Hot Water Cylinder Questions

JKenH

1961Nick wrote: »

There are plenty of youtube clips that demonstrate what can happen..

https://www.youtube.com/watch?v=iIm6JXQQ8kw

I haven’t tried one of these but it reminded me of my attempt to warm an aerosol can in boiling water. I ended up with a hole in the kitchen ceiling and a new satin black colour scheme in the kitchen. My wife went off on holiday for 2 weeks while I cleared the mess up. That was just a 300ml can, I can’t imagine the havoc a 200 litre cylinder could wreak.

Just on the insurance point though, unless you have a specific endorsement on your household policy relating to the installation, you probably would still be covered.

Reed_Richards

mmmmikey wrote: »

1. I've been doing some testing and found that in my installation, once the thermal store is up to 75 degrees it will give me sufficient water for normal use, including good hot showers in the evening, without any more heating for 2 days.

2. I can't really quantify this yet, but my instinct is that I'm losing quite a lot through standing heat losses, both as a result of the lagging of the adjacent pipework and controls, and as a consequence of running the cylinder at quite a high temperature. I'm going to continue to experiment a bit, starting by running it at 60 degrees or so for a week or so and see what effect that has. Once I've done that, I'll lag the pipes better and see what difference that makes.

I was looking at the spec. for a Megaflow "Eco Solar PV Ready" cylinder and saw that they give a figure for the heat loss per day (in kWh). Is there something specified for your cylinder?

zeupater

Reed_Richards wrote: »

I was looking at the spec. for a Megaflow "Eco Solar PV Ready" cylinder and saw that they give a figure for the heat loss per day (in kWh). Is there something specified for your cylinder?

Hi

There's an issue with taking standing heat-loss as anything other than a comparative measure.

The figure used is derived from a test procedure requiring the total volume of the cylinder being heated to a standard temperature and allowed to soak at that maintained temperature for a standard time in a room with a maintained standard ambient temperature before a standard period of monitoring the energy required to maintain the content's temperature ... the replacement energy being the standing heatloss.

The problem with this is that the ambient temperature used in the test procedure represents that of the internals of a typical household, not the raised temperature which would be normal within an airing cupboard or small confined space, it also assumes a constant temperature, not a reducing one as heatloss actually occurs ....

Effectively there's lots that can be done to reduce cylinder standing losses through slowing heat transfer into & through the building's structure - ie, insulate the area containing the cylinder ... our own standing losses are well below the nominal data for the cylinder, with that being achieved at a contents temperature above that in the test procedure ...

Anyway, take care of the published data, some manufacturers use kWh/day (energy) whilst others use kW(power), so be prepared to multiply or divide one or other by 24!

HTH
Z

mmmmikey

Thanks both.

Quoted standing heat loss for the cylinder is 1.68kWh per day. In addition to the factors that Z mentions, I would anticipate that the quoted losses only relates to the cylinder and not the adjacent pipes and controls which is where I think I'm losing most of the heat from (although as current regulations require the controls to be provided as part of the unit this would be a bad way of testing, but who knows?)

The most practical thing to do would be simply to add insulation which I will do. I'm playing with the storage temperature first simply because it's easy to do while I focus on other jobs and it all helps build understanding. Turning down the storage temperature reduces heat loss but at the expense of energy storage capacity, so I'll probably end up increasing the temperature again. At this stage it's really about developing an understanding / reconciling theory and practice.

My latest data shows energy consumption of 5.52 kWh/day at a storage temperature of 74 degrees and an estimated use of 60litres/day. This is measured with a plug in monitor on the input side of the immersion controller, so it shouldn't be affected by varying output power, but will include losses in the controller itself. According to my calculations, if I assume 60litres of hot water usage per day and a temperature rise of 35 degrees this should take about 2.5kWh without losses - hence my reason for thinking I'm losing more than I should.

None of this is particularly significant from a cost perspective because most of the time I'm losing "free" solar energy, but it feels like a job half done at the moment. Also, this loss beccomes more significant now I have battery storage as I could be using it to charge the battery in the shoulder months,

zeupater wrote: »

...our own standing losses are well below the nominal data for the cylinder, with that being achieved at a contents temperature above that in the test procedure...

