In an earlier contribution to this discussion,
John Rumm wrote:

Set Square wrote:

a system from scratch or how to convert a vented system (apart from
mentioning a couple of pre-requisites).

Here was my plan of action for that (going from conventional boiler to
combi, original tanks in the loft):

Boiler Replacement Procedure

Determine pipe arrangement
Determine gas pipe sizes
Calculate quantities required
Establish pipe routing for CH
Add desludging agent to existing system
Order boiler and enough fittings and pipe to make sure all
eventualities are covered!

Prep

Ensure hot water at full temp
Turn off boiler

Loft Work

Tie up ballcock in CH header
Drain system - ensure 3 port valve is in centre pos to ensure heating
loop is drained.
Cut CH feed and return in loft and loop back. Include gate valve on a
tee in loop
Cap pipe ends to prevent seepage

Kitchen

Take static gas pressure reading and ensure loss is zero
Turn off gas
Pull fuse from boiler FCU
Disconnect boiler electrics and remove programmer
Disconnect CH feed and return
Disconnect gas feed
Open boiler
Remove from wall
Remove old flue
fill old flue hole with bricks and mortar
Using template mark out positions
Core bore new flue exit
Fit boiler mounting frame (remove tiles as required)
Connect CH feed and return
Remove existing gas feed pipe to under worktop
Cut new hole through worktop for DHW feed and return, 22mm gas,
condensate Install 22mm gas feed from existing ¾" pipe
Turn off water
Cut in new DHW feed and return and connect with service valves
Install filling loop
Mount boiler on tray
Install new flue
Install condensate drain

Loft

Tie off cistern ballcock
Drain remains of hot water
Disconnect hot water output from cylinder and cap off
Disconnect input mains water from cistern and CH header tank
Disconnect cold shower feed from cistern and connect to rising main

Bathroom

Remove bath panel and identify CH stat wire - join to existing down
wire and disconnect from remainder. (Leave remainder available for
pulling through new wires later!)

Kitchen

Connect boiler electrics and stat
Carry out any pre checks as per install manual
Purge gas meter and pipework
Carry out gas soundness test
Commission boiler as per manual
Flush as required
Treat with corrosion inhibitor
Remove tanks etc. and cylinder from loft.

Job Done!


All in all getting on for four days work!

So you need to be able to estimate the water volume. If you know how
much pipe there is of each size, that bit's not too difficult. Then
there's the

If in doubt, err on the side of too large for your expansion chamber,
since too small is a problem too big is not.


[Back to my calcs. How does this grab you? Assume you have a 10 litre
pressure vessel, and prime it with air at 0.7 bar (1.7 bar
absolute). Then

Sounds ok.... here is a table from the install manual for my boiler:

Assuming pre charge pressure between 0.5 and 0.75 bar

System volume Expansion vessel
(litres) volume (litres)
25 1.8
50 3.7
75 5.5
100 7.4
125 9.2
150 11.0
175 12.9
190 14.0
200 14.7
250 18.4
300 22.1
For other system volumes
multiply by the factor across 0.074

Are there any major flaws in my reasoning?]

Not obviously.

Thanks for that. It looks like 10% is a bit conservative and that 7.4% is
ok.

Your todo list looks pretty comprehensive - and goes further than I
envisaged. I plan to keep the existing boiler and stored hot water system,
simply replacing the F&E tank with a pressure vessel and associated bits.

Some rules of thumb for estimating radiator/boiler/cylinder coil capacities
would still be nice - as would guidance on acceptable locations for the
pressure vessel.

--
Cheers,
Set Square
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