24kw system boiler, 15mm pipes.

housebuyer143

Can anyone recommend a system boiler than is 24w and can run on 15mm pipework?
I need a more powerful boiler and I was advised by an engineer that there were one or two boilers they supplied that would be compatible due to the small pipes. 

He told me one was the worcester greenstar 4000 24kw system boiler, but does anyone know of any others?

Is what I was told actually correct? Does having the 15mm pipework mean I'm limited on boilers? The pipe labelled gas going into the boiler is 22mm.

jonnydeppiwish!

What pipework are you talking about? I have an Ideal 32 and, except for the gas pipe, all the pipes are 15mm

housebuyer143

jonnydeppiwish! said:

What pipework are you talking about? I have an Ideal 32 and, except for the gas pipe, all the pipes are 15mm

The ones that go around the house. The gas engineer made a big deal how these were too small to support a 24kw system boiler... Looking into it now, in not sure if this is true or complete fiction.

TadleyBaggie

I think he was trying to drum some business. 15mm is fine.

nofoollikeold

I think some misunderstanding may have arisen.  
There are typically three sets of pipes involved:
1. The gas pipe.  The diameter of this pipe depends on the distance from the meter to the boiler, the length being the actual distance (including bends etc, not "as the crow flies") plus an allowance for each bend, tee, elbow etc fitted in the run.  The true calculation will also allow for other gas appliances.  Unless the boiler is within a few metres of the meter, you are almost bound to need 22 mm pipe, possibly 28mm pipe, and in extreme circumstances, 35 mm pipe.  Because gas is compressible, the full run can be considered as a series of sections, so you might have, say, a 22 mm section, then a 28 mm section and a short final 15 mm section.  Provided any drop in pressure at full usage is less than 1 millibar between meter and boiler it will suffice.  Note again, the full calculation must include all gas appliances.
Combi boilers heat domestic hot water "on the fly" so require much greater power, albeit for relatively short times, than system or open vent boilers, which heat water in a cylinder.  In consequence combi boilers generally require bigger gas pipes than other types.
2. Boiler primary pipes.  These carry hot water from the boiler to the space heating system, and for system and heat only boilers, also to the hot water cylinder.  They should be sized properly, but a good and very widely used rule of thumb is to have 22 mm pipes from the boiler to form a "spine", with individual radiators fed from the spine by 15 mm pipes.  The main alternative is the microbore system, which often uses a manifold upstairs and another downstairs the manifolds are connected to the spine, and microbore pipe (usually 10 mm) runs from the manifold to each radiator.  The primary pipework to the hot water cylinder is usually in 22 mm.  Its doesn't always have to be, but its a good working rule.  Very old boilers which use gravity circulation for hot water will usually have 28 mm pipework to the hot water cylinder.  
3. Domestic hot water pipes from combi boilers are usually 15 mm.  The water is at mains pressure, so a reasonable flow rate can be achieved with 15 mm pipe.  However, don't expect this to fill a bath too quickly!

The fact that your current gas pipe going into the boiler is 22 mm doesn't mean the whole run from the meter is 22 mm,  If it is smaller your engineer may have already worked out that it is too small for a modern boiler.  

If your existing radiators are based on a microbore system, the engineer may be warning you that in his view you need to upgrade to the 22 / 15 spine and feeds system.  Microbore works very well in many instances, but if the system is not well and regularly maintained, is more prone to clogging up.  It should also be pointed out that if we all have to move to lower temperature systems like heat pumps, its unlikely that smaller pipes will have sufficient heat carrying capacity.

Perhaps worth going back to the engineer and asking him to explain in more detail what he is recommending and why.  In any case you would be well advised to get quotations from 3 different firms, and pick the one you are most comfortable with.  

housebuyer143

nofoollikeold said:

