Board logo

Cental Heating Weirdness, After changing the circulation pump.
bigbravedave - 7/7/13 at 09:02 AM

I had a local plumber out to service the boiler and as he was coming out I thought he could swap the noisy circulation pump that sits in the airing cupboard to save me a job. Its a pretty traditional indirect boiler (not a combi) with a 3 way change over valve and a circulation pump.

He swaped the pump over first and left the heating and hot water running to bleed out any air. Then the boiler clicked off as normal.

With the boiler set to off on the controller he took the cover off to service it and it powered up by itself. The switched live on the boiler was permanently live despite being off at the controller. I changed the 3 way change over valve encase one of its micro switches was sticking. Then turned the hot water on on the boiler and the relay inside barely clicked compared to the central heating relay.

I took the controller apart and took the cover off the relay (LZ-24HS - 5amp rated?) to find its switching terminals fused together, I prised them apart with a tiny screw driver, and the switched live is now switched again.

I have run the hot water up 3 times the following 2 times it stuck the switched live terminals together in the first 30 seconds.

Its been running for an hour now and all is fine.

What I don't understand is how changing a pump that is rated at 0.5 amp max has fused together a relay rated at 5 amps and the main 3amp fuse on the isolator is fine.

I have tried two identical timers and its done the same on both. It now seems ok.

Was the pump just a bit tight? Me and the plumber are confused, He's offered to swap the pump. I might stick a multimeter in the circuit and see what its pulling.

Any ideas? (the boiler doesn't have a frost stat that overrides the switched live either)

Thanks in advance

big-vee-twin - 7/7/13 at 12:32 PM

If the pump was a little stiff before water got through it the starting current was little higher than normal, when a motor starts it takes a lot more current, if it was stalling it would take more.

Maybe now pump has bedded in it will be ok.

Fuses are designed to take account of starting currents so wouldn't be affected, relay has probabily been value engineered to death so its only just rated enough under normal conditions?

I would just keep an eye on it for a few days

bigbravedave - 7/7/13 at 01:05 PM

I spoke to soon its welded its relay contacts together again!

big-vee-twin - 7/7/13 at 01:24 PM

Once a relay contact has welded it really needs replacing

gremlin1234 - 7/7/13 at 07:22 PM

quote:

Once a relay contact has welded it really needs replacing

absolutely,

also the 5A rating on the relay is for a resitive load, motors are inductive, that relay /should/ cope with a 1/8th hp load, (about 100W) and I guess the pump is near that load.
I recommend, replace relay with a slightly higher rated one if possible.

mangogrooveworkshop - 7/7/13 at 10:28 PM

And stick a large capacitor across the terminals to stop the arc on the relay

scottyboy235 - 25/9/13 at 10:13 PM

Unbelievable, I have experienced the exact same fault.

My trusty Danfoss timer which has been in operation since 2007 welded its contacts together after the pump was replaced during the summer. Nothing wrong with the original Grundfos pump (been in use for at least 15yrs) - replaced it with another Grundfos pump when the system was power flushed. Don't know when the timer fault happened but I have a feeling it wasn't immediate.

Comparing the ratings (inductive load, current etc) for the pump and timer reveals that the timer contact ratings are way in excess of the demand generated by the pump. Bah!