Does it matter if boiler is slightly over sized?

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I have to choose a new gas boiler for a very small 3 bed terraced centrally heated house. Full double glazing and 6” loft insulation.
I have a Y plan system.
I posted with various questions about this system recently but have got to the main question now.
I have been warned about oversized boilers being inefficient. The problem is that according to the size calculator on the IDHEE website I only need 4.2 kw to heat the house, not including the hot water allowance of 2kw, and I am confused about the relative importance of sizing the boiler, non condensing mode, short cycling, etc. A couple of other calculators I tried still don’t go above 6kw.

I am interested in something like the Intergas HRE 18 SB or OV, which modulates down to 5.6 kw
The Veissman 100 W compact would seem to modulate from 13kw max to 1/3 of that, ie 4.3kw.
Baxi have models that go down to 2kw to 12 kw

I understand that even with these low kw boilers, there will still be a majority of times in autumn and spring where even 2kw will not be required so even an exactly sized boiler will spend most of it’s time running inefficiently. Is this correct?

Can anyone explain exactly what would happen that ideally shouldn’t happen if I use the larger two, Intergas and Veissman boilers? From what I can see on the ‘net I am forming the impression that it is accepted for condensing boilers only to condense efficiently after switch on while the house is heating up. Is there a level of inefficiency at which the system worsens from non condensing due to return being too hot, to actual short cycling, and in reality do both these situations occur in warmer weather?

Incidentally on the SEDBUK boiler efficiency database, in the tables for gas boilers, the lower wattage figure that the boiler can modulate down to is not given, only the higher figure, which is displayed twice, ie "modulating (28.4-28.4 kw). I would expect two values to be given, the lower and the higher, why is this?
Also does anyone know how quiet these boilers are in operation?
 
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the boiler will cycle excessively...but they all do that when you use on-off controls, for the simple reason that they are working to a fixed flow temperature...

You are better off using compensation controls that adjust the flow temperature to match demand..

all decent boiler have control strategies to deal with cycling...but only when you use compensation controls...for example temperature controlled running of the pump, and lower temperatures as the room temperature appoint set point...
 
the boiler will cycle excessively...but they all do that when you use on-off controls, for the simple reason that they are working to a fixed flow temperature...

You are better off using compensation controls that adjust the flow temperature to match demand.
But even if you have (weather) compensation controls, you will still have the boiler cycling if the output necessary to maintain the required water temperature is below the lowest modulating output.

Let's face it: modulation, until recently, has been seen as a way of automatically range-rating a boiler. i.e setting the maximum output. No thought was given to the lower output required for 90-95% of the time.
 
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the boiler will cycle excessively...but they all do that when you use on-off controls, for the simple reason that they are working to a fixed flow temperature...

You are better off using compensation controls that adjust the flow temperature to match demand..
Whilst modulating controls will undoubtedly be better in this regard, they are still limited by the lower output power of the boiler which, in so many cases even with highly regarded models, is way too high for many a modern home.

[Edit: Apologies, missed D_Hailsham's post where he's already covered this]

Mathew
 
You can set the parameters on the Intergas as you can with some others, however because of the design the Intergas will condensate whereas the other will not.

Alec once again mentions Compensation controls, but they are found wanting with an oversized boiler.

All you need to stop the boiler cycling, is a simple in-line stat controlling the low temperature.

So, the compensation controls the room temperature, and the in-line stat says wait till the temperature is a bit lower, could be air temperature or return temperature.
 
'mmm

if they all have the same lower figure, and will all modulate down to it, what's the point of having the four models?
 
Variations in cost, not to mention ensuring candidate representation in the marketplace at all levels.

Mathew
 
just a marketing ploy you mean? plus the cost of making four slightly different models and carrying different parts...

still can't see any true advantage.
 
As far as I am aware the Worcester 12Ri is the smallest output boiler commonly used.

Not sure what the minimum output is though.

They are very quiet in output too!

Tony
 
just a marketing ploy you mean?
More of a strategy than a ploy.

In any highly competitive market every little difference counts. This, coupled with high volume sales, has a direct correlation between success and failure of a business and its products.

If they only sold the 26kW, and assuming it works as efficiently as say their 13kW model at the same lower output, then they'd have to charge the same high cost for all boilers sold. Being able to shave off even a few tens of pounds by selling smaller models can make all the difference when pitched up against competitor's offerings.

Furthermore, there is a very real danger when producing a 'one size fits all' model because it carries the risk of being snubbed by the marketplace. If someone has a heating requirement of, say, 10kW they may not even stop to consider a 26kW offering even if unbeknown to them it can deliver 10kW just as efficiently as a competitor's 12kW model.

still can't see any true advantage.
You don't have to - it's their business and they obviously do. Their bean counters wouldn't allow it to happen if it didn't directly correlate with a sales benefit.

Mathew
 
There is usually little difference in manufacturing costs between different power outputs.

Some makes set the power in software or with a plug in identity module.

On those the low power models are then sold for less.
 
I can well believe it. It is very common in consumer electronics - the very same innards will be shared amongst a variety of models throughout a range and will be selectively crippled as part of production in order to produce the desired lineup.

Mathew
 

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