Which size bore?

[Sye]

I have decided to install a Worcester Condensing Combi boiler and new radiators to replace a 30yr old gravity fed system. As a precaution I will replace the pipework which may not be able to cope with a pressurised system. Can anyone tell me the benifits and diasadvantages of using 15mm 10mm and 8mm pipework please.
Thanks.
Sye.

 

[breezer]

10 & 8 mm is known as micro bore, each radiator has a pair of your chosen size and it has to go back to a manifold. it is used because it is small and easy to hide etc, touble is in my opinion it gunges up too quick

 

[oilman]

I agree with breezer, use 15mm or bigger, to rads.
Why a combi? If you like having baths you need to think further. If you like watching baths fill then a combi will give you hours of entertainment over a year. Combi's supply 1 tap at a time. Washing-up when someone is in the shower gives rise to unexpected conversations.

What about getting the existing pipework pressure tested? If it's ok, it could save you days of work.

 

[MANDATE]

As I understand it pipe work offers a resistance to flow.The longer the pipe the greater the resistance and the smaller the bore the greater the resistance.
It is normal for pipes leaving a boiler to be 22mm bore until it gets nearer the radiators or divides at a tee junction when it is reduced to 15mm.
The same applies on the return route, 15mm at radiator then up to 22mm back to boiler.
Other than the micro bore system I can't see 8mm ever being used.
I do believe 10mm is sometimes used on a conventional system( because I've seen one), because it is supplied as a coil, so less joints and easy to bend, ideal behind plasterboard. Maybe its ok for short runs providing the pump can cope.

 

[Sye]

Thanks for the advice 15mm it is then. As for the flow rate, the wife wants a combi but she also wants a bath. The boiler has been chosen because it has a decent flow rate. 14.5ltrs per minute doesnt seem much I know, but compared to most of the others (between 9 1nd 11ltrs) its not too bad.
Sye

 

[oilman]

Can your mains supply provide a flow rate of 14.5 litres/min at peak times, and what is the quoted temperature rise for 14.5l/m. The mains water gets damn cold in the winter, and your baths may be a trifle cool if you want to fill it quickly

If you want a bath, the supply can be better provided by a hot water tank. There ar three choices, "standard" vented tank, thermal store, and unvented tank. The first you probably have now, the others allow you to stare a lot of heat for high demand situations and they also allow you to have mains pressure hot water at a flow rate generally better than any combi. The thermal store can be sealed or open vented and is comparitively low cost of ownership. It can be installed without notifyng the local authority. The unvented tank MUST be notified to the local authority before installation and MUST be installed by a certified operator and should have an annual service.

You may have looked at these options, but if not, I strongly you do a bit of research before you alter your system as you may find they will provide a better solution. They may make the property more saleable too.

 

[masona]

A mate of mine said the combi is not a lot cheaper to run than non-combi but now got a bigger water meter bill because when filling the bath or sink they have to run it for a while until the water heat up the correct temperature !

Funny old world isn't it !

 

[breezer]

he must have a dodgy combi then, my other half often has a bath (we have combi) the water is hot within 20 seconds of turning on the tap (thats a guesstimate) and she fills the bath up and its very hot.

your mate is not pulling your leg is he?

 

[masona]

No,the only thing I can think of is,they have got low water pressure which the combi wouldn't be any good.

My mum who live on her own which is ideal to have a combi and was told the water pressure is very good for her to have a combi but sadly it wasn't and now find it quicker to boil the kettle for hot water for the washing up.

I think the biggest problem is,some plumber's can't be bothered to find out if the combi is suitable for their need and if the water pressure can cope with it.It a shame when it damaged the reputation of good plumbers

As I said before they're a lot of happy combi owners out there and those who are not has been misled.

 

[AnonymousImport]

I feel sorry for your mum. All the installer had to do was turn on the cold tap in the kitchen and you can usually tell by looking at it, but if in doubt all he had to do is stick a flow cup under and that would have told him the water volume.

Someone out to make a fast buck no doubt

 

[masona]

My mum's never like to bother me and I only live round the corner

Out of interest,do you know what the minimum flow rate volume in measure for combi requirement?