Archives for posts with tag: immersion heater

An immersion heater is, simply put, a number of elements, usually 3, elements, a screwplug and a IP rated terminal box to protect the connections.


3 of these (preferablynot chopped up) and

screwplug sizes

…one of these, drilled through, for the elements to be braised or welded into…

Terminal box

…and one of these to attatch to the screwplug, to house the terminals and wiring.

And, in the best Blue Peter traditions, here’s
one we made earlier…

 Immersion Heater

PII Industrial Immersion Heater

The heater is then screwed throught the side of the tank using a weld boss…

weld boss

Mild steel weld boss

Mild steel tanks require mild steel weld bosses, and likewise, stainless steel tanks require horribly priced stainless steel weld bosses. I’ll tell you what brass bosses are used for when I find out.

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?

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”. The gaffer 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 PEA Range is 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

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

various screwplug sizes

1.25″BSP, 1.75″BSP, 2″”BSP, 2.25″ SCREWPLUGS

Examples of some of the various screwplug sizes available, these are undrilled, therefore can be used as a blanking plate or for our customers who are unsure of their requirements, to screw into their socket before anything is manufactured bespoke. Very useful!

This is the simple version,

1/2 .825
5/8 .902
3/4 1.041
7/8 1.189
1 1.309
1 & 1/4 1.650
1 & 1/2 1.882
1 & 3/4 2.116
2 2.347
2 &1/4 2.537
2 & 1/2 2.95

…and this is the complicated version,

Thread Form Type Major Diameter mm d=D Pitch mm p Threads per inch tpi Pitch Diameter mm d2=D2 Minor Diameter Male Thd. d3 Thread Height H1 Tap Drill Diameter mm
1/8 “ BSPP/BSPF 9.728 0.907 28 9.147 8.566 0.581 8.7
1/4 “ BSPP/BSPF 13.157 1.337 19 12.301 11.445 0.856 11.6
3/8 “ BSPP/BSPF 16.662 1.337 19 15.806 14.95 0.856 15
1/2 “ BSPP/BSPF 20.955 1.814 14 19.793 18.631 1.162 19
5/8 “ BSPP/BSPF 22.911 1.814 14 21.749 20.587 1.162 20.75
3/4 “ BSPP/BSPF 26.441 1.814 14 25.279 24.117 1.162 24.5
7/8 “ BSPP/BSPF 30.201 1.814 14 29.039 27.877 1.162 28
1 “ BSPP/BSPF 33.249 2.309 11 31.77 30.291 1.479 30.5
1 1/8 “ BSPP/BSPF 37.897 2.309 11 36.418 34.939 1.479 35
1 1/4 “ BSPP/BSPF 41.91 2.309 11 40.431 38.952 1.479 39.5
1 3/8 “ BSPP/BSPF 44.323 2.309 11 42.844 41.365 1.479 41.5
1 1/2 “ BSPP/BSPF 47.803 2.309 11 46.324 44.845 1.479 45
1 3/4 “ BSPP/BSPF 53.746 2.309 11 52.267 50.788 1.479 51
2 “ BSPP/BSPF 59.614 2.309 11 58.135 56.656 1.479 57
2 1/4 “ BSPP/BSPF 65.71 2.309 11 64.231 62.752 1.479 63
2 1/2 “ BSPP/BSPF 75.184 2.309 11 73.705 72.226 1.479 72.5
2 3/4 “ BSPP/BSPF 81.534 2.309 11 80.055 78.576 1.479 79
3 “ BSPP/BSPF 87.884 2.309 11 86.405 84.926 1.479 85.5
3 1/4 “ BSPP/BSPF 93.98 2.309 11 92.501 91.022 1.479 91
3 1/2 “ BSPP/BSPF 100.33 2.309 11 98.351 97.372 1.479 97.75
3 3/4 “ BSPP/BSPF 106.68 2.309 11 105.201 103.722 1.479 104
4 “ BSPP/BSPF 113.03 2.309 11 111.55 110.072 1.479 110.5
4 1/2 “ BSPP/BSPF 125.73 2.309 11 124.251 122.772 1.479 123
5 “ BSPP/BSPF 138.43 2.309 11 136.951 135.472 1.479 136
5 1/2 “ BSPP/BSPF 151.13 2.309 11 149.651 148.172 1.479 148.5
6 “ BSPP/BSPF 163.83 2.309 11 162.351 160.872 1.479 161.5

I hope these are useful for reference purposes.

More importantly for us, on a daily basis, are the following limitations as to what will fit onto the smaller screwplug sizes. We deal with up to 2.25″ only, with 2.5″ being accommodated by fitting a brass adaptor to 2.25″ screwplug.


