myson midas

[phil170756]

my myson midas boiler just makes loud banging noise and shuts down when switched to hot water and i need to press the reset button to restart it, works perfect on heating, any ideas?

 

[weargas]

midas,
b?
si?
sfi?

 

[ChrisR]

That the one with a pump for HW and a pump for CH? HW pump would be obvious suspect.

 

[weargas]

simple test for the pumps on a midas.
1 switch off electricity supply to the boiler at the spur.
2 unscrew pump shafts and pull forward to locate into impellor
3 switch spur on while watching pumps.

when a midas si and sfi first get power they spin both pumps together for 1 second. if only one pump spins, one is faulty. not sure if the midas b spins the pumps though.

 

[phil170756]

both pumps spinning when power is switched back on

 

[weargas]

pumps are ok then. which midas?

 

[phil170756]

its a midas sfi

 

[kevplumb]

just a suggestion but the couple of these beasts ive worked on seemed very prone to collecting air but it was only apparent when in hot water

could be worth a try venting it

 

[weargas]

the sfi keeps a small amount of dhw stored. when you switch on the dhw it will automatically fire up to heat this water. it sounds like the sensor isn't telling it to shut down when the water is hot enough which is causing it to boil and overheat.

 

[phil170756]

can the sensor be tested if not is it easy to replace

 

[weargas]

Have a look at the boiler when it fires for dhw preheat. Does it start to sound like it's boiling after only a few seconds or does it take a minute or so?

I can never remember which has which sensors between the si and the sfi but they are different. I'll tell you about both.
One type will be a wet sensor screwed into the pipe work underneath the secondary heat exchanger. It will have blue plastic where the electrical connections go. This sensor is a thermistor and you'd be testing the resistance across it with the leads pulled off.
The other is a dry sensor fitted to the 3/4" pipe coming out of the right hand side of the secondary heat exchanger. This sensor makes and breaks the circuit so you'd test for continuity.

 

[croydoncorgi]

Just to cheer you up, see http://www.lancs.ac.uk/staff/strongs/creativewriting/graham1.htm.

 

[phil170756]

can see two sensors one on mains water with two blue wires the other is on domestic hot water but it is red with two red wires

 

[weargas]

The red sensor controls the heat of the dhw whilst it's running. The blue sensor is the preheat sensor. Usually when that type of preheat sensor goes wrong it just keeps the boiler in preheat, the result being that ch never comes on. I've never ever had one of those blue sensors cause an overheat. I've a feeling you're gonna be looking towards the control pcb.

 

[phil170756]

the blue sensor looks pretty corroded, what happens if the red sensor fails and regarding the pcb can this be tested or is it a straight replacement or they expensive