Planning for winter - can I run my combi boiler off a genny?

Discussion in 'Electricians' Talk' started by jimwillsher, Aug 18, 2011.

  1. Chris_T

    Chris_T New Member

    Thank you. You've given me a hint of where to look next.

    I've contacted Bosch Worcester technical help line but they were extremely unhelpful. They just said they don't make generators so they couldn't give any advice on the subject.

    However, I've found something very interesting on an American website :

    "Gas furnaces use an electrical return path through the flame and the chassis of the burner to prove the existence of a flame. This requires that the power supplied to the furnace has a "hot" and a "neutral" connection and that the neutral be "bonded" to the equipment ground conductor. This is a normal connection when on utility power.

    Some generators do NOT have this neutral-ground bond and therefore there is no differentiation between hot and neutral conductors, both are "hot" in reference to the equipment grounding conductor. You need to ascertain if your generator has a "floating neutral" (not bonded) before proceeding. You can do this one of two ways. If the generator is running use an analog voltmeter or a solenoid-type tester from the smaller slot in a 120 volt receptacle on the generator to the ground connection. If you get a voltage reading close to 120 then the generator IS bonded. If you get no reading or a reading significantly different from 120 volts your generator is NOT bonded."

    The Honda generator I am trying to use has a neutral ground bond and I left this alone as everything I read said this was the right thing to do. In fact, I have never come across equipment that deliberately uses the earth wire as part of the electrical circuit, it is always a safety connection to the metal case of equipment.

    I will try an experiment when it gets light tomorrow and let you know.
     
  2. MGW

    MGW Screwfix Select

    In the USA they have a odd mixture of supply methods including using delta and earthing one winding in centre. Also a 110 - 0 - 110 volt supply so I would not look on any USA websites.

    I would say in general if plugging in a standard socket tester the supply passes in the main it will work. Because generators can be grey imports there are a few which don't follow normal UK standards I have found 230 volt outlets which were wired 120 - 0 - 120 and 50 - 0 - 190 the latter was a Honda so to say simply yes it will work is clearly wrong.

    However the Honda was duel voltage and I would not expect to find this with a single voltage model and the other one was a grey import from USA.

    I stand to be corrected as I don't use plug in testers I have proper meters but I would think a simple plug in tester will highlight any odd internal wiring.
     
  3. Chris_T

    Chris_T New Member

    OK, some success at last. I started up the 2.5 KW generator and took a wire from a 13 A wall socket earth to an earth point on the generator. The boiler now works.
    I then did the same thing with the Honda, but it didn't work. I'm going to call Honda technical help and if they say anything interesting I'll post it, but for the moment it seems to me that running a boiler off a generator needs a good waveform as has been pointed out, and the correct earth connections for the installation.
    Thank you to everyone that contributed.
     
  4. Lectrician

    Lectrician Screwfix Select

    That does make sence. Many boilers will not work with reverse polarity. What happens on a high resistance TT though? I guess the boiler manufacturers have considered this.

    Small gennys do float, rather than being tied to earth. You can connect the neutral to earth at the output of the genny, and rod this connection down, effectively creating a TN-S (not TN-C-S as many think) earth. Many generators document this in their manuals, while some do not.
     
  5. Chris_T

    Chris_T New Member

    This was it ! Thank you. I contacted Honda but they were useless.
    My central heating finally runs from the Honda generator as well as the older one I borrowed. For the benefit of anyone having similar problems, the simple fix is to connect the neutral wire in the mains plug from the generator directly to the house wiring earth. I did this using a 2 inch long piece of wire in the socket I fitted next to the boiler for the generator supply.

    The reason the borrowed generator worked was because it was an older type and the neutral had been connected to the generator casing by the manufacturer as standard practice so when I ran a wire from the generator earth to the house earth, everything started working.

    My Honda generator is a newer type and contains a built in inverter. The neutral is not connected to the generator casing so when I connected the a wire from the generator earth to the house earth nothing happened.

    This will happen to anyone trying to use an inverter from a battery to run their central heating.

    Here is a copy of the full explanation that I found on another site :

    The boiler flame failure device requires an "earthed supply" but the inverter provides a "floating supply. In simple terms, the flame failure device, ionisation probe, and monitoring circuit work as follows :

    Alternating current is supplied to the probe, small electrode, current flows from the probe through the ions in the flame to the large electrode, the burner.

    The large and small electrodes rectify the AC into DC, meaning current will only flow from small to large electrode.

    The monitoring circuit sees the DC,which has to be at the correct polarity, so knows there is a flame present.

    If there is no voltage, it know there is no flame present so it shuts down the gas.

    If it sees AC, it sees a short circuit and again shuts down the gas.

    The monitoring circuit can’t work if the supply from the inverter floating.

    In most cases the flame failure device won't work with transposed live/neutral inputs, though everything else in the boiler isn't fussed either way.

    Virtually every electrical item in a household will still work with live and neutral transposed, deadly though, due to the fact that even though the item is switched off, all neutrals within it will now be live

    Most mains generators and inverters provide a floating supply and some types of flame failure detection devices will not work with a floating mains supply. All that is required for the boiler to function normally, is that the generator/inverters neutral is bonded to earth, so that the boiler sees a true live feed with respect to neutral/earth.
     
    Welshdragon1 likes this.
  6. Martin Brown

    Martin Brown New Member

    I am a bit concerned about connecting the E&N wires because it would prevent the generator’s own RCD working. However, it is certainly what many people recommend and it seems to be essential for some boilers. Don’t blame me for the spelling mistakes but this extract may be useful:

    “The Briggs & Stratton manual transfer switch is ideal for this application.

    This should be fitted between the electricity meter and the building consumer unit. The switch connects the building to either the mains supply or to a lead which can be plugged into the generator.

