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I use a step down transformer to deliver 1.5 kW into the immersion heater when there is sufficient 'spare' PV power available.
Do you know what voltage the step down transformer is outputting at?

I'm just wondering if it's possible there's been some misunderstanding, and it's actually outputting at 110V, which counter intuitively would only give you 660W of power into the immersion, as explained in this calc I nicked via google.
Ohms Law V=IR and I=P/V
An immersion heater rated at 3kW on a 230V supply will use approx 13A of current. (I=3000/230).

So, using Ohms Law the resistance of the element would be 230/13= approx 18 ohms.

Using the same element on a reduced voltage using Ohms Law again 110/18=approx 6A

P=IV

6 x 110=660 watts. So if you reduce the voltage to 110V the power is reduced to approx 20%, not 50%. So the answer is 0.66kW not 1.5kW.
[credit to unphased]

I'd suspect that at only 110V, and with only 660W of input that the majority of the heat output is then confined to the upper part of the immersion heater, though I'm not entirely sure on that.

If I'm right, then the same would also apply but to a lesser extent if it was stepped down to a voltage that did give 1500W output - ie the heat would be more concentrated in the upper section. I could easily be wrong about that though.
 
Grundfos also do a range of circulation pumps suitable for potable water Grundfos Comfort | Grundfos

A couple of caveats to all of this. Please be aware of the L8 legionella regulations, and their impact on hot water storage.
If you are displacing gas as the energy source, you will have to save at least 2200kWhrs of gas for every £100.00 of cost to break even on what you are doing. (based on 4.5p per kWhr)
 
We're on oil.... Only thing the massive Rayburn does during the summer is drain our oil tank at a rate of knotts to heat the water, we'll make our return in saved oil in under two years. Last summer we turned the oil off and used the paid for leccy at 9.5p (plus VAT) / unit, so ignoring the oil, I'm saving 9.975p/kWh, so need to save 1003 kWh to save £100.

The Solar King, you're right, if you are using a natrual gas condensing boiler to heat your hot water at present, then the savings could take a long time in coming - It's not a panacea for everyone.

http://www.hse.gov.uk/pubns/priced/l8.pdf
 
Clause 158 is the one you need.

This also means you need to have a TMV fitted to the tank outlet to avoid scald risk. On a vented tank this can restrict flow and fitting TMVs at point of use may be an alternative. Best practice would be the latter and would allow compliance with TMV2 which will come in to force for domestic properties in the not too distant future.

There is debate as to the efficacy of clause 158. If sized properly and used regularly, the tank is not a standing body of water in which bacteriological growth can occur for long enough to create a hazard. You can only be infected by inhaling contaminated aerosols. Infection in domestic premises is most likely to occur from a shower that hasn't been used for an extended period. (water sat in the hose loop). If you are not meeting the guidelines of clause 158, always give your tank a hot blast and run your showers before use on return from holiday .

Best advice if fitting the kind of gear discussed here for a customer is to ensure the boiler brings the tank up to temperature every day as per clause 158. If the tank is already at 60deg C, the boiler will not fire. This is also the guidance in CE131, the solar thermal equivalent or DTi 2nd edition. The last thing you want is to be blamed for a Legionella outbreak.

There may be new guidance over tank disinfection in the revised BS5918 (Solar heating systems for domestic hot water – Code of practice) when it is published. It has just been out to consultation.
 
Grundfos also do a range of circulation pumps suitable for potable water Grundfos Comfort | Grundfos

A couple of caveats to all of this. Please be aware of the L8 legionella regulations, and their impact on hot water storage.
If you are displacing gas as the energy source, you will have to save at least 2200kWhrs of gas for every £100.00 of cost to break even on what you are doing. (based on 4.5p per kWhr)
why do they need to break even in a single years usage on equipment that should be in place for 10-20 years plus?

L8 - well, sort of. L8 regs only fully apply to units in a workplace environment, and are mainly (but not exclusively) intended for hotels, hospitals etc with very long pipe runs, rather than domestic set ups. Not that I'm saying that legionella risk can be ignored, and if you do apply L8 regulations to domestic set ups then they'll obviously be protected, but it's overkill in most circumstances and not required by the regulations.

As confirmed by this paragraph in the overview of the latest changes to L8 from HSE

Domestic systems can present a risk, depending on the circumstances of use, but the ACoP will only apply to systems from which the risk arises in relation to any work activity.
[hse]

Interesting to note that they've actually removed the 300l limit that previously applied to L8 though, I wasn't aware of that until I just read this document. That potentially brings a whole array of hot water systems within the L8 scope that previously weren't.
 
Problem is L8 is referenced in standards relating to domestic installation. You have exactly the same situation of under heating a tank due to insufficient input using PV as you do in a normal ST system. If there wasn't a risk the guidance for ST would not be given. Whilst the risk may be small it is the same in both circumstances. I would consider it imprudent not to take this approach until there is clarification in BS5918 which will replace CE131.

There are other advantages in storing at higher temperature. You do not need as large a tank to meet daily hot water requirements at normal usage temperatures. To get the full benefit of any kind of solar input, ideally you only want auxiliary heating input once a day. On days with good solar gain, solar input will bring the tank up to 60deg C without auxiliary input (at least it will with ST) giving a greater energy input than with lower storage temperatures. Sized correctly, the auxiliary energy input is the same for a smaller tank heated to 60deg as it is for a larger tank heated to 45deg.

I appreciate that all nuances of system design cannot be taken into account when dumping excess PV energy into a tank, but feel it rash to ignore what is already established as best practice.
 
Immersion heaters are constructed from resistance wire running their whole length, so regardless of the input power, they deposit heat uniformly over their whole length.

I have a 170V custom transformer which deposits 1.5kW into the heater.

Note that the UK mains is 240Vrms, irrespective of what anyone tells you. Put a voltmeter on if you do not believe me.
 
Don't really see what difference it will makes myself as long as its not touching the bottom/sides/cylinder itself, happy to be proved otherwise though
 
I imagine the manufacturers have put the label there because a 36 inch heater would touch either the sides, bottom, or more likely the indirect coil.
 

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