New combi sizing - overspecced??!

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Hi all,

I know the word overspec is always a bad one but bear with me.....

I have 3 bar pressure at the garden tap and roughly 13-14L /min at the kitchen tap. At 1 bar I have around 111/12L /min.

Considering a combi and the flow rates stated are for 35/40degree rises all advice I have seen is to go for a combi (if I do pick one) that supplies this rate.

The thermostatic shower I want to use states minimum hot of 52 degrees which in summer will be ok but in winter could be some way off and need higher temperature rise and lower flow.

So.........

My questions are 2...

1) When a combi says that x l/min hot water rise by 40 degrees I am guessing the total output flow will increase that figure at the tap as cold will need to be added to hot water? Temperatures dependent of course but flow should increase when you add cold no?

If so....

2) Based on the calculation for winter that for example 12l/min rising by 50 degrees would need 42kw output (far in excess of advice for current flow rate boiler size) I calculate that 42kw would provide this. Based on 12/60 x 50 x 4.2 joules = 42kw.

Although 42kw is incredibly overspecced, would it not mean that my puny 13/14l coming in would all be 50-60 degrees? Hence no flow drop required in winter?

Theoretical this is .........
 
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Although 42kw is incredibly overspecced, would it not mean that my puny 13/14l coming in would all be 50-60 degrees? Hence no flow drop required in winter?

Not sure about that....

But you may end up with the boiler turning itself on and off when just keeping the house warm because the boiler cannot modulate ( reduce ) its heat output down low enough to match the low heat load required to keep the house warm.
 
You are right Dan it wasn't all the advice I received, but it was about 50/50 split on here and RGIs that I have had round.

The 42kw thing is just a question in my mind, theoretical only.

Although on the combi front I have figured out I probably actually cant afford an unvented cylinder (certainly not if my 16 year old boiler packs up anytime soon) I have since seen an installation of an unvented and it looks like an oil rig to me and I still dont have enough flow of water into my house to accommodate it.

I know it is the best thing since sliced bread but I cant see it fitting my house and the gravity fed system needs to die, I cant raise the cold anymore and even if I added a shower pump the kitchen and bath take a month to fill!

I cant see anyway out for me other than a combi!

Help me Dan!!!!!
 
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There are alternatives but most RGIs do not like anything out of the normal.

This provides mains pressure showers without the need for G3 qualified plumbers to install and maintain it. The cylinder is a heat store for the shower as well as the hot water supply to taps in bath and basin where mains pressure can be too much pressure.

A 6 kW boiler can take 20 minutes to heat the cylinder and then the shower can extract 30 kW from the cylinder for a 4 minute shower. ( These times and values will vary depending on size of cylinder and the boiler output )

It is not a sludge bucket ( as some heat stores are ) because the water in the cylinder is not the water circulating around the radiators and hence does not have chemicals and products of corroding radiators in it. That dirty water ( shown dark brown ) is confined to the lower coil in the cylinder.

water shower.jpg
 
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Hi all,

I know the word overspec is always a bad one but bear with me.....

I have 3 bar pressure at the garden tap and roughly 13-14L /min at the kitchen tap. At 1 bar I have around 111/12L /min.


I presume you mean you have a dynamic flow rate of 11 li/min @ 1.0 bar.

That will not be adequate to get the maximum flow rate from a high power combi.

But there are usually ways to increase flow rates by upsizing internal pipework and externally if that is not sufficient.

But for a user the difference between 11 li/min and 14 li/min is not very great.

In my view your budget ( meaning what you want to spend ) should not be part of the specification. You should choose the best system for your needs and then arrange any finance required.

But for a property with any more than two people and always from more than one bathroom then I would always advise a storage hot water system sized at 50 li plus 50 li per person.

Because you have chosen to start a new thread, I am replying solely based on what you have posted in this one. That may not have been the most wise decision you have made.

Tony
 
Absolutely tony and apologies my forum etiquette isn't what it should be re starting a new thread. Should have known that.

11l per min dynamic is correct. I have blue pipe coming into stop tap under stairs and then 15mm on from there. Would I benefit flow wise from running 22mm from stop tap to boiler? If so by how much and how would you legislate for that in choosing a boiler?

Main thing for us is shower usage. We have 1 at a time and at random times of day. We have a tiny second bathroom with an electric shower which is currently used as main but will e relegated once this is resolved.

Would love to lose the cylinder as could do with the space. Vented cylinder is 90l currently and not enough if we were pump it.

I know it kills you guys to hear idiots like me when you know what you're talking about and you've all been ace but as an ignoramus the attraction is less space taken up, don't need to worry about setting it on or off or running out. Had one before and got used to the one tap at a time thing easily.
 
A full flow lever valve has less resistance but not thought to be the best type on mains water inlet.

You should really upgrade in 22 mm to at least the last cold connection.

Or possibly a separate 15 mm from boiler to the stopcock.

But you ask what benefit, but don't tell us the length from stopcock to boiler!

Don't worry we deal with customers all day long!
 
Haha but you make a living from them mate! Well to be fair once I understand this someone will make part of their living from me and that could be you!

It's from the front door straight out under the floor boards via kitchen and downstairs shower I would say 7 metres from tap to boiler. You've also got me thinking now toilets dishwasher washing machine and cwt are all fed off that piece of string!
 
We are now also going back to basic physics but....

If you have a pressure of 3 bar for example. If you add another pipe from the mains feed in do you just get two pipes at the same pressure and flow rate as the original one? Assuming resistance and pressure drop through pipe is consistent etc.

And If you add a third do you get 3 pipes producing the same pressure and flow until the mains pipe in starts being exceeded capacity wise?
 
You will only run at the capacity of the original /smallest connection.

Pressure is unaffected, but you may improve a little the flow at that given pressure. But, as your assorted threads have developed, it seems you are pishing in the wind trying to get an expensive shower to work on budget equipment.

The stock phrase is "Champagne ideas, beer money".

As is common for us in the business, people are more concerned with spending money on the visible / point of use items than they are the gubbins in the background that make it all work.
 
I take your point Dan but the shower I want is only £150....it's a triton excellente. Hardly champagne fella but it's hard when you haven't got a bloody clue what you need to do!

How much flow increase would you expect for upgrading from 15mm to 22mm?

See what I also come back to is I currently get 14l/min and the last thing I or an installer wants is to go invented upgrade and then say ah, that's still not great...

I don't know whether to expect 1 l/min more or 10l/min more wen the installer as per his written quote runs a 22mm dedicated to the loft for the uv cylinder or does the same for a combo feed for that matter!

It's not so much the idea of spending money I fear mate, it's wasting it!
 
Quotes I have had for the unvented cylinder plus relocation or the combo install have differed by £200, absolutely nothing really.
 
Go Unvented then, with the 22mm upgrade, will out perform any combi all day.

We can't give you exact figures because there are too many variables.
 
Thanks mate i do appreciate the advice.

I appreciated it last time also but then read everyone (including people on this thread) saying if you haven't got 22l/min dynamic then you're "pishing in the wind" and when you're unsure of yourself and it's not what you know it's easy to be blown all over the place cos everyone swears they're right!
 

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