    Most buildings now have an RCD built into the consumer unit. This is configured to operate from the mains supply with an earthed neutral, and not from a generator with a floating earth. To utilize this protection device, it is necessary to modify the generator so that it is configured in the same way as the mains supply.

    This is a simple modification for a qualified electrician, involving adding a link wire from the neutral terminal to the earth terminal. It is recommended to make this connection in the plug that is to be used to connect to the generator. This ensures that the generator is unmodified when it is disconnected from the house, and therefore remains safe.

    The plug should be labeled "Do not connect to mains: Neutral-Earth link fitted". The lead between the generator and the transfer switch is not protected by the RCD, it is therefore recommended to use a steel armored cable for this connection. Finally a local low-impedance earth spike needs to be installed.”

    See: http://www.handkcrawfordelectricalltd.co.uk/page/back-supply-connecting-your-generator-to-your-home


    I have read other electricians telling us NOT to connect the E & N wires but I have yet to understand why not. Normal house wiring always has the earth and neutral wires connected – as does the mains supply generator. Look out for (TN) TerraNeutral i.e. (C) Combined Earth and Neutral, which is then (S) Separated again to provide a different Earth and Neutral wire. The Earth wire is used in the RCDs etc.


    PRESUMABLY, and I don’t know this for sure, you could link the E & N at the first plug into the generator, which would disable the generator’s own RCD, but you could then have a separate RCD on a very short, very tough lead so that the connecting lead is RCD protected, apart from that very short (150mm?) lead.
     
  7. Tony Goddard

    Tony Goddard Screwfix Select

    Martin, this thread is an antique, David Cameron was PM when it was last touched!
     
  8. Mr Rusty

    Mr Rusty Screwfix Select

    Antique it may be, but I just read it for the first time and learned something :)
     
  9. spinlondon

    spinlondon Screwfix Select

    I was thinking of getting a generator so I could charge up my electric car.
    Which would be greener, petrol or diesel?
     
    Ind spark and Tony Goddard like this.
  10. Dougal Brown

    Dougal Brown New Member

    Appreciate we are now 7 years on from your post but thank you so much for this info!!

    After powercuts at the start of the year i went and bought the "IMPAX IM1800IFG 1800W INVERTER FRAME GENERATOR 240V (843HP)" from screwfix and had an electrician in to put our ideal logic combi esp1 boiler onto a 3 pin (round pin/5A style) plug but alas although the boiler switched on it would not ignite instead displaying F2 and LN fault codes.

    Today i made up the following adapter - so a lead from the generator to a loose wall plug, then linked the neutral wire from this to the earth pin in another plug going to the boiler wall socket.

    Fired right up and we have heating and hot water working!

    Thank you so much as we are now sorted for winter.

    [​IMG] [​IMG] [​IMG] [​IMG] [​IMG]
     
    Abbadon2001 likes this.
  11. Abbadon2001

    Abbadon2001 Screwfix Select

    isnt this is what is commonly know in the US as a "suicide lead"? exposed pins on the end of a cable intended to connect to a genny? just be careful how you connect up and who has access to this area when connected :)
     
    The Happy Builder likes this.
  12. unwoken

    unwoken New Member

    Looks like a "widowmaker" because of the 13 A plugs with live exposed pins, and the neutral deliberately connected to earth, just because something works electrically doesn't mean It's safe, this rig needs a rethink and should not be used.
     
    The Happy Builder likes this.
  13. Dougal Brown

    Dougal Brown New Member

    Calm down - i'm not selling these or anything, its just for personal emergency use.

    Obviously everything will be plugged in before the generator is turned on so no scope for touching live pins, and the generator will be earthed down via a rod.
     
  14. Abbadon2001

    Abbadon2001 Screwfix Select

    I didnt say dont use it - but just be careful, it is/was commonplace in the US to have something like this as a feed for a temporary generator to house, but a bit sketchy from a safety perspective. it could also be dangerous if left plugged in to house, after disconnecting the generator, and making the CH circuit live again. Human error is not 100% preventable. Be safe :)
     
  15. arrow

    arrow Screwfix Select

    Hope you fitted a fused 5 amp plug and not an unfused one on the boiler.

    There is no actual exposed pins, there is nothing connected to the live and neutral of the white plug, if that was accidentally plugged in the black plug would not become live.
     
  16. The Happy Builder

    The Happy Builder Screwfix Select

  17. Dougal Brown

    Dougal Brown New Member

    The electrician did that part and looking at it now i think its un-fused, although that plug is on its on dedicated circuit back to the consumer unit with its own MNTN106 MCB which is rated to 6A?

    Not especially happy with that, and the use of the weird 5A plugs, considering the boiler was originally hardwired to a 3A switched spur per the manual.

    Just popped over to screwfix so tomorrow will swap the boiler plug out to regular 13A plug with a 3A fuse in it, and swap the face plates on both the wall and adapter to switched ones.
     
  18. The Happy Builder

    The Happy Builder Screwfix Select

    you are a complete and utter idiot, your wiring is ridiculous and dangerous leaving yourself and others without any protection from electric shock.
     
  19. The Happy Builder

    The Happy Builder Screwfix Select

    a few more Prime Ministers can be added to the list!
     
  20. Dougal Brown

    Dougal Brown New Member

    I thought the whole point of forums like these is offer advice and help?

    Calling someone an idiot without first offering any of those 2 says more about you than it does me to be honest - with respect you come across like an utter fanny.

    If you read back you'll see the only wiring i've actually done is to bridge the neutral to earth by way of the white plug, and everything else (including making the extension lead) was done by an electrician. The bridge is required in order to get the boilers flame detection to work and what the original poster recommended, and also several others on this thread.

    If you know a better way then please share, i'm all ears?
     
    Last edited: Nov 4, 2022

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