Immersun Perfomance Estimation

sorry, not meanting to hack you off.


It takes 9,5Kw to heat 180l of water by 45deg multiple that by 365day and you cab see you need about 3500kwh to heat hot water in many houses. Not sure how the government got to a 2400 watt figure from as it is not my experience. We personally run our boiler at 55 deg for instance.

Yes but you don't raise the temperature of the whole 180L by 45degC every day! This is where the lower figure comes from.

EWP said:

sorry, not meanting to hack you off.


It takes 9,5Kw to heat 180l of water by 45deg multiple that by 365day and you cab see you need about 3500kwh to heat hot water in many houses. Not sure how the government got to a 2400 watt figure from as it is not my experience. We personally run our boiler at 55 deg for instance.

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The assumptions you are making mis-understand those used in SAP and other calculations of hot water usage.

SAP is based on a usage temperature of 47degC which is on the high side. Most hot water is used for showering. Get in a shower at 47 deg and you will get out again bloody quick! Most people shower between 38 and 42 deg. Anything above this is pretty uncomfortable. Humans are very sensitive to contact heat above body temperature. There is also an inherent assumption that water is heated to 60deg for Legionella disinfection.

Calculation of daily hot water consumption is still open to debate. The calculation in SAP2005 was based on 25litres a person plus a 36litre overhead. Your 180litres is enough by this calculation for 5.76 people. SAP2009 and draft 2012 both use a calculation based on floor area. This came about from a survey that showed the average occupancy of a domestic dwelling is 3.1 people. An algorithm was then built to reflect this. Whilst this may be OK for the general purposes of SAP, it is completely bonkers for use in planning/designing domestic heating systems or renewable heating systems. The Draft of the revision of MIS3001 'Requirements For Contractors Undertaking The Supply, Design, Installation, Set To Work, Commissioning and Handover of Solar Heating Microgeneration Systems' goes some way to address this by allowing use of the SAP2005 formula if average occupancy is known. Practical experience suggests that this still understates usage. I personally work on 40 litres per person with no overhead.

These parameters can then be input into the SAP hot water energy requirement. This is seasonalised to take into account variance in water inlet temperature and usage. Using the parameters you state gives a figure of 3450kWh. However, using the SAP2005 usage formula and the SAP standard temperature gives 2145kWh. Using what I consider more realistic parameters (4 x 40ltrs @ 45deg) gives 2387kWh or to more closely align with SAP (4 x 40ltrs @ 47deg) gives 2523kWh. As such you will see that the given figure of 2400kWh is entirely justified.

This is based on usage and not storage volume. This can make tank sizing a black art when taking into consideration heating to 60deg (required in solar thermal standard), standing loss from tank, and pipe losses. As alluded to the simple act of replacing a tank leads to increased savings due to reduced standing loss.

Blithely selling or fitting an Immersun or other proportional controller without taking in to account the factors outlined in my original post is doing no one any favours. The purpose of this thread was to generate constructive comments on how to better what I had already put together to allow evaluation.

IME usage is very dependent upon whether the system is mains pressure / pumped for showers, or open vented just pressured from the header tank.

If it's from an open vented system then the shower flow rate is probably in the 2-4 l/minute range, whereas mains pressure will be at around 8-12l/minute, so a mains pressure shower will probably use around 3 times as much water as a vented system, and showering is the main hot water use in most houses.


These figures are from memory, but IMO you really ought to factor this in to any calcs on hot water consumption levels as with either solar water heating or immersun you ought to know what system you're working with.

This is partly why I generally advise customers for solar water heating to stick with open vented if they have it already, from an energy saving perspective there's not much will increase your energy consumption faster than switching to main pressure showers... other than actual electric showers.

Wise words Gavin.

It underlines the need to take holistic approach when altering hot water systems. I tend to favour unvented systems due to their many advantages. However, we measure shower flow rates post installation and advise on upgrading to modern mixers and shower heads designed for use with unvented systems to overcome the potential problems you highlight.

If a property already has an vented system, keeping it the same does make installation more straight forward.

As with so many aspects of renewables, the devil is in the detail. To serve customers well, knowledge and experience are paramount. We all see too many examples where at best an installer or sales person has only had their eye on the profit, not the interests of the customer.

Totally impossible to calculate as this will differ from household to household. However, in general you can suggest that if they leave the house empty during the day and dont use any kind of home automation - they would be foolish NOT to have one of these types of devices fitted at the right price of course.

However the more savvy PV owner will have a monitoring device and/or use PVO telling them exactly in data or graph format what their consumption and generation levels are and when - the rest and the resultant decision is IMO a no brainer based on knowing this info.

I would agree, you need to make a lot of assumptions, so I have been careful about publishing anything.

I am just happy to have hot water the vast majority of days for free.

danesol said:

Totally impossible to calculate as this will differ from household to household.

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EWP said:

I would agree, you need to make a lot of assumptions, so I have been careful about publishing anything.

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Welcome to the world of Solar Thermal. System sizing is everything. Too small and the benefits are greatly reduced. Too large and excess heating of auxiliary volume is required when solar gain is unavailable. There have always been approximate measures of usage and effort is constantly made to refine methods of estimation.

Returning to the Immersun, which is where I started, you can be certain someone out there will fit these without due regard to the issues originally mentioned. We have seen too many examples in renewables where the reputation of excellent products can be tarnished by inappropriate installation.

As with PV performance, I prefer to err on the side of caution. This still shows an excellent ROI for the customer. If I have made an oversight I am still OK and if I haven't the system over-delivers, equaling one very happy customer. My approach to the Immersun is the same. This is one of those excellent products where a little forethought ensures complete customer satisfaction. Hence the reason for my original post.

The Intelligent Immersion Ltd model i2 and its prototypes and derivatives have been in service for up to 2 years now. We predated the Immersun. Several have comprehensive internet based monitoring systems on them so we are able to quantify their benefit. We have proved that the statement made by some competitiors that £250 per annum will be saved is quoting the top end of the scale as if it applied to all and I think it discredits us all to quote such figures. We conservatively quote payback periods of 1-4 years. But we do not have any data for systems in the north of England or Scotland so am interested in any data from there.

Hi I was interested in your thread & have daily input monitoring figures for my pv system if you are interested.
Have you found the immersun to be effective as i am thinking of installing one.

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Hi I was interested in your thread & have daily input monitoring figures for my pv system if you are interested.
Have you found the immersun to be effective as i am thinking of installing one.

- - - Updated - - -

Hi I was interested in your thread & have daily input monitoring figures for my pv system if you are interested.
Have you found the immersun to be effective as i am thinking of installing one.

See here for real life examples of it in action (data anonomised)