Why do some OLDER boiler heat exchangers have four ports?

[Kesterlester]

I know that many of the modern thin-plate heat exchangers in boilers have four ports -- and I know that on many of these, one pair of ports serves as an in+out for hot water, while the other pair served as an in+out for the central heating.

But why do some older boilers which (I presume) have only one water path through their heat exchangers still have four ports?

For example, I attach two pictures of a cast iron heat exchanger currently being sold on ebay (item number 383864605259 ) for an older Ideal boiler.

The casting for the above heat exchanger looks to me to be very similar to the one I have in my Ideal Classic NF50. Mine definitely has only one water path running through the heat exchanger, from one diagonal corner to the other, and a similar path is suggested by the diagonally opposite placement of the the open-ended pipes on unit whose pictures I have attached. I'd therefore be very surprised if the pictured heat exchanger being sold above had more than one path through it [Though perhaps it doesn't?] as the modern heat exchanges don't use diagonally opposite ports in pairs precisely as they do need to maintain separate water flows.

I had previously assumed that the four holes on the back of my NF50 heat exchanger (and similar ones) were just so that a plumber or customer could (if he really wanted to) swap the flow and return pipes around while keeping the flow from the top of the exchanger and the return from the bottom. This would help with cases where space constraints made it hard to work with flow and return the standard way round.

However, the item in the pictures has clearly been made made all four ports having specially shaped pipes attached -- and two if them have very special hemispherically capped ends -- a sort of capping I've not seen before. The manufacturer would have to go to a lot of trouble to make those special closed ends, so (I imagine??) the maker of this part didn't expect most people to cut off the rounded ends. Perhaps only a small nubber of customers have to cut them off?? Or do they serve some other purpose?

Basically, what is the purpose of the second pair of capped pipes on this heat exchanger?

 

[Kesterlester]

Update:

Since posting I have spotted elsewhere on this forum a diagram showing a heat exchanger like the above in a boiler.

The link to the diagram is here:
https://www.diynot.com/diy/media/untitled.35425/full

That diagram calls the hemispherically capped pipes the "gravity flow pipe" and "gravity return pipe". This is in contrast to the open pipes which it calls the "pumped flow pipe" and "pumped return pipe".

How were these "gravity" pipes supposed to be used?

Was the idea that most people would not use these gravity pipes at all ..... but that a small number of customers would cut off the hemispherical caps and piumb this secondary circuit to the coil of a nearby HW cylinder? The latter group of customers would get HW cylinder heating without pump noise, and while (say) the radiators for the rest of the house required flow from the pumped circuit. Still, the apparent expectation that they will need a pump anyway makes me wonder what benefits such a system really had.

 

[ianmcd]

Google gravity Hot water systems

 

[aesmith]

How were these "gravity" pipes supposed to be used?

Was the idea that most people would not use these gravity pipes at all ..... but that a small number of customers would cut off the hemispherical caps and piumb this secondary circuit to the coil of a nearby HW cylinder? The latter group of customers would get HW cylinder heating without pump noise, and while (say) the radiators for the rest of the house required flow from the pumped circuit. Still, the apparent expectation that they will need a pump anyway makes me wonder what benefits such a system really had.

That's exactly how they would be used. Two pipes on one side of the boiler providing gravity feed to a high level HW cylinder, and the pair on the other side pumped for heating. The house I grew up in was set up that way, converted from original coke boiler. Like you I can't see any benefit over a fully pumped system, except possible for retro fitting to houses that previously had solid fuel. Or I suppose it might be seen as simpler with fewer parts to fail (no zone valves).