Solenoids and thermocouples

[PetersDIY]

John - Thought about an earth rod but not sure how that affects the MCB and RCD trip. The boiler appears insulated apart from the earth connection. My guess is it would work if I made a permanent connection to the house earth and used the generator to supply live and neutral. However it’s not my simple “plug in” solution that would not have the generator sharing wires with the house.

Petet

 

[JohnW2]

I suppose it could do - both poles of the thermocouple have the possibility to go the the gas valve, isolated from earth.

From what I recall from dissecting them, I think that there is just a single insulated conductor inside the metal tubing, that tubing (which is in electrical continuity with the boiler metalwork) being used as the path for the 'other pole' of the thermocouple.

Kind Regards, John

 

[Jackrae]

I think you'll find a thermocouple flame failure device does not tie into the electrical control system. Rather, the current from the thermocouple is used to directly operate on a small solenoid contained within the main gas valve. The thermocouple output voltage is quite low but has a fairly high current capability, 'just' sufficient to hold open the gas valve. It cannot open the gas valve on it's own accord and this is the reason why, on older appliances, you have to manually hold the valve open, either by rotary knob or press button, until the thermocouple is sufficiently hot enough to produce the hold-in current.

 

[JohnW2]

I think you'll find a thermocouple flame failure device does not tie into the electrical control system. Rather, the current from the thermocouple is used to directly operate on a small solenoid contained within the main gas valve.

Yes, that's always been my understanding.

The thermocouple output voltage is quite low but has a fairly high current capability, 'just' sufficient to hold open the gas valve. It cannot open the gas valve on it's own accord and this is the reason why, on older appliances, you have to manually hold the valve open, either by rotary knob or press button, until the thermocouple is sufficiently hot enough to produce the hold-in current.

Again, yes, that is my understanding. "Quite low" for the voltage is perhaps a little of an understatement, a typical voltage (at least, with mine', when 'in a flame' being around 25 mV or so - and I'm not at all sure about 'fairly high current capability' (I suppose that depends upon what one regards as 'fairly high'), and I've never bothered to measure it.

.... It cannot open the gas valve on it's own accord and this is the reason why, on older appliances, you have to manually hold the valve open, either by rotary knob or press button, until the thermocouple is sufficiently hot enough to produce the hold-in current.

I don't think that is 'the reason' for having to manually hold the valve open. I think that is always necessary since, even with the thermocouple only operating a very modest solenoid, it cannot do even that until the thermocouple has got up to the 'full flame temperature', which takes at least a few seconds.

However, the point I've been making is not about any of that but, rather, about the electrical connection between the sensor (thermocouple) and whatever it is operating - which, as I've said, appears to be "single and earth" - i.e. a single insulated conductor inside of an 'earthed' copper tube.

Kind Regards, John

 

[ericmark]

I am sure on my boiler flame detection is a photo cell, but I use oil.

 

[JohnW2]

I am sure on my boiler flame detection is a photo cell, but I use oil.

You may well be right. I know nothing of, and never have any dealings with, oil boilers.

If it's a photocell, do I take it that it is somehow protected from 'extraneous light' - or is it perhaps really a 'thermocell' - i.e. sensitive to IR but not visible light?

Kind Regards, John

 

[ericmark]

The photo cell is inside the burn chamber. There is a controller which gives a fixed time for the burn to start, other wise it locks out.

The bits inside an oil boiler are very similar to those found in a space heater.

 

[Harry Bloomfield]

From what I recall from dissecting them, I think that there is just a single insulated conductor inside the metal tubing, that tubing (which is in electrical continuity with the boiler metalwork) being used as the path for the 'other pole' of the thermocouple.

Kind Regards, John

Correct, the thin tube is like a single core Pyro cable. They only generate a tiny voltage/current, so the connections have to be good, where they connect to the coil.

 

[Harry Bloomfield]

until the thermocouple is sufficiently hot enough to produce the hold-in current.

It doesn't hold it in it just operates a latch.

 

[Harry Bloomfield]

Again, yes, that is my understanding. "Quite low" for the voltage is perhaps a little of an understatement, a typical voltage (at least, with mine', when 'in a flame' being around 25 mV or so - and I'm not at all sure about 'fairly high current capability' (I suppose that depends upon what one regards as 'fairly high'), and I've never bothered to measure it.

I tried it and noted a similar result. The current is also tiny, if you check the hold in coil, they have a massive number of turns of very fine wire. They do not possess enough energy to directly hold a gas valve open, rather they operate a mechanical latch to retain the valve open.

