How to measure the remaining HW in an unvented cylinder

[Adsibob]

2 hours ago, marshian said:

 

Forgot to add a Picture

 

 

So just to understand this a bit better: what is the typical stratification of a cylinder? Is it linear such that the 39 degree delta between the top and the bottom here means that if the cylinder is 156cm tall (39 x4) every 4cm there is a change in temperature of 1 degree?

[Adsibob]

9 minutes ago, SimonD said:

may be why you're experiencing times when the cylinder doesn't reach desired temp.

This is not a symptom we are experiencing. Everything works well. The only issue we have is that we don’t always keep tabs of unusually high use, such that when somebody has unusually high use, they don’t top up the cylinder. That’s why I wanted to investigate automating it. I think I should have just got a mixergy.

[SimonD]

21 minutes ago, Adsibob said:

That’s why I wanted to investigate automating it. I think I should have just got a mixergy.

 

But Mixergy won't solve this problem because it learns your regular patterns of use and therefore you'll still need to manually top it up following unusual hot water usage.

[ProDave]

1 hour ago, Adsibob said:

This is not a symptom we are experiencing. Everything works well. The only issue we have is that we don’t always keep tabs of unusually high use, such that when somebody has unusually high use, they don’t top up the cylinder. That’s why I wanted to investigate automating it. I think I should have just got a mixergy.

Please explain why you don't want to leave the hot water on all day letting the thermostat top it up when needed.  Not wanting to do that is forcing you to devise a complicated strategy to measure when an unusual amount of heat has been extracted from the cylinder when the tank thermostat will do just that, if the heating were on to allow it.

[SteamyTea]

1 hour ago, Adsibob said:

So just to understand this a bit better: what is the typical stratification of a cylinder? Is it linear such that the 39 degree delta between the top and the bottom here means that if the cylinder is 156cm tall (39 x4) every 4cm there is a change in temperature of 1 degree?

Not really, it is much more complicated than that as it is not really stratification at all.

There is a mean temperature gradient, but that changes with the mean temperature.

Why I suggested that you look for another solution.

[ProDave]

1 hour ago, Adsibob said:

So just to understand this a bit better: what is the typical stratification of a cylinder? Is it linear such that the 39 degree delta between the top and the bottom here means that if the cylinder is 156cm tall (39 x4) every 4cm there is a change in temperature of 1 degree?

Normally an UVC will start off full of hot water heated to the set temperature.  As it is used, a hot / cold transition moves up the tank.  If no more heat was added, you would get almost the same temperature of water coming out of the taps right up to the point where the hot / cold transition reaches the top of the tank and the water would suddenly go very cold.

 

If the cylinder thermostat was say half way up, then at the point the transition reaches the thermostat the boiler would fire up.  At that point the heat input to the tank would stir up convection within the tank and continue to do so while it is heating.  With the heat source being a big boiler, it can probably re heat the tank as fast as you can empty it.

[Adsibob]

16 minutes ago, ProDave said:

Please explain why you don't want to leave the hot water on all day letting the thermostat top it up when needed.  Not wanting to do that is forcing you to devise a complicated strategy to measure when an unusual amount of heat has been extracted from the cylinder when the tank thermostat will do just that, if the heating were on to allow it.

Instinctively, it just seems like a very inefficient use of gas. Even if I’m wrong about that (am I wrong?), is it not going to increase the wear and tear of the boiler?

[ProDave]

10 minutes ago, Adsibob said:

Instinctively, it just seems like a very inefficient use of gas. Even if I’m wrong about that (am I wrong?), is it not going to increase the wear and tear of the boiler?

Why is it inefficient?  The boiler would be working hard to re heat the tank as quick as it can, not short cycling and would shut off as soon as the tank reaches temperature.  Yes the more times it does that it has to re heat the water within it and the water in the connecting pipes but that is a small volume.  Mut unike room thermostats, a tank thermostat usually has quite a large hysteresis, so it would not be firing up to re heat just because the tank has lost 1 degree of water temperature.

[Alan Ambrose]

>>>

Normally an UVC will start off full of hot water heated to the set temperature.  As it is used, a hot / cold transition moves up the tank.  If no more heat was added, you would get almost the same temperature of water coming out of the taps right up to the point where the hot / cold transition reaches the top of the tank and the water would suddenly go very cold.

