Replace your old boiler with a condensing boiler or a heat pump?

Your old boiler consumes a lot of energy. It may be on the verge of failing, or even already broken down, or you are anticipating a little by thinking in advance what you are going to replace it with to save money on your heating bill.

Whether you are fuel oil or natural gas, you wonder if investing in a condensing boiler is a good idea or not. For the moment these two energies are at a (relative) low point in their price curve, but… how long will this last?

Your plumber told you about the condensing boiler, a marvel of technology that displays a yield greater than 100% thanks to the energy it recovers in the fumes, and which would make it possible to save up to 30% in energy costs. energy…
… but what is it really?

Condensing boiler, how does it work?

The combustion products that escape from your boiler through the flue can reach 200°C. Your current boiler expels these fumes without exploiting this temperature, this latent heat. A condensing boiler is supposed to recover part of this energy, which, according to the manufacturers, allows energy savings of up to 30%.

How? By exploiting the condensation of water vapor contained in these combustion products. Let’s see this in detail.

When you boil water, steam rises above the pan. If the steam comes into contact with a very cold window, the water vapor condenses on the cold wall of the window: you then see the water return to the liquid state and flow along your window.

Physically, what’s going on? The water droplets contained in the steam are hot: they contain energy. When they come into contact with the cold wall, this causes them to change state: the water becomes liquid again. In passing, the energy is dissipated. This is what happens when a body changes state, here from the gaseous state to the liquid state.

Well in a condensing boiler, the same thing happens. The fumes are passed around a condenser which plays the same role as the glass wall in our example. This condenser must contain a liquid as cold as possible so that the water vapor, in contact with it, condenses and gives up its energy. This “cold” liquid is your central heating water. The so-called “return” water, that is to say, the water which returns to your boiler after having circulated in all the radiators of your house.

Marvelous! … On paper

You will have understood that the return water from your central heating must be as cold as possible for the phenomenon of condensation to occur. Of course, it doesn’t need to be really cold. In practice, this water must reach the so-called dew point temperature: this is the temperature from which the water vapor will condense. If we take the example of natural gas, this dew point is 53°C.

In other words, if your return water has a temperature higher than 53°C, condensation cannot occur.
When does condensation really occur?
If you have soft heat radiators, condensation will work. We speak of soft heat when the boiler sends water at 55°C-60°C into the radiators, and the radiators are large enough to operate well at this temperature. In general, the return water from the central heating returns to the boiler at 8-10°C less than on the way. Thus, if it returns to the boiler at 53°C or less, the condensation phenomenon will occur correctly, and the condensing boiler will produce its effects. Otherwise… you will have a nice condensing boiler that won’t condense anything at all! And the only savings you will make on your heating bill will be those due to the difference in performance between your old boiler and a newer boiler. Most often, barely 10% gain.

The ideal situation: underfloor heating
With a water regime of 30-35°C, underfloor heating is perfect for the phenomenon of condensation to occur. Under these conditions, you will indeed save money with a condensing boiler. But… much less than with a heat pump which, on underfloor heating, displays a COP greater than 4, that is to say, efficiency greater than 400%!

Soft heat radiators?
The radiators of houses in the past were sized to operate with heating water at 75°C. They were small since, at this temperature, a small exchange surface with the part to be heated is enough for the heat to be transmitted to the air in the room. The power labels of the radiators are indicated for this temperature regime.

We talk about delta T 50 . Quesaco?

The delta T is the temperature difference between the water in the radiator and the air in the room. For example: if the water reaches 75°C in the radiator, it comes out at 65°C. The average temperature in the radiator is therefore 70°C. The comfort temperature in the room is 20°C. The delta T is, therefore: 70°C – 20°C = 50°. Delta T 50. CQFD.
If you want the return water not to exceed 53°C, the boiler must send water at around 60°C. This means that the average temperature in the radiators is 57°C. In this case, the delta T is therefore 37.

