New Boiler: Getting contradictory advice about size

[jweaver]

Thanks all... The point many of you made was also made by the fitters. If a 12kW has been fine, why upgrade... But the point is, I don't know if its been as good as it can be and whether it could be better and/or heat the house quicker as I have nothing to compare with..

So I am swaying towards an 18kW, just so I match the final (identical) houses that were built... But I still wonder if I should go with the recommendation of 15kW? But is it worth stressing over 3kW?

 

[jweaver]

Do you get new everything when a fault occurs?

Was that aimed at me? Believe me, I run everything into the ground and fix anything I can until its beyond repair. This boiler would have been scrapped years ago, but I sourced cheap parts to keep it running.. But its finally reached end of life.. It seems that in the 10 years before I moved in, it was never serviced so the heat exchanger is a block of scale/coke.. And there is a leak in the gasket between the burner and heater, resulting in scorch marks on the casing.

Its had it.. Its 15 years old and has had a rough life.. And I believe I managed to keep it running a lot longer than it could/should have.. I think I am the last person in the estate to get it swapped, so I have done quite well.

Why do people feel the need to be so condensing when a genuine question is asked? Not everyone in life is an idiot you know!

 

[vulcancontinental]

Consider the Vokera 20s, fantastic bit of kit. Will run as low as 1.5kW.

Thank you for the mention (I work for them so there's no ambiguity on my bias) but the minimum is 1.77kW and it can be range rated to minimum so 20 limited to 11 or anything else would be no issue.

 

[denso13]

Thank you for the mention (I work for them so there's no ambiguity on my bias) but the minimum is 1.77kW and it can be range rated to minimum so 20 limited to 11 or anything else would be no issue.

I know mate, I recommend these whenever I can.

 

[CountryFan]

Was that aimed at me?

Yes. If you read what you wrote, maybe you would think the same.

However, its finally developed a fault time for a new one,

No mention of what you have just written above

Why do people feel the need to be so condensing when a genuine question is asked? Not everyone in life is an idiot you know!

I don’t know the relevance to condensing, maybe you meant condescending? I don’t think you are an idiot, but you didn’t give much to go on.

 

[jweaver]

Yes. If you read what you wrote, maybe you would think the same.

No mention of what you have just written above

I don’t know the relevance to condensing, maybe you meant condescending? I don’t think you are an idiot, but you didn’t give much to go on.

Bravo!

 

[flameport]

But all this said, my HE12 is fine.. It can easily heat the house/water on the coldest of days.

So it's probably oversized already.
On the coldest of days, it should be working flat out to just about maintain the required temperature.

But the point is, I don't know if its been as good as it can be and whether it could be better and/or heat the house quicker as I have nothing to compare with..

The boiler you have easily heats the house on the coldest of days.
There is no quicker/better with a a bigger boiler - it's already doing what it's supposed to do.

Theoretically if you installed a massively oversized boiler and also replaced all of the radiators with oversized ones and upsized most of the pipework, then it could heat up the building from cold faster.
However that just leaves you with a massively oversized and inefficient mess, plus a huge bill for all of that unnecessary work, all so you can heat the house up a bit quicker on the one day a year when you return home from holiday with the temperature outside at -6C and the heating hasn't been switched on for a week.

So I am swaying towards an 18kW, just so I match the final (identical) houses that were built...

What other people have is irrelevant. Very likely their boilers are grossly oversized as well. Plenty of boilers are, due to persons that just sling in the largest piece with no consideration of what's actually required.
The same culprits also leave the boiler at the default flow temperature of 70C or whatever, to ensure that the maximum amount of gas is used and their 'new' condensing boiler never does.

 

[vulcancontinental]

The only benefit from having an oversized boiler is if is capable of being range rated to the actual heat loss for the coldest weather and modulating low enough to match the load at 10⁰C - 14⁰C outside. When the heat exchanger surface area is bigger than the load the ratio improves.

 

[jweaver]

So it's probably oversized already.
On the coldest of days, it should be working flat out to just about maintain the required temperature.



The boiler you have easily heats the house on the coldest of days.
There is no quicker/better with a a bigger boiler - it's already doing what it's supposed to do.

Theoretically if you installed a massively oversized boiler and also replaced all of the radiators with oversized ones and upsized most of the pipework, then it could heat up the building from cold faster.
However that just leaves you with a massively oversized and inefficient mess, plus a huge bill for all of that unnecessary work, all so you can heat the house up a bit quicker on the one day a year when you return home from holiday with the temperature outside at -6C and the heating hasn't been switched on for a week.



