Unvented cylinder swap immersion heater weird

[dcawkwell]

This is just plain weird.

Swapped a system from vented hot water tank to direct twin immersion heater unvented tank.

Electric only house so added timer and a top/bottom switch so they can swap between upper immersion and lower immersion. This is powered from the old immersion heater switch.

Now comes the weird bit. Switched on and nothing. Checked with a multimeter 240v at the immersion.

The house does have electric underfloor that uses off peak but I don't think this is involved.

Checked all the way through thermostat is on immersion checks out resistance. Wire the immersion up directly to a normal plug socket. Immersion comes on and draws current I can see this on the meter.

Wire back up to the old immersion heater switch but use a 1.5kw blow heater as a load. Blow heater comes on no problem?? Back to the immersion and nothing.

Luckily the plug is convenient so wire the whole thing up to that and it works fine. Something is weird about the old immersion heater switch.

All I can think is a problem in the earthing at the old immersion heater switch? Immersion heater switch itself with a high resistance? Old immersion heater cable?

 

[PrenticeBoyofDerry]

At what points of reference are you getting 240V?
Live to earth and live to neutral. You may have a faulty neutral across the switch.

 

[securespark]

Yeah. Get a DMM (tenner from Maplin http://www.maplin.co.uk/store-locator/yorkshire-and-the-humber-region) and check continuity across DP switch.

 

[bernardgreen]

Are you qualified to work on un-vented hot water cylinders and understand the need to have pressure and over heat safety sensors fitted to the tank that will cut off power to the immersions.

Without these the tank may explode in a fault. Not only from an immersion remaining powered and the water boiling but also from an explosive mixture of hydrogen and oxygen from electrolysis of water from a corroded element.

 

[dcawkwell]

Are you qualified to work on un-vented hot water cylinders and understand the need to have pressure and over heat safety sensors fitted to the tank that will cut off power to the immersions.

Without these the tank may explode in a fault. Not only from an immersion remaining powered and the water boiling but also from an explosive mixture of hydrogen and oxygen from electrolysis of water from a corroded element.

Yes thank you.

 

[dcawkwell]

At what points of reference are you getting 240V?
Live to earth and live to neutral. You may have a faulty neutral across the switch.

Live to neutral. I didn't check the live to earth.

 

[PrenticeBoyofDerry]

At what points of reference are you getting 240V?
Live to earth and live to neutral. You may have a faulty neutral across the switch.

Live to neutral. I didn't check the live to earth.

Did you check at the supply or load side?

 

[dcawkwell]

At what points of reference are you getting 240V?
Live to earth and live to neutral. You may have a faulty neutral across the switch.

Live to neutral. I didn't check the live to earth.

Did you check at the supply or load side?

At the load immersion heater.

 

[PrenticeBoyofDerry]

So you measured, 240V at the load side of the immersion heater isolation switch across L-N with switched closed?
If that the case can only assume that you had not terminated the load side to the switch contacts correctly, could have been insulation trapped or loose in terminal.

 

[dcawkwell]

So you measured, 240V at the load side of the immersion heater isolation switch across L-N with switched closed?

No I measured 240v at the immersion heater L and N connection.

 

[PrenticeBoyofDerry]

So at the control thermostat then?

 

[dcawkwell]

So at the control thermostat then?

A live and neutral connection on the immersion/themostat. The is only a L, N and E connection here. There was 240v between L and N.

 

[ricicle]

If you have 240/230V at the element terminals and the resistance reads approximately 19Ω then the heater should work. Have you carefully tried a clamp on ammeter ?

 

[Chri5]

So you measured, 240V at the load side of the immersion heater isolation switch across L-N with switched closed?
If that the case can only assume that you had not terminated the load side to the switch contacts correctly, could have been insulation trapped or loose in terminal.

I'd go with the above.

Have you checked the switch and made absolutely sure the LNE supply and load sides are correct, have you used continuity on a MM and checked the switch isn't duff?