Odd central heating problem - relay or thermostat switching a motor

[Gefreiter Krueger]

When I moved in 13 years ago, there was a central heating system utilising only radiator stats. I fitted a room thermostat (the basic bi-metallic strip variety), which connected live to the motorised valve, which in turn switched on the pump and boiler. For some reason, it would often arc when switching off, which heated the bi-metallic strip up a little and it never managed to go off. Problem solved by using an electronic thermostat which contained a relay, causing cleaner switching.

Similar problem now, with the heating system extended so it does the garage, aswell as the house, with a seperate electronic thermostat. The problem this time though, is because I could only find a motorised valve for the garage which did not have contacts inside it (only one which fitted my odd diameter of pipe), I had to fit a relay to switch the boiler and pump on and off (if I'd connected the output of the garage stat straight to the pump, boiler, and valve, then the valve would open every time the boiler and pump ran for the house too). Anyway, it worked fine for a year, then the relay started behaving like the old bi-metallic strip thermostat - it arced when switched off, and took ages to do so, making an alarming noise. So I replaced the relay with a big industrial contactor which is working for the moment. The relay I replaced says it can take up to 2HP motors, way more than my pump. Any idea what's going on?

[harryagain]

Arcing is caused by stored emergy in the system (usually coils/electric motors.) You will often get a tiny spark when switching motors, this is normal and not enough to affect a bimetal strip. Most switches have a "snap action" to minimise this. Just what do you mean by arcing?

[Brian Gaff]

I have seen so called snubber networks across motors and when I asked they told me it reduced sparking on switch overs, but whenpressed to explain they were more vague, but certainly you will get a pulse after disconnection of any inductive load. Brian

[Dave Liquorice]

Bi-metalic thermostats are renown for not operating quickly and sitting there partially open with and arc sometimes for many seconds. Sort of surprised a mains relay has developed what appears to be the same problem, maybe the small, inevitable, arc when switch a motor has damaged the contacts sufficiently for the gap not to be large enough quick enough to break the arc and it damages the contacts even more...

When switching reactive loads, like motors, you need to look at the spec of your relay to see how badly it is derated. Contacts rated for

240 V AC 10A resistive load may well fall to 1 A with reactive loads.

[Gefreiter Krueger]

The inductive rating of the relay is well above that of the motor. Anyway, I've decided the relays are fine, it's the electronics that's buggered in the thermostat. I blast the floor clean in there (it's converted to a parrot room) with a jetwash every week. I guess the damp mist from it has knackered it.

[fred]

In article , Gefreiter Krueger writes

You may get more help if you re-post using one of your less troll related sock puppets.

[Gefreiter Krueger]

Trolling is in the eye of the beholder.

[Dave Liquorice]

Think you have the logic inverted there. If the arc is warming the strip it ought to be forcing the contacts open, off, satisfied not to closed, on, call for heat that requires a cooler strip.

Don't think I've ever seena a pump with a snubber network. They are normally fitted at the switch not the reactive load.

[Gefreiter Krueger]

Ah, my bad memory. The problem was it couldn't switch the heating on. As the room cooled, the contacts tried to close, and just as they were closing there was an arc and it caused the strip to warm up and bend away again. It would be intermittent at first, then as the room got even colder it became continuous.

The switch? The thermostat is the switch.

[Gefreiter Krueger]

Although that doesn't explain the burning smell from the thermostat, which has no damage inside it.