 

[Harry Bloomfield]

You may well be right. I know nothing of, and never have any dealings with, oil boilers.

If it's a photocell, do I take it that it is somehow protected from 'extraneous light' - or is it perhaps really a 'thermocell' - i.e. sensitive to IR but not visible light?

They have to be kept out of the light, so they can see only the flame. I'm not sure why they use PC rather than a thermocouple, unless maybe an oil flame doesn't get hot enough - it could also be because an oil burner cannot use a pilot light (?)

 

[RandomGrinch]

I am sure on my boiler flame detection is a photo cell, but I use oil.

...and apparently they will be UV detectors, in forthcoming Hydrogen boilers!

 

[Jackrae]

I tried it and noted a similar result. The current is also tiny, if you check the hold in coil, they have a massive number of turns of very fine wire. They do not possess enough energy to directly hold a gas valve open, rather they operate a mechanical latch to retain the valve open.

The ones I discected had a few thick turns within the 'solenoid' section which held open a sping-return plunger style pilot valve within the main gas valve. The pilot valve being what controlled the main gas port operation. There are no doubt many variations on the theme of gas valve design.

 

[Harry Bloomfield]

The ones I discected had a few thick turns within the 'solenoid' section which held open a sping-return plunger style pilot valve within the main gas valve. The pilot valve being what controlled the main gas port operation. There are no doubt many variations on the theme of gas valve design.

I don't know how a 'few thick turns' could possibly work, or would even be necessary, with such a low voltage and current. It is certainly at odds with what I would expect or found when I explored a burnt out coil on a gas valve.

 

[martygturner]

Bit more description for the dimmies pleaseMarty.

Wow a lot of have not tried and telling every man as his dog how it can't be done, whilst here on planet earth the rest of us just do...

Its pretty simple firstly you need to isolate the mains from the consumer unit, ...use a genny switch or and dp isolator. Wire in the genny to one side of the genny switch and the mains to the other then they consumer unit is feed from the genny switch. I have used this and they have an earth connector to provide an earth to the genny, use some 4 core arctic cable to break out the earth to the gen earth point, do not rely on the earth in the plug that's normally just floating around in the gen.- I use the house earth...we have TNS not a neutral earth.


BS7430
7.1.3 Unearthed generators (rating below 10kW) supplying a fixed installation
Where an unearthed generator is to supply a fixed installation it is recommended that ADS is adopted as follows:

a) One pole of a single phase generator should be connected to the installation MET- ie the generator earthing point to the main earth terminal in the house.
b) The MET should be connected to an earth electrode - so an earth rod (TT) at the house or via TNS. If I had a pen earth system whereby the the earth came in via the neutral I would be adding an earth rod.
c) The installation should conform to BS7671 with all exposed conductive parts and all extraneous conductive parts connected to MET
d) The installation should be protected by RCD's

The earth electrode should have a resistance to earth not exceeding 200Ω

The RCD will not provide protection for faults on the generator side of the RCD, and consequently precautions should be taken. - ie the bit from the gen to the main consumer unit with rcd or if you are using a extension lead ( hmmm thats a no from me) its all risks no rcd... ie do not trust a genny that says it has an RCD...it probably does not and if it does will if trip quickly enough.... But a small suitcase one has few exposed metal pieces and some simple precautions - ie don't turning it off or on in bare feet in a thunderstorm...

We have used a dp isolator and and earth rod in the past at a far more rural site with no issues..same earth rod supplied the house and the genny.... In reality most boilers have a mains transformer and that is usually a standard 50/60hz job so it can be supplied all over europe. As for flame detection requiring an earth...give the unit its own earth if its that fussy, if boilers were supplied needing a neutral earth to work half the continent would not have heating.


The quality of the feed we get from the genny is far better than the feed we get from the mains, we use a little clark unit 1100w they put it at 50hz +/-0.1% , my hz meter in the solar inverter puts it rock steady at 50HZ.. the mains is all over the place +-0.14% the solar inverter switches off mains feed in at 0.09% differential. The legal limit is 0.5% variation. A really cheap gen set with no sine wave rectifying would probably have issues...you need a stabilized supply from a genny..we have used a small 500w genny 12v supply + 12v inverter in the past when we we doing a welsh hills build and living in our caravan...even that managed to power a biasi combi on LPG over winter.



Check out the http://www.gridwatch.templar.co.uk/ a couple of times a day to see the variation in in the hz.

These are pretty good resources...https://www.electricaltechnology.org/2013/11/how-to-connect-portable-generator-to-home-supply.html