Converting to combi pipework

I currently have a open vent conventional boiler which will be replaced with a combi, I'm planning on removing the tanks and cylinder and modify the pipework ready for the plumber to fit the new boiler

Cylinder is in the bedroom cupboard
Boiler is in the kitchen

Having read a few posts, if I have understood correctly are the following procedures correct?

I want to ensure the pipework is most efficient for the flow of the taps and heating and do not want any unnecessary dead pipework where water has to travel unnecessarily.

- Remove f+e tank and remove all pipework in loft down to cylinder
- Remove cold water feed tank and remove all pipework in loft down to cylinder
- Remove and cap CH flow and return from cylinder
- Remove and cap mains water feed from loft down to cylinder, do the same with hot water and do the following instead of routing new pipework:

Rather than extending the hot water pipe and main cold pipe from the cylinder down to the kitchen, is the following acceptable?
- Tap into the kitchen taps to obtain cold water main and hot water

I would appreciate your comments

Al@n said:

I currently have a open vent conventional boiler which will be replaced with a combi,

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Why ?

bernardgreen said:

Why ?

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Need the space, doing a loft conversion, its a 2 bedroom property with one bath. The decision has been made, no going back now

Al@n said:

Need the space, doing a loft conversion, its a 2 bedroom property with one bath. The decision has been no going back now

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Ignore Bernard, he's a DIYer who is a great fan of archaic systems. I think he'd quite like it if everyone had a coal stove with a back boiler to keep them warm....

Your description is broadly correct, although you will need to join some pipes together in the airing cupboard as well, unless your boiler installer is going to do that? A photo of the pipework in your cylinder cupboard would help

muggles said:

Ignore Bernard, he's a DIYer who is a great fan of archaic systems. I think he'd quite like it if everyone had a coal stove with a back boiler to keep them warm....

Your description is broadly correct, although you will need to join some pipes together in the airing cupboard as well, unless your boiler installer is going to do that?

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Cheers, I'm planning on joining the pipes together myself, a detailed explanation would be welcomed

Is it acceptable to get the mains and hot from the kitchen? do i still need to loop the loft mains water feed into the bottom cold feed or can i just cap both pipes off?

Yes, you can take your hot and cold from the kitchen. You'll need to join the rising main to the cold feed to any low pressure pipework you have.

Ideally you should remove all 22mm hot and replace with 15, if at all practical to do so. It'll reduce the waiting time for water to get hot at the taps, and reduce gas consumption too

muggles said:

Ideally you should remove all 22mm hot and replace with 15, if at all practical to do so. It'll reduce the waiting time for water to get hot at the taps, and reduce gas consumption too

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Rising main being the feed to the water tank, join the top pipe to the bottom feed pipe of the water tank?

Even though I'll be capping it off and tapping the hot from the kitchen? So are you saying convert it to 15mm then cap it off? Any ideas how much of the hot will be in 22mm?

muggles said:

Ignore Bernard, he's a DIYer who is a great fan of archaic systems. I think he'd quite like it if everyone had a coal stove with a back boiler to keep them warm....

Your description is broadly correct, although you will need to join some pipes together in the airing cupboard as well, unless your boiler installer is going to do that? A photo of the pipework in your cylinder cupboard would help

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Open vented systems have some quite massive advantages over combi systems, but from scratch one would be very expensive to install. That disadvantage doesn't exist, if you already have the system. Depending on lifestyle, another advantage is running economy for both water use and gas.

I have had both and presently have an open vented system. Nothing would persuade me to replace it with a combi, other than the unlikely need to rip it all out and replace with new..

Harry Bloomfield said:

Open vented systems have some quite massive advantages over combi systems, but from scratch one would be very expensive to install. That disadvantage doesn't exist, if you already have the system. Depending on lifestyle, another advantage is running economy for both water use and gas.

I have had both and presently have an open vented system. Nothing would persuade me to replace it with a combi, other than the unlikely need to rip it all out and replace with new..

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Not even if you needed the space for a loft conversion......

nwgs said:

Not even if you needed the space for a loft conversion......

