My 160L Joule cylinder is rated at 79W, which I presume means 79W/h, so 1.896kW per day. I have no idea what temperature (delta) this is at, but obviously the hotter the water in the cylinder (or the larger the dT between cylinder temp and ambient temp), the greater the loss.

We heat our cylinder to 45C each afternoon, and around 18h later (following morning), it has dropped by 6C to around 39C, during which time there has been some light usage (washing up mainly). If the tank temperature were uniform, that would correspond with a 1.12kW loss (to reheat from 39C to 45C), so 24/18 * 1.12kW = 1.49kW including some minimal usage (so your figures seem very reasonable). Our actual loss of ~1.5kW is presumably lower than the manufacturers stated figure as their losses are presumably measured at a higher tank temp (maybe 50C, 55C or 60C)

Besides better insulation, the best way to minimise losses is not to heat the hot water hotter than required, (although legionella may be more of a concern to you if you are not on mains water).

We don't heat our hot water when away, so I do not have comparable data for scenarios with no usage.

Edit: using an online calculator, I see that heating a 150L volume of water from 40C to 48C requires 1.4kW, so you only have 400W of losses if using 1.8kW which seems pretty good.

Posted by: @old_scientist

↑

My 160L Joule cylinder is rated at 79W, which I presume means 79W/h, so 1.896kW per day. 

No it means 79W (power) which is 1896kWh (energy) per day.

Posted by: @jamespa

↑

Posted by: @old_scientist

↑

My 160L Joule cylinder is rated at 79W, which I presume means 79W/h, so 1.896kW per day. 

No it means 79W (power) which is 1896kWh (energy) per day.

Thank you for correcting my units, but you may want to check your own as you seem to be 3 orders of magnitude out.

Posted by: @old_scientist

↑

Posted by: @jamespa

↑

Posted by: @old_scientist

↑

My 160L Joule cylinder is rated at 79W, which I presume means 79W/h, so 1.896kW per day. 

No it means 79W (power) which is 1896kWh (energy) per day.

Thank you for correcting my units, but you may want to check your own as you seem to be 3 orders of magnitude out.

 

You are correct, I missed a very important decimal point.  1.896kWh/day (or 1896Wh/day)

Can the heat easily travel up your pipework?

Obviously copper pipes will conduct heat away from the tank, but pipes which go upwards from the tank connection will allow more heat to convect up through the water in the pipes as well. Can you tell how much pipework is hot/warm when there's been no hot water demand.

The temperature difference between your DHW temp and the surrounding air will definitely be a factor.

Posted by: @davidb

↑

The reheat energy is 1.8kW (not power used).  I know my cylinder is marketed as loosing 1kWhr per 24hrs.  So the extra 0.8 is either through attached pipe losses or the cylinder is loosing more than advertised.

First please check (and share) the cylinder documentation. For example while searching for "Vaillant 150l cylinder" I came up with this VIH RW 150/5 C
SLES which specs 1.4kWh/24h. You have to get the exact reference because heat loss depends on the cylinder dimensions. With the same volume, a tall and skinny cylinder has more surface area, thus more losses, than a short wider cylinder. So it's easy to get confused.

Then, heat loss is proportional to temperature difference between water inside the cylinder and air outside, which raises the question: at which water and ambient temperatures is the heat loss is specified? The documentation should mention this. If it is specified for 85°C water and 20°C air (65°C delta), and you have 45°C water and 20°C air (25°C delta), then your actual heat loss should be in proportion, ie 25/45 or 38% of what the manual says. However if the manufacturer is wildly optimistic and quotes heat loss for 40°C water and 20°C air, and your cylinder is in the attic with 45°C water and 5°C air, then your actual heat loss will be much higher than what the manual says! Again, even more confusing...

Then there's thermosyphon. You can have it in a single pipe, either horizontal or vertical, with two water flows in the pipe, and that will suck the heat out of your cylinder. It's easy to check: if the pipes coming out of the cylinder are hot to the touch further than about 50cm, then you have a thermosyphon problem.

