Informational content provided by Jamie at  Call on 07897 246779.

I was going to provide a link to the best definition of watts density available on the web, but they are in very short supply, so here goes.

Watts density is the amount of heating energy emanating from any given amount of surface area of the hot part of an element.

In Imperial Britain and the States, this is usually described in terms of watts per square inch, but could be expressed (to keep the Eurocrats happy) as watts per square cm, mm, or indeed, meter. But why bother?

For 8mm diameter elements, simply divide the heating power output (in watts) of the given element by the length of hot section, ie. subtracting the cold ends at either end of the element, which can be 2″, 4″, 6″ or any custom length.

So, for the benefit of a simple example, if we have a 1kW, 54″ element with 2″ cold sections, we first take 4″ (2 ends of 2″) of cold off the total length of the element to give our hot section, 50″. Then we divide the power output of 1000w by 50″ to give us a watts density of 20w/in².

Now imagine we have a 1kW, 104″ element with 2″ cold ends (c/e), giving a hot length of 100″, do the maths as above, and you should get a watts density of 10w/in². Now, picture an inch square in your mind, or draw it if you prefer. Now, fill the box with 10 or 20 little boxes, happy faces, “watt monsters”, whatever crumbles your cookie. The size of the square doesn’t change between the 2 examples, but the size of the contents do, more watts per square inch means more squashed, little watts trying to get out of the same space. Now draw a box, mentally or on paper, and squeeze 40 of your little watt icons in that box. While your at it, have think about how long a 1kW element with 2″ c/e, would need to be to give a watts density of 40w/in²?

For the less imaginative among you, somebody kind drew this –

Watt Inch

Different watts densities are requires for different applications. Some are set in stone, for obvious reasons, such as oil heaters needing to be no more than 12w/in², others are a little more flexible and open to “interpretation”. ElementYoda always told me 5w/in² for elements in still air, but as can be seen below, grill elements can work at up to 42w/in². Fortunately, we don’t supply many of them, cos there wouldn’t be much of a guarantee at that top end. We are mainly suppliers of immersion heaters for water, with soft water being fine between 50-75w/in², but hard water being better suited to 40w/in² or below.

Our (Element Air) EA Range is rated at 27w/in², designed for forced air heating units, where the airflow needs to be 2m³/s. The simple way this is achieved is that every meter of element equates to 1kW, so if you want 3kW you have to bend/coil a 3m element into your available space.

Below is a detailed table of watts density in inches and equivalent cm, plus details of colour change and suitable applications.

W/in2 W/cm2  
94 14.5 Immersed only
83 12.9 Immersed only
73 11.3 Immersed only
63 9.7 Immersed / High Arflow / Machined Fit
52 8.1 Immersed / High Arflow / Machined Fit
42 6.5 Equivalent to Grill / Radiant / Orange
31 4.8 Glow Red
21 3.2 Colour Change
10 1.6 Little or no Colour Change Still Air
9 1.5 Little or no Colour Change Still Air
8 13.0 Little or no Colour Change Still Air
7 1.1 Black Heat
6 1.0 Black Heat
5 0.8 Black Heat
4 0.6 Black Heat
3 0.5 Black Heat
2 0.3 Black Heat

And for the visual learners among you, here is that same info in colourful form that I “borrowed” from a friend –

Colour Change Chart

To convert to other element diameters, please divide your solution as above by the following factors.  If you think of the 1kW, 54″ element with 2″ cold sections that we started with, only this one is 12mm in diameter, share our 20w/in² initial answer by the factor of 1.484, gives a watts density of 13.48w/in², which makes sense as a wider element will have more surface area for the 1000w to “escape” from.

8 0.99
8.5 1.05
9.5 1.175
10 1.24
11 1.36
11.5 1.42
12 1.484
12.7 1.57
13.4 1.657
16 1.98
20 2.474
50.8 6.28

When you understand all this, and realise that glowing ain’t good, you can see the sense in squeezing as much element into the space available, cos simply put, the lower the watt density, the longer it’ll last, whatever it is heating.

Thanks for visiting, please leave a comment.

Informational content provided by Jamie at ImmersionHeaters.UK or 07897 246779.


Click HERE for our online water heat up time calculation page. Or here.

