Combi Boiler size - does it matter?

[jesteh]

What is more important when you decide on size of your combi boiler - radiator output or the flow rate?
Speaking to Worcester Bosch tech support, they are telling me that you shouldn't look at radiator output but 'how many taps you want to use at the same time'. And yet, there is plenty of recommendation in forums and energy saving websites that you need to choose as small a boiler as you can get away with based on radiator output. Our radiators would only need 15KW output so it seems daft to choose 30KW our plumber recommends. And yet, Worcester Bosch web quiz recommends same as the plumber. Further, on asking WB tech support, they tell me that WB boilers can modulate down and this does not affect efficiency of the boiler at all.

So, that would mean that only flow rates matter on modern boiler and I should choose one based on how many bathrooms I have and people liekly to be using them at the same time. Is that true?

 

[Inky Pete]

You'll need to size it for whatever your maximum load on it is.

Usually this will be for your hot water demand.

No point having a small boiler which will do fine for the heating but won't support your hot water usage.

 

[Gasman77]

Choose a vaillant combi, you can range rate the heating down to what you need and. Still have a 30kw of power on the hot water side of the combi

 

[45yearsagasman]

Realistically you cannot use a combi to supply more than one tap at a time and expect good performance.
The flow rate is the most important thing to consider, so 30 Kw is good, but it still won't supply 2 taps at the same time.

 

[bengasman]

Boiler sizing for ch demand became pretty much obsolete when modulation became the norm, which makes it a matter of decades ago. Around 10% increase in price on the whole job covers you against all future increases in demand. Apart from large projects where every pound counts, it makes it a no-brainer.

The fact that you are asking this question on the forum, shows that you have not found a good installer. Finding a good installer is really the only thing you should be spending your time on: he will answer all your questions for you and a lot better than we can from behind a keyboard.

 

[D_Hailsham]

Boiler sizing for ch demand became pretty much obsolete when modulation became the norm, which makes it a matter of decades ago.

That's OK provided the boiler can modulate down below the calculated CH demand.

I assume that, when only fixed output or range-rated boilers boilers were available, you would have selected the nearest one above the calculated CH demand. So why do differently just because it is a combi boiler?

If you size purely on the HW requirement/flow rate you could end up with a boiler which is considerably oversized for the CH as it cannot modulate low enough.

 

[bengasman]

I assume that, when only fixed output or range-rated boilers boilers were available, you would have selected the nearest one above the calculated CH demand.

Like I said, that became obsolete around 30 years ago.

So why do differently just because it is a combi boiler?

One of the best attempts you have made so far in demonstrating your lack of understanding about boilers and central heating in the real world.

If you size purely on the HW requirement/flow rate you could end up with a boiler which is considerably oversized for the CH as it cannot modulate low enough.

My god, you really don't have a clue, do you?
Even the biggest combi commonly used in he domestic market will modulate down to about the same level as a very small set rate old fashioned boiler.

 

[jesteh]

Thanks for all the replies.
As far as modulating - I looked at the specs before asking the questions and I agree with bengasman - even the biggest boiler goes down to 10KW (which is less than the smallest one you can buy). 10KW is around requirement for a small 3 bedroom house.
My only remaining question is around the efficiency when the boiler has modulated down - whether it remains the same or not.
As far as asking my plumber all these questions - I didn't want to ask him as he may misconstrue it as lack of trust that I question his choice, where its more of an active interest that what I am getting is right for me.

 

[jesteh]

Bengasman - related to your comment and why I don't want to question plumber too much. I have asked the plumber to breakdown the quote he's given me for central heating and bathroom installation as he only told me what boiler he's using and how many radiators/bathrooms he's fitting. So far he's not very happy breaking it down further than that.

I am not sure whether its me or him who's being odd. What I wanted to know was size and type of radiators he's putting into rooms, type of TRVs, what he's flushing system with and general overview of materials he's using - not down to every pipe but just a fairly good idea - he must know as he's quoted on basis of it. He says that shows I don't trust him to put right size radiators, etc. I can't disagree with him but I want to make sure what he's fitting is right for me. The way I see it, if I see what materials he's supplying, I can't question any of his choices later on. He sees it as lack of trust and wants me to let him get on with it to best of his judgment. I am happy to let him get on once he's told me exactly what with. I guess we just haven't 'clicked' so far - its a shame as he seems decent and a good plumber .

 

[mickyg]

should be no problem with him telling you what materials he's using, i put this information on all my quotes anyway.

 

[D_Hailsham]

10KW is around requirement for a small 3 bedroom house.

It will all depend on how old the house is and how well insulated it is. A modern small 3 bed mid-terrace built to the latest standards for insulation will only need about 5kW to heat it. If you install a boiler which can only go down to 10kW, it will spend all the time in on/off mode and the advantages of modulation will be lost.

 

[commercialman]

5kw/17k btu ........................3 beds to heat , living room/diner , hallway , bathroom , kitchen..... i can guarantee with outside temperatures in the region of -1 degrees c the householder WILL be wearing an overcoat. ..............it aint happening in the real world.

 

[BGEngineer]

A modern small 3 bed mid-terrace built to the latest standards for insulation will only need about 5kW to heat it. If you install a boiler which can only go down to 10kW, it will spend all the time in on/off mode and the advantages of modulation will be lost.

If this is the case the OP might as well go down argos and pick up a couple of electric fan heaters 2.5Kw each for 9.99 each and be done with it lol

 

[Servotech]

Bengasman - related to your comment and why I don't want to question plumber too much. I have asked the plumber to breakdown the quote he's given me for central heating and bathroom installation as he only told me what boiler he's using and how many radiators/bathrooms he's fitting. So far he's not very happy breaking it down further than that.

I am not sure whether its me or him who's being odd. What I wanted to know was size and type of radiators he's putting into rooms, type of TRVs, what he's flushing system with and general overview of materials he's using - not down to every pipe but just a fairly good idea - he must know as he's quoted on basis of it. He says that shows I don't trust him to put right size radiators, etc. I can't disagree with him but I want to make sure what he's fitting is right for me. The way I see it, if I see what materials he's supplying, I can't question any of his choices later on. He sees it as lack of trust and wants me to let him get on with it to best of his judgment. I am happy to let him get on once he's told me exactly what with. I guess we just haven't 'clicked' so far - its a shame as he seems decent and a good plumber .

I know plenty of good but grumpy grumpy plumbers - infact it's almost a trade requirement

 

[bengasman]

...Bengasman - related to your comment and why I don't want to question plumber too much. I have asked the plumber to breakdown the quote he's given me ...What I wanted to know was size and type of radiators he's putting into rooms, type of TRVs, what he's flushing system with and general overview of materials he's using - not down to every pipe but just a fairly good idea - ...

Personally, I quote an all in figure, and don't break it down price wise.
I do discus with people what boiler they get and kind/what size rads.

He may find it a pain in the ass, but you are entitled to know what he is going to install, as he is unlikely to change anything once it's there. Must admit I find it a bit odd he didn't give you any choices, unless it is a budget install; beggars can't be choosers.