@bobflux thanks for your focused thoughts.  The tank is the Standard Unistor - not the slimline.  The information plate is shown below

The standing heat loss is given as 42W so I assume that is 1008 W per 24hrs which is roughly in line with the sales literature - from memory.  As you say, there is no info on test temperature differences.  Our tank is fully inside the thermal envelope of the house.  Max DHW temperature set at 48 and ambient at 19.

Your point re thermosyphon was unconsidered by me and one I checked.  The pipe from the top of the tank is horizontal before rising to the bathroom etc.  While warm close to the tank connector, the pipe is essentially cold before heading up (about 40cm) - so I would assume no thermosphon problem.  The contractor installed pipe insulation on the flow and return is not optimal and I plan to upgrade it over the summer but can’t think the associated loses losses would account for 800W or so.

Since starting this topic I have wondered about both the accuracy of the Vaillant app quoted heat figures but also when the start and stop on those figures it set.  Maybe others can assist re Vaillant accuracy.  I guess the heat required for my top up includes the energy to get the pump up to full flow temperature but I have no idea how long that takes.  Also, at the end of the DHW cycle, I know the system diverts the higher DHW flow into the heating system.  Maybe that also contributes to energy used but not transferred to the tank but again how much?

Any reflections on any of the above would be really appreciated.  I am trying to get to understand what should happen vs what is happening and might be improved.  I know the £ involved are small but over a lifetime will add up.  Incidentally, on DHW I am getting a COP of roughly 3 but that was while the heating was also on.  I assume that will reduce during periods when the heating is off - something I hadn’t really thought about pre installation.

@davidb Here is a plot typical of what my 200l Vaillant cylinder does.  Up to 20:00 there was some water draw off for dishwashing/hand washing but no shower or bath.  Temperature sensors typically are accurate to about 0.5C but this is generally a more or less fixed offset, thus I would expect relative temperature to be much better than this figure.  

200l of water has a heat capacity of 230Wh/C, so an 80W loss (the spec for mine is probably a bit higher) is roughly 0.4C/h which is very roughly what I am seeing allowing for draw off. I heat to 48C and the loss will likely be speced at perhaps 60C.  In other words its broadly consistent.

Whether you look at energy in (as you appear to have done) or temperature (as I have done), you need to be aware of measurement uncertainties caused by (a) the fact that the tank is stratified, but you measure only at one point and (b) the measurement sensors. 

I would say temperature will suffer less because the tank is likely to cool fairly uniformly and you are comparing readings from the same sensor.  Measuring reheat is rather more suspect because you are comparing two different sensors (flow and return temperature), multiplying it by another (flow rate) to get energy, and reheat will not be anything like uniform (because of convection currents) nor will the tank sensor necessarily represent any kind of average.  Then you have to account for energy to heat the pipework, water in the pipework, and the bits in the ashp itself.

@jamespa thanks James, seeing your graph is similar to what I seem to be experiencing is comforting. I appreciate your sharing the level of detail provided by your software.

in looking at your sample data, can I assume you are on a time of use tariff and hence reheat overnight?  Secondly, unless that night was unusual, why the temperature dropped post midnight with the specific low just before reheat?

Posted by: @davidb

↑

@jamespa thanks James, seeing your graph is similar to what I seem to be experiencing is comforting. I appreciate your sharing the level of detail provided by your software.

in looking at your sample data, can I assume you are on a time of use tariff and hence reheat overnight?  Secondly, unless that night was unusual, why the temperature dropped post midnight with the specific low just before reheat?

Yes I am on a TOU tarrif and yes I reheat at night.

I don't actually know what the explanation is for the dip just before the reheat, but I imagine its an artefact, probably related to convection currents starting up.  Its worth remembering that the bottom of the tank will have cold water in it whilst the top will still be warm, with the temperature sensor being somewhere in between.  In a well designed tank the temperature of the water at the bottom may be only 20C or thereabouts, maybe even colder.  When the convection currents start up its at least plausible that a stream of part-reheated water flows past the sensor causing it to see an initial drop, which is sustained until the heating takes the temperature of the part heated water rises above the temperature that the water at the level of the sensor was prior to the commencement of reheat.

@jamespa thank you - that makes sense.