12 Litres Per Minute - OK for a combi?

[Madrab]

That really seems to me to be a 'back of a fag packet' design to me. It really isn't difficult to run 2 specific test around dynamic deliveries and that will provide a solid starting ground to which a practical and sensible design can be constructed. That coupled with what the district mains actually delivers. That is then coupled with the site specific requirements in the way of what is needed to satisfy the outlets and then add the clients 'requirements' .

It really isn't rocket science and to me the starting point is, what can the cold mains deliver to the property if the clients were using the outlets in a typical daily fashion and then balance that against peak possible requirements. With you it would be - if the washing machine was on and someone wanted to use hot water for dishes downstairs and at least 1 person was in the shower if not both were being used, what would the worst case be as far as supply needed to deliver that and how could that be done, if at all.

That then drives possible designs. To me it's not just about 'what's the biggest or most expensive boiler that can be jammed in there', it's more about the whole hot and cold water system design, especially where the mains seems to be as limited as yours may be.

 

[BlueLoo]

No dynamic readings given.

We have 1 bath, 2 showers, 15 rads.

The whole experience of getting a boiler has been frustrating, mainly becasue every man and his dog seem to advise something diferent and it's hard for a layman to know what's best. We've had it all.

Anyone asked if you actually need a combi?

 

[Agile]

Its such a pity that 50 years after combi boilers became popular, most installers do not understand what the dynamic flow rate means or how to measure or estimate it.

An open pipe volume flow rate has little use apart from showing what the maximum flow rate might be tending towards. Nothing more.

Most are driven by the desire to get their quote agreed and do the installation.

In reality you will probably need to not use any other outlet while a shower is being taken.

 

[Madrab]

In reality you will probably need to not use any other outlet while a shower is being taken.

Cant agree with that I'm afraid. Especially in a family home with 2 teenagers. 2 showers suggests 2 bathrooms so another toilet too, could easily have a washing machine or a dishwasher on at the same time & depending on time then sink outlets etc could be getting used .....

Using my personal example, my good lady could be showering, my eldest is too, whilst the washing machine is running, I've just shoved the dishwasher on and I'm running H/C Water in the sink. That happens with us more often than not. Now I have >3bar and 24 odd L/Min dynamic with unvented HW, so supply isn't a problem. If I had a combi on a mains delivering 12L/min single outlet with 1.8bar static, then that could never be realised but with a fair amount of storage and a well thought out and clever H&C system design, then no reason it couldn't.

 

[BlueLoo]

I thought it a universal law that nobody can go for a dump when a combi is running a shower?

 

[Agile]

There is no law about it.

Just a fact of life for most combi boiler users.

 

[BlueLoo]

There is no law about it.

Just a fact of life for most combi boiler users.

So a poor way to provide heating and hot water for a normal house?

Iianm, the Combi is only a thing because flats, etc where head tanks and hw storage are not possible?

We have an industry flogging them because they are a cash cow when the bog std, vented, boiler and head tank in a loft/attic would be a far better performer.

System boilers too for that matter.

 

[Agile]

Well Blue Loo, the fault lies with the installer choosing to fit a substandard installation just to get the job!

 

[gerbils]

Anyone asked if you actually need a combi?

As mentioned earlier in the thread, everyone offers something different. One offered a System boiler + unvented tank, but another says we don't have the water pressure for that. It was £1500 more anyway so not a goer for us.

 

[gerbils]

Cant agree with that I'm afraid. Especially in a family home with 2 teenagers. 2 showers suggests 2 bathrooms so another toilet too, could easily have a washing machine or a dishwasher on at the same time & depending on time then sink outlets etc could be getting used .....

Using my personal example, my good lady could be showering, my eldest is too, whilst the washing machine is running, I've just shoved the dishwasher on and I'm running H/C Water in the sink. That happens with us more often than not. Now I have >3bar and 24 odd L/Min dynamic with unvented HW, so supply isn't a problem. If I had a combi on a mains delivering 12L/min single outlet with 1.8bar static, then that could never be realised but with a fair amount of storage and a well thought out and clever H&C system design, then no reason it couldn't.