 

If the cylinder thermostat was say half way up, then at the point the transition reaches the thermostat the boiler would fire up.  At that point the heat input to the tank would stir up convection within the tank and continue to do so while it is heating.  With the heat source being a big boiler, it can probably re heat the tank as fast as you can empty it.

<<<

 

Yeah, the behaviour of DW cylinders is quite a complicated business if you think about it - stratification / mixing / heat loss / statistical usage patterns etc. The simpler way would be no storage tank and use a heat source powerful enough to heat the water to the required temperature instantaneously at the required flow rate. @SteamyTea will be along shortly to tell us what instantaneous heat power is required for a typical shower . That system would be much more efficient because you would only heat the water you need / there would be no 'waste' heat when the cylinder cools while it's waiting for your next shower / there would be no need to heat up more water than you need not knowing how long your shower was going to be. However, the instantaneous power requirement is more than most heat sources can provide - hence the tank.

 

The storage tank lets a lower power heat source heat over a longer period and then the heated water is delivered in a much shorter period. Sort of heat-based impedance matching if you get my drift.

 

Given that the tank thing is all a bit of a fudge anyway, why not just heat a bit longer until you don't run out of hot water ever. Yeah, there's going to be some waste, but using a cylinder in the usual way implies some waste anyway.

 

The other system nobody uses would be to have one or more vented tanks which didn't refill with cold water until the start of a new heating cycle had been determined, say 3 am? The control / safety system might be more complicated though. A bit like a more modern - 'heat the kettle on the stove and fill the bath in the parlour system'

 

Edited January 27 by Alan Ambrose

[SimonD]

2 minutes ago, ProDave said:

a tank thermostat usually has quite a large hysteresis, so it would not be firing up to re heat just because the tank has lost 1 degree of water temperature.

 

This is where I'd like to know the set up of the system, the controls being used and how it's wired up because on some systems, like the Evohome system I have installed, I have full control of this so I can set a differential, pump over-run time, minimum run time etc., and why using something like the Viessmann cylinder stat wired directly to the boiler on a 4 pipe system is designed to make this approach as efficient as it should be. Hence why I'm confused about how the PDHW has been approached in this installation, but also why the system isn't being used as it is suggested it's been installed. @Adsibob?

[ProDave]

12 minutes ago, Alan Ambrose said:

will be along shortly to tell us what instantaneous heat power is required for a typical shower

I find a 10kW shower woefully inadequate, that is based on showering with an electric instant electric shower, and the 10kW instant gas water heater we had in the static caravan.  So I would guess for a proper decent shower you are looking at 25kW or more?  I think the 10kW shower was rated at less than 5 litres per minute.  I can get almost 15 litres per minute from an UVC fed shower.

[SimonD]

58 minutes ago, Adsibob said:

Instinctively, it just seems like a very inefficient use of gas. Even if I’m wrong about that (am I wrong?), is it not going to increase the wear and tear of the boiler?

 

No, it should actually improve the efficiencies and reduce wear and tear on the boiler. One of the reasons for this is that if you're heating up your cylinder for a longer period of time from empty, the flow rate throught the cylinder and boiler is high enough for the return temperature to go above 54C. Therefore, there is likely to be a long period of time where the boiler is not condensing. It's also more likely you'll see higher return temps causing boiler cycling. If you run PDHW and allow the boiler to top up the cylinder frequently, the time period where the return is above 54 is reduced, therefore efficiencies are improved. This is partly the case because with modern cylinders manufacturer's don't tend to advise that flow is balance using a gate valve so you have full boiler/pump flow rates without much restriction.

 

You can further improve your efficiencies by reducing cylinder temp and thus required related dhw flow temp to get the cylinder to required temperature IYSWIM.

Edited January 27 by SimonD

[SimonD]

5 minutes ago, ProDave said:

So I would guess for a proper decent shower you are looking at 25kW or more? 

 

Generally speaking with 25kW you'll get to see about 9-11l/min with an instantaneous temperature lift of 30-35 degrees C, sometimes you can be lucky to see a bit more.

