Central Heating: Replacing 3/4" Steel Pipes with 15mm C

[Credulous999]

Hi,

Still doing the bathroom. Recently plumbed in the towel radiator and to my horror found that the main outward and return central heating pipes were 3/4" mild steel with copper solder ring fittings being used to connect them!

Currently the pipes go under the bathroom floor to a far corner, where they junction to the front bedroom radiator and the downstairs lounge radiator.
At the point of the junction they reduce from 3/4" to 1/2" / 15mm.

The supply and return pipes from the boiler (which is a replacement combi boiler to the original system) are currently 15mm copper and arrive at a central junction in the central heating system (under one of the back bedroom floors) where they are connected by a plastic push fit set of junctions to:

i) the branch of the central heating to the bathroom, front bedroom, and lounge radiator.
ii) the branch of the central heating for the two back bedrooms;
iii) the branch of the central heating to the loft conversion radiator.
iv) the branch of the central heating to the kitchen and downstairs hallway radiator.

Branch iii) and iv) are already copper
Branch ii) I can't currently see but suspect is mild steel.
Branch i) is currently a hybrid of copper and mild steel (due to plumbing in the towel radiator, and replacing the lounge radiator a year back. The joins between the steel and copper are currently compression fittings.

Given the precarious state of the mild steel elements of branch i) (one of the joints between two mild steel lengths is currently slow leaking, I've read up on electrolysis and its scared me some, some of the already replaced mild steel was ridiculously corroded where it met the plastic push fitting), and plans to put down a hard tile floor in the bathroom, I intend to replace the remaining mild steel elements in branch i) with copper components, all soldered.

I currently have six 2m lengths of 15mm copper from the remainder of the bathroom project. My question is whether I can use this to replace around 1.5-2m of each of the 3/4" steel pipes up to the junction where the flow splits to each of the two radiators or whether I need to spend yet more money on 22mm copper pipe to reach this point?

Given that the feed from the boiler is already 15mm I can't see any harm in this, but given I'm not a heating engineer any information would be useful. For example would the system need rebalancing?

I will likely replace branch ii) with copper at some point in the future (if it is indeed mild steel), but this isn't as pressing as it is isolated from the copper by the plastic central junction, and will be under soft floor fittings making future access less of an issue.

Thanks in advance for any help.

 

[muggles]

My best advice would be to repipe everything, with 22mm flow and return connections to the boiler, and as you've had corrosion issues this should be either followed by a full powerflush of the system, or alternatively done in conjunction with the removal of all rads for manual flushing out with a hosepipe outside. It sounds like you have a real mishmash of a system that may not have been installed very well/

Incidentally, I suspect your 3/4" steel is actually 1/2".....

 

[Agile]

1/2" steel is that size INSIDE and about 3/4" on the OUTSIDE!

 

[Agile]

But what you have is more likely to be the steel thin pipe which was used as a substitute for copper when Zambian copper was unable to cross Rodesia when Ian Smith declared UDI about 1971.

Tony

 

[Credulous999]

My best advice would be to repipe everything, with 22mm flow and return connections to the boiler, and as you've had corrosion issues this should be either followed by a full powerflush of the system, or alternatively done in conjunction with the removal of all rads for manual flushing out with a hosepipe outside. It sounds like you have a real mishmash of a system that may not have been installed very well/

Incidentally, I suspect your 3/4" steel is actually 1/2".....

Thanks very much for the response. It is definitely 3/4" past the plastic junction (assuming I'm right that 3/4" is the 'almost' imperial equivalent of 22mm).. You are saying that the pipe should be 22mm all the way to the boiler then? Currently has 22mm out the boiler for about a metre before reducing down to 15mm. Probably runs like that for two to three metres before meeting plastic junction and becoming 3/4" steel.

The system does need a good clean - black as anything inside.

 

[Credulous999]

But what you have is more likely to be the steel thin pipe which was used as a substitute for copper when Zambian copper was unable to cross Rodesia when Ian Smith declared UDI about 1971.

Tony

Agree. The pipe is thin walled and looked just like regular copper pipe to my untrained eye under the thirty to forty years of accumulated grime on its surface, especially when factoring in the soldered yorkshire joints.

Given the advice given (I've thanked you all) I will splash out the extra 50 quid or so on 22mm pipe and pipe 22mm all the way to the split for the radiators. 15mm from there to the rads.

Given having a fully functional bathroom is the priority I will worry about the remaining steel later. Dragging all the rads outside for a hosing (which I agree is definitely needed) can also be done later in the summer.

Any further information on why 15mm pipe feed from the boiler is incorrect would be most welcome in factoring in whether I sort that out in the future.

 

[Nige F]

But what you have is more likely to be the steel thin pipe which was used as a substitute for copper when Zambian copper was unable to cross Rodesia when Ian Smith declared UDI about 1971.

Tony

Is that who caused the shortage . I always thought Smithy was a IUD or something very near to one

 

[Credulous999]

As an update I have now replaced all the pipework in branch i). Have left the floorboards up for now above any joints I made and can see (and feel when running finger underneath) no leaks. Thought job was a good un....

However boiler pressure keeps dropping (has dropped 0.3 bar since last weekend). Suspect remaining steel pipe work in branch ii) (if indeed it is steel as I suspect) has been disturbed and is slow leaking.

No staining of ceilings though (although the kitchen, being the room below branch ii) ) has a sub ceiling made from plastic which would I guess hide such a slow leak).

Does a 0.3 bar pressure drop sound consistent with a slow leak (I'm going to be raising the floorboards above branch ii) at the weekend anyway, just to check), or could it just be the system settling down/equalising pressure from refilling etc?

With it being a slower drop in pressure does this mean less likely to be pressure relief valve related?

Thanks in advance for any help.

 

[Agile]

When you have disturbed a system, the residual air takes a few days to dissolve. That causes the pressure to reduce and may need to be topped up a little in the first few days.

That thin steel pipe usually creates a big leak when it fails!

An experienced engineer will test it with a magnet when quoting for heating work. I once identified some and refused to replace the boiler unless all the pipework was also replaced. The owner would not agree so I presume a less experienced person did the job and had or risked a problem!

Tony