Vaillant 630 bypass question

[Last Plumber]

I understand that this still should be put in place however with the use of variable speed modern pumps set to reduce their speed as the pressure increases the likely hood is the by-pass would never open.
The more you think about the modern system designs the more I see the case for hydraulic separation, the cost of using Close Coupled Tees is small compared to LLH but it may explain why Vaillant are pushing them in the M.Is.

Many thanks to all for your input to this.

There are many ideas that are slowly making their way form the Commercial side to domestic but obviously on a small scale. Headers of various types, for example, have been used in the Commercial side for many years.

 

[fishpond]

There are many ideas that are slowly making their way form the Commercial side to domestic but obviously on a small scale. Headers of various types, for example, have been used in the Commercial side for many years.

Indeed - I always install LLHs in domestic installations where there are more than 2 zones or when cascading boilers.

 

[fezster]

It seems to be that with some boiler's (eg. large KW output Vaillant's with high resistance heat exchangers and software which requires high flow rates before the boiler has a chance to modulate down), many domestic installers are using/relying on an ABV to overcome the problem of insufficient flow in the CH system. The task of the ABV should be to operate in the conditions described above (i.e. closing down of TRVs or ZV's), but many are set to allow the minimum flow rate required whilst these boilers operate at a high output before modulating down, at the expense of bypassing under normal conditions also. This has been my experience.

The correct solution seems to be CCT's or LLH. I think Fig 6 of this article explains well how the boiler is able to maintain it's required delta T with the use of an LLH:

Best Practice: Low Loss Headers