26 or 30 kW boiler?

Ellie77

I am looking to replace an old Worcester Bosch 24 CDi boiler in an end-of-terrace c.1905 property. It has a living room (1 radiator), dining room (1 radiator), one bathroom (1 radiator), two bedrooms (1 radiator in each) and conservatory, so 5 radiators in total. The boiler is located upstairs in the second bedroom, which is located next to the bathroom and has a bath and separate shower. I have been advised that a 26 kW boiler will suffice, but the 30 kW (the next model up) is only just under £50 more. The water flow rate and temperature is not particularly good in the bathroom, which I guess could be due to the age of the current boiler and the fact that it is on the same floor.

I have read a blog post by a heating engineer that states higher kW boilers do not cost more to run because they are "fully modulating", however this seems to be contradicted everywhere else. My thinking is that if I go for the 30 kW, I know that I've done what I can kW-wise to improve flow rate and temperature upstairs as well as future-proofed myself should I decide to install more radiators or a more powerful shower. However, I obviously don't want to be paying for what I do not need. Online boiler calculators are in favour of a lower kW (24–27) for my size home. Any advice would be greatly appreciated

Tom99

Go for the bigger one. That's what I did base on the same reasoning as you.

FreeBear

Ellie77 wrote: »

The water flow rate and temperature is not particularly good in the bathroom, which I guess could be due to the age of the current boiler and the fact that it is on the same floor.

Flow rate will have little to do with where the combi-boiler is located. It is more likely an issue with the mains water pressure.

ComicGeek

But flow rate of hot water (and the supply temperature) from the combi does depend on the kW rating of the combi boiler. A 24 kW combi will have a pretty low DHW flow rate - a 30 kW combi would have a higher rate. Whether it will still be sufficient depends on a number of factors.

The combi boiler is always going to be sized on hot water supply rate, and not space heating.

Given the age of your property, it's likely that you only have a small diameter supply pipe, which limits flow rates for both hot and cold water connections. Do you have a cold water tank in the loft?

Ellie77

Thanks for your replies. I have been told that my mains pressure is very good (good enough for pretty much any kW boiler). Water pressure is excellent downstairs in the kitchen, it just isn't as good upstairs. No, there is no cold water tank in the loft. It is a straightforward combi for combi installation.

vw100

Compromise with a 28KW if you don't want the 30KW one. Then you have the best of both worlds

tacpot12

Go for the bigger one. It will help to produce better flow rates of hot water (at the expense of using a little more gas), but the boiler will modulate its output down to the heat output required by the central heating system when it is running to heat the house.

However, no boiler modulates down to zero; they all have a level at which they won't modulate down beyond any further. Normally the larger boilers will have a higher minimum that they modulate down to, but this is not always the case. Nor are manufacturers very good at publicising the modulation range, you might have to download the Installation Manual for each boiler model to find the figure. Ultimately a higher minimum for modulation will result in the boiler cycling a little more, but this is unlikely to be a significant problem.

phill99

Always future proof the boiler. Always have more capacity than you actually need. The installation costs are the same, but you will pay a bit more for the boiler. The additional running costs will be non existent. ,

GDB2222

Water pressure is excellent downstairs in the kitchen, it just isn't as good upstairs.

Is that where the problem lies? The difference in water pressure between ground and first floor is around 0.25 bar. That's almost nothing.

If you have excellent water flow downstairs, then there's a problem with the water feed from downstairs to upstairs. Perhaps as simple as a valve partly turned off.

Ectophile

With a combi boiler, the flow rate is often deliberately throttled. Otherwise, especially in winter, you would get a really good flow of slightly warm water. Reducing the flow gives the boiler more time to heat it up.


So with a higher wattage boiler, you can increase the flow rate and still get an acceptable temperature.

Ellie77

tacpot12 wrote: »

Go for the bigger one. It will help to produce better flow rates of hot water (at the expense of using a little more gas), but the boiler will modulate its output down to the heat output required by the central heating system when it is running to heat the house.

However, no boiler modulates down to zero; they all have a level at which they won't modulate down beyond any further. Normally the larger boilers will have a higher minimum that they modulate down to, but this is not always the case.

Thanks for this, it's 4.7 kW for both the 26 and 30 kW.