Adding a secondary return to a system

[Aire]

They must have a reason to say must not be used.
If you think of worst case scenario
Cylinder immersions set to 70
On 24/7 and return pump on constant

The plastic pipes goes a bit floppy and you could have a fitting blow off.

At least most CH systems are sealed so you would only get a few buckets of water.

Only reason I can see is the British standard thing. If it had been tested and found the pipe fails under a secondary return system then surely they would state that.
They say their pipe will function correctly for 50 years at 90°c temperature and at 6bar so even in the most extreme cases of an unvented system breakdown, the pipe should be fine.

 

[EvilDrPorkChop]

I believe it's something to do with the heat and it being oxygenated water. In a sealed system the water isn't fresh.

However I've never come across a proper answer to the question.

I only really found out about it when a few years ago a plumber I know had an issue with Polypipe fittings failing due to the circulating return. The fittings were just crumbling to bits, it was the second time in this house they'd had a flood because of the issue.

However as the system was fitted before 2007 the plastic manufactures hadn't realised by that point the circulating returns caused an issue. So they honoured the warranty, however I think since 2008 they've strictly stated that they cannot be used for cirulating returns.

 

[Aire]

I believe it's something to do with the heat and it being oxygenated water. In a sealed system the water isn't fresh.

However I've never come across a proper answer to the question.

I only really found out about it when a few years ago a plumber I know had an issue with Polypipe fittings failing due to the circulating return. The fittings were just crumbling to bits, it was the second time in this house they'd had a flood because of the issue.

However as the system was fitted before 2007 the plastic manufactures hadn't realised by that point the circulating returns caused an issue. So they honoured the warranty, however I think since 2008 they've strictly stated that they cannot be used for cirulating returns.

Crumbling fittings sounds extreme, I wonder how many systems there are out there that are ran in plastic. I know my bosses house has plastic secondarys which have been fine for 10+ years.

 

[Jim Goodenough]

This is a very interesting topic, does it have anything to do whether you are in a soft or hard water area?

 

[EvilDrPorkChop]

This is a very interesting topic, does it have anything to do whether you are in a soft or hard water area?

No.