Condensing boiler efficiency condensing temperature

[Reddwarf4ever]

I understand that a condensing boiler, is only really efficient when the inlet temperature is below 55oC, when burning natural gas.  Would the condensing / heat recovery and thus efficiency increase as the inlet temperature decreases.....e.g. 45oC, or is the maximum achieved at 55oC

thanks

[Bufofrog]

My understanding is that the efficiency will increase with the lower inlet temperature, because more heat will be extracted from the exhaust.

[Reddwarf4ever]

6 minutes ago, Bufofrog said:

My understanding is that the efficiency will increase with the lower inlet temperature, because more heat will be extracted from the exhaust.

Agreed, but how does the efficiency improve downwards from 55oC if any ? Thanks

[Bufofrog]

6 minutes ago, Reddwarf4ever said:

Agreed, but how does the efficiency improve downwards from 55oC if any ? Thanks

??  Like I said, the inlet temperature lower than 55 will remove more heat from the exhaust increasing the overall efficiency.

[Reddwarf4ever]

But  to what point ? I asked this on a plumbing forum and got the same answer, thought I could get a more scientific answer on here 🤔

wonder if there is a graph of temperature Vs heat recovery / efficiency.....maybe need to ask a process engineer 🤔

[swansont]

The maximum efficiency of a heat engine is 1 - T_c/T_h

Temps need to be using an absolute scale (e.g. Kelvins)

But the actual efficiency is going to be less.

[Reddwarf4ever]

Sorry you lost me, more interested in a graph that shows heat recovery vs condensing temperature, I found that 55 or 57oC is the start point of condense mode, want to know how much is recovered as the temperature falls, if significant or a straight line, will aim for a lower temperature 

[studiot]

42 minutes ago, Reddwarf4ever said:

Sorry you lost me, more interested in a graph that shows heat recovery vs condensing temperature, I found that 55 or 57oC is the start point of condense mode, want to know how much is recovered as the temperature falls, if significant or a straight line, will aim for a lower temperature 

 

1 hour ago, swansont said:

The maximum efficiency of a heat engine is 1 - T_c/T_h

Temps need to be using an absolute scale (e.g. Kelvins)

But the actual efficiency is going to be less.

 

swansont was (correctly) quoting actual thermodynamic theory.

unfortunately boilers such as condensing boilers can appear to havve efficiencies greater than 100% (according to advertisers and politicians) due to the way they measure it.

The correct name for this term is COP (for Coefficient of Performance.)

 

Edited January 16, 2021 by studiot

[Reddwarf4ever]

Still would like to see a graph or chart of condensing temperature vs heat gained 

[studiot]

6 minutes ago, Reddwarf4ever said:

Still would like to see a graph or chart of condensing temperature vs heat gained 

But you are asking for a graph of a meaningless term.

[Reddwarf4ever]

Wel....i actually got what I needed from my ‘ plumbers forum’ really thought that this forum would be the place to get my answer, but unfortunately was not the case.....

So here  is the graph I needed........and FYI it’s NOT meaningless 🤯

[studiot]

52 minutes ago, Reddwarf4ever said:

Wel....i actually got what I needed from my ‘ plumbers forum’ really thought that this forum would be the place to get my answer, but unfortunately was not the case.....

So here  is the graph I needed........and FYI it’s NOT meaningless 🤯

 

Thank you for your reply, and the graph which does not display the quantity you asked for and I said was meaningless.

You asked for

Quote

Still would like to see a graph or chart of condensing temperature vs heat gained 

 

You also wanted to be 'scientific' and cast various nastertiums about this forum.

Your graph does show temperature (but not condensing temperature) but it does not show heat gained (which is the meaningless term) it shows efficiency.

I was going to offer you some real world discussions, including practical facts and figures,  from a 'plumber's' site  -  actually a very valuable resource,

But it is important to actually be scientific and understand what you are talking about.

 

It would be more profitable to discuss the science of condensing boilers, particularly in relatation to your excellent graph (+1 for that), rather than continued sniping.

In particular your graph show boiler 'efficiency' falling dramatically with rising water inlet temperature, which begs the question of what is meant by efficiency?

 

Edited January 17, 2021 by studiot

[Reddwarf4ever]

Hello

I thank  you for your responses, obviously I speak a different language, I am not being rude, I just think, how I see and explain things is different to how things are interpreted on a scientific forum.

Important thing , is I got what I needed

[MigL]

What happens at the far left of that graph, when the inlet water temperature drops to 0⁰ C ?
You would have a nasty discontinuity ?

[Reddwarf4ever]

Finding it difficult to get my boiler inlet temperature below 55oC so that isn’t a problem 😂😂😂

[Endy0816]

1 hour ago, MigL said:

What happens at the far left of that graph, when the inlet water temperature drops to 0⁰ C ?
You would have a nasty discontinuity ?

Literally, as ice will start forming lol.

5 hours ago, studiot said:

In particular your graph show boiler 'efficiency' falling dramatically with rising water inlet temperature, which begs the question of what is meant by efficiency?

The lower the inlet temperature, the more heat can be recovered via phase change.

 

[studiot]

1 hour ago, Endy0816 said:

Literally, as ice will start forming lol.