For the heat delivered in the room to be the desired one, the exchange surface must be larger. Much bigger. Here is an example :

A 1000 W delta T 50 radiator is approximately 90 cm long by 50 cm high. This same radiator, in delta T 37, only develops 665 W. In other words, to have the same power of 1000W, in this case, you need a 1500 W radiator with nominal power delta T 50.

Okay, so let’s say you have soft heat radiators, so the condensing boiler can really condense: is it worth investing in a condensing boiler?
A condensing boiler has an efficiency of 110%. The 30% savings announced by the manufacturers rely on the replacement of a boiler with an efficiency of less than 80%. This is indeed the condition to achieve these 30% maximum savings.

What if instead of opting for a condensing boiler with an efficiency of 110%, you choose a heat pump with an efficiency of more than 300%?

Certainly, the energy used by the 2 systems is not at the same price: natural gas or fuel oil for the condensing boiler, and electricity for the heat pump.

But even under these conditions, the heat pump can save 30% compared to a top-level natural gas boiler with 100% efficiency, and if you replace an old boiler with 80% efficiency – as in the comparison made by condensing boiler manufacturers – the savings will be around 45%!

If the replaced boiler is an oil boiler, the saving is 40% if the boiler efficiency is 100%, and more than 50% if the efficiency of the replaced boiler is 80%.

The price difference between the two solutions is quickly amortized by the much greater savings made thanks to the air-water heat pump.

Wait… a return cannot be greater than 100%!
It’s true, that’s what we learn in school.

The trick of condensing boiler manufacturers is to calculate the yield starting from the higher calorific value, where the yields are normally calculated on the lower calorific value.

Well, okay, it’s not very clear… An example to understand? So :

A conventional boiler produces 0.90 kWh of heat for 1 kWh of gas consumed. Its efficiency is therefore 90%. This yield is calculated on the PCI, the lower calorific value of the gas, which does not take into account the energy contained in the fumes. A condensing boiler that produces 0.95 kWh of heat for 1 kWh of gas consumed will have a yield of 95%… but on the PCS, the Superior Calorific Value of the gas, which takes into account the energy contained in the fumes. Where condensing boiler manufacturers are clever in marketing terms, it is that they compare the energy produced by a condensing system with the energy produced without condensation: 0.95 kWh instead of 0.90 kWh, i.e. 0.95/0.90 = 105.5%. This allows them to say that the efficiency of their condensing boiler is 106%. You will have understood that this is not entirely true 😉

In the case of heat pumps, we no longer really talk about efficiency, but about the coefficient of performance, the famous COP. We can still, to understand it better, liken it to a yield: it displays the quantity of heat produced from a paying unit of energy. Clearly, how many kWh of heat a heat pump produces from one kWh of electricity? Since the energy is not produced by magic, all the “yield” greater than 100% of the heat pump is in fact extracted from very real energy… but free: the energy contained in the outside air, which the heat pump extracted thanks to its outdoor unit, and that its compressor increases by taking advantage of the virtues of the refrigerant. Kind of like your fridge, but upside down.

In summary
What financial aid for the air-water heat pump?

The condensing boiler takes advantage of the paid energy contained in the gas but until then evacuated without recovery in the flue gases.
Actual yield: 95%.
It only really condenses if the return water from the radiators is below 53°C.
The heat pump takes advantage of the free solar energy it collects from the atmosphere!
Yield: 300% and more.

The price difference between a condensing boiler and a heat pump is quickly amortized by the much greater savings made thanks to the air-water heat pump.
Savings are allowed by replacing a standard boiler with an efficiency of 80%
If the replaced boiler is natural gas:
heat pump: 45% savings
condensing boiler: 30%… if it really condenses, 10% in other cases
If the replaced boiler is fuel oil:
heat pump: 50%
condensing boiler savings: 30%… if it really condenses, 10% otherwise

In any case, the heat pump is much more economical than the boiler, even if it is condensing!


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