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FAQ - Oil Heating, Hot water production and plumbing “funnies” in France

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Opel Fruit....what a very knowledgable and helpful guy you are and also gifted with the ability to explain things in an understandable fashion! To join french 16mm to british 15mm I use a french 16mm compression fitting; on tightening the 15mm  side the 15mm olive protrudes around 2/3 mm out of the backnut but a simple wipe of solder sorts that.I make up short tails like this then go to Screwfix Speedfit (I wish to god Screwfix delivered here ; they are French owned after all!)But once again many thanks, your post is more than helpful and unselfish. Rog
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Yes, it was not lost on me! [:D]  So who came first - Allen or Alain?  Frenchified or Anglicised?

Just checked Wikipeia - seems it was American to begin with:

"Allen wrench" was originally a trademark

of Allen Manufacturing Company in Hartford, Connecticut in 1943. In

non-English speaking parts of Europe, it is usually known as an

"Unbrako key" (also often misspelled as "Umbrako"), which is a brand

name established in 1911. The brand is owned by SPS Technologies. In

Germany and Netherlands, this type of screw is known under the name Inbus™, named after the company that invented them in 1936, Bauer & Schaurte Karcher in Neuss (Innensechskantschraube Bauer und Schaurte). (It is still known as Inbus-key - or often misspelled as Imbus-key - in parts of Europe.) In Italy it is known as brugola, after Egidio Brugola, who invented it in 1926.[1]

Acknowledgment to Wikipedia.org.

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I have an economic question rather than a technical one (but I have already learnt a lot from this section!)

We have a house with 21 radiators and it seemes to be generally agreed that we need a boiler of about 30KW.  The current system is an oil-fired boiler for the heating and a 300l electric water heater.  Boiler needs replacing NOW.   Elec heater is old and would need replacing in due course but works OK at present and I have just calculated it costs us approx 150 euros a year TTC to heat the water at heures creuses. (Solar water-heating options would therefore have too long a payback period to interest us.)

Most successful quote to date is from plumber who is recommending (and we like the idea of) a Geminox oil-fired condensing boiler, but he also wants us to go for the version that heats the water as well.  This obviously involves larger boiler including a 100l or 120l hot water ballon. 


1. Regardless of any difference in installation cost (on which I am still trying to get a precise answer!) is this likely to end up being more economic to heat water than keeping an elec water heater? 

2.  Would installing the two-function boiler mean a lot of messing around with the existing hot water feeds and returns, making installation that much more expensive?

3.  Would the boiler benefit from being in use all year round, rather than having the sudden shock of coming back on after a long, hot idle summer?

4. Are these boilers reasonably reliable or does the dual function make them more prone to breakdown than a boiler for central heating only?   (In the case of breakdown assume there would be no way to back up a hot water supply - of course in UK we had a hot water tank with an emergency immersion heater option.)

5.  Is there anything glaringly obvious that I'm not thinking about???

By the way, regarding whoever it was who has an old defunct oil tank in the garden, I was quoted 1100euros plus TVA at 19.6% (logic of 19.6 rate escapes me) last month to de-gas, cut up and remove a 5000 litre tank - but ours is in a known location of course, and above ground. 

Do hope you can stop my mind from whirling......

Chrissie (81)

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Your 300l electric water heater costs you 150E per year. What is the difference in price between the basic CH boiler and the larger one (including the new ballon) ?. What replacement life do you anticipate (amortisation period)? What is the additional oil cost for that DHW compared to heures creuses ( greater I suspect)?

It should be noted that condensing boilers only operate in 'condensing mode' at full load or thereabouts, not at low / part load. Hence, in summer, with just DHW not CH+DHW it will not be in condensing mode and hence any perceived efficiency increase will not be there.

I dont believe that boilers suffer 'shock' when being returned to service after summer, at least no more thermal stressing than any other start up from cold.

The summer shut down allows you to plan the routine pre-winter servicing.

Hope that gives you some ideas.

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You cannot beat Heures Creuse for water heating, by either oil or gas.