Anything smaller than a 1.25″ screwplug will be limited to 1 element, having immediate implications for the achievable watts density.  Likewise, being limited to U bends only, makes reducing the watts density more problematic. Many domestic and commercial plumbing applications use 1.5″BSP, and are thus limited to 3 hairpins and only 1 stat pocket. maximising the immersed length of those U’s is all that can be done to reduce the watts density. To overcome the issue of only 1 stat pocket, we find ourselves increasingly selling dual control and cutout stats.

By hairpin, we mean a U bent element, and trombone indicates double looped. To determine what element is suitable, for U’s you merely double the maximum immersed length available, ie, 24″ immersed length will use u bent 48″ elements. However, if double looping is possible due to the screwplug size and other factors relating to the application (cleaning, etc), that 24″ maximum immersed length could be multiplied by up to 3.5, allowing the use of 84″ elements. See the watts density article to appreciate what a difference that will make. 48″ to 84″ almost double the hot element available, so is almost going to halve the watts density. A big issue on little screwplugs.

What is Ingress Protection?

IP Explanation and Ratings
EN 60529 outlines an international classification system for the sealing effectiveness of enclosures of electrical equipment against the intrusion into the equipment of foreign bodies (i.e. tools, dust, fingers) and moisture. This classification system utilizes the letters “IP” (“Ingress Protection”) followed by two or three digits. (A third digit is sometimes used. An “x” is used for one of the digits if there is only one class of protection; i.e. IPX4 which addresses moisture resistance only.)
Degrees of Protection – First Digit
The first digit of the IP code indicates the degree that persons are protected against contact with moving parts (other than smooth rotating shafts, etc.) and the degree that equipment is protected against solid foreign bodies intruding into an enclosure.

  • 0 – No special protection.
  • 1 – Protection from a large part of the body such as a hand (but no protection from deliberate access); from solid objects greater than 50mm in diameter.
  • 2 – Protection against fingers or other object not greater than 80mm in length and 12mm in diameter.
  • 3 – Protection from entry by tools, wires, etc., with a diameter of thickness greater than 1.0mm.
  • 4 – Protection from entry by solid objects with a diameter or thickness greater than 1.0mm
  • 5 – Protection from the amount of dust that would interfere with the operation of the equipment.
  • 6 – Dust tight.

Degrees of Protection – Second Digit
The second digit indicates the degree of protection of the equipment inside the enclosure against the harmful entry of various forms of moisture (e.g. dripping, spraying, submersion, etc.)

  • 0 – No special protection.
  • 1 – Protection from dripping water.
  • 2 – Protection from vertically dripping water.
  • 3 – Protection from sprayed water.
  • 4 – Protection from splashed water.
  • 5 – Protection from water projected from a nozzle.
  • 6 – Protection against heavy seas, or powerful jets of water.
  • 7 – Protection against immersion.
  • 8 – Protection against complete, continuous submersion in water.

Remembering all of the aspects and considerations discussed on the “How to choose an immersion heater” page, water hardness must be taken into account when designing a water immersion heater.

A detailed description of water hardness can be found HERE, but for the purposes of this blog, we will say that hard water has a higher concentration of calcium and magnesium which leads to limescale, which we are all familiar with in kettles, etc, within the home. Whilst merely being an inconvenience in the home, (if I can’t taste it in my tea, and remember not to look in the kettle when filling it, I can pretend it’s not there) it can be a real issue in commercial and industrial applications.

The problems are caused by the limescale deposits building up on the hot part of the elements. Once this process starts, it becomes self perpetuating, and indeed self accelerating, if not removed by jet washing or other means. Basically, the limescale build up prevents the heat getting away from the element, acting like an unwanted insulating jacket, increasing the element surface temperature, which in turn, attracts more deposits. An example of which is shown below, courtesy of, which would perhaps be better used as a christmas tree than for making coffee!

heatingelement limescale

The easy way to maximise the working life of your immersion heaters, whether in a hard water area or not, is to minimize the Watts density. The article explains in detail a simple concept which we are using every day, maximise the element length THUS minimising the watts density THUS maximising the working life of the element, all other things being equal.

Remember all of the stuff that applies to all water immersion heaters, plus for oil heaters our target is to get the watts density down to 12w/in² or below.

POIL Oil Imersion Heater

POIL Oil Imersion Heater

The POIL Range has many benefits, including bespoke screwplug size, immersed length, double stat pockets, etc, all with a 2 week leadtime. The 8mm elements can be double looped to fill your maximum available immersed length with a large overall length of element. As can be seen in the fifth image on our PII web page, the elements drop from the screwplug to your maximum available immersed length, before looping back up to within a few inches of the screwplug, before dropping back to full immersed length, then returning back up to the screwplug. This allows us to fit nearly twice the length of element possible in a plain U bent element. For example, in 24″ of maximum immersed length, we could use an 84″ element double looped.

if you have a very large tank with a massive maximum available immersed length, you may choose not to double loop, but to have plain U bent elements over a meter in length.