What other people have is irrelevant. Very likely their boilers are grossly oversized as well. Plenty of boilers are, due to persons that just sling in the largest piece with no consideration of what's actually required.
The same culprits also leave the boiler at the default flow temperature of 70C or whatever, to ensure that the maximum amount of gas is used and their 'new' condensing boiler never does.

I take on board everything you say.. But there are a couple of things I didn't mention.

1.. Whilst it heats the house up to temp, I am not sure it does this as quickly/efficiently/effectively as it could as I have nothing to compare it too... All I know is that in the 2 years it took to build the estate, someone must have made a decision to put in 15s and then 18s as they went around..

2. I only live in the house with my partner and have an Evohome which can control 12 "zones".. This means that several of my rooms/zones are not active and only around 10-12 of the rads are actually in use.... I want to make sure I am future proofing by ensuring that the boiler is fit for purpose, should my circumstances change, or I sell the house.

And to me, it just feels like a 12kW isn't quite enough...

 

[trojanhawrs]

And to me, it just feels like a 12kW isn't quite enough...

The boiler can only work as hard as the radiators let it. If you could be bothered you could add the wattages of all your radiators and see what it comes to. Nonetheless, the majority of the time that max rating will be far more than what's required and the boiler will cycle if it doesn't have the ability to modulate.

 

[denso13]

And to me, it just feels like a 12kW isn't quite enough...

As I said previously an 20kw Vokera Pinnacle, will cover any expansion and will also modulate to less than 2kw.

 

[drives]

And to me, it just feels like a 12kW isn't quite enough...

12kW is a lot. Even going by what you describe (5 bed, 18 radiators, albeit what appears to be a newish build), it will likely be too much.

We’re a 5 bed with 17 radiators and quite poor insulation in parts. Our heat loss is around 8kW. We have a 12kW unit and it’s plenty. It’s an ASHP, so its output at -2°C is slightly lower than the badge rating, which is why we need a bit of headroom. Obviously that doesn’t apply to a boiler. 18–20kW is getting into the realms of what you’d need to heat a large mansion. Advice that you need a 24kW- these companies clearly have no idea what they are doing.

Fundamentally, why would you pay more for a higher-rated unit when you don’t need it?

 

[jweaver]

I did a bit of digging today and found something.. All of the (identical) houses around me which were built in the same 18 month period have 18kw boilers.... Mine is the only house with a 12kw... I found the original documentation today and someone has torn off the label stuck at the top which has the model and serial number.

I spoke to some of the neighbours and there is a suggestion of foul play by the builder when they were built with staff not getting paid and let go at short notice.. And there was general bad feeling on the site when they were being built.. So it was suggested that its no surprise that mine had the wrong spec put in... I then spoke to the original owner and he knows all about this. He wasn't happy on day one that they only put a 12kW in, but the builders wern't willing to do anything about it.

So there is a suggestion that the fitter at the time may have just put the wrong boiler in, against the design of the houses.

That doesn't mean that a 12kw isn't suitable, but my gut feeling has always been that this is too small for the size of house and its looking that way.. This explains why everyone else has had their replaced with 18 or even 24kw boilers.

The problem now is that the fitter who is doing this is nervous that there might not be sufficient gas pressure to drive an 18kW and wants to stick with what is there now (12).. But I don't like this as its just doubling down on a prior mistake..

I am just going to go with the 18 and see what happens when its commissioned.

Thanks all for your help and thoughts with this.

 

[Mister Banks]

I think you will find that it’s all the houses with an 18kw boiler that have the wrong one, even your 12kw boiler is possibly too big. What does the survey say your homes heat loss is?

 

[jweaver]

I think you will find that it’s all the houses with an 18kw boiler that have the wrong one, even your 12kw boiler is possibly too big. What does the survey say your homes heat loss is?

Survey? What survey? When i bought the house? If 12kW is fine for my house, it must be WAY over sized for the countless small houses they are fitted in and they don't have a problem...

This is what I don't get... If a 12kW is right (or too big) for a 120m2 house (with 240m2 of floor space over 2 floors) with 5 bedrooms and 18 radiators.. What would you put in a small 2 bedroom house? Its not like I am talking about a 30 or 36kw like the calculators say I need... Its "Only" 18kW.

What are the implications of putting a 18 over 12? Just gas usage (boiler price is the same)? If so, I can live with that?

The mistake I made here is when I said that my current 12kW can heat the house on the coldest of days.. Thats true.. But it takes time and thats with 30% of the radiators off... I have nothing to compare it too and don't know whats possible.. But for me, it doesn't feel like there is enough 'umph', in the current system. It gets there for sure, over time.. But I believe its struggles in extreme cases.