Immersun Perfomance Estimation

[EWP]

Have finally found the reference to 130w start up threshold on the Immersun. So just how 'all wrong' are the figures I have used?

Ours is effectively Zero. Now that is low.

As for the figures being all wrong, i only remarked on the 130 watts threshold. With that corrected, i would need to look at how you calculated it in detail. It is very hard to get a 100% accurate figure as there are so many variables in each home. For instance an immersion heater often only heats the top of a tank.

I have tried calculations with the SOLiC 200, but you have make a few assumptions and not everyone will agree those are the ones they would use.

We have accurately measured the power put though to the immersion circuit, in our original test installation over a 12 month period and came out with a saving of over £260 a year vs oil, using the SOLiC 200.

 

[Earthstore]

How did you quantify the cost of oil to heat your DHW?

Sorry that should be calculate.

 

[EWP]

By knowing how much energy was put into the tank, you can compare how much oil you would need to do put the same amount of energy into the tank then multiply that by the average cost of oil for the period. Including a 95% average boiler efficiency for the boiler from memory.

 

[Gavin A]

95%?

You should be going into boiler manufacture if you can get that efficiency from an oil boiler

 

[The Solar King]

These calculations with regard to oil all seem a little odd. If oil is 60p/litre and you get 10kWh per litre, thats 6p/kWh. Divided by the seasonal performance factor of a high efficiency condensing oil boiler gives you approx. 7.95p/kWh. A £260.00 annual saving would mean 3270kWh of fuel saved. As this is only going to be a partial contribution to the total hot water heating requirement, was this a hot water tank or a swimming pool? Was it even insulated?

As I mentioned in my original post, the figure given as a hot water requirement comes from draft SAP2012, 2400kWh being not untypical for an average household, based on a daily requirement of around 160 litres at a usage temperature 45deg C. Even getting a 60% contribution from a proportional controller would mean a saving of 1440kWh worth £114.48.

On the same basis a £260.00 saving would mean a hot water heating requirement of 5450kWh and a daily hot water consumption of 320 litres, enough for 8 people. I should add that the 40 litres a day figure is higher than that in SAP2005 (25litres a person plus 36litres overhead) or SAP2009/2012 which uses a totally bonkers formulation based on floor area.

Hot water consumption and hot water cylinder sizing can start to become a black art, on which there is little agreement. There are even calculation differences in different MCS standards depending on technology. This is why objectivity is so important, and the real purpose of me starting this thread. I do get a little hacked off when somebody tries to hijack it for the purpose of promoting their own product.

 

[EWP]

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.

 

[methley]

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

 

[The Solar King]

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.

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.

 

[Gavin A]

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.

 

[The Solar King]

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.