PV immersion heater proportional control

[yorkshiremike]

Brian, do you think your 17.6kWh of gas saved daily is a bit optimistic? For example, how many kWh has your Immersun indicated that it's putting into the HWC each day? The reason I'm asking is that I'm wondering how inefficent (in terms of gas energy to energy in hot water) the indirect gas water heating is. Certainly a modern condensing combi boiler can manage 80%+, but it is directly heating the water so I was sort of assuming that a modern condensing system boiler would be making 70%+ after allowing for heat loss in pipes to the HWC etc. But this would still mean that 17.6kWh should be more than enough to fully heat the HWC from cold twice a day. Energy to heat water in Wh at 100% efficiency = 1.16 x number litres x temperature rise, so 130 litres through a 35 deg temp rise = 5.28kWh. The immersion should be close to 100%.

Mike

We've fitted several Immersuns now, though a couple of the customers needed rewiring as they had other stuff connected onto the immersion circuit. I've one fitted at home, but it is only heating a towel rail - we have a combi - currently planning to have a thermal store fitted, unless anyone has a better idea.

 

[sly_dog_jonah]

I went through some very similar calculations yorkshiremike when weighing up whether to invest in an immersun. I think Brian must be basing his calculations on reduced gas usage, not the amount dumped into the immersion by the immersun. But I'll let him answer your question.

Our hot water usage via a modern Gas boiler (non-combi) averages 13/14kWh/day for 1 shallow bath (for a toddler) and 2-3 showers/day. Our dishwasher and washing machine both have cold supplies only. The tank's thermostat (for controlling the boiler) is set to around 45C. Since we have a large tank I'm hopeful that sufficient heat will be retained from good PV days even if they are followed by rubbish days, and negate the need for any gas water heating except for during poor weather spells. However I'm realistic and expect that on most days of the year we'll have to use some gas, but less than we currently do.

 

[brian wright22]

Brian, do you think your 17.6kWh of gas saved daily is a bit optimistic? For example, how many kWh has your Immersun indicated that it's putting into the HWC each day? The reason I'm asking is that I'm wondering how inefficent (in terms of gas energy to energy in hot water) the indirect gas water heating is. Certainly a modern condensing combi boiler can manage 80%+, but it is directly heating the water so I was sort of assuming that a modern condensing system boiler would be making 70%+ after allowing for heat loss in pipes to the HWC etc. But this would still mean that 17.6kWh should be more than enough to fully heat the HWC from cold twice a day. Energy to heat water in Wh at 100% efficiency = 1.16 x number litres x temperature rise, so 130 litres through a 35 deg temp rise = 5.28kWh. The immersion should be close to 100%.

Mike

We've fitted several Immersuns now, though a couple of the customers needed rewiring as they had other stuff connected onto the immersion circuit. I've one fitted at home, but it is only heating a towel rail - we have a combi - currently planning to have a thermal store fitted, unless anyone has a better idea.

Yes it is the amount of Gas in kwh that I am not using by having the Gas central heating boiler on water heating only.This was based on the gas bill from EDF over the period of time that the boiler was not used for the Central heating.
The thing to remember here is that the Gas boiler loses a significant amount of heat through its casing anf the 1 inch pipes that feed the tank indirect coil.It was originally thermo syphon but is now fully pumped but not a condensing boiler.
With the Immersion heater every bit of power goes into heating the water.
From my EDF Gas Bill, the meter is measuring the gas by cubic M, this is then converted to Units and the value equals Kw/h with a calorific value of 38.4. Whereas the electric bill quotes the Units are equal to Kw/h.The units of Gas does not equal units of Electricity
I hope this helps

 

[brian wright22]

I went through some very similar calculations yorkshiremike when weighing up whether to invest in an immersun. I think Brian must be basing his calculations on reduced gas usage, not the amount dumped into the immersion by the immersun. But I'll let him answer your question.

Thanks Sky, you are correct it is based on my reduced gas consumption.

