Boiler size - conflicting engineer advice

[Steelmasons]

No it wasn't .

Absolutely naff all point installing a high powered combi. Look at the other thread that's popped up this evening with his 32kW Wooshitter.

Take a break Dan FFS...

 

[Dan Robinson]

I always recommend a 28 kW or 32 kW because they can provide more flow in the winter.

Why is the flow different in the winter? The flow will be exactly the same. The end result of the temp rise might be different though.

 

[Dan Robinson]

//www.diynot.com/diy/threads/is-there-something-wrong-with-my-boiler.464120/#post-3698234

 

[Steelmasons]

Can only speak from experience Steel .

Something I lack Dan..

 

[Agile]

Because the flow rate has to be reduced at the tap or shower to get the hot water at about 43 C which most people expect for their shower.

 

[Razor900]

//www.diynot.com/diy/threads/is-there-something-wrong-with-my-boiler.464120/#post-3698234

I would say that thread is more about hydraulic gradient than it being a combi Dan

 

[bernardgreen]

Boilers with a large modulation range have a lower minimum power output, and so are more efficient when running your heating.

Something I have never seen ( in the public domain ) are efficiency ratings for boilers that are modulating. Maybe a boiler is 90% efficient at its full output of say 24 kW but how efficient is it when modulated down to say 6 kW ? Can these figures be made avialable to people who are choosing a new boiler ?

 

[J_N30]

I'd go unvented myself.

 

[D_Hailsham]

Maybe a boiler is 90% efficient at its full output of say 24 kW but how efficient is it when modulated down to say 6 kW ?

It will all depend on how the boiler is controlled.

When a boiler is quoted as having an efficiency of 90% that is for a flow temp of 80C and return of 60C, i.e above the point where condensing begins. If the boiler is controlled to a fixed flow temperature - not weather compensated - it will still be running with the same temperatures when modulated down to minimum (not in on/off mode), so efficiency should not change.

If weather compensated, the required flow temperature will reduce as the outside temperature rises, so the boiler will modulate down to maintain the lower flow temperature. The efficiency will increase the further the return drops below 55C.

If the boiler goes into on/off mode, who knows what effect that will have on efficiency? If the boiler is designed to always start at maximum and the modulate down, efficiency will drop considerably; but if the boiler start at minimum and the modulates up, the efficiency may not change very much.

 

[bernardgreen]

When a boiler is quoted as having an efficiency of 90% that is for a flow temp of 80C and return of 60C, i.e above the point where condensing begins.

In which case the efficiency when the return is below the dew point (where condensing begins) will be ( or should be ) several percent points higher than 90%. I do not think that is the case, I believe the quoted 90% is when the boiler is condensing and that the efficiency drops ( far ) below 90%. This seems to be borne out by the figures below where summer seasonal efficiency drops to 79.4% compared to ( quoted ) 90.1% winter seasonal efficiency

Some data from a reliable source http://www.ncm-pcdb.org.uk/sap/

For a boiler that some on this forum consider to be rubbish

Fuel mains gas
SAP 2009/2012 annual efficiency (%) 89.1
SAP winter seasonal efficiency (%) 90.1
SAP summer seasonal efficiency (%) 79.4
Comparative hot water efficiency (%) 58.0
SAP 2005 seasonal efficiency (%) 90.7
Efficiency category SEDBUK based on certified data
SAP equation used 102
Output power (kW) 16.0
Electrical power when firing (W) 10
Electrical power not firing (W) 0

And for a boiler that is highly praised by some on this forum

Fuel mains gas
SAP 2009/2012 annual efficiency (%) 88.9
SAP winter seasonal efficiency (%) 89.9
SAP summer seasonal efficiency (%) 79.2
Comparative hot water efficiency (%) 57.9
SAP 2005 seasonal efficiency (%) 90.1
Efficiency category SEDBUK based on certified data
SAP equation used 102
Output power (kW) 18.2
Electrical power when firing (W) 130
Electrical power not firing (W) 2

 

[Agile]

Amazing what you can find on the internet.

Interpreting it may be a little harder though.

 

[bernardgreen]

The results tends to agree with results obtained by some measurements on boilers intalled in houses as part of a university project.

 

[DP]

Tony please stop under estimating other people. It is getting tiresome.

Bernardgreen must have something useful to say, he certainly has a better thank you score than your good self

 

[Agile]

Bernardgreen must have something useful to say, he certainly has a better thank you score than your good self

Ask Dan to identify one!

 

[D_Hailsham]

In which case the efficiency when the return is below the dew point (where condensing begins) will be ( or should be ) several percent points higher than 90%. I do not think that is the case, I believe the quoted 90% is when the boiler is condensing and that the efficiency drops ( far ) below 90%. This seems to be borne out by the figures below where summer seasonal efficiency drops to 79.4% compared to ( quoted ) 90.1% winter seasonal efficiency.

I wasn't talking about the theoretical efficiency, as calculated using the SAP equations, but the measured efficiency as given in manufacturers' specifications.

As for the two examples you quoted (pity you didn't identify the boilers), when the data was available on the Sedbuk website there was a warning that differences between boilers was only significant if greater than 3%.

The formula used for calculating the annual/winter/summer efficiencies can be found in SAP 2012 Appendix D.