Advice on pipework for central heating

[Milo22]

Hi guys so just gonna throw this question out there... i maybe get slated here as it often can happen ive seen on forums where trades chip in saying call a plumber or doing us out of work etc... but im gonna ask away as ive had five plumbers round my house and not had one quote from them as of yet.... sooooo im looking to do the work myself basically i have a old oil boiler tank system in at the moment and looking to convert it to a gas combi system keeping the existing 22mm ring main with 15mm tails to the rad bringing a new flow and return over from the new gas boiler location aswell as a single hot pipe to feed kitchen tap and bathroom so my question is this

is a central heating loop on combi system literally a loop of 22mm pipe coming out the boiler and returning in 22mm to a inlet in the boiler ?

and with regards to the hot tap feeds is it soley a 15mm 1 way feed from the boiler to the last tap ? or would this be 22mm with 15mm tails off to the taps ?

would the cold feed into the boiler be straight 15mm feed or a 22mm cold feed ?

and along the way would there have to be any fancy bits of kit such as expansion vessels or tundish sort of things ?

obv i wouldn't be fitting the actual boiler but if i got the pipe work all to the boiler location then i would be almost there and just get a trade to pipe up the gas and boiler

any advice or help would be great apart from saying leave it to the professionals :-D

many thanks

[Someone-Else]

Just my POV (I am NOT a plumber or heating engineer) Depends how it is installed. My rads it leaves and returns in 22mm, but as soon as it leaves There is a T left and right that are only 15mm. My boiler has an expansion vessel in its cabinet (So you can't see it) I also have a "magnaclean"

I honestly think you should find a heating engineer first and ask him what you can/can't do. Put yourself in his shoes, would you work on a job when the owner has done what he thinks should be done, only for you to have to say its wrong.

I would say when the plumbers came round, you never made them a cup of tea.

[ericmark]

I am also an electrician, not a plumber. With oil, the boiler does not modulate, it just turns on/off. There is an output temperature sensor, but often no return sensor, so it does not matter if hot water is returned to the boiler, but with a gas modulating boiler for it to gain the latent heat the return water temperature is important, it needs to be cool enough, so setting the lock shield valves, and use of TRV's instead of a simple wall thermostat also becomes important.