Confused by DHW Boiler Sizing

[mgns]

Hi,

After getting some quotes for a new combi boiler, I'm a little confused as to which size boiler - DHW wise - would be appropriate?

The 2 choices are:

Vaillant 831 @ 12.7 l/m
Vaillant 837 @ 15.2 l/m

The house:

Incoming mains measures at just over 16 l/m and pressure at 3.5 bar.
3 bed semi
1 bath + 1 cloakroom
10 rads
but a loft conversion with ensuite may happen in the future.
(going for a combi to get rid of the loft tanks)

I'm wondering if the 837 would be overkill taking into consideration the
incoming mains and that the extra installation/running costs would add no
benefit.

My initial thoughts are that by taking into account mixing hot and cold
water for a bath/shower, I wouldn't get any benefit from having DHW @ 15.2 l/m
on a mains that is 16 l/m.

Thanks for your advice.

mgns

 

[Agile]

there are not just two possibilities! You can have any size combi between 24 kW the smallest and about 37 kW which is the normal largest.

This 16 li/min is almost certainly the open pipe flow rate.

Whats actually needed is the dynamic flow rate! Obviously your installers dont even understand what that is or why its relevant.

Try a bit harder to find a good installer or is it that you only choose cheap ones?

Tony

 

[Stamina]

What is dynamic flow rate??

 

[45yearsagasman]

That's a bit harsh Tony that last bit about only choosing cheap installers!
I would go for the 837 to cover for future extensions to your property.
The flow rate is marginal as measured but you may have to do something about incoming main when you extend.
You probably won't get 15.2 from the 837 as is so why not preplan and upgrade main supply now,and be ready for extensions?Just a thought.

 

[mgns]

Thanks for the info guys.

I did look at upgrading the water mains to go down the unvented route but I struggled to find enough space for a cylinder. So, if I leave the mains as is, would it still be advisable to go for the 837 or is it a waste of money seeing as I wouldn't get full usuage?

In reply to Agile, the reason I highlighted the 831 was because the other quotes were for similar sized Baxis and Worcesters. The quote/advice for the 837 was a one off, so I was wondering what people's opinions were.

Having tried a number of recommended installers/companies, none have done anything different - they've all stuck a pressure gauge on the outside tap or used a cup which measures both pressure/flow rate. I'm in the South East and not one of them has come in under £3000 for a new install + 10 TRVs. I don't think I'd see anything different if I tried a couple more installers.

 

[Agile]

Thanks for the info guys.

Having tried a number of recommended installers/companies, none have done anything different - they've all stuck a pressure gauge on the outside tap or used a cup which measures both pressure/flow rate. I'm in the South East and not one of them has come in under £3000 for a new install + 10 TRVs. I don't think I'd see anything different if I tried a couple more installers.

I have not seen a combined cup and pressure gauge! That does not mean that they do not exist but just that I have never heard of one. I doubt that they do though!

The dynamic pressure ( sometimes called working pressure by shower makers ) would typically be quoted as 11 Li/min @ 1.0 Bar for example.

That then gives a 1.0 Bar pressure to overcome the resistance through the boiler and through a shower head.

Only a nupty installer does not understand the consequences of these measurements.

Many ( most ) just measure the open pipe flow rate. Depending on the supply pressure in the street the dynamic flow rate is often between 50 - 70% of the open pipe rate.

Also be aware that this is not just the demand on the hot water. The same flow supplies both hot and cold so flushing a toilet or a washing machine will also deplete the flow of hot water.

Fitting a more powerful boiler than you can use will sometimes give some issues with heating as the minimum CH power will be too high and cause cycling. But a more powerful boiler would enable you to uprate the water supply in the future.

Those seem only slightly expensive quotes. My rule of thumb would be £350 for 10 Drayton TRV4s ( the best ) but I would usually prefer to fit new lockshields at the same time for £150. Then say £1100 for the boiler and flue (?) £840 for fitting, £300 to upgrade gas supply pipe if required.

If converting from a cylinder open vented system if thats what you have then another £400. So perhaps not much different from your quotes?

Tony