Correctly Sized Boiler

[stw1974]

I've come to realise that the boiler I have fitted is oversized for my needs. It's a Vaillant Ecotech 428. However, any sizing calculation I do comes out with a maximum heat requirement of about 18kW.

My question is, is it less efficient to run the 428 with the output dialled down than to run a more appropriately sized boiler near flat out?

 

[oldbuffer]

The 428 will modulate down to a minimum heat output (CH) of about 5 1/2 KW, so it should sense that the load you've got on it doesn't require full output. Also, you can adjust parameter d.0 to limit its maximum output. (See installation manual).

I'd say your maximum heat requirement is comfortably within the boilers modulating range, and it should be just as efficient as a smaller boiler.

 

[Dan Robinson]

Bit late to worry about it now.

 

[ScottishGasMan]

To answer the question, yes, its more efficiant to run a boiler that is sized correctly (ie a 418) but not so much that your going to notice much difference in the gas bills (certainly not enough to ever justify paying to swap it now unless the size of the current one is giving you problems)

Do you have an Unvented hot water cylinder? remember if you do, the coil in these alone can handle over 20kw in many cases? old indirect cylinders much less (4ish kw)

 

[stw1974]

Thanks for the helpful answers.

I've currently got an old indirect cylinder but was planning on replacing it with an indirect fast recovery one. I don't want an unvented cylinder as the inlet pipe to my house is small bore and whilst the pressure is high enough, flow rate is low.

 

[Agile]

As a matter of interest, how did you come to choose to fit such an oversized boiler?

Tony

 

[vulcancontinental]

The thing is that your max requirement is 18KW but only when the whole house is cold and it#s -1 or -3 degrees outside.

Most of the time the house is up to temp so the boiler is combating the loss only and the weather is not extreme.

At those times your loss is around 3KW so your boiler output will be just under twice the size it needs to be for most of it's life.

That's why the manufacturer's are trying for better modulation, to cut down cycling.

 

[gas112]

An indirect cylinder ? are you sure about that

 

[stw1974]

As a matter of interest, how did you come to choose to fit such an oversized boiler?

Tony

The fitter originally suggested a combi boiler, 38kW. After testing the water pressure and flow rates I told them I wanted to stick with a cylinder and use a pump for the showers. I trusted the fitter with their choice of the 28kW alternative instead of at least having an idea about it for myself!

 

[stw1974]

An indirect cylinder ? are you sure about that

Pretty sure. The boiler is system, not combi.

 

[Agile]

If only you had read here first and in the FAQ then you would have seen just how bad oversizing is.

Very unfortunately many boiler installers still dont know how to properly size boilers and some I can only guess dont trust their judgement and knowingly fit an oversized boiler.

It can be very bad too. I saw a 32 kW boiler fitted in a house with a heating loss of just 11 kW.

Another a 28 kW ( combi admittedly ) fitted in bedsitter with a single 2 kW rad !

Tony

 

[Agile]

If only you had read here first and in the FAQ then you would have seen just how bad oversizing is.

Very unfortunately many boiler installers still dont know how to properly size boilers and some I can only guess dont trust their judgement and knowingly fit an oversized boiler.

It can be very bad too. I saw a 32 kW boiler fitted in a house with a heating loss of just 11 kW.

Another a 28 kW ( combi admittedly ) fitted in bedsitter with a single 2 kW rad !

Tony

 

[vulcancontinental]

A study in Germany (early 2000's) resulted in the conclusion that condensing boiler efficiency is more like 78% rather than the 90 plus the current test methods predict. This is due to unrepresentative test procedures and oversizing.

 

[gas112]

stw74 ignore me i read it as direct cylinder even though for some reason i wrote indirect

 

[stw1974]

stw74 ignore me i read it as direct cylinder even though for some reason i wrote indirect

OK, thanks for letting me know