My central heating and hot water has a Honeywell ST699 control unit. The display went blank and so the boiler doesn't come on. I changed the 2 amp fuse which was blown in a second sealed box next to the controller. I also changed a blown 5 amp fuse in the FCU. I turned the power on and the display came back. As soon as I moved the hot water sliding switch the fuses blew again. I took the front off of the control unit to access the wiring to the control unit. There are 5 wires attached plus an earth wire which is not attached. Is there a wiring issue or should I replace the control unit?
Power issue to central heating control unit
- Thread starter SmallTalk
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photos ?? is the CPC insulated or bare and hanging loose ? what is the 2 amp fuse within ? could be wiring or programmer
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Wiring issue indicated. Could be rodent damage to a cable.
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you have an electrical fault. The fuses are attempting to protect your appliance. If you keep replacing the fuses without identifying and correcting the fault, you will do more and more damage.
Not being a boilerman, if it was me I would start by looking for a water leak dripping onto an electrical part, perhaps around the pump or a motorised valve if you have one, which you might be able to fix with a simple plumbing repair, or around the cylinder thermostat. As an inexperienced amateur you should not be taking a gas boiler apart. You can also look for obvious damage to the cables, such as rodents (as above) or a heavy object crushing flex.
If you insist on powering up the boiler when you already have a fault you might damage the electronics, which will be an additional expensive repair on top of whatever fault you already have.
Not being a boilerman, if it was me I would start by looking for a water leak dripping onto an electrical part, perhaps around the pump or a motorised valve if you have one, which you might be able to fix with a simple plumbing repair, or around the cylinder thermostat. As an inexperienced amateur you should not be taking a gas boiler apart. You can also look for obvious damage to the cables, such as rodents (as above) or a heavy object crushing flex.
If you insist on powering up the boiler when you already have a fault you might damage the electronics, which will be an additional expensive repair on top of whatever fault you already have.
I shall have a better look for a leak. I had a brief scan with a torch before and everything looked dry and the floor is dry. A lot of the pipework is behind stud walling so fingers crossed it's nothing too far in. Gas is way out of my league so I'd obviously leave it to a professional.
I would remove wire to cylinder stat , then that rule out programmer
Then connect wire back and remove wire to m/ valve
Than as before , then remove pump wire
Paul
Then connect wire back and remove wire to m/ valve
Than as before , then remove pump wire
Paul
Not if you want it fixed, Sparks are useless at CH wiring!!
Sounds like a motor is shorting out, so you're looking at the circulation pump, zone valve motor, fan etc etc.
Unless you've got a multimeter & know how to use it, call an experienced time-serviced Gas Safe heating engineer.
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Multimeter is largely the wrong tool for the job
@aptsys , how do you think that ? I would use a multi-meter every time in this instance and never failed to find the cause yet, interested in how you would use other methods, happy to find other ways of any task
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Depends on the fault - if it's a straight short then no problems. Many faults don't appear until biased though and the 2 or 3V typically used on the ohms range isn't good enough. Something like an MFT or meter with 250 or 500V IR test is better for finding the fault.
H
Hot&Cold
As what Puller has said,get an experienced gas tech to sort out your fault.
No need to call an electrician unless the gas tech advises you.
No need to call an electrician unless the gas tech advises you.
I never found the need for anything other than a multimeter in 37 years as a gas engineer, and If it is blowing fuses in the order that the op says I am in agreement with @PullerGas , a very easy diagnosis for someone who knows how heating systems operate
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