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Been there, done that, still got space for the tanks.

muggles said:

Ignore Bernard, he's a DIYer who is a great fan of archaic systems.

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Combi boilers were developed to provide hot water and heating in flats and small houses where there was no space to install a hot water cylinder.

http://www.thnet.co.uk/thnet/BF/BFPages/overhist.htm

The efficiency of a combi is often quoted as being higher than a convential boiler and thus the combi will be cheaper to run.

That is possibly true if you draw off 50 litres of hot water in a single draw but if you draw off 50 litres as 50 draws of 1 litre with a few minutes between them than a combi can be a low as 40% of its claimed efficiency.

Combi boilers need a lot of gas if you want to get a reasonable flow of "instant" hot water. This may mean that you find ( late in the process ) that a larger gas pipe has to be installed, This pipe may have to be an un-sightly copper pipe run along the outside of your house.

The gas meter is approx 5 metres away which is in direct line of sight to the boiler, it will require upgrading, but will cause minimum disruption.

I've had a combi in a previous house and have been more than happy, this debate for the pros and cons of either system could go on forever.

I'm past the point of no return, so i an interested to hear and learn about the pipe configuration changes required to fit a combi

bernardgreen said:

The efficiency of a combi is often quoted as being higher than a convential boiler and thus the combi will be cheaper to run.

That is possibly true if you draw off 50 litres of hot water in a single draw but if you draw off 50 litres as 50 draws of 1 litre with a few minutes between them than a combi can be a low as 40% of its claimed efficiency.

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The visitors we get, who are used to combi boilers at home, are amazed at just how quickly they get hot water when they turn the tap on and the fact that both hot and cold can be run at the same time. A combi boiler needs much higher heat capacity, just to deliver hot water at a reasonable speed - that usually means it needs to have much more heat capacity than is needed for central heating. Too large a boiler, means the heating system is less than optimal efficiency most of the time.

A local retired friend had a new boiler installed and allowed himself to be persuaded to have his system converted to a combi, not properly understanding the implications - he regrets it now.

Combi boilers are much cheaper to install from scratch, ideal where there is no space for tanks, work well where the place might not be occupied much of the time and hot water might not be needed for long periods, or irregularly. For everyone else - open vented work best and more economically.

Measurements on installed combination boilers

Domestic hot water production
Hot water production was reviewed for summer months alone where no space
heating demand was expected. This discussion of efficiency for DHW production
must be treated with caution as the heat meters used were found to under record
very short water draw offs (less than 15 seconds).
For summer months, the regular boilers were efficient for generating DHW to the
cylinder, but recorded kWh heat delivered to taps was much lower. As little as half
the energy delivered to the cylinder was recorded as drawn off for use by the
householder.
Poor energy balances recorded for combination boilers in periods of very low
consumption led to a laboratory investigation. This identified higher than estimated
losses from ignition and fan overrun. In response to the poor performance of heat
meters for short draw off patterns and the higher losses an adjustment of 25% was
made (for the purpose of summer DHW assessment alone) to increase heat
recorded by the heat meters to attempt to reflect the true heat used. After adjustment
the combination boilers gave an estimated overall mean hot water efficiency of 73%.
The efficiency of DHW production was found to be dominated by different factors
when considering regular and combination boilers. With regular boilers, efficiency is
dominated by standing losses from cylinder and primary pipework and therefore very
small total draw-off means poor efficiency. In contrast, efficiency of combination
boilers is dominated by the length of individual draw-offs, thus very small individual
draw-off (e.g. hand-washing) gives low efficiency and larger draw-offs (e.g. a bath)
giving efficiencies comparable to those achieved when providing space heating. It is
therefore important to choose a standard draw-off pattern when comparing the
efficiency of DHW production. Currently SAP uses approximately 100 litres of
DHW/day (~5.8kWh) for a 100m2 property and these investigations show no
substantive reason to change this.

https://assets.publishing.service.g...toring_of_condensing_boilers_final_report.pdf

I've opened a can of worms, appreciate all the concerns and advice not to convert

I appreciate if you could respect my decision just looking for advice on the pipework nothing more, nothing less