Resistance of gas valve solenoid

[YAPH]

Bit of a head-scratcher today. Couple of days ago I was called to look at a newish Ferroli boiler[1] (DOMIcompact F24D) which seemed to have its gas valve packed up: went through normal start-up sequence up to point of sparking to light the gas but gas was there none. Checked 240V on wiring to gas valve present when it was sparking to boiler electrics OK; checked resistance of solenoid - open circuit.

Today picked up new gas valve. Sanity check: test resistance of coil. Open bloody circuit! Call Ferroli[2] who assure me coil resistance should be

5.9 MegOhm!! (Yes, Megs not K - I asked, tech bloke assured it was so.) My multimeter only goes up to 2M FSD. Could have put the megger on it - technically that resistance would have passed an insulation resistance test! WTF: even at DC that would only pass 40 micro-amps from 230V; with ac the inductance would make it less still. How does the damn' thing even work? Or has it some electronics in it?

Work it did, though: gas, heating, happy customer.

[1] funnily enough one I'd been asked to commission a few years ago by the unqualified cowboys installing it. Usual response: "I've spent many hundreds of pounds and weeks of my time getting and maintaining my gas qualifications, and you want me to put that on the line ticketing up this boiler for you for HOW MUCH DID YOU SAY YOU WERE OFFERING?" [2] 8p/minute 0871 number which they spun out with blah blah press 1 for this etc and no proper mobile-friendly geographical number for engineers (I asked). So think of that if you're thinking of buying a Ferroli (as I pointed out to their phone droids!)

[Bob Minchin]

That is only 10mW (VA) - not enough power to move anything.

I wonder if it uses a capacitor to drop a whole load of volts without heat and for safety, they have a high value resistor to discharge the cap. So when you measure DC ohms, you see a coil in series with a resistor.

Do post if you find the answer. I'm intrigued!

Bob

[geoff]

John, chuck us the old valve when you next return an item so I can do a post mortem on it

[Skipweasel]

If it's the gas solenoid, then it isn't moving anything. It's just holding open the valve that's already been pushed open by your thumb.

Indeed, it's quite likely that the lump is already magnetised but not quite enough to hold against the spring - the small current from the thermocouple is just enough extra to hold open.

Like I said when this came up last month - the solenoid is miles and miles of very fine wire.

[geoff]

In message , Skipweasel writes

I think he knows that

gas valve solenoids are usually in the range of a few kohms

we're talking a factor of 1000 difference here

[John Walliker]

You will probably find that it has a bridge rectifier inside. The forward voltage drops of two diodes in series are probably greater than the operating voltage of a multimeter, which is why the resistance appears to be very high or open circuit.

John

[YAPH]

Modern condensing boilers[1] are pretty much thumb-free zones.

What is this thermocouple of which you speak? When C21 gas valves like this open it's a half-inch pipe into a 24kW burner with an ignition electrode doing its best to make sure it all lights up or shuts off again PDQ!

Anyway the old thermocouple-operated hold-in solenoids were very *few* turns of *thick* wire - tens of millivolts but loads of current.

[1] or even so-twentieth-century technology standard-efficiency boilers with a secondary heat exchanger bolted on to get up to mid-noughties efficiency standards, which this Ferroli is.

[Michael Chare]

Are you not using DC to determine how much AC current a device might pass.

I use a flow switch to control my boiler. I was rather disappointed to find that my new replacement flow switch still had a resistance of about 50kohms even when there was a flow. However I was subsequently pleased to find that the flow switch would fire the relay that starts the boiler. I think that the reason for this is that the flow switch uses a triac.

[Skipweasel]

Sorry - I've just driven to Kent and back and my synapses are feeling a bit battered.

[Skipweasel]

Mine wasn't when I took it apart. Miles and miles of the stuff, it was.

[Andy Wade]

That was the mains-voltage coil for the main gas valve, rather than the millivolt-level coil for the pilot port. A multi-function valve of that type will of course contain one of each.

[Harry Bloomfield]

Skipweasel has brought this to us :

Even miles and miles of thin copper wire will not have a resistance in the M Ohm range. Usually solenoid resistances are measured in the low K Ohm range. Only if there is a device in series with it, will it measure in the M Ohm range.

[Harry Bloomfield]

I checked a failed one and it was not miles, but a great deal of very thin wire.

[Harry Bloomfield]

Andy Wade presented the following explanation :

Not in my case, very thin enameled copper wire on the thermocouple solenoid.

[geoff]

Pilot valves are normally 240v

Main valves are normally 240v

Modulating valves are normally 0-24V

Lately some main gas valves have been designed to run off DC as rectified mains

Vaillant have taken to putting a bridge rectifier in the loom between the pcb and the gas valve