 

[sly_dog_jonah]

Brian, The Immersun can display it's daily/weekly/monthly/lifetime kWh's delivered to the immersion: Main Menu>View Savings then cycle through the time periods). Would be interesting to compare those figures with your Gas savings over the same period (or similar period depending on when your meters were read by you/EDF).

http://www.immersun.co.uk/downloads/immerSUN Installation and User Guide v1.0.pdf Section 5.4

 

[brian wright22]

Brian, The Immersun can display it's daily/weekly/monthly/lifetime kWh's delivered to the immersion: Main Menu>View Savings then cycle through the time periods). Would be interesting to compare those figures with your Gas savings over the same period (or similar period depending on when your meters were read by you/EDF).

http://www.immersun.co.uk/downloads/immerSUN%20Installation%20and%20User%20Guide%20v1.0.pdf Section 5.4

Yes Sly, Its been averaging 3.3kw hours per day for the past 32 days.


By this comparison, either the gas boiler is very inefficient or the calculation of the Immersun is somewhat inaccurate? I must say that using recognised formular to calculate the heat required to raise the amount of water in my tank seems to indicated that I need more kwh's than the Immersun indicates!

 

[pwboston]

I have dual immersion as only source of heating hot water, on E7

so i can easily tell each day what my water took to heat it. currently using between 4 and 8kw per night at 6p per unit to heat my water, so my savings wont be that huge, and its a lot harder to guage what gas or oil would be saved, but this system does give an accurate saving on electric (if i installed one)

 

[pwboston]

Incidentally with this being a fit after an install job does it still come under the 5% VAT?

 

[brian wright22]

Incidentally with this being a fit after an install job does it still come under the 5% VAT?

Very interesting question. Immersun are certainly adding 20% vat!!! via their suppliers. If it does qualify for a 5% vat then they are making a 15% extra profit.

 

[yorkshiremike]

By its self, it doesn't qualify for the 5% rate (see HMRC site):
[h=2][/h][h=2]The reduced rate applies to installations of:

• controls for central heating and hot water systems (see paragraph 2.6);
• draught stripping (see paragraph 2.7);
• insulation (see paragraph 2.8);
• solar panels (see paragraph 2.9);
• wind turbines (see paragraph 2.10);
• water turbines (see paragraph 2.11);
• ground source heat pumps (see paragraph 2.12);
• air source heat pumps (see paragraph 2.13);
• micro combined heat and power units (see paragraph 2.14); and
• wood-fuelled boilers (see paragraph 2.15).

[/h]


If you argue that it is ancillary to fitting PV and the Immersun is fitted at the same time it's possible that it should be 5%. So far, we've just fitted for existing customers who have already had PV, so charged at 20%. If the bill you get shows VAT, then the legally the rate shown is what they have to account to the Tax man at.

 

[brian wright22]

Thanks Yorkshiremike. Retro fit its quite clear 20% vat then.

 

[Gavin A]

Thanks Yorkshiremike. Retro fit its quite clear 20% vat then.

top line - hot water controls.

it's 20% for the equipment only, 5% if professionally installed.

 

[sly_dog_jonah]

My understanding is the same as Gavin A's, 5% if it's installed professionally as it qualifies as a hot water controller.

pwboston, thanks for the your comment about amount of E7 you use. A couple of questions if you don't mind: How big is your tank? What temperature do you heat it to?

 

[yorkshiremike]

Thanks Gavin, I hadn't considered the heating controls paragraph, I guess the Immersun is an 'electronic timer'. On the HMRC site paragraph 2.6 is:

[h=2]2.6 Controls for central heating and hot water systems [/h]Central heating and hot water system controls include manual or electronic timers, thermostats, mechanical or electronic valves, including thermostatic radiator valves.

 

[sly_dog_jonah]

Thanks Gavin, I hadn't considered the heating controls paragraph, I guess the Immersun is an 'electronic timer'. On the HMRC site paragraph 2.6 is:

2.6 Controls for central heating and hot water systems

Central heating and hot water system controls include manual or electronic timers, thermostats, mechanical or electronic valves, including thermostatic radiator valves.

The Immersun is much more than an electronic timer, although that is one of its many functions. It's primary use-case is as a dynamic load controller, optimising diversion of exported power to a local load (usually an immersion) to maximise benefit from microgeneration while remaining grid-independent. Given the finite capacity of any heat storage, the device can't eliminate export, but it can certainly reduce a household's carbon footprint by reducing gas consumption (or electric heating).

On that basis it should certainly qualify for the 5% VAT.