1KW Immersion Heater

[Orty]

I know this subject has been covered many times but my simple question is:
We use an oil fed boiler for central heating to our home and for hot water.
During spring and summer we usually switch off the central heating and just use the oil boiler for supplying our hot water. Usually fired up for a few hours in the morning and in the evening.

We have a 4kw PV system installed and I was thinking about replacing a 3Kw immersion with a 1Kw element and using this for our hot water requirements.
In the past we have only used the immersion heater for emegencies but with the price of oil as it is, I am thinking of using the immersion on a more regular basis.
Question is
"Will a 1Kw elelment cope with heating a full tank to a usable temperature."
I have fitted a timer to the immersion heater to switch on around mid day when the panels are most active.
Or later I may consider using the multi function relay on the SMA 4000TL to do this automatically.
All comments appreciated.
Orty

 

[sharpener]

Question is
"Will a 1Kw elelment cope with heating a full tank to a usable temperature."

Probably not. Measure how long it takes to heat the tank from cold with the present 3kW element and then multiply by 3. For a normal size tank this will probably give an answer in the region of 10 hours. So with typical usage of at least two showers per day there will not be sunshine for long enough even at the height of summer for a 1kW element to re-heat a full tank. Hence the Parsons switch and its imitators will not save you much money.

I too have oil (with an ultra-efficient condensing boiler) and a recent calculation showed the price of heating water with oil or off-peak electricity to be almost exactly the same.

You also need to be careful that you do not import electricity at peak rates via your timeswitch, at this time of year you will be lucky if you ever see 3kW out of a 4kW PV system (mine is in Devon and faces due south; the output just reached 3kW for 10 mins in the middle of 19th February and has not been that high since). As in the main thread on this subject you really need to have a proportional controller which will feed everything available into a 3kW element.

 

[Gary K]

I was also considering a 1kw heater.
My thoughts are it may not heat a tank from cold but every little helps. If you can prewarm the water,you normal water heater will not take as long to bring up to normal temperature.
A 1 kw immersion heater will cost about £65 + whatever devise you decide to use to direct power to the element when available (cheapest I have found £35).
How long will it take to recoup this outlay?

 

[liquidity]

If my maths is correct 1 kWh will heat 859 litres of water by 1 degree C. Or to put in in terms of a normal hot water tank of 150 litres that 1 kWh will heat the contents by 5.7 degrees C. So if your tank is at 40 deg C after the morning’s use you need 4-5 hours of electricity at 1 kWh to heat it back up to 60 deg C. Note this doesn’t take into account any heat loss from the tank.

 

[sharpener]

Duplicate posting deleted

 

[sharpener]

Thanks liquidity, that's pretty consistent with my estimate of maybe 10hrs from cold.

For my Ariston Primo 150 l pre-insulated pressurised tank they quote a reheat time of 175mins for a delta T of 50C @ 3kW, which implies 3.5 hours for 20 deg @ 1kW so pretty much agrees with your calcs.

They also give the heat loss at 60C as 2.2 kWh per 24 hours (i.e. just under 100W continuously).

So when the house is occupied I use the timeswitch to make sure (at off-peak rates!) that the tank is hot first thing in the morning, then after two showers the proportional controller can re-heat it to the extent permitted by the solar PV output. If it is a sunny day we can run the dishwasher as well and it automatically turns the immersion heater down/off to suit. When we are away we leave it just on solar, at this time of year we have about a 50% chance of more than 2kWh on any given day and of course the tank temperature does not decay completely in 24 hours, so there is always some hot water when we get back.

Incidentally there is usually some stratification in a hot water tank i.e. the water from the element rises to the top without a great deal of mixing so you have the benefit of a smaller quantity of hot water rather than a large quantity of tepid water. The same re-heat calculations apply in principle whether or not mixing actually takes place.

 

[Orty]

Interesting views from everybody here.
As some of you mention, I would not always be heating the tank from cold, merely topping it up after initial showers etc.
The decision is now wether to pull out the 3Kw element and replace it with a 1Kw or stay with the original 3Kw?

 

[langstroth2]

I guess the other point is that even heating a tank from say 25C to 35C will mean less heating that the oil boiler has to do after sunset - so you'll still save something even if you still need to leave the oil boiler on.

We use water solar panels (as well as PV). I tend to find that anything above 45C is actually warm enough for most uses**. HW tanks stratify quite significantly and quickly, particular if the heat source is near the top; so it's usual to have 15C water in the bottom third and 60C water in the top third, with a steep temperature gradient in between.

**I've seen concerns expressed regarding legionella if a tank isn't heated to 60C everyday. Personally I don't consider it a risk to be concerned about - providing that the HW tank water is completely changed each day anyway thru natural use. But you need to make your own decision on that one.

 

[Orty]

Interesting views from everybody here.
As some of you mention, I would not always be heating the tank from cold, merely topping it up after initial showers etc.
The decision is now wether to pull out the 3Kw element and replace it with a 1Kw or stay with the original 3Kw?

