Am I too focused on DHW flow?

We are going to replace our ageing Vokera Compact SE 28 combi with a new condensing boiler. I'm trying to decide which boiler is the most suitable. I have four options (2 Vaillants and 2 Worcesters), but I'm not sure how much attention do I need to pay to DHW flow rate. I don't want to start argument Vaillant vs. Worcester vs. something else. This is purely about water flow so I'm using primarily flow rather than make/model in the examples below .

Our current boiler is supposed to do about 11.5 l/min (at the standard 35C rise). I measured the flow upstairs and I'm getting about 14-15 l/min from the cold taps and about 10 l/min from the hot ones. I'm reasonably happy with the flow I'm getting now from the showers (provided nobody else is using any water at the same time).

Now, the boiler I like can provide 14.3 l/min (Greenstar 34 CDI Classic), but the installer I like most suggested 11.5 l/min (EcoTec Pro 28 ) or 12.7 l/min (EcoTec Plus 831). The fourth option is 12.3 l/min (Greenstar 29 CDi Classic).

The main question is if I go for 12.5 or 12.7 l/min will I regret it? Or will I just waste my money if I go for the 14.3 l/min one?

I'm also considering the installation of a water accumulator in the future. However, I don't want to feed two showers with it but want to even out any drops in pressure/flow when somebody flushes the toilet or uses the taps for a short period of time.

Does anyone have any thoughts about this and wants to share them with me ?

As long as you get the rate nto the boiler then you'll get it out, higher is better i think, Combi's will never run more than one shower at a time unless it's a storage ombi such as a Vaillant 937, go with the ecotec plus rather than the pro as the warranty is longer, also take a look at the new Ideal range, very good boilers, good warranty and not like the old Ideals,

As important as the output rate/temp rise is to you, quiz the installer about system water quality and the importance of correct gas pipe sizing.

Pretty much all boilers with say, 28kw output to hot-water will quote a very similar figure, then it comes down to things such as casing size, warranty, build quality and most importantly installation quality...., not price alone - I hope.

Dreadnought

Thanks guys.

The Vaillant installer ofers 5+ year on both pro and plus models, but yes I'm more inclined to go for the plus model.

I'm not entirely sure what you mean by water quality drednought, but yes we've discussed the gas pipe size (22mm already). Casing size is roughly similar to the current one. There is enough space in the cupboard for any of those boilers. Warranty between 5 and 7 years depending on installer. I guess I'll see the installation quality once they start installing it. That's the worrying bit .

Anyway back to my main question.
If my main provides 15 l/min and the ratio of cold/hot water used by the shower is between 1:1 and 1:2 the boiler will need to heat between 7.5 and 10 l/min. Is that correct calculation? So there isn't much point to go for boiler that can do more than 12-13 l/min?

By water quality I mean cleanliness of the water in the boiler/radiators (that does not ordinarily change). The existing system will need thorough cleansing BEFORE the new appliance is connected (or flushed though so that the new appliance is isolated). The BS is rather low in this respect, If I can assume your installer is in the Vaillant installers regime I am sure it will be chemically cleansed as well as jet/powerflushed.
Filters (strainer and/or magnetic types) are also increasingly required it seems, along with suitable corrosion inhibitor chemicals that are intended to minimise electrolytic corrosion).

If your mains water is in a 'hard' water area, a suitable scale inhibitor should also be considered.

As to flow rate, during say Jan/Feb cold inlet temperature will be circa 3 degrees, so a 35 degree temp rise will raise it to say 38 degrees (blood temp only) at the given flow rate, you may not need to add any cold water at all.

In july /Aug it may be 19 degrees plus, quite a difference.

Combi's are not the only heating or hot-water solution available. (is still fit 4 combi's out of five boilers though.... )

Dreadnought.