I think some misunderstanding may have arisen.  
There are typically three sets of pipes involved:
1. The gas pipe.  The diameter of this pipe depends on the distance from the meter to the boiler, the length being the actual distance (including bends etc, not "as the crow flies") plus an allowance for each bend, tee, elbow etc fitted in the run.  The true calculation will also allow for other gas appliances.  Unless the boiler is within a few metres of the meter, you are almost bound to need 22 mm pipe, possibly 28mm pipe, and in extreme circumstances, 35 mm pipe.  Because gas is compressible, the full run can be considered as a series of sections, so you might have, say, a 22 mm section, then a 28 mm section and a short final 15 mm section.  Provided any drop in pressure at full usage is less than 1 millibar between meter and boiler it will suffice.  Note again, the full calculation must include all gas appliances.
Combi boilers heat domestic hot water "on the fly" so require much greater power, albeit for relatively short times, than system or open vent boilers, which heat water in a cylinder.  In consequence combi boilers generally require bigger gas pipes than other types.
2. Boiler primary pipes.  These carry hot water from the boiler to the space heating system, and for system and heat only boilers, also to the hot water cylinder.  They should be sized properly, but a good and very widely used rule of thumb is to have 22 mm pipes from the boiler to form a "spine", with individual radiators fed from the spine by 15 mm pipes.  The main alternative is the microbore system, which often uses a manifold upstairs and another downstairs the manifolds are connected to the spine, and microbore pipe (usually 10 mm) runs from the manifold to each radiator.  The primary pipework to the hot water cylinder is usually in 22 mm.  Its doesn't always have to be, but its a good working rule.  Very old boilers which use gravity circulation for hot water will usually have 28 mm pipework to the hot water cylinder.  
3. Domestic hot water pipes from combi boilers are usually 15 mm.  The water is at mains pressure, so a reasonable flow rate can be achieved with 15 mm pipe.  However, don't expect this to fill a bath too quickly!

The fact that your current gas pipe going into the boiler is 22 mm doesn't mean the whole run from the meter is 22 mm,  If it is smaller your engineer may have already worked out that it is too small for a modern boiler.  

If your existing radiators are based on a microbore system, the engineer may be warning you that in his view you need to upgrade to the 22 / 15 spine and feeds system.  Microbore works very well in many instances, but if the system is not well and regularly maintained, is more prone to clogging up.  It should also be pointed out that if we all have to move to lower temperature systems like heat pumps, its unlikely that smaller pipes will have sufficient heat carrying capacity.

Perhaps worth going back to the engineer and asking him to explain in more detail what he is recommending and why.  In any case you would be well advised to get quotations from 3 different firms, and pick the one you are most comfortable with.  

Thank you for this detailed response. The house is only 20 years old so I am not sure if that has any bearing at all. 
I'll go back to the engineer and ask for more info on why he has recommended that specific boiler and get additional quotes. 

FreeBear

Count up the radiators and the sizes.  Use this web site to find out what the output rating is for each radiator -> http://www.simplifydiy.com/plumbing-and-heating/radiators/power

If the total comes to more than 24KW, then you do indeed need a bigger boiler. But for the majority of people, a 12-18KW system boiler is plenty big enough. If you do find that you need a 24KW+ boiler, the you will probably need to install bigger pipes (perhaps as big as 28mm for the main ones).

markin

Unless you're extending the house its odd for only a 20 year old system to be sized incorrectly?

tacpot12

The size of the pipes that are needed to distribute the heat doesn't really depend on the boiler, it depends on the size of the radiators (or more correctly on the heat load in each room). However, pipes that are too small can be made to work with a stronger pump. This is probably why he is suggesting certain combi boilers because he knows that the pump in them has a bigger head (i.e. the pump is stronger). 

Running your heating system with pipes that are too small will cost you more in electricty as the pump has to work harder. The difference might only be 50w per hour, but this might cost an extra £100 a year (assuming the heating system is on for half a year). It might be worth the investment of getting the pipes correctly sized now.

housebuyer143

markin said:

Unless you're extending the house its odd for only a 20 year old system to be sized incorrectly?

It's a large 4 bed detached house but what I find is the boiler isn't capable of powering both the hot water and heating at the same time. If you do this the radiators at the end of the run go cold and the hot water only heats to luke warm. I wanted to upgrade the upstairs radiators to doubles also as they are ridiculously small. 

I'll get some more advice though as if a new 18kw can do the job then I'll take it, but when I explained my problem to the last engineer he instantly said we needed to upside. 

ThisIsWeird

H143, we need some basic info, please:

1) Your house - type, and how many beds? Extended?
2) Distance of boiler from DHW cylinder?
3) Size of current boiler? (Make & model, please)
4) Issue you have having with current boiler?

And photos of your DHW cylinder would be helpful too, showing the pipes and valves around it. And of the underside of your current boiler showing the connected pipes.