At ImmersionHeaters.UK, (Call Jamie on 07897 246779) we have 2 types of customer.  There are those who tell us what they want as they know their exact requirements.

Then there are others who need guidance and advice as to what they need.  One question which comes up time and again is “How many kW do I need to heat up my tank?”

If we can calculate the volume of water and the required temperature rise, we can answer this question.

The following formula is used to calculate the power of heating element needed to heat a specific volume of water by a given temperature rise in 1 hour.

volume in litres x 4  x temperature rise in degrees centigrade / 3412

(4 being a factor and 3412 being a given constant)

for example 100 litres of water, to be heated from 20ºC to 50ºC, giving a temperature rise of 30ºC would give –

100 x 4 x 30 / 3412 = 3.52

meaning that the water would be heated in 1 hour by 3.5kW of applied heat.

Also we can use this information to extrapolate both ways.  To heat the same water volume in half the time (30 minutes) would need twice the heating power, ie, 7kW.

Converesely, if we only use half the heating power, 1.75kW, it will take twice as long to heat up to desired temperature, ie, 2 hours.

If we only have a 1kW element available, we will expect a heat up time in excess of 3 hours.

Also we can use this formula as the basis of similar calculations for heating oil.  Generally, oil heats up in half the time of water, due to its density.  However, oil needs a much lower watts density element than water, as described in the “How to choose an oil heater” article.

As a supplementary to the page “How to calculate the kW required to heat a volume of water in a particular time

which requires you to know your volume of liquid to be heated, I thought it’d be useful to share how to do that.

In a rectangular vessel it’s just a matter of multiplying length x depth x height.

And this is where Metric comes into its own over Imperial, as lengths mm/cm/m equate to volumes ml/litres/Hectoliters.

Due to talking to customers about IBC’s, I know a 1m cube of water is 1000 litres and weighs 1000kg or a metric ton.

So, I figure a 10cm cube is a liter. And there are 1000 of those 10cm³ in a 1m³.

So, if we take our measurements in cm, multiply the three together, then divide by 1000, it should work.

100 x 100 x100     =  1.000,000     =   1000 litres.

1000                         1000

And it does. And I shall add an automated calculation to our Heating Calculations website. Til then, use this.

Now for a cylinder. We need the area of the circle (π × radius²) multiplied by the height.

Pi is 3.142 & is mathematical magic. Radius is the centre of the clock to the circle outer.

Let’s keep it simple and use a radius of 1m, (so a 2m diameter cylinder), and keep height as 1m, and hope for a larger volume than 1000 litres.

Again, if we take our measurements in cm then divide by 1000, it should work.

3.142 x 100² x 100   = 3142 litres.


And it does. And I shall add an automated calculation to our Heating Calculations website. Til then, use this.

Any suggestions contact Jamie on 07897 246 779 or email

Thanks for visiting.

For full details, options and buying information, click here.

Sighted as regularly as rocking horse do-doos, a 1.75″BSP immersion heater has a circumference of 168 mm and a diameter of 51mm, thus it has options element-wise.

If only single phase is required, you could employ 2 looped elements and one stat pocket. Two elements commoned up at both ends, job done.

If 3 Phase is required, needing 3 elements, then the 3 elements would only be U bent. It is not possible to loop the 3 elements to reduce the watt density, as the resulting element bundle would not fit thru the hole. On the plus side, this arrangement would enable 2 stat pockets to be fitted, if needed.

If you are unfortunate enuf to be tied to 1.75″BSP, call Jamie on 07897 246 779 to discuss your requirements.

Shop for Industrial Immersion Heaters

Close, but no cigar, 2″BSP is a good enuf screwplug size (circumference of 187 mm and a diameter of 57mm) in that it is not limited as to number or looping of elements, as are it’s smaller brethren, but due to it’s scarcity, you are unlikely to find it on the shelf, in stock, for quick delivery.

My best suggestion is to be very accurate in your G##gle search term, ie, ‘2″bsp 3kw 20″ single phase immersion heater’ and try and find some cheap, eastern manufactured, one element tat, for about £30, and cross your fingers.

Problem is, as with typing that previous sentence, the inch symbol confuses G##gle with the “IT MUST HAVE THESE WORDS EXACTLY IN THE BLOODY SEARCH RESULTS” speech marks, which doesn’t help.