The lower the inlet temperature, the more heat can be recovered via phase change.

 

I think it is rather more complicated than that.

The lower the input temperature the more heat you need to raise the water to the fixed output temperature.

It also need to be considered whether the boiler is running in DHW or CH mode.

But it is good to discuss these things to gain understanding.

[Endy0816]

3 hours ago, studiot said:

I think it is rather more complicated than that.

The lower the input temperature the more heat you need to raise the water to the fixed output temperature.

It also need to be considered whether the boiler is running in DHW or CH mode.

But it is good to discuss these things to gain understanding.

Think ideally your output temperature should vary based on heating load. If return temperature isn't low enough you won't be able to steal back the latent heat locked away. Does take the correct setup to really see the benefits.

[studiot]

9 hours ago, Endy0816 said:

Think ideally your output temperature should vary based on heating load. If return temperature isn't low enough you won't be able to steal back the latent heat locked away. Does take the correct setup to really see the benefits.

Every degree above the optimal represents wasted heat and possibly dangerous conditions.

Do you like being suddenly scalded in the shower ?

 

[Endy0816]

20 hours ago, studiot said:

Every degree above the optimal represents wasted heat and possibly dangerous conditions.

Do you like being suddenly scalded in the shower ?

 

Sorry, maybe should have been more clear but you need lower temperatures in general.

You want to go from a gas back to a liquid. You need the return line at a cool enough temperature for that.

 

 

Edited January 19, 2021 by Endy0816

[John Cuthber]

You can choose the "cold" temperature anywhere you like - even above 100C (though that's no longer a condensing boiler).
The merit of a condensing boiler is that it recovers the latent heat from the water vapour produced by burning fuel.

The amount of energy that is present depends on the amount of water vapour present, and the amount you reclaim depends on the amount of water you condense.

The colder you get the outgoing air, the more heat you can recover because more of the water condenses.
But, the law of diminishing returns sets in.

By the time you have cooled the air down to 55C you have reduced the vapour pressure by about 85% compared to 100C.

If you had a perfect mixture of methane and air you would end up with this

2CH₄ + 4 O2 + 12 N₂ --> 2CO₂ + 4H₂O +12 N2

(I have included the nitrogen in the air, even though it doesn't react.)
The outgoing flue gas would be 4 /(2+4+12) =4/18 i.e. 2/9 water vapour by volume. (and very hot) That's 22.2% water vapour

You could run that through a heat exchanger.
If you were making steam you could run the heat exchanger at 100C (not very efficient but...)
The flue gases would be at 100C
At that temperature there's going to be no condensation (at 1 atmosphere pressure)
Imagine you add a second heat exchanger to warm up the ingoing water.
What happens when you cool the gas mixture.
Well, initially not a lot. It just cools down.
You can use the sensible heat (that's a technical term) to heat the water.
But, when you get the temperature down to about 63C the water starts to condense out 

(that's the temperature where the vapour pressure is 22.2% of normal atmospheric pressure- there's a table here)
https://en.wikipedia.org/wiki/Vapour_pressure_of_water

As the gas gets cooler, more water condenses out.
When you are down to 55C, you have reduced the water content to about 15.5% or so.

And you have reclaimed (very roughly)  22.2-15.5 % of the latent heat. 7% or so.
If you cooled it to 45C you would drop the outgoing water concentration to about 9.5%
And that would let you recover more of the latent heat- 22.2-9.5 % i.e. about 12%

OK, that's better than only recovering 7% of it, but there's a price or two to pay.

First, you need a bigger heat exchanger.
Secondly you need to run the system with a cooler return.

But running a hot water system with colder water is a potential problem.

So there's a cut-off where it's not considered worthwhile.

That's an economy decision and, at the moment, the compromise between heat recovery and size (and cost) of the exchanger and the difficulty of getting a hot water system to cool the return water is about 55C

With bigger, cooler, radiators in the house, you could do better. But that would also be expensive.

So there's nothing fundamentally magical about 55C.

It's a compromise.

(Incidentally, I have ignored the heat that goes into or comes from the air, and also the fact that it expands when hot. I'm lazy and the maths is not helpful here.)

 

On 1/18/2021 at 10:47 AM, studiot said:

Do you like being suddenly scalded in the shower ?

We are talking about the temperature of the cool water return to the boiler.
It's the hot water from the boiler that scalds people.

Edited January 19, 2021 by John Cuthber

[studiot]

22 minutes ago, John Cuthber said:

We are talking about the temperature of the cool water return to the boiler.

I wasn't and the OP stated

On 1/16/2021 at 12:11 PM, Reddwarf4ever said:

inlet temperature decreases

 

You misunderstand although you posted a fair description of the partial working of a condensing boiler, it is a pity you have to always contradict me.

The situation is much more complicated, for instance there may be no 'return' flow to a boiler running in DHW mode.

But certainly if the inlet water is 100^oC or greater then the output water would scald you. In fact it would scald you at a much lower temperature.

 

The point is that the inlet water temperature is not fixed.

But the output temperature will be set (there are often adjustable controls in a boiler for this) for both DHW and CH mode for efficiency purposes.