Central heating pipe layout

[Jim_cam]

Hi all -- I'm adding a towel radiator in the upstairs bathroom so I've got some floorboards up and I wanted to check that a couple things I noticed are normal.

i) The feed and return pipes come up out of the combi boiler into the bedroom, where there's immediately a radiator with 15mm pipe going into the feed at one side and the return at the other. I guess this is ok but does it mean that hot water is going to be returning to the boiler immediately? Is that an problem?

ii) About halfway along upstairs there's a 15mm pipe that directly connects the feed pipe to the return. It's 2 t-junctions so the pipes also continue on to supply the other upstairs radiators. I guess this is to ensure that the water can flow round the system even if all the radiators are shut off, but might this mean that not enough hot water gets to them?

Maybe these aren't a problem at all -- I'm not a plumber -- but I thought I ought to check while the boards are up and it's all easy to fix.

 

[stem]

i) The feed and return pipes come up out of the combi boiler into the bedroom, where there's immediately a radiator with 15mm pipe going into the feed at one side and the return at the other. I guess this is ok but does it mean that hot water is going to be returning to the boiler immediately? Is that an problem?

That's normal. When the radiators are 'balanced', the one closest to the boiler will have its lockshield valve almost closed, so that the majority of the water goes to the rest of the system and only the amount needed flows through the radiator.

ii) About halfway along upstairs there's a 15mm pipe that directly connects the feed pipe to the return. It's 2 t-junctions so the pipes also continue on to supply the other upstairs radiators. I guess this is to ensure that the water can flow round the system even if all the radiators are shut off.

A by-pass is normal, and you are right, it is to maintain the required flow of water through the boiler. However they are usually regulated to some extent. Older systems have a manual gate valve cracked open half, to one turn, so that most of the water flows around the radiators and not through the by-pass. Newer systems have an automatic by-pass valve [building regulations requirement] that will open up and regulate the flow through it as the radiators shut down.

 

[Jim_cam]

That makes sense, thank you. The situation with the by-pass is actually slightly different from what I'd thought: the feed and return pipes don't join together where I thought they did, but rather the pipes I thought were connecting them instead disappear off through a hole to supply the radiators downstairs. So there's still no ABV, but then I looked up the model combi boiler I have (Vitodens 100-W) and it said that it has an internal bypass which will do instead.

*BUT* it also said that if I have TRVs on any radiators, I might need to add an additional ABV. Is it the case that if the rads in some rooms have TRV and don't in others (e.g. towel rail in bathroom, downstairs by the thermostat) that I won't need one, since there will always be a possibility for water to flow if the boiler comes on?