If you don’t want to go the disposable route, and are happy to part with a coupla hundred quid, and wait a coupla weeks for bespoke heaters, full details, specification & purchasing advice are available at Immersion Heaters UK. Call Jamie on 07897 246 779.

Needing a 2.5BSP Immersion Heater is highly unusual & rather unfortunate circumstance to find yourself in. My condolances.

There is no benefit to the extra size (circumference of  236 mm and a diameter of 72mm), over & above 2.25″BSP, which is the Industrial standard here in the UK.

In my experience, nobody makes 2.5″ heaters, cos they are as rare as hens teeth. By all means, let me know if you find some, & I will share on here.

Instead it is normal procedure to take a standard, stock 2.25″BSP heater, and screw one of these onto it…

Full details, specifications, buying advice, etc, can be found here, so Call Jamie on 07897 246 779 for a consultation.

For full details, options and buying information, click here.

As rarely sighted as rocking horse poo, a 1.25″BSP immersion heater has a circumference of 131 mm and a diameter of 42mm, thus it is only possible to fit 3 x 8mm U bent elements and 1 stat pockets onto the screwplug, and thus it could be a Single or Three Phase immersion heater, 1-27kW. It is not possible to loop the elements to reduce the watt density, as the resulting element bundle would not fit thru the hole.

If you are unfortunate enuf to be tied to 1.25″BSP, call Jamie on 07897 246 779 to discuss your requirements.

Shop for 1″BSP Immersion Heaters.

The smallest screwplug immersion heater I have ever sold, and as rare as a pile of Rocking Horse Sh1t, a 1″BSP Immersion Heater comes with some serious limitations

With a circumference of  104 mm and a diameter of 33mm, it is only possible to fit 2 x 8mm U bent elements and 1 stat pocket onto the screwplug, and thus it would have to be a Single Phase immersion heater.

Full details can be found here, and if you wish to discuss a design & purchase of a bespoke item, look here, then call Jamie on 07897 246 779.


Shop for 1.5″BSP Industrial Immersion Heaters

The domestic standard screwplug size here in the UK, 1.5″BSP Immersion Heaters are not our stock in trade, 2.25″BSP heaters are, but we can do 1.5″ to your exact specifications in a coupla weeks leadtime.

With a circumference of  150 mm and a diameter of 48mm, it is only possible to fit 3 x 8mm U bent elements and 2 stat pockets onto the screwplug, and thus it could be a Single or Three Phase immersion heater. It is not possible to loop the elements to reduce the watt density, as the resulting element bundle would not fit thru the hole.

With these inbuilt limitations, and a host of folks selling cheap, one element, massive watt density heaters online for peanuts, I often advise potential customers to go buy something cheap, not cos I want to live in a disposable society, but my stuff is heavy industrial gear which is not going to come cheap. It could cost 10 times as much as some of the heaters I have seen, but I cannot in all good faith tell you it will definitively last 10 times as long. Particularly when people are proving a theory or application, do it on the cheap, I would, cos I’m half Scottish, Half Jewish & half Yorkshireman. But live in Sunny Tammuff.

Full details and buying guide can be found on our webpage here.

Call Jamie on 07897 246 779 to discuss yout 1.5″BSP needs, so we can co-design the best heater for your application.


Starting with not necessarily the biggest, but certainly the best & most common, 2.25″BSP Immersion Heaters are our stock in trade. As the industrial standard in the UK, most people want 2.25″ so that is what we keep on the shelf. As 2.25″ accommodates 3 elements, and thus can be Single or Three Phase, stock is basically the 3 times table, 3, 6, 9, 12, 18, 24 & even a 27 if needs must.

Shop for 2.25″BSP Industrial Immersion Heaters

Full details and purchasing options can be found here, but basically…

With a circumference of  206 mm and a diameter of 65mm, it is possible to fit 3 x 8mm LOOPED elements and 2 stat pockets onto the screwplug, and thus it could be a Single or Three Phase immersion heater.

  • Available at any kW loading from 1000w up to 27kW.
  • 2.25″”BSP available in brass and Stainless Steel 316.
  • Available with 2 thermowell at 7, 11 or 18″ for thermostats / cutouts.
  • 2 M25 cable entry glands as an option.
  • Available with no thermowell if specified by the client. If you don’t need it, omit it from the build, as it is an unecessary weak point.

