MCB Failure modes / ageing?

[Adrian Brentnall]

HI Folks

Our domestic central heating is run by a ground-source water heatpump - most of the time this is very successful, it was installed about 12 years ago.

In the last 24 hours, the MCB that feeds the heatpump circuit has been tripping out. When it's manually reset, the heatpump fires up again and runs.. until it trips out again.

Wiring is as follows Main CU has a D40 MCB that feeds a smaller CU in the workshop, where the heatpump is housed. A D20 MCB in this smaller CU feeds the heatpump - which is nominally 3kw, but takes a big switch-on current.

As I say, this has all worked well for the past 12 years - until last night.

So - do MCBs age over time? Am I seeing an MCB failure, rather than a heatpump failure?

I have a call into the local engineers anyway - but just wondering... I'd rather replace an MCB than a heatpump!

Thanks Adrian

[Ray]

Some can do.

Only way to be sure is measure its power use or swap the MCB and see if that fixes the problem.

No way to say for sure without measuring or swapping the MCB

[The Natural Philosopher]

I'ts a lot cheaper to replace to see... Can we confirm this is not in fact an RCBO and earth leakage is not involved?

MCBS can age. But 12 years is not long...

[Adrian Brentnall]

Thanks - that's what I was reading on the 'net

I know that the 'running' current for the heatpump is around the 10 -

13A range, but I remember that a standard 20A MCB would trip out on the initial switch-on current, when the compressor starts - which is why they fitted a type 'D' - 'slow-blow' MCB in the first place.

Yes - I think MCB-swapping is a good place to start. Now just got to find a supplier that has some in stock. Thanks Adrian

[Adrian Brentnall]

That's what I was thinking. Need to find a local supplier with some in stock... thanks

Yes - it's an MCB

The heatpump itself is rated 3kw, but takes a fair 'over-current' when the compressor kicks in. Initially the installer had a standard 20A breaker on there, but that kept tripping, which is why the Type D MCB was installed.

Thanks Adrain

[ARW]

I would expect ageing to occur faster if held close to the maximum current for long periods or in an overheated CU ie between two heavily loaded MCBs.

What's the MCB you require? There might be one on the shelf at work and if there is one you can have it almost free if you want it - P&P costs must be donated to charity....

[Adrian Brentnall]

That's very kind of you Adam - but the narrative has moved on from this morning..

Drove to the wholesaler in the next-town-but-one - collected a couple of D20 single-pole MCB's. Got back home, all cocky, like - installed a new MCB and the darn thing wouldn't run for more than a few minutes. In fairness, wholesaler fellow said it was quite possible that the MCB could have been the problem - it just wasn't..

Decided to apply a bit of science (perhaps a bit late!) and measured the current into the heatpump at 35A (!) - which would explain why the MCB was tripping.. - should have been nearer 13A.

Contacted the heatpump guys, under instruction from them checked the current to the 'motor run' capacitor - which was 9/10 of sod-all.. and should have been somewhere in the 6A range. Other diagnostic was that the compressor sounded to be running a bit 'rough'.

Drove back to the next town and collected a new 50ufd / 400v motor run capacitor from the heatpump guys. Swapped it in, and the meter showed 6A through the capacitor and 12 - 13A to the entire unit.

Problem seems to be solved.. Heat-pump supplier pleased that it's fixed, regretful that he can't sell me a new compressor!

Thanks everybody - and particular thanks to Adam for the generous offer...

[Andy Burns]

What size of house are you heating with a 3kW heatpump?

Even with a COP of say 4, then 12kW output sounds low relative to an "average" 24 to 30kW combi-boiler.

[Brian Gaff (Sofa 2)]

Sounds like a small heat pump what does it actually achieve? Brian

[Adrian Brentnall]

It heats our (fairly) small house quite nicely!

Very happy with it - when it's working well - which, over the last 15 years has been most of the time.

Sucks heat out of the ground via 3 x 100m coils of plastic pipe buried under our polytunnel and produces circulating central heating water to feed our radiators.

The manufacturer reckons that the system produces 3 - 4kw of heat for every kw of electricity consumed.

[harry]

You have to include the electricity used in the various pumps/fans round the system. Looks a lot less good then.

You can get to the point where off-peak storage heaters are more economical.

Storage heaters need virtually no maintenance and are much much cheaper to install. And more reliable (no moving parts except possibly a fan.

There were lots of cowboy installers around in the early days of heat pumps too.

[The Natural Philosopher]

I had my house which is pretty big assessed and they reckined that 10Kw would run it and its been on a 12Kw boiler since built.

Its insulated to 2000 specs...

[tony sayer]

Those things are a Pile of poo removed more of them that i care to remember!...