Another way that has just come to mind would be to leave the 3Kw element in place and fit a changeover switch in circuit with a 100v transformer.
This would give me the choice of high or low settings, best of both!
Low for when the panels are generating and a high setting for emergency use if caught short without hot water.
What's your thoughts?

 

[TechyTim]

Hi,
As "liquidity" says a 1kW immersion will take about 4 hours or so to heat a reasonably sized tank.
What you need is my SolarHeat PV controller which monitors the PV output and then switches on the immersion heater circuit when the output exceeds 1.5kW.
Yours for £205 plus VAT.

Tim Smith
SolarHeat Controls
01202 625070

 

[Orty]

Hi,
As "liquidity" says a 1kW immersion will take about 4 hours or so to heat a reasonably sized tank.
What you need is my SolarHeat PV controller which monitors the PV output and then switches on the immersion heater circuit when the output exceeds 1.5kW.
Yours for £205 plus VAT.

Tim Smith
SolarHeat Controls
01202 625070

I thought the multifunction relay in the Sunnyboy 4000TL could do that!

 

[thushar]

Both 1 kW / 1.5 kW immersion heater and solar immersion heat controller now available on ebay ! Business Building Kits items in Fair Trade Hub Shop store on eBay!

 

[TechyTim]

The SunnyBoy relay can't cope with 13A 240Volt switching - it is designed for volt free switching of mA currents, i.e. an external contactor or monitoring device.

The E-bay switching unit is cheap and cheerful, but probably has no hysteresis or delay on switching so likely to burn out with rapid contactor chatter.

:nonod:

 

[Orty]

The SunnyBoy relay can't cope with 13A 240Volt switching - it is designed for volt free switching of mA currents, i.e. an external contactor or monitoring device.

The E-bay switching unit is cheap and cheerful, but probably has no hysteresis or delay on switching so likely to burn out with rapid contactor chatter.

:nonod:

If I am correct, the Sunny Boy software can be set up to cope with eratic switching of a contactor

Orty

 

[Glen Powell]

The big problem is you lower your voltage, will that not increase your Ampage required, in turn will that not potentially blow your stat controller.

The way a solar cylinder should work, if designed correctly is to take a temperature reading from the bottom of the cylinder and feed it back to the control stat, however I am taking it you do not have solar water heating.

To do this economically as electricity is more expensive than gas (at present) it may be worth looking at some form of relay system, some will say this is an expensive way of doing things, however it depends on the person designing the system. A set light dependent diode can switch a 2 port valve and disable electricity from going to the boiler, with another relay attached to a timer you could then bypass that circuit to ensure you have hot water when you need it.

For example simple programmable stat £50 attach that to an NC Relay, attach that relay to an NC and NO relay, you will need a few more bits and bobs in between but I am sure you get the picture.

 

[Glen Powell]

If I am correct, the Sunny Boy software can be set up to cope with eratic switching of a contactor

Orty

Contactors or known for electrical feedback on electricalequipment and that is why snubbers are needed in a lot of cases wherecontactors are installed, be aware 18.4kw is required to heat a 210 litre tank from 5 degrees to 65 degrees, so the volume of water you require will determin the heating mechanism (my calcs are based on an unvented cylinder with heat loss of 1.8kw per 24 hour period, usual ambient temp, ext temp, altitude, etc)

 

[EXC]

I have a 'Parson Switch' with a single 1kW immersion heater element (designed for boats I think) installed in my 200L heat store ( linked to a wood burning stove) which pre heats the water before going through the combi boiler .
I have found this is enough to keep the tank heated to 70deg C on a sunny day.

 

[thushar]

Interesting views from everybody here.
As some of you mention, I would not always be heating the tank from cold, merely topping it up after initial showers etc.
The decision is now wether to pull out the 3Kw element and replace it with a 1Kw or stay with the original 3Kw?

You could also use a 3 kW power reducer (small & compact, with no transformer) available from http://solarimmersionheaterswitch.co.uk . It reduces the existing 3 kW immersion heater to a 1.5 kW. This enables you to use the existing immersion heater for solar heating without changing it. Help to save the plumbers cost as well.

I have been using this power reducer together with the automatic solar switch available from the same site for the last few months and it's working perfectly.

 

[EXC]

Hi thusbar
I like the idea of it being adjustable ,cheap too. Wish I had seen this before getting a Parsons switch. The reducer is a good idea as well.

 

[thushar]

Hi thusbar
I like the idea of it being adjustable ,cheap too. Wish I had seen this before getting a Parsons switch. The reducer is a good idea as well.

Cost matters for better ROI. Looks like the switch from http://solarimmersionheaterswitch.co.uk cost less than £150 and the 3 kW immersion power reducer make the thing simple to install as it uses the existing 3 kW or 2 kW immersion - Saving you the plumbers cost and the trouble of changing the existing immersion.