Is anyone aware of the effect oversizing boilers has on reducing efficiency? I 'm particularly interested in the numbers.

Eg. Assume a domestic hot water demand of 30kW. if a 40kW 90% efficient condensing boiler is installed, what is the maximum efficiency likely to be reached?

Hi Matt, I should have said we are talking about a combi boiler. I have heard that about modulating boilers as well. I don't know if its true either - anyone?

Re the oversizing - As far as I know , combi boiler size is promarilly detemined by flow rate, with the other factors being constant [am I wrong?]. If I'm right, an oversized boiler cannot ever reach full capacity because there is simply insufficient flow of water! Therfore it will only ever run at part-load and not reach optimum efficiency [at full load]

Posted By: Mike George As far as I know , combi boiler size is promarilly detemined by flow rate, with the other factors being constant [am I wrong?].

I prefer to think of it the other way around. The max flow rate depends on the max boiler power. If you have multiple showers you may need all the flow rate you can get. Info on the web says that most people consider 15L/min to provide a good drenching shower. Around 7-8L/min is the governments idea of a low flow water saving shower. Check out the specs for the combi you have in mind. Do you need to be able to run two showers or one shower and the washing machine at the same time?

I've heard of gas & oil boilers having efficiencies of over 100% before, but I've never understood how that works.

Can anyone explain the maths behind that?

Cheers

Impossible for a boiler to run at 100% efficiency, that would mean no losses and and all of the input energy i.e. gas is directly turned into heat - achieve that and we have perpetual motion. Combi boilers are pretty much always oversized when it comes to the heating load because the high output is needed for the hot water flow rate. Boilers are also traditionally oversized because they are used for spike or intermittent heating. Oversizing of fully modulating boilers doesn't really matter as the controls look at the flow temperature and modulate the output down when the set point flow temp is being approached. The boiler then matches it's output to the system load. However, the downside is that from initial switch on the boiler will run at maximum output which will give a very rapid heat up of the system and when the return water temp. exceeds 52'C the boiler will no longer run in condensing mode. To get the most from a condensing boiler in "the perfect world" either the boiler needs to be undersized or the radiators oversized - the boiler needs to be working hard to maximise the running time in condensing mode i.e. return water temp below 52'C. Sorry, can't give you any figures as to efficiency but what I can say is that it will always be less than the quoted SEDBUK rating. I would suggest a minimum efficiency of 5 % below SEDBUK.

how about showing them the EST whole house boiler sizing guide ,let them work it out themselves with this
there is a online version

sorry, forgot about the combi bit ,
Do modern combi boiler modulate in heating mode , if so then its not such a problem as Dantenz mentions
(do they modulate in HW mode ?)

Any idea Dantenz?

http://www.worcester-bosch.co.uk/homeowner/products/gas-fired-boilers-and-systems/greenstar-27cdi/literature
Have a look at the tech/spec on page 6-7
"central heating mode ,the boiler initially fire at minimum output before modulating upwards to meet the actual system requirements"
"hot water mode electronic controls modulate the boiler output downwards to ensure HW temperature remains between 40-60oC"

From the manifacture gumf, but it looks like this combi does modulate in CH and HW mode
also note the minium max outputs in both modes on page 4

Yes, combi boilers (gas that is) modulate in both CH & HW mode. As for the comment about it not being such a problem then I would say this. A fully modulating boiler that is better matched to the peak heating load will always return a better efficiency than one that is oversized. You have to realise that a 30 kW condensing boiler firing into a 18 kW load will run efficiently for only a short period of time, that is until the return temp reaches 52'C and then it doesn't condense anymore. On further temperature rise the burner output will then modulate down to match the load of the system albeit, still no longer condensing. This is likely to happen after only some 15 mins operation or so. Compare this to an 18 kW boiler which then operates in condensing mode for longer (perhaps 30 mins) = improved efficiency before it too modulates down. You could argue the savings against an oversized modulating condensing boiler compared to those of a correctly sizied modulating boiler are small however, there is a case to argue.

Posted By: jamesingramDelboy, sounds impossible to me, how could you get more than 100% energy out of something unless it was coming from somewhere else?

It's not possible for a boiler to achieve 100% or more due to the laws on COE. HOWEVER figures in manufacturers literature of 102% are valid due to the way the "efficiency" is defined and calculated...

The problem is working out how much energy is going into the boiler. eg How much energy does the fuel contain? In the past it wasn't possible to extract the latent heat of vaporization from the flue gas - so that was ignored when calculating the energy content of the fuel. This makes it look like the fuel contains less energy than it really does and it makes non-condensing boilers look slightly better than they really are. Back then this didn't matter as the figures were under 100%.

These days it IS possible to extract this energy (eg using condensing boilers) but we haven't corrected our assumption about the amount of energy that exists in the fuel. So the result is that some boilers appear to be just over 100% efficient.

If we wanted to correct the problem we would have to rewrite all the manufacturers data sheets and web sites and then state if the figures were calculated using the old or new method.

Pushing this concept one stage further... One could argue that the energy content of the fuel is a lot higher still. For example Einstien said E=MC^2 ..but we don't use that method of calculating the energy content of the fuel because it would be rather difficult to extract it in a regular boiler!

Hi,
Just to clarify - I had to look it up
Net eff is with the fuel burnt but exhaust is basically vented.
Gross eff is with the fuel burnt but with all the exhuast giving up its latent heat throught eh condensation process. This is more often quote in europe on the basis that everything is condensing.
Basically the condensing part is just that, its a waste heat recovery unit what in industry has always been called an economiser.
Add on units to do this are begining to come on to the UK market (gas only). Oil has a much lower dew point at 47deg so to be meaningful the return temp would have to be 45ish deg, whereas with gas its about 55deg so you get a reasonably higher radiator return temp and hence a better delivery temp.
An alternative to heating the boiler feed is to use the exhuast gas to heat the incoming combustion air. some boiler manufacturers do this.
How complete the condensing would be is difficult to say in practise.

I thinkl Combis always get a default lower rating becuase of their irregular operration eveen including the heat store type.

Cheers, Mike up North