[Adsibob]

57 minutes ago, SimonD said:

 

This is where I'd like to know the set up of the system, the controls being used and how it's wired up because on some systems, like the Evohome system I have installed, I have full control of this so I can set a differential, pump over-run time, minimum run time etc., and why using something like the Viessmann cylinder stat wired directly to the boiler on a 4 pipe system is designed to make this approach as efficient as it should be. Hence why I'm confused about how the PDHW has been approached in this installation, but also why the system isn't being used as it is suggested it's been installed. @Adsibob?

I’m not really sure the answer to this. I didn’t realise boiler setups were that complicated. My boiler and heating system and tank was installed by an electrician and a gas engineer. I d did some research and came to a few conclusions like:

1) Veissman together with a Veissman low loss header would prevent the need for a buffer tank and would not short cycle. 
2) 300L cylinder should just about meet our needs

 3) PIR activated secondary loop would be beneficial.

4) I like tado.

5) different pipe diameters for different applications, eg 22mm for bath, 15mm for shower, 10mm for basins.

So to the extent there was any design, that was it. I conveyed that info to the gas engineer/plumber, asked him if he was Veissman registered, he said he could get approved and registered. A couple of months later he showed me the Veissman certificate for his registration, and after that I let him get on and do the work.

My electrician asked me how I wanted to control my heating and HW and I said tado and he did the rest, choosing which wiring systems etc.. It all works, and my bills have been pretty low considering it’s a 5 bed house with fairly high HW use and probably would only score a C, maybe a B, on a EPC.
I have a tado controller wired to the boiler, and that tells the boiler to heat the cylinder, save that it is overridden by the cylinder’s stat when the cylinder stat decides it’s reached temp.

 

With retrospect, I should have got the system properly designed, as then proper insulation for the secondary return/loop pipe would have been specified, and we might have a more sophisticated HW heating system but given everything is working well, I’m not sure how much I would have saved in the long wrong by paying for a professional design.

Edited January 27 by Adsibob

[TonyT]

Is tado smart cause you can use a smart phone to control it?

or does it do anything else?

[Dan F]

3 hours ago, SimonD said:

 

But Mixergy won't solve this problem because it learns your regular patterns of use and therefore you'll still need to manually top it up following unusual hot water usage.

 

Mixergy you can tell it to top-up when charge is 10% or 20% or whatever.

[Adsibob]

Thanks everyone for the feedback. It seems I should have just accepted my installer’s recommendation to leave it running all the time. Seems so counterintuitive, but I guess that’s what our boiling water tap does in our kitchen (re boils for 20 sec every 20 min to let the water at a constant 98C). So if the theory works for that, then maybe it will work for a cylinder that is 60 times bigger as well.

 

I have now reset my HW timer to do the below schedule,  and will drop the thermostat by a couple of degrees.

 

Final query is whether to have it running 24/7 as opposed to only when we are awake. Noise isn’t an issue.

 

 

 

[Adsibob]

25 minutes ago, TonyT said:

Is tado smart cause you can use a smart phone to control it?

or does it do anything else?

You can control it from your phone, regardless of where you are. We use that feature a lot. It also does geolocation so that the heating on can be overridden to off when you are away. You can set a radius so when you are within a certain distance of your house it will turn back on.

 

 Other smart feature is lots of data gathering in terms of humidity and temperature graphs. There is also an open window/door detection feature which works well in an airtight house. If it senses a draft at the same time as the heating is on it will tell you and ask you if you want to turn the heating off in the relevant zone.

 

 Finally, and this is the only feature that requires a subscription, it can do weather compensation for you, if you pay a £3 monthly fee. We don’t bother with this, though would have been helpful for when the temperature suddenly drops. I might sign up for a couple of months to try this feature out.

[marshian]

4 hours ago, Adsibob said:

So just to understand this a bit better: what is the typical stratification of a cylinder? Is it linear such that the 39 degree delta between the top and the bottom here means that if the cylinder is 156cm tall (39 x4) every 4cm there is a change in temperature of 1 degree?

