Vaillant 630 bypass question

[Last Plumber]

I went right through the MI's & I couldn't see anything about fitting external by-pass being required.

There is nothing in there you're right, as the internal one along with the new pump 'should' - as Frog 67 said - be sufficient. As I was saying earlier, it's only my own thoughts and experiences and I do take each system on its own merits. That's another important thing for apprentices to learn, each system has its own peculiarities. Some of the Engineers I have spoken to from Vaillant in particular agree it's sometimes necessary but not always.

 

[Chuck]

But I'm not talking about 'always-on bypassing', which wastes energy, but about the situation where the boiler is cycling on and off because the heat requirement is less than the boiler's minimum output.

The bypass valve would need to be very smart to recognise and only open under these conditions. Although you can get more sophisticated valves and controls on commercial installations, the type of valve being discussed here responds only to differential pressure, not temperature, boiler power or cycle time.

You seem to be talking about the situation where the boiler is running flat out and it is suddenly turned off, e.g by the room stat or programmer.

Yes. A short-lived 'over-run' state, which can damage the boiler if there is insufficient flow to constrain temperatures to within safe limits while it occurs. You can watch this happening by attaching some thermocouples to either side of the bypass valve and then turning down the thermostat to shut the zone valves.

In normal operation, however, the external bypass should be closed so that the return flow temperature is kept low and the boiler is running as efficiently as possible.

Not all boilers require an external bypass, it depends on their design. As always, the manufacturers' instructions trump anything you read on the internet.

 

[chris watkins]

So to sum up, if I may, (so I can pass this on to trainees).

1. With this boiler & I assume others with the same kind of HeatX & builtin bypass the likely hood is it will be able to deal with the heat dissipation required if the system shuts down suddenly on the programmer or stats on systems without a 3 port (mid or diverter) valve or hydraulic separation.

2. The boiler manufacturers instructions state minimum flow rates that are required to allow correct modulation & pump speed control. (In this case 1290 l/h) If the system deign is such that this can't be guaranteed (due to valves closing such as TRVs & power heads on U/F) then we would install a automatic by-pass set up to open when the pump pressure reached this point.

I normally give some rough guidelines like, on a small 7-8 rad system we would expect the hall to not be fitted with a TRV so I would balance the system, wind the by-pass fully closed, shut down all the TRVs except one (plus the hall), then slowly open the by-pass feeling the pipe until it goes hot (indicating valve open) then half a turn back so hopefully any less than two full open rads or the equivalent in the system flow would open it. (3 rads for larger system.
As has been sated before the rush of hot water from the flow to the return should quickly cause the boiler to turn off on the boiler stat & enter anti-cycling mode.

 

[chris watkins]

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.

 

[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.

So to sum up, if I may, (so I can pass this on to trainees).

1. With this boiler & I assume others with the same kind of HeatX & builtin bypass the likely hood is it will be able to deal with the heat dissipation required if the system shuts down suddenly on the programmer or stats on systems without a 3 port (mid or diverter) valve or hydraulic separation.

2. The boiler manufacturers instructions state minimum flow rates that are required to allow correct modulation & pump speed control. (In this case 1290 l/h) If the system deign is such that this can't be guaranteed (due to valves closing such as TRVs & power heads on U/F) then we would install a automatic by-pass set up to open when the pump pressure reached this point.

I normally give some rough guidelines like, on a small 7-8 rad system we would expect the hall to not be fitted with a TRV so I would balance the system, wind the by-pass fully closed, shut down all the TRVs except one (plus the hall), then slowly open the by-pass feeling the pipe until it goes hot (indicating valve open) then half a turn back so hopefully any less than two full open rads or the equivalent in the system flow would open it. (3 rads for larger system.
As has been sated before the rush of hot water from the flow to the return should quickly cause the boiler to turn off on the boiler stat & enter anti-cycling mode.

Sounds OK to me.

 

[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