Circuit debugging

[Jon Connell]

After an unrelated problem with my hot water, I spotted that the immersion heaters in my pressurised hot water cylinder have no voltage going to them. I'm guessing that explains why the hot water temperature has been a bit variable for the past few weeks. The heaters are each connected to their own FCU.

Our heating system was installed 3 years ago and since then I've had the consumer unit replaced, so I am wondering whether the guy who did this missed these out.

Can anyone offer some hints to start diagnosing the problem beyond my speculation?

[Dave Plowman (News)]

If you have gas heating, immersions are generally only a back up, and left switched off until actually needed, like say with a fault on the heating system. It being more expensive to heat water by electricity.

No neons on the immersion FCUs? Most prefer to have these - so they know if it's been left on.

It one of those posts which suggests you'd be best to get an electrician in to check it out, rather than DIY. ;-)

[Tim Watts]

I would say they are either working or not working and should have no bearing on your water temp - you are heating normally with gas/oil?

Yes - trace through with a voltage tester or multimeter.

The "volt stick" is worth having here - it's non contact and will detect a live cable, so you may even not have to open anything up. Just touch onto the cable going into the FCU - or the FCU plate to see if there's power from the CU. Then go from there depending.

[tabbypurr]

Trace the circuit from one end to the other, seeing where it has & hasn't power, using a multimeter or voltstick. Make sure you know how to not get electrofried first. Divide & conquer is the basic plan for electric or electronic fault finding.

But it does sound like you don't need immersion power, assuming you have HW.

NT

[spuorgelgoog]

Immersions are best on a 20 amp switch with neon, rather than an FCU.

Owain

[Jon Connell]

Heating with oil, timed to come on a couple of times a day. My expectation is that the immersions will keep the temperature maintained when the tank is not being refilled.

Aye, it's a stick tester that tells me there's no power to the immersions since it has no neon. My CU isn't particularly well labelled and no MCBs are tripping, so I am at a loss as to where the fault is given that all the cables are buried.

[John Rumm]

Its normal in these cases to have the immersions switched off, and set the central heating programmer to replenish the cylinders often enough to not need any further topping up.

(some programmers even have a "cylinder boost" setting to give the hot water a recharge on demand)

First test the tester on something that is known live. Then you can check the FCU and the cable to the immersion. Chances are the immersion(s) will be on a dedicated circuit at the CU. If there is one heater, then its likely a 16A MCB. For a pair it may be one 32A MCB.

Normally you would expect there to be a switch somewhere (is the FCU switched?)

[Tim Watts]

In that case, test you have power coming *into* the FCU.

Given you do not know how to isolate the circuit, I'd recommend going and buying a volt stick and touching the face place in various places - that will indicate if anything is live inside.

[Jon Connell]

That's precisely what I did and it's not live. And given the the CU has no tripped MCBs, it makes me wonder where on earth they're cabled to.

[Jon Connell]

OK, useful to know that the immersions are not adding much value.

The FCUs have no power to them and all the MCBs are "on". Do MCBs fail without tripping off?

[Tim Watts]

Ok - sorry, misunderstood...

Well - that is weird...

[Chris French]

Did these heaters definitely work when first installed?

Did they work do you know after the new CU was installed?

right now my money is on them being missed out somewhow when the CU was replaced (or when they were installed) I thyink you need to try and track the cable.

after we bought the house I realised after a bit that the main thermostat didn't seem to work.

investigations shew that whilst the was a cable, it wasn't actually connected to anything at the boiler end

[John Rumm]

They are worth having as a backup.

Also depending on your heating system design, it may work out cheaper to heat the water electrically in the summer rather than run the boiler just for that.

(this tended to be truer of older low efficiency boilers hooked up to traditional cylinder with a slow recovery. Modern unvented cylinders can often swallow close to the maximum output from the boiler anyway, so there is little loss of efficiency using the boiler for just the water heating.

Yes they can do... what brand are they?

Try turning one off then back on a couple of times and then test again.

I take it you have checked the fuse in the FCU?

What you do next really depends on whether you have a multimeter and how comfortable you are using it to carry out some measurements.

[John Rumm]

It sounds like the first job is to label all the circuits in the CU - a bit of trial an error turning MCBs off and seeing what stops etc.

[Jon Connell]

Yeah, the electrician who installed the CU did a crappy job of that. The things he did label were not overly accurate. Fortunately, I'm sensible enough to check a circuit is not live rather than trusting those labels.

I should relabel...

[Tim+]

Hmm, have you ever knowingly used your immersion heaters? I just thinking that it's not uncommon to have an immersion heater switch somewhere between your fuse box and your airing cupboard, often in the kitchen or near a bathroom.

Tim

[charles]

Or, possibly, a time switch.

[tabbypurr]

It's usually cheaper to rely on oil alone, no electric top-up.

A voltstick is hardly adequate at this point. A multimeter will tell you wh ere each of L, N, E do & don't reach to, and that's what you need to know. Do appreciate though that it's not hard to go very wrong with a multimeter, you need to know how to use one, which isn't hard, and what things to neve r do. Really, it's what you need at this point. Prices are £2 upward, so save your pennies :)

NT

[Chris French]

And making sure that they do turn things on an off.

I had an MCB in the old CU, had a pair of cables going into it. No clear label.

Appeared to be a ring main, but they didn't seem to actually power anything. Have never yet worked out what the cables might have gone to once. Disconnected them on CU replacement, and everything in the house still worked.

Another thought, is there a switch hidden somewhere. We had a rented house where there was a rather random switch for the upstairs immersion hidden away behind a cupboard in a pantry. Presumably so someone could switch it on and off from downstairs.

[ARW]

And of course the electrician left you with an electrical installation certificate and did the Part P notification.