Quick gravity heating circuit question. Multiple diameters?

[Rendelf]

Hi all! Hope autumnal things are progressing well.

 

I'm doing some work on our gravity/thermosyphoning heating circuit. It's in 28mm copper, and very simple. Today I noticed that the end section farthest from the stove, which is vertical and transitions the circuit 180 degrees (the beginning of the return run) is in 15mm copper.

 

I'd have thought that this would introduce friction into the system. Seeing as I'm doing some plumbing at that point anyway, is it worth swapping it for 28? Or is there something about the thermosyphoning process I don't know?

 

Cheers!

[Bee]

Can't see any reason for 15mm, I would do it in 28 if theres a problem but if it works ok I would leave it alone.

[MtB]

I'd hazard a guess that it was probably originally 28mm, then replaced with 15mm copper in a vain attempt to 'divert' some of the circulating heat into one or more radiators which don;t really get hot.

 

Just a guess really, but I can't imagine any other reason for introducing a length of 15mm into the circuit. It won't have made any difference. Are there any rads that don't heat up properly?

[Rendelf]

Thanks peeps,

 

Have since discovered a tap beneath the insulation. Presumably the 15mm was to allow this to be added. It's not a drain cock....so unsure of its purpose: closing the system would just cause water in back boiler to boil, I'd have thought.

 

As I need to get at the pipes there anyway, I think I'll replace it.

 

We only have two radiators, and they're both good - though it looks like there was one a third in the salon, which was removed.

[OldPeculier]

If the 180 degree bend is done in 15mm, it could be that the fitter didn't have the ability to bend 28mm pipe or didn't want to pay out for 28mm elbows.

[bigcol]

I did put a post on re air in a gravity system.

 

I now have removed the circulation pump, which I never used,off the system

I have had drain off valves put on the last 2 rads both ends of the system.

I've a automatic air vent placed on the 22mm pipe at the front.

And lots of air was released.

 

Hence now All radiators hot, and all top 22mm pipes and hot,

but the lower pipe (which I imagine is the return)is cool through out,

accept for 2ft both sides of the Rayburn which is in the middle Of the boat

 

I through the forum, are now sitting here, very warm with central heating down to gravity.

 

But why would the bottom pipe be cold,

 

Surely the system is a loop?

 

Col

[David Mack]

I did put a post on re air in a gravity system.

 

I now have removed the circulation pump, which I never used,off the system

I have had drain off valves put on the last 2 rads both ends of the system.

I've a automatic air vent placed on the 22mm pipe at the front.

And lots of air was released.

 

Hence now All radiators hot, and all top 22mm pipes and hot,

but the lower pipe (which I imagine is the return)is cool through out,

accept for 2ft both sides of the Rayburn which is in the middle Of the boat

 

I through the forum, are now sitting here, very warm with central heating down to gravity.

 

But why would the bottom pipe be cold,

 

Surely the system is a loop?

 

Col

 

The bottom pipe is cold because the water in it has given up all its heat when it passed through the radiators. Without a pump in the circuit, the thermosyphoning will be quite slow, and so the water has time to cool down to quite a low temperature. As long as the radiators are hot there is nothing to worry about.