About how hot do you run your store and roughly what are you losing through standing losses? Based on your experience what kind of losses should I be aiming for? I have a notional target in mind of 3kWh per day for water heating, to be achieved primarily through reduced heat loss and (once I've got this right) reducing the shower flow-rate slightly. Does this sound reasonable?

Thanks, Mike

zeupater

mmmmikey wrote: »

Thanks both.

Quoted standing heat loss for the cylinder is 1.68kWh per day. In addition to the factors that Z mentions, I would anticipate that the quoted losses only relates to the cylinder and not the adjacent pipes and controls which is where I think I'm losing most of the heat from (although as current regulations require the controls to be provided as part of the unit this would be a bad way of testing, but who knows?)

The most practical thing to do would be simply to add insulation which I will do. I'm playing with the storage temperature first simply because it's easy to do while I focus on other jobs and it all helps build understanding. Turning down the storage temperature reduces heat loss but at the expense of energy storage capacity, so I'll probably end up increasing the temperature again. At this stage it's really about developing an understanding / reconciling theory and practice.

My latest data shows energy consumption of 5.52 kWh/day at a storage temperature of 74 degrees and an estimated use of 60litres/day. This is measured with a plug in monitor on the input side of the immersion controller, so it shouldn't be affected by varying output power, but will include losses in the controller itself. According to my calculations, if I assume 60litres of hot water usage per day and a temperature rise of 35 degrees this should take about 2.5kWh without losses - hence my reason for thinking I'm losing more than I should.

None of this is particularly significant from a cost perspective because most of the time I'm losing "free" solar energy, but it feels like a job half done at the moment. Also, this loss beccomes more significant now I have battery storage as I could be using it to charge the battery in the shoulder months,



About how hot do you run your store and roughly what are you losing through standing losses? Based on your experience what kind of losses should I be aiming for? I have a notional target in mind of 3kWh per day for water heating, to be achieved primarily through reduced heat loss and (once I've got this right) reducing the shower flow-rate slightly. Does this sound reasonable?

Thanks, Mike

Hi

The issue is that we don't have a thermal store so don't need to maintain high temperatures throughout the water column in order to transfer energy ... our stratification boundary layer simply moves up the cylinder thus reducing the heated volume & therefore heatloss.

To save a few pages of posts, there was a discussion covering this about 3 years ago which may be of interest - have a look at the posts from the link below to post #545 ...

zeupater wrote: »

Hi

Agree ..... If you have the DHW storage capacity to not need to fire up the boiler every day you can hugely impact on the seasonal efficiency. Of course, this not only depends on the volume of DHW, but also the level of insulation and the functionality available on a heating programmable controller (unless DHW is controlled on a manual basis as required).

For example, as short winter days approach our solar thermal system moves from providing all of our DHW, through needing a manual boiler 'top-up' about once/week, through to having two 'burns' a week in December/January .... unless we experience an extended period of consecutive dull days, even in deep winter the thermal system usually provides some heat to the cold fed water at the bottom of the cylinder which, although not enough to heat the DHW to usable temperatures, still reduces the GCH fuel demand ...

HTH
Z

... as you'll see, the test procedure is to maintain a standard 65C ... when we heat the DHW using gas we heat to 60C as anything higher simply extends the boiler heating cycle or requires the boiler Delta-T (thermostatic setpoint) to be raised, with the solar thermal currently being set to mid 70sC (75C before stagnation) - it has been run higher but was reduced in order to help extend mechanical component & system lifespan (as well as failure related safety!) ...

All of the pipework around the cylinder is well insulated as is the cupboard it's in, which used to be an airing cupboard, but now has no room for airing ... (:shhh:, I know, MrsZ wasn't too happy either!) ... but I did run a 48Hr no draw-off heat requirement test some years back which suggested losses at a 75C cut-off setpoint (so 73C/74C real) were considerably lower than those as per the test procedure (from memory around 70%, but can't currently find the spreadsheet to support this!), but in general use over a year we lose <1.5kWh/day, so say a rounded ~500Wh/100litres per day as that accounts for volume differentials ...

HTH
Z

mmmmikey

Thanks Z - that thread makes interesting reading and together with your comments gives me some confidence that I'm heading in the right direction.