Reducing energy consumption in old buildings

Hello everyone,

we are buying an old building with energy class G. The current owners have demonstrably done a lot in recent years, including installing new windows and redoing + insulating the roof. There is also a solar system for hot water generation.

We are therefore surprised why the energy consumption is so high. The broker and the owners explained that the energy consumption certificate only documents how high the actual consumption was in the last few months. And since nine people (= energy consumers) lived there, that much was consumed. It would be different with us (soon three of us :-) ).

Can one generally say that? And are there any experiences where most energy is lost in an old building (100 years old)? I am considering getting an energy consultant to help us.

Thanks for all answers!

Best regards, Ramos

Is there a consumption certificate or an energy demand certificate? It’s best to look up the difference online.
But it sounds like a consumption certificate.

Consider whether you need less heating with three people than with nine. The heating energy demand depends more on behavior than on the number of users (only electricity and water will be less with fewer users).

G would mean 200-250 kWh per m of usable area, so 300 kWh per m of living space.

With new windows and roof, I would expect rather D to E.

Is the value for primary energy or final energy?
What kind of heating system is installed?
What kind of walls? Basement ceiling? How old are the windows really (the silver spacer in the edge of the glass has the date printed)?

With such consumption and the mentioned points, an energy consultation can already make sense. The cheapest first approach would be, for example, the consumer advice center for an initial assessment.

Hello dertill,

thank you for your detailed answer. We have a consumption certificate, we know the difference to the demand certificate and it was also explained to us. Therefore, the broker and the owner also said that it is probably due to the high number of residents.

I can imagine it that way. Although one should basically not "freeze" any room regardless of whether there are two or nine people, and therefore heat a little everywhere, it does make a difference whether everyone turns the heating up full in their own room or not. At least as a layman, that is how I explain it to myself :-)

The final energy demand is 210 kWh/m²*a, the primary energy demand is 234 kWh/m²*a. The windows were replaced in the last one to three years. I cannot say anything specific about the walls. The central heating system is powered by gas, there is a solar system for hot water production and a water-carrying fireplace.

Best regards Marius

Would consult an energy advisor. Considering the few euros relative to the purchase price, it is definitely worth it.

While one should basically not let any room “freeze” regardless of whether there are two or nine people, and therefore heat a bit everywhere, it does make a difference whether everyone turns the heating up full in their own room or not.

With complete (!) use of a building, it does not matter whether 3 or 9 people live in it. If you do not use a "wing" of the building, consumption will of course decrease, but not significantly.

If you lower the room temperature permanently from 21°C to 17-18°C (any lower hardly makes sense and is also physically dangerous in terms of building physics), you save about 5% of the energy amount per °C for the cooler part. Example: 50m² with 200 kWh/m² from 21°C to 18°C = 50m² * 200 kWh/m² * 0.05 * (21-18) = 1500 kWh/year.

Based on 40,000 - 60,000 kWh (how big is the house?), that is not much.

If the current occupants prefer to live at 23°C throughout the whole house, there is naturally another 10% to be saved on the total amount.
What is added on top is a reduction in the hot water area (approx. 500 kWh/year per occupant), which here is low, since probably about 50% is covered by solar heating.

Is the hydronic fireplace included? It is stated in the certificate under fuels. Probably not, since primary energy is higher than final energy.

What you must not forget: More people also produce more waste heat (100-200 W per person) and more technical devices (dishwasher, washing machine, TV, oven/stove) too.

I wouldn’t have high illusions about it, but rather talk to a professional. You will not be overheating the place; however, the figures indicate that there are still slight amounts of energy to be saved somewhere, or that the current users simply want to live all year round like they did last summer.

But if you first plan to move in without renovating (I don’t know the overall condition of the house), you can just live there for a year and see what the heating does.

Forget all statements from the broker and owner - they want to get rid of the thing and not save energy for you. In a 100-year-old house, energy literally leaks everywhere. Energy class G would have to be over 200kWh/sqm*a, not really fun for your wallet...