Oversized boiler - efficiency issues?

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Install larger radiators.
Set boiler thermostat to 60c.
Check/tweek CO2% in order to maximise flue gas dew point temperature.
.....or maybe chill out and go get a beer.:rolleyes:
Measuring condensate discharge volume is another route you may want to take in order to establish maximum efficiency.
 
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Different people!

But I am surprised that David has not noticed that this is a highly efficient boiler with a good turndown ratio and a minimum which would have been the same as the previous model 18 kW boiler which should have been installed to match the heat load of the house.

Whatever is done is going to make only a very minor difference to the operating costs.

Weather compensation ( ignoring manufacturer's hype ) is normally only considered to give an improvement in gas consumption of about 3-6%.

Even Alec is unlikely to claim more!
 
Is Waldorv and D_Hailsham one of the same

No, we're not. Strange question.

But my original question seems to get lost in the branches of this thread.

Basically:

If flow over heat exchanger is over 1200 l/h, with a differential of around 5 degrees
but
flow through radiators is only around 700 l/h, with a differential of around 11 degrees

that is because

bypass opens up to let hot water back into return. This is with bypass closed to the max for the boiler.


Q1: Does that negatively affect efficiency?

and if so

Q2: What could be the cause and how to improve?

I think the third Q, (will WC help anything), has been answered as only barely being cost effective on the medium term. So that's probably not worth the trouble.
 
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Not sure why your being so anal with regard to efficiency?...


Sorry missed this one.

Quite simple: I live in a 5 bed detached house with bedrooms kept around coolish and only 3 rooms at 21. But my annual usage is 37000 kW. Bit steep I think.

It's double glazed, cavity insulated, loft insulation ok; house stays warm for hours once it's warm and heating is turned off so feels like it's properly insulated.
But costs a fortune to run.


Waldo
 
Basically no it won't affect efficiency, your problem is your heat exchanger is too hot to cool the flow gases down. Don't worry about temperature difference, worry about the average temperature of the heat exchanger. Ideally you'd build your house out of radiators and have the flow as 21c and the return as 20c therefore getting the best efficiency as the flue gas would be <100c and fully condensed
But clearly your heat demand is not great so just whack the flow temperature and therefore return down and put in some bigger radiators.
Hopefully that answers your questions
 
Below the dew point just means that some of the water will condense as the air is saturated. You actually want as much to condense as possible, so the lower the better. But you definitely want the latent heat of vaporisation which would be when it drops below 100c, as that's a huge amount of energy for the temperature drop.
 
I mean to say you want saturated air only to leave the flue, not steam. Any additional condensation by reducing the temperature further is a bonus.
 
But my original question seems to get lost in the branches of this thread.

Basically:

If flow over heat exchanger is over 1200 l/h, with a differential of around 5 degrees
but
flow through radiators is only around 700 l/h, with a differential of around 11 degrees

that is because

bypass opens up to let hot water back into return. This is with bypass closed to the max for the boiler.


Q1: Does that negatively affect efficiency?


I have to disagree with some of the advice given.

The efficiency of boilers depends to a large degree on the return temperature being low to allow more condensing of the combustion gases.

Ideally the return temperature should be around 55 C which with a differential of 20 C would provide a flow temperature of 75 C which is high enough for any heating purpose.

If the boiler is bypassing then that will raise the internal return temperature and significantly reduce the efficiency.

Others may be able to comment further on why your boiler seems to be allowing bypassing. On a large system like yours the system flow resistance would usually be quite low if the pipework sizes are correct.

It also sounds as if you may have been fiddling inside the boiler. We do not advise DIYers to open a sealed boiler for safety reasons.

Tony
 
I have to disagree with some of the advice given.

It also sounds as if you may have been fiddling inside the boiler. We do not advise DIYers to open a sealed boiler for safety reasons.

Tony

Don't worry, the Gas Certified service repair man adjusted the ABV, on advice from the Vaillant helpdesk by telephone.

Thanks for the input @Agile, it looks like opinion is divided, but I think you are correct, it shouldn't bypass and it's bad for performance.

Problem is, how to find out what is causing this. I think I'll have another try with a vaillant certified installer around here, here's hoping they can shed some more light on the matter...
 
I am always suspicious of any RGI who needs to call a manufacturer over routine matters.

Why was he contemplating changing the ABV setting?

They come from the factory set so there is no bypassing on a low resistance heating circuit and with red paint on them to indicate if they have been changed. This factory setting is close to fully screwed in. So any adjustment is normally to allow bypassing at a lower pressure by unscrewing them.

Their purpose is to allow a minimum flow through the boiler if there was little externally. Such as if all TRVs had closed.

But on a large system I cannot see any reason why anyone would want to change it from the factory setting.

So what was he trying to achieve?

Tony
 
@Agile if I recall correctly this was trying to push more flow through the radiators, we had a discussion about house a bit slow warming up during his maintenance visit. I asked about powerflush and if that would help but he said the system didn't seem sludged up and was coping well with the boiler on full.

Agree that calling helpdesk didn't make me feel he knew all the details about these boilers.

I will have a look at the vaillant website see if they can recommend a local specialist.
 
Do you know which way he turned the adjustment screw?

If he felt he had to call Vaillant, it makes me wonder if he turned it the wrong way!

But since he was not the installer, we presumably don't know if the installer had fiddled with it !

Tony
 

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