This would cost a lot more though so maybe that's why combis are popular. It's my understanding that we could stick with a standard boiler but we'd need a new hot water new cylinder with it and we'd also need some kind of shower pump or digital shower installing to get something workable.

Likewise a system boiler apparently needs our driveway and all digging up to fit a new direct pipe from mains, plus a flashy unvented tank. That's quite a bit more expensive, plus another told us this wouldn't work and we'd need an accumulator which was quoted at an added £1800.

From a consumer point of view, there just doesn't seem to be any agreed consensus as to what is best - given an average budget. Clearly if money is no object we'd be doing something diferent. We could get a heat pump and new rads for £4k which is cheaper than all our quotes thus far, but we have microbore so that rules that out.

 

[Madrab]

It all comes down to what you want. If you are happy to make sure that if someone is in the shower no one else uses water in the house then that would be the lowest budget answer probably. Anything else then you need to look at other options.

As it currently stands, going on a guesstimate from the figures that we have, no an unvented is not an option. If a combi is fitted then if anyone is using a HW tap then practicality wise, no one else is, that would be the shower etc and there will probably be a drop in delivery if anyone else uses any tap, hot or cold, unless 12L/min is a truly dynamic figure.

If the requirement is to feed those 2 showers at the same time or 1 shower and a hot tap and cold tap or any other permutation where multiple outlets were needed to be used at the same time, then stored Hot and cold would need to be used. Yes, that would need extra work which is extra cost but again it all comes down to what you will find acceptable. What are you happy to live with?

I think you also need to find out from your water transporter what the mains is capable of, that's just a call to say you're having an issue with your mains supply and they'll come out and test it FOC. Whatever is wanted though, unless its the combi and the restrictions that comes with, then there will be extra cost and we're back to what you want from your system.

 

[BlueLoo]

A system boiler is just a standard boiler with the pump and (for sealed systems) an expansion tank included.
I have no idea why the drive would be bothered about this lol.

If water supply to the house is an issue, then surely a buffer type system with a head tank in the attic and a HW storage tank with either an electric shower, or a pumped shower would be best.
In fact, if you have attic space and an existing HW tank, why on earth would you look to put a combi or system boiler in at all?

A std boiler is cheap, reliable and simple. Std boiler allows the other stuff to be put seperate making it cheap, reliable and simple.
A HW storage tank will give you great flexibility for when there is high demand.

But, i'm not an expert so perhaps there are other reasons
Also, you can add solar PV to heat up the hot water tasnk (electrically) during the day if you tie this in as well..

 

[vulcancontinental]

Look at what you have at the sink cold tap, is that, or a tiny bit less acceptable to fill a bath?

12l/m you'd want a 30kW boiler if you wanted the incoming cold water temperature raised 35 degrees.

If your family like hotter water then a more powerful combi boiler will be able to raise the incoming cold more than 35 degrees.

One hot tap at any time only with a combi, families live around it, they get used to it; not saying it's wrong or right, it just is the way it is.

For simplicity and reliability if you have the head and roofspace a vented hot water cylinder with immersion heater fed by a cold water cistern in the loft and correctly sized pipework is hard to beat. Add a heat only boiler or system boiler with external controls, job done.

 

[gerbils]

For simplicity and reliability if you have the head and roofspace a vented hot water cylinder with immersion heater fed by a cold water cistern in the loft and correctly sized pipework is hard to beat. Add a heat only boiler or system boiler with external controls, job done.

I do like this idea because it's risk free (we have old 8mm microbore so I understand there's a risk going pressurised?).

We also have the space already (big huge water tanks in loft + a cylinder in the airing cupboard)

however... the small amount saved (£500) on a standard boiler is outdone by the need for a bigger and more efficient cylinder and 2x digital showers, (or shower pumps).

Maybe I need another quote....

 

[Agile]

I would not expect any leaks on an 8 mm microbore system.

But achieving enough flow can often be a problem.