Restoring Belling 942 Electric Fire - request for help

[TJGillott]

Hi all

I am trying to restore a Belling "Type 942" vintage electric fire, and can find very little information about it. It has two independently operating open-coil elements (three rows each), with the coils placed in removable ceramic insulators. These elements are selected by a front-mounted toggle-type switch. The fire is clearly marked "Belling and Co., Enfield, Middx", so must be pre-1955, as after this all Belling fires were manufactured in Burnley, Lancs. It is black cast-iron, portable with a handle cast into the top. The top is also vented. Cast-in decoration looks vaguely Art Deco, so it could be late Twenties/early Thirties.

So, to my problem. The original element coils gave up after many years, and I am trying to replace them using 25 swg nichrome wire hand-wound coils, which seemed to be the closest I could get to the original coil diameter/wire gauge. The issue is that these coils only last a matter of weeks before developing hotspots and blowing. I went to a larger wire diameter nichrome wire (24 swg) in the hope that this would solve the problem, but the coil just blew even more quickly. I have no idea of the nominal output, all the back plate says is "200 volts" and "6 Amps". I would estimate it at about 2kW, though. Is there anyone out there who can tell me anything about the coils I should be using, size, wire diameter, etc.? Any help gratefully received!

Cheers

Tim Gillott

[TonyDuell]

200V at 6A is 1.2kW. An electric heater is going to be essentially resistive, so issues of power factor don't come in.

I would assume that's the total power so each element should be 600W. Each element would draw 3A from the 200V supply so should have a resistance of 66+2/3 ohms

The thicker the wire, the lower the resistance, so the more current it will draw and the hotter it will get. You might have better luck with thinner nichrome wire.

Have you measured the resistance of one of your replacement elements? Or the current it draws from the mains.

From what I remember on such elements it is important to get the turns equally spaced. If the turns are closer in one part than another you will get a hotspot in the former location. Probably wind the wire as evenly as you can (tightly spaced), divide the resulting coil in 3 (with cardboard markers or similar) for the 3 rows in the imsulator and stretch each part separately to fit in said insulator.

[Ed_Dinning]

Hi Tim, also note that there are a range of resistance wires that seem to go under the generic "nichrome".
Some idea of the diameter of the original wire and the number and size of the turns should allow us to work backwards and find out what was originally fitted.
I may be able to help you with the wire as I have a small stock of different types; you may even get lucky and find a hardware shop that has on or the old spiral elements used as spares on "infra-red" wall mounted fires.

Ed

[Nicklyons2]

I may have a NOS spiral - I'll have a look. The alternative is to buy a 1KW old ceramic fire bar and 'pinch' the wire from it; now they came in lengths from around 9" - 14" and I would suspect the shorter ones may be your best bet as they have the shortest wire length - therefore the highest resistance/metre. Unwind the wire from the ceramic and onto a former of a bit less than the required diameter (as it will uncoil a bit). I think 8" and 9" elements were also available as 750W versions BUT BEWARE the shorter ones around 6" are often 110V/125V intended for series pairs and so will have too low a resistance.

[Lucien Nunes]

I haven't tried this but if you can't find NOS bare spiral elements you might be able to use a spiral from inside a silica tube element, which are still readily available. The turns are more closely spaced inside the tube but with care might be stretched out to the correct overall length for your heater. Various wattages are available, I rapidly found one at 650W and one at 500W, perhaps 600W exists too. There's quite a selection here: Silica-tube elements at Easyspares

[julie_m]

In order to work out the resistance of the wire required, you need to know two formulas. Firstly, Volts = Amps * Ohms, E = I * R; and secondly, Watts = Volts * Amps, P = E * I.

To get 1 kW at 230 V, you need to use V = 230 and P = 1000, then rearrange the equations as necessary to solve for the unknowns. I = E / P, so we have a current of 4.348 A; then using R = E / I, we get R = 52.9 Ω. If you want each element to give 1 kW, then you will need to use enough wire to have a resistance when hot of 52.9 Ω; otherwise, if you want the total power of both elements to be 1 kW, then you will actually need each coil to have a resistance of 105.8 Ω.

Knowing the ohms per metre of the resistance wire you are using, you will be able to work out how much wire you are going to need to use to wind your element coils. You might even be able to make a temporary paper former to support them and space the turns evenly apart while winding; which can then be burned out with a lighter afterwards, leaving just a free, self-supporting coil. (Unleash your inner pyromaniac! )

The simplest and safest way then of measuring how much power the fire is converting to heat would be to use one of those plug-in energy monitors. By pressing one button you can cycle the display between voltage, current, frequency, power (which we are really interested in), power factor (which will be 1.00 here, or else something is very wrong), time and kWh. This is a lot simpler and less error-prone than trying to lash up two multimeters (which would work every bit as well, though).

But that's just the beginning of it, because you will find that the resistance of the wire actually will decrease with rising temperature. You could do a lot of scary-biscuits maths and most probably make a mistake anyway, or you could just simply keep shortening the element a bit each time and measuring again .....

[TrevorG3VLF]

"you will find that the resistance of the wire actually will decrease with rising temperature."

No, just the other way, the resistance goes up with temperature.
The variation will depend on the wire composition. Bulbs which have tungsten filaments and run over a much higher temperature range can vary their resistance by a factor of eight.

I would try wire with the original diameter.

To reduce the power, you need to lengthen the wire you have which may be difficult to put in a lot more turns evenly.

[Nicklyons2]

I've found a NOS spiral if anyone's still interested.

[julie_m]

Originally Posted by TrevorG3VLF

"you will find that the resistance of the wire actually will decrease with rising temperature."

No, just the other way, the resistance goes up with temperature.

Gah! Blame the lateness of the hour when I typed that. Yes, of course the resistance will increase when it's hot, it's the power that will decrease. You need to shorten the wire to get it right, anyway.

[Herald1360]

Originally Posted by julie_m;852792Yes, of course the resistance will increase when it's hot, it's the [I

power[/I] that will decrease. You need to shorten the wire to get it right, anyway.

Or not. Presumably if it runs a bit cooler it'll last longer as a working display piece.

[TJGillott]

Wow! So many helpful replies! As you have all guessed, I am something of an electrical numpty (I get along with motorcycles OK, but farm out the electrickery).

Thanks for the clue on the wire gauge. I may use thinner wire, but I reckon the problem is the hand winding. NickyLyons2 - I would be very interested in the NOS spiral, please could you get in touch via my email, which is tim<at>tngk<dot>vispa<dot>com (symbols replaced to combat spambots).

For interest, I source all my wire from The Scientific Wire Company in Essex, which has an unbelievable range of nichrome wire in every conceivable gauge, prices are good too. I have had good luck with 25 swg, will look into better (or pro) winding.

Once again, many many thanks all.

Cheers

Tim Gillott