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TOPIC: Sealed Central Heating Expansion Tank

Sealed Central Heating Expansion Tank 19 Oct 2021 16:56 #126374

  • Richard Cooper
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I've done a few simple and effective gravity heating systems on boat stoves. top pipe from boiler rises all the way, then falls all the way back to the bottom outlet on the boiler, rads spaced along the way. I always fitted the expansion tank to the very highest point of the pipe run, this was of course to the 'hot' pipe just as it turned the corner to become the 'cool' return. However once I fitted the tank to the top of the 'cool' pipe probably some sort of awkward access problem- only a matter of a foot or so of pipe but it really did not work at all. Much bubbling and gurgling and the whole lot tried to circulate backwards - the bottom pipe was really hot and the top pipe was cool. I guess the open header tank made it easy for the hot water to climb. Absolutely agree that if it works then quietly close the door on it and tiptoe away.

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Sealed Central Heating Expansion Tank 19 Oct 2021 09:15 #126354

  • Paul Hayes
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Conventional Feed and expansion tanks are always connected to the pump inlet side.

If connected to the outlet side of the pump it's liable to "prime over" e.g. pumping up the expansion pipe, if the hydraulic balance (resistance) in the circuit alters, e.g. a TRV or two close.

Therefore, it follows that convention (and often manufacturers instructions) say that a pressure vessel should be connected to the same place, as it takes the place of an F&E tank. However a pressurised system does away with an expansion pipe.

By putting it on the flow side an increased load may be put on the pump reducing its life. Also some hydraulic balance problems may occur.

In practice however, it doesn't make a great deal of difference if it's on the flow or return.

The lifespan of the membrane may be reduced as it can get hotter on the flow.

If it's worked for 15 years let sleeping dogs lie.

Paul Hayes

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Sealed Central Heating Expansion Tank 18 Oct 2021 17:57 #126343

  • Balliol Fowden
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I don't know the answer for certain, but would guess that the only difference between tee-ing the expansion tank off the flow rather than the return would be that the tank will be absorbing and disseminating more heat, which may not be where you want the heat. Might also have an impact on the life of the membrane I suppose, but it has worked for fifteen years.......

So fas as pressure is concerned was it Newton that said something about that?


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Sealed Central Heating Expansion Tank 18 Oct 2021 17:42 #126341

  • Colin Stone
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Thought I'd check the expansion tank pressure which I haven't looked at in 15 years. So made the mistake of reading the tank's instructions to check the original pressure. I also found that I put the tank in the wrong pipe. It is in the boiler supply pipe rather than the return pipe, probably as it was an easier route and neater.
It would be too difficult to change to return pipe, but I wonder if I really need to??
I can only think that with the tank in the return pipe, there is slightly reduced pressure change from cold to hot system. But whether of any significance, I know not.
But someone probably knows??

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Colin Stone
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