Central Heating tripping RCD

[kwangomango]

Apologies in advance if my terminology or explanation is poor but i only have a rudimentary understanding of how the system works.

So i have a fully pumped CH and HW system - there is a Glow Worm Fireblaze back boiler in the kitchen and in the loft is a hot water cylinder, grundfos pump and 3 port valve. I have a programmer with separate controls for the hot water and heating.

I generally have HW on full time and CH on a timer.
About 2 weeks ago the RCD tripped whilst the both HW and CH were on. I turned the power back on and thought nothing more of it. It tripped again a few days later. Then it started tripping once or twice a day. I tried to eliminate any other electrical appliances by gradully turning off all power sockets. I got to the point where it would only trip when the HW/CH was on.

So for a few days i had HW turned on 24/7 and kept CH off. No tripping occurred. Turning the CH on caused the RCD to trip within 30 mins - just as the rads had reached a nice temperature. This was happening regardless of whether i had HW on or not, so i concluded that it must be something to do with the CH.

Throughout all of this i was immediately able to turn the power back on at the consumer unit. Up until a few days ago i the following pattern was occuring -

HW on and CH off - No problem.
Turning CH on - RCD trips within 30 mins.
Immediately turn power back on and put HW on - No problem.
Turning CH on - RCD trips within 30 mins
etc

So i tried turning the CH for 20 mins, turning it off for 5, turning it on for 20, turning it off for 5 etc. No tripping occurred. This was making me think that something was getting too hot after 30 mins and tripping the RCD, but less than 30 mins was OK?

On Tuesday i had an CH engineer pay a visit. He checked a few things but couldn't decide where the problem was. Just before he left he started playing with the thermostat on the boiler. When the rads were cool and the HW and CH were on, clicking around with the thermostat did nothing. However, if the CH had previously been on but then tripped and the rads were still warm, now clicking with the thermostat could fairly consistently trip the rcd. This would occur even when the thermostat was disconnected from the gas valve.

So he seems to think it is a faulty thermostat in the boiler but hasn't ruled out that it could be the pump in the loft. What do you think?

Today it seems to be getting worse and the pattern has changed again. After the CH has been on and caused a trip i can still turn the power back on immediately but if i then put the HW only immediately back on, it trips within a few minutes. I need to wait about 20 mins and then i can put HW and have no problems.

Could it be a fault occuring when the pump is too hot? It does feel incredibly hot to the touch when it is running.

I hope all of that makes sense and would really appreciate any advice!

 

[J_N30]

A pump is a common cause of tripping RCDs or popping fuses but not always and tends to trip as soon as power is supplied to it.

 

[Gas2Air]

Check wiring to boiler isn't burnt and shorting. You need an RGI to do this.

 

[Homesafe]

Your pump will need changing more than likely.
But thats just a guess.
If you get someone competant in they should be able to test this.

 

[Agile]

A simple way to check the pump is if you are competent to rewire it to a plug fused at 3A if there is a non RCD protected power source available.

Even if all your circuits are protected by an RCD then perhaps you neighbour might give you a non protected feed ? Not strictly kosher because taking an extension across a garden should be from an RCD fed supply.

If I was fault finding then I would be measuring the earth current from the pump. Thats easy for me with my clamp on meter.

Tony

 

[kwangomango]

Thanks for the replies guys.

I've turned the boiler thermostat down to zero and had CH and HW on for a good few hours with no trip. Obviously it is just pumping cold water around. The pump feels very slightly warm to the touch.

Does this prove that the pump is OK? Or could the pump be creating an earth fault when it gets hot pumping heated water? When it is pumping heated water it is extremely hot to the touch - i can only hold my hand on it for a couple of seconds.

Could an electrician easily prove one way or another if it is the pump. Is it easily done with a meter even though the trip only occurs after 30 mins of pumping heated water.

 

[kwangomango]

Another update:

I've set the boiler thermostat to just above zero, as low as i can get it, and put both HW and CH on. So the rads are slightly warm.

The RCD hasn't tripped since i did this a few hours ago. I've just gone into the loft to feel the pump and it feels warmer than the rads but i can easily hold my hand on it.

So at a lower temperature the RCD doesn't get tripped.

 

[kwangomango]

Bumping.

 

[Agile]

A simple way to check the pump is if you are competent to rewire it to a plug fused at 3A if there is a non RCD protected power source available.

Even if all your circuits are protected by an RCD then perhaps you neighbour might give you a non protected feed ? Not strictly kosher because taking an extension across a garden should be from an RCD fed supply.

If I was fault finding then I would be measuring the earth current from the pump. Thats easy for me with my clamp on meter.

Tony