If you really want to copper your bet, ask your plumber to quote you for a bi-fuel ballon, which can be heated either by the boiler or electricity. (Simply, it has a second dedicated calorifier - heating coil -  inside, run from a domestic hot water circuit from the boiler).

There is no real downside problem in shutting off central heating for the Summer, provided, of course, it is fully protected with anti-corrosion fluid.

Firing up a central heating boiler merely to provide domestic hot water is extremely expensive and inefficient. Firstly, the system has to overcome the latent heat requirement of the boiler casing, the static water and etc.

Personally, I would stick with a dedicated boiler for heating: and a cheap rate ballon for hot water.

One final point: the heat output of your boiler will be predicated by various factors, which include the volumetric capacity of the rooms; the number and surface area of windows (and the attendant heat loss); the level of insulation of the rooms (and the attendant heat loss); how many levels the house is on, as heat rises; and the ambient temperature you want for each room. For example, living rooms are normally kept at circa 24-5 degrees and bedrooms 18-20 degrees.

Finally, the radiator size is critical as is their placement: rad size is predicated by the total heat output required to raise and maintain each room at the desired temperature, allowing for continuous heat loss.

It is critical to work out the demand potential, maximum. A larger boiler than maximum is always a good idea as the boiler is then not working flat out to meet the requirement.

In my experience, French plumbers are not very good at this!

Worth checking.


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As Gluestick says, a larger boiler than max demand but.......not too much larger, especially if condensing. See comment about part load etc.

The primary factor is, to paraphrase TB    Insulation, insulation, insulation.

There is absolutely no point in pumping in heat that is going to go to waste.

The maths of designing a properly sized system are not that difficult, time absorbing perhaps and quite repetitive but not difficult (unless you dont have a calculator).

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quote"It should be noted that condensing boilers only operate in 'condensing mode' at full load or thereabouts, not at low / part load. Hence, in summer, with just DHW not CH+DHW it will not be in condensing mode and hence any perceived efficiency increase will not be there."end quote

Now that is something I'd never heard of (why would I?)  Another nail in the coffin of a dual system.  My gut feeling all along has been to stick with an electric ballon, and it looks like this is what we will do.

Funnily enough, I am fairly happy with the way they have all calculated the boiler size required.  Each set came with proper measuring equipment and there was a lot of discussion of window location and double glazing etc - our house is pretty well insulated, thank heavens.  The old radiators are not going to be replaced or moved in a hurry, but they were installed by a (wealthy) chauffagiste and apparently this was well done.  There was also discussion about future plans etc.  My only "problem" has been this surfeit of bright ideas and desire to fit new technology for which, on a different budget, I would be all in favour!

Many thanks to those who replied so helpfully

Chrissie (81)

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I am looking into alternatives to firing up a big oil boiler just for domestic hot water in the summer

By "Bi Fuel Ballon" I take it you refer to what in the UK would be the traditional jacketed indirect hot water cylinder with immersion heater(s) and internal coil fed from the boiler ? 

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Yes, analogous to the uk system. Its a ballon with a water / water heating coil for when your boiler is running and also an electrical heating element which you use (during summer) when your CH boiler is shut down / not required. The electrical heating element supply can be on heures creuse if its available to you. Its just left electrically isolated during winter CH time.

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A ballon is indeed a hot water store but in my system the one which was there was purely electric and not heated from the boiler at all. This would be OK for the summer but not really acceptable otherwise.

A new boiler would be far more efficient and quieter than my ancient monster although of course one would expect a new replacement French one to be also.

Maybe the way to go is have a new ballon fitted which is fed from the boiler because storage, by whatever means, is what I need to overcome a lot of the innefficiencies.

I think I'll have a word with the local chuffage engineer who has been maintaining the system for the previous owner.


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Heures Creuses vary - they are 8 hours per day.  Ours are something like 2.00am to 8am and 12.30 to 14.30 but other people will have other regimes.


You have to opt for Heures Creuses - most people do because if you have hot water heating it generally works out cheaper, especially if you have other appliances on timers (départ différé) so you can run washing machines, tumble driers, dishwashers etc. during the cheap period.

It will be shown on your last bill if you are on Heures Creuses.

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