Informational content provided by Jamie at Immersion Heaters UK Ltd. Let’s Collaborate.

Use 2.25″BSP weld bosses, as it is the industrial standard and more available off the shelf.

If you are one of that rarest of breeds, the Rich Brewer, you can go for DN80 or even DN125 hygenic connections, which allow you to remove the heater fairly quickly and easily for cleaning purposes. BlogPost to follow when time & life allows, but in the meantime, have a shufti here – Removeable Brewery Heaters

Heat rises, put the entry ports low, then it’ll agitate itself to a degree.

BONUS ITEM – if heaters are low in the vessel, you can switch them on as soon as they are covered with water, heating a small volume of water, which has gotta be a good idea and save a fair chunka time, no?

Don’t position the entry ports on the same level, as this will cause elements to clash in middle when inserted.

How not to do it – (2.5″, in the belly not the bottom, clashing like jousters)

A better job –

Any suggestions, input, constructive criticism, etc, greatly appreciated.

Let’s Collaborate.

Thanks for visiting.

I started a post about my BI18 Brewery Immersion Heater, which rapidly twisted to being about “The Holy Grail of Brewing – Easy Clean Immersion Heaters”, so I shall replicate here in a seperate post, in the hopes of finding more collaborators via serendipity & G##gle’s algorithms.

This is something I have been playing at and toying with for years, which now deserves some serious focus and attention. If we can work together to improve the unfortunate end to the otherwise beautiful brewing day, life will be better, brewers will be happier, hangovers will be shorter, tax will be lower, politicians will be honesterer…..

Sorry, I went off on one then!



Here is my friend Alex from the Green Duck Brewery in Sunny Stourbridge using one for a guitar solo. Worry not, it’s not plugged in! Mind you, neither is he.

Having spent a little time with Alex & Nathan brewing, trying to learn as much as I can about the process to be better able to understand & problem solve, two things struck me.

Firstly, banging heavy rock music is a subtly essential part of this process.

Secondly, the worst part of the day is the last part of the process, cleaning that awful grey sludge off the elements, in a process that I think Nathan refered to as “Winking the Elephant” whatever that means?

This involves some serious welly trouser type things, 3M Scotch green scouring pads & wire wool, and an awful lot of elbow grease & swearing. To have that “Turd in the Swimming Pool” of a job hanging over your head all day, somewhat spoils an otherwise pleasurable, if damp, day.

So I have made it my mission, my Holy Grail, to make this better, to remove said turd from the horizon. I’m not clever enuf on my own, so let’s Collaborate. Please call or email me with mad ideas from other areas of industry or applications, share ideas you have tried that failed, & why, or just to tell me how you do the cleaning process. I have spoken to Brewers cleaning with acids, lemon juice, caustics (FFS, NEVER USE CHLORINATED CAUSTIC! Nameless Brewery did, shiny new heater very dead inside a month), but surely the prize for Best in Show thus far, albeit with an expensive DN80 removeable Heater, goes to Nial Fitzgerald at Stone Barrel Brewing in the Emerald Isle for this inspired creation of purest beauty…

My Collaborator In Chief, Dave, and I had visions of shiny stainless steel tubes, munsen ringed to a wall, sold for huge sums of money to that rarest of breeds, the rich & shiny obsessed brewer. And Niall goes and achieves the same easy cleaning with a bit of drain pipe & some gaffer tape! I love it! Basically, the idea is to take the heater out of the vessel, (intermittently or every brew is question 1), soak it overnight in ???, then jetwash/steamclean the bugger off in the morning. Hopefully with nonchalant ease.

My first question to anybody brave enuf to read this far is, what precludes the same thing from being done with a screw in (considerably cheaper) heater such as my BI18 fan favourite, with which this ramble started, way back when?

If it is the faff of disconnecting power, could one of these not be fitted?

The only other idea currently percolating around my head is this…

A heater with elements bent in such a way as to allow flexing for insertion/removal, supplemented by possibly a cleaning block with the correct sized holes to clamp a scouring pad around the 2 legs of the element. To facilitate easier winking 😉

Alex the Guitarist, above, is a willing volunteer & test pilot, but many heads make light work, and I want to build Win-Win relationships with as many folks, particularly brewers, as possible. So, gissa call on 07897 246 779. Cheers, Jamie.