Understanding Boiler Efficiency

paperclap

Hi all,

We have an old non-condensing conventional Potterton Profile 40e boiler. 12 kWh output. It's probably 70% efficient. 

This means it uses 12kWh of gas per hour to run. However, being 70% efficient, it wastes 30% of the energy. Correct? Therefore, it takes longer for the boiler to heat the water to the required temperature, to flow around the pipework of the house, and heat the home to the desired temperature. Right?

Hypothetically speaking, let's say we were to replace our boiler with a new, 12kWh output boiler, which was say 95% efficient. It wastes 5% of its energy. It will still use 12kWh of gas per hour to run. But, it will heat the water to the required temperature faster (25% faster), and in turn heat the house (25%) faster.

Is my thinking correct?

And yes, we could go into a bucketload of variables. Current internal temperature, external temperature, humidity, insulation, draft proofing, etc. But, let's not.

Perhaps I'm oversimplifying this, or perhaps I'm not.

Thanks in advance.

FreeBear

paperclap said: Hypothetically speaking, let's say we were to replace our boiler with a new, 12kWh output boiler, which was say 95% efficient. It wastes 5% of its energy. It will still use 12kWh of gas per hour to run. But, it will heat the water to the required temperature faster (25% faster), and in turn heat the house (25%) faster.

The speed at which your house heats up is limited by the size of the radiators and the temperature of the water. All things remaining equal, switching to a new condensing boiler won't get the house up to temperature any faster.

Replacing the boiler with a modern one comes with other potentials for saving energy - Assuming you only need 8KW to heat the house, the boiler will modulate down and use less gas. With a smart(er) thermostat, as the house approaches the set temperature, the boiler can be commanded to modulate down even further so that you don't get a massive overshoot.

Doozergirl

Pretty much, but it's 25% less efficient which makes the new boiler ~35% more efficient.  

FreeBear

Doozergirl said:

Pretty much, but it's 25% less efficient which makes the new boiler ~35% more efficient.  

With a programmable thermostat that allows you to set different temperatures throughout the day, there is potential to save a further 10% of gas.

tacpot12

Add in weather compensation to reduce the flow temperature so that you extract the maximum amount of enegy from the condensation process, and you should save more than the raw efficiency figures suggest. 

Ectophile

Ultimately, the efficiency is about how much of the heat from the gas flames ends up heating the water, and how much disappears out of the flue.

If you put a pan of cold water on a has hob, light the hob, and hold your hand above the saucepan, you'll realise that a lot of the heat misses the pan entirely.  The same happens with gas boilers.

Modern condensing gas boilers improve this by having two heat exchangers.  One directly over the flames, as normal, and a second one in the flue.  The incoming water goes through that second heat exchanger, which uses heat from the flue gases to pre-warm the water.  Then the already warmed water goes throgh the main heat exchanger.

macman

It may have been 70% efficient at install, so probably rather lower now.
Since it's now at least 18 years old, the question of efficiency may not be the main factor in deciding whether to replace it; rather more the question of how much longer it will be reliable, and how much longer you can still get spares for it.

Rodders53

paperclap said:

This means it uses 12kWh of gas per hour to run. However, being 70% efficient, it wastes 30% of the energy. Correct? Therefore, it takes longer for the boiler to heat the water to the required temperature, to flow around the pipework of the house, and heat the home to the desired temperature. Right?

No wrong.

The heat input when new depended on the gas pressure/flow rate set during installation:
Minimum was 11.63 kW in (output 8.79 kW into the circulating water).
Medium was 13.29 kW input (10.26 kW out)
and Max in was 14.93 kW (11.72 kW out).

So it more depends on what settings yours is at and what the house needs.

Those cast iron heat exchangers were fairly bulletproof, I believe.  But DO keep a CO monitor in that room:  the fan flued system is positive ventilation and a combustion chamber / case seal leak will push combustion gases into the room.