# Calculating DPE

Does anyone know how a house’s energy consumption is calculated? I assume it’s a combination of wall insulation, double glazing etc but I haven’t been able to find an authoritative sourse.

Depends if you want a very precise figure. The simplest way to get an idea for your home if it was built pre-war is to take an average of last few years’ energy bills and divide by surface area of the house. That gives you a ball park of which energy band you fall in.

There are online simulators that I guess will take you through the details.

1 Like

To be honest, my experience of having had this survey done is that it is computerised guesswork.
The guy who came to do ours was only interested in floor area, area of outside walls, area of party walls, ceiling height, type of heating, type of windows, and manner of hot water provision.
I had made up a detailed list of the type and amount of interior wall insulation that had been installed, and was told that it was irrelevant !
The estimator made no attempt to check any of the insulation whatsoever.
We were marked down for having an electric ‘Ballon’ for the hot water on the grounds that they leak heat into the atmosphere, despite the fact that it is sited within the inhabited area so that any lost heat warms the living space.
We were also marked down for having a 20 year old oil fired central heating boiler on the grounds that it must be inefficient due to it’s age, though the reputable company that services it says that it is 87% efficient.
Also marked down for having an uninsulated concrete floor over the cellar on the grounds that the cold would come up through the floor in the winter. What cold ? The boiler is in the cellar !
Mainly, the whole process was just a joke that cost me €300.

I understand that there have been instances of different estimators classifying the same house as being in Group C and Group E.
They make it look good with a fancy computer print-out with some graphs, but in reality it seems to be little more than a guesstimate.

7 Likes

I fet exactly the same way when the Pompe à Chaleur people came to do their “study” of a suitably dimensioned heat pump - au pifomètre !

5 Likes

Thanks - I assumed it would be like you describe, @Robert_Hodge!

The issue I have with these 1/2 a day how to use our Ipad app brigade (dont question the app) make the real engineers look bad. Most heat loss is from unisulated budings that are also leaking air. A project I am working on requires air tightness testing after we seal the room. That is one area that multifoils fitted correctly can really reduce losses but the wally’s with their Ipads are clueless about.
In short, both sides of the channel it is a tick box exercise, no sign of a thermal imaging camera or proper calculations.

For those with swimming pools, these potentially use more energy than other items in the home but are not considered at all, still perfectly possible to run a pool with 90% less electricity than a standard setup and it increases the water quality.

3 Likes

reading this might help,might not of course

1 Like

That’s helpful - mentions the inclusion of the last three years’ bills, as @JaneJones thought, too

But what if you improved your insulation 1 year ago, do they still average which will be wildly innacurate.
Our building went from 4C above outside ambient to comfortably warm for most seasons without heat source. Do they ask for receipts of stere’s of wood burnt which reflects how good the stove is not neccesarily the building.

Or even if - like us - your consumption is diminished for another reason?

1 Like

My mother used to “sing” that - she loved flanders and swan

(Strangely reminded of them a few days back as went to Oscar Kokoscha exhibition, and he was Alma Mahler’s lover for a while and I recalled the F&S song “Alma, tell us, all modern women are jealous. Which of your magic wands hooked Gustav, and Walter and Franz…”

I’m interested by the concept of air tightness, and am wondering how this fits in with reasonable ventilation to allow for the expulsion of humidity caused by cooking, showering, and doing the laundry etc. Surely there has to be some means of expelling cooking smells and odours generated in other rooms ?

Yes via mechanical ventilation and heat recovery.

1 Like

So presumably there have to be some vents to allow fresh air in ?
I can hear our bathroom extractor fan labouring more when the door to the room is shut, as opposed to when the door is slightly ajar to allow replacement air into the room. If the whole building is sealed then where is the replacement air going to come from ?

Incoming air would come from a vent mounted in a suitable place, that would feed into a cross flow heat echanger so the warm but stale air would pre heat the fresh incoming air as the stale air is exhausted. The amount of air is controlled from CO2 monitors usually but temperature and humidity would also. be adjusted as required. Mechanical ventilation units are quite common in france, you see them in bricos. This just takes it up a notch with the cross flow heat exchanger and smart variable flow motors.

This is how they work
Comment fonctionne une VMC double flux ?).

1 Like

does one need electricity for these mechanical ventilators…
and, if so, isn’t simply opening a window more eco friendly… and free ???

That’s going to be interesting for us - we heat our house and gites almost entirely with wood, largely without any money changing hands… I wonder what the rating will be for a 10-bedroom property using only one small heated towel rail…

1 Like

Pretty much no Stella, whilst opening a window in the right conditions is fine 99.9% of the rest of the time no. Opening a window allows uncontrolled loss of heat and gain of humidity. Yes powered by electricity but can also be passive system (that the type we are fitting) still have a backup fan. The tiny amount of power consumed is nothing to the wasted heat energy to either heat or cool as required in an old building/room.

I guess it depends on the person and how they organize their affairs.
We do have an extractor fan which runs automatically if one switches the light ON in the walkin shower at the end of the main bathroom. Left running while one showers, then switched OFF.

Likewise, there is an extractor hood over the gas hob… I use that sparingly, as and when needed. The knack is not to have stuff boiling away like mad and sending steam into the room.

Apart from that… we use windows…

Obviously not if a north wind is blasting away in the window direction…
but, as an example…
when the morning sunshine warms the metal balustrade on the bedroom balcony (even in winter)… that’s the time to open the huge windows and air the bedding and the room.
No heat is wasted, as the radiator is OFF long before then (… if it’s been ON).
With the corridor door shut… the bedroom retains/attains a gentle natural warmth… until the sun does a runner…
We have excellent air-quality here and it’s nice to make good use of it for free… when we can.