 

No - that was bath night for SWMBO That was after the bath had been poured

 

The tank I have stratifies quite well - there was about 40 L left in the tank at 57.7 Deg - there will be a slight zone where the temp gratuates  but then everything below that was at 18.6 Deg (cold water tank temp)

 

I've just had to manually do my DHWP as our water usage today was higher than normal (SWMBO left the bath tap on a very slow trickle after the morning cleaning session - ooops) it took the tank down to 22 deg top and bottom

 

36 mins with the boiler shoving between 10 to 12kW in thro the 3 kW coil and the top is at 51 and the bottom 50.

 

Best delta I get heating water is 16 Deg due to the coil being a std 3kW one

[SimonD]

4 hours ago, Adsibob said:

I have a tado controller wired to the boiler,

 

With your tado, where did you get it? And when you bought it did you ensure that it was definitely a version with digital control possible of the boiler?

[Adsibob]

2 hours ago, SimonD said:

 

With your tado, where did you get it? And when you bought it did you ensure that it was definitely a version with digital control possible of the boiler?

I got it from the Tado store. There isn’t much data on its exact specs online, but with all the cable ports it comes with (see second picture, here) I would be surprised if it’s missing much functionality.

[SimonD]

9 hours ago, Adsibob said:

I got it from the Tado store. There isn’t much data on its exact specs online, but with all the cable ports it comes with (see second picture, here) I would be surprised if it’s missing much functionality.

 

Are you aware that this product does not do load compensation with the boiler, i.e. manage flow temperatures to get full benefit of your modulating boiler? It only does relay switching for both hot water and central heating?

 

I'm a bit of a critic of Tado given that in the UK and also from their professional installer's shop that ships from Germany, you can't get a controller that does digital control and modulation for system/heat only boilers (S & Y plan), only for the combi. And if you want modulation via Opentherm E-bus or Km-bus for a combi you pay a hefty premium, £200 versus about £89. I now always spec other controllers because of this. For single zones, as a contoller, the Honeywell T6R-HW is far better because Opentherm is supplied as standard, as is the process of setup compared to the Tado (IMHO).

 

If you want full efficiencies from your boiler and heating setup, I'd recommend you change your controllers. Your heating engineer really should have told you this given the importance you put on modulation.

[SimonD]

17 hours ago, Adsibob said:

I didn’t realise boiler setups were that complicated.

 

Most people don't. When I quote for installs, I take time to try to explain to customers what it takes to install and configure a new system that is as energy efficient as possible. Often they're surprised because none of the other companies quoting even mention anything like this. Just like the heat pump market though, the boiler companies and controller companies don't make things easy. It takes a long time to understanding what's on offer for which make of boiler and how to configure it to work properly and then you get the big names like Tado and Hive, for example, marketing their controllers that apparently produce massive savings but neither do any modulation control in the UK, just relay.

[marshian]

13 minutes ago, SimonD said:

 

Most people don't. When I quote for installs, I take time to try to explain to customers what it takes to install and configure a new system that is as energy efficient as possible. Often they're surprised because none of the other companies quoting even mention anything like this. Just like the heat pump market though, the boiler companies and controller companies don't make things easy. It takes a long time to understanding what's on offer for which make of boiler and how to configure it to work properly and then you get the big names like Tado and Hive, for example, marketing their controllers that apparently produce massive savings but neither do any modulation control in the UK, just relay.

 

Superb Post

 

Nails the problem with both customer expectations (don't want to pay for the extra effort required to set up the system to work at it's most efficient) and poor knowledge of boiler installers (who throw the biggest boiler at the wall and set it to a much higher temp than needed on the grounds that they won't ever get a call back for a house not heating up).

 

My neighbour won't change his flow temp from the boiler ECO setting becuse the gas engineer he uses says that's the best setting for the boiler all year round - I've measured the actual flow temp - it's 68 deg C - his return is 54 deg - it's only condensing in the warm up phase once the house is up to temp and the delta between flow and return narrows it's never going to condense.

[SteamyTea]

7 minutes ago, marshian said:

My neighbour won't change his flow temp from the boiler ECO setting becuse the gas engineer he uses says that's the best setting for the boiler all year round

When he drives a car does he accelerate in the lowest gear possible up to the red line, then hit the brakes, then does it over and over again.

I do the long journey up to Buckinghamshire every couple of weeks, always amazed me, on the motorway, the number of cars that have no traffic in front of them that keep apply the brakes, for no obvious reason.