temp and then use the switch facility as an output signal to fire HW as you would be about to run out of HW at that point?

[SimonD]

13 hours ago, Adsibob said:

It is set up for priority domestic HW and when the boiler fires, it heats the cylinder pretty quickly. But I would like to automate the topping up when there is a surprise exhaustion of the HW, or if that’s not possible, an early warning system that warns me we’re about to run out. But I don’t see how your suggestion fixes my issue. We already have the system boiler cylinder fire up for about 45 min in the morning and 40 min in the evening. At weekends we also have it come on for 20min in the middle of the day, as we tend to use more HW on weekends.

 

As @ProDave really.

 

This is not really priority hot water, you keep the hot water on all the time to top it up automatically, but you can obviously turn it off over night.

 

And also look at your hot water recirc. and the timing of the pump. This is where it's important to understanding usage patterns, particularly short draw offs as that's where the energy losses can be greatest with long runs and poorly insulated pipework.

 

13 hours ago, Adsibob said:

No idea which one was installed. I can take a picture, but I didn’t specify or discuss thermostats with the installer, I just assumed a thermostat for a cylinder was a standard piece of kit.

 

 What’s special about something like this?

https://viessmanndirect.co.uk/Catalogue/Accessories/Vitodens/Vitodens-100-W-System-Boiler-Up-o-35kW/DHW-Cylinder-Sensor-7179114

 

It connects straight into the boiler and is a low voltage connection - it tells the boiler by how much the cylinder is below set point and then by how much it is above it to manage firing up and switching off. It also manages boiler dhw output according to set temp of the cylinder to reduce re-heat time: 

 

Quote

The burner, the circulation pump and the 3-way
diverter valve are started or changed over if the cylin-
der temperature lies 2.5 K below the set cylinder tem-
perature.
In the delivered condition, the set boiler water temper-
ature is 20 K higher than the set cylinder temperature.
If the actual cylinder temperature exceeds the set cyl-
inder temperature by 2.5 K, the burner shuts down and
circulation pump run-on begins.

 

You don't get this if you have a normal cylinder stat connected via relay connection within a wiring centre. All that then happens is you have to manually set to DHW output temp. at the boiler and then it fires up according to the cylinder stat. If you have sufficient cylinder coil output, then it is better for the boiler to manage the temperature differential between boiler output and target cylinder temp.

 

3 hours ago, Adsibob said:

2) in winter I can heat the cylinder at the same time as running the central heating, to get better efficiency from the boiler. Questionable whether this makes a difference though, given the Veissman boiler is very efficient and modulates to 1/17th of its max output.

 

This is definitely not PDHW. With PDHW you want a higher boiler output temp, which is above the desired DHW temp, and with PDHW the system should close off all flow to the central heating system during a cylinder heat cycle to permit the great flow temp. You don't want to boiler modulating for CH during cylinder heating. Then when hot water demand is satisfied, the system is able to reduce flow temps according to either weather comp or load compensating controls to your central heating circuit.  If you are mixing these two functions, you're undermining the whole pdhw approach and you may end up providing flow to the DHW cylinder for very long wasteful periods and it will never fully get to desired temperature, which may be why you're experiencing times when the cylinder doesn't reach desired temp.

 

I think we need to know exactly how your system has been configured with the motorized valves and wiring because something doesn't ring right here.

 

Edited January 27 by SimonD

[Adsibob]

3 hours ago, ProDave said:

Your recirculating water pipes will be losing far far more heat than the tank, and that is where your attention should be, e.g make the circulating pump only come on when hot water usage is likely, e.g by motion detectors in the bathrooms.

Yes, this is already in hand. The pump only comes on when certain PIR sensors are triggered, and even then it only comes on for 30seconds. The worst heat loss will be when someone is washing up, as the PIR sensor above the kitchen sink will trigger continuously, though maybe there is a clever setting on the Shelly app that can stop that.