Condensing Boiler Efficiency

[bernardgreen]

If all the Water Vapour condenses in the boiler, then the boiler is working as efficiently as it can. (Presumably the manufacturer quoted efficiency)

This is a can of worms.

In central heating mode the boiler cannot condense all the water vapour if the temperature of the water returning from the radiators is above the dew point temperature. In most houses except for the first few minutes of heating the return water temperature is above dew point temperature

For domestic water heating a combi boiler could be fully condensing as the water supply is well below the dew point temperature. However the design of most domestic combi boilers is such that the domestic hot water is heated in a secondary heat exchanger which is heated by the water used to heat the radiators but diverted from the radiators to feed the secondary heat exchanger. Hence the heat exchanger heated by the combustion does not receive the supply water ( below dew point temperature ) but instead receives water returning from the secondary heat exchanger. This is likely to be above dew point temperature.

The test rig used to "prove" the claimed efficiency may be rather different from the real life situation where the boiler is actually installed.

 

[Steelmasons]

CO2% in flue gas plays a considerable role.

Bernard , have you any input on the above??

 

[Steelmasons]

Why not offer constructive critique of the information rather than purile attempts to discredit the person.

Why should you be given credit for something you have copied off the internet???

 

[transam]

1 cubic metre of natural gas will in "theory" produce 1 litre of condensate

afaik ??

basically there are 2 types of heat (boilers)

1 = sensible heat . which is heat that can be sensed by the boiler.

2= latent heat. latent comes from the latin word (I think ?) Latarre which means hidden . So latent heat is hidden heat.

A standard efficiency boiler can only extract sensible heat

a condenser is able to extract both types. latent heat via flue heat recovery.


(dunno if the above is relevant to the O.P but I just thought I would mention it )

1 Kw of energy will raise the temp of 1 litre of water by Hmmm forgot is it 5 degs c ??????

 

[oilboffin]

Now when you run a new high efficient boiler,you have to have a high return temp because
if you don't you can reach what is called the dew point.this is when you get condensation forming in the combustion chamber,now being that this dew would be acidic this can cause corrosion,which is why the high return temp is required to stop this,so with the higher temp there is reason to think
that the higher the temp then probably you would get less condensate.

 

[Steelmasons]

Now when you run a new high efficient boiler,you have to have a high return temp because
if you don't you can reach what is called the dew point.this is when you get condensation forming in the combustion chamber,now being that this dew would be acidic this can cause corrosion,which is why the high return temp is required to stop this,so with the higher temp there is reason to think
that the higher the temp then probably you would get less condensate.

Eh????
I think you are getting confused with the standard efficiency boilers whereby low return temperatures caused premature failure due to back end rot........

 

[oilboffin]

No I was told at Worcester that the old back end rot can still affect the new boilers

 

[Dan Robinson]

I wouldn't be surprised of a Wooshitter. But are you not confusing gas and oil?

 

[vulcancontinental]

Transam it's more like 1.465 litres condesate per cu m methane. Latent heat is released when the molecules in the vapour condense. If they condense in the flue the heat is not transferred to system water. Approx .9kw latent heat potentially available per cu m but you'd be doing well to get between .2 and .5 kw per cu m. manufacturers figures are just that; figures. Volkswagen operate a test facility for boiler operation.

 

[bernardgreen]

If they condense in the flue the heat is not transferred to system water.

But in a concentric flue some of that heat might be recovered in warming the air being taking into the combustion chamber.

manufacturers figures are just that; figures.

as the Building Research Establishment found back in 2008.

 

[vulcancontinental]

But in a concentric flue some of that heat might be recovered in warming the air being taking into the combustion chamber

Yes, I understand Viessmann use this arguement but the air usually enters a combustion box before mixing with fuel and entering the chamber. I look at it differently, it cools the warmer ambient temp in the box just not as much. It's all miniscule amounts anyway.

 

[Dan Robinson]

I'll test it later, but I'd wager POC transfer feck all to the incoming air. My flue is a out 3m vertical.

 

[bernardgreen]

A finned stainless steel separator in the flue could recover a lot of the heat from the water vapour. But probably not cost effective in a domestic boiler

it cools the warmer ambient temp in the box just not as much.

but if it reduces the temperature of the flue gases leaving the property then regardless of where the heat energy goes less heat energy has "escaped" from the building.

It's all miniscule amounts anyway.

true in the domestic market

 

[Dan Robinson]

A finned stainless steel separator in the flue could recover a lot of the heat from the water vapour. B

Wow Berny, why didn't someone think of that before?

 

[bernardgreen]

Wow Berny, why didn't someone think of that before?

They did, way way back in the 1960's, and probably even before that. Never the less thank you for thinking I had "invented" the idea but the reality is I am only passing on knowledge I had aquired before even the internet was invented.