Vaillant 630 bypass question

[Last Plumber]

Yes, only one day a week now.
They have to connect up an S plan (un-vented cylinder, regular condensing boiler, sealed system kit etc,), wire it up, then turn on test & commission it.
None of them have a clue about setting the by-pass which must reflect on what is going on in the real world.

Possibly.
I think a lot of people out there, pipe up, switch on and walk away.
I'm glad you're discussing it with them. People in your position play a vital role in putting things in the minds of those learning. Hopefully that makes them ponder over the subject matter and ask questions on. Especially if they ask questions on site and want to know 'why' something happens.
What percentage of your class would you say were genuinely interested in the work?

 

[Chuck]

Your description of what happens is okay until here:

The true minimum flow is therefore that required to produce the maximum permitted differential at minimum output. This means that you only need the bypass to operate when the boiler is cycling at minimum output because the heat requirement is less than the boiler output.

where you draw the wrong conclusion. During normal continuous operation the external bypass valve should not be passing flow. The regs haven't permitted 'always-on bypassing' for years because it wastes a lot of energy.

An external bypass circuit is not able (or intended) to dissipate 5kW continuously. It needs to have sufficient heat capacity to absorb heat from the HX at a rate of 20kW for the short period between when the zone valves all shut, which shuts down the burner, and when the heat exchanger has cooled sufficiently to avoid damage due to overheating the 'cold' section. This so-called 'over-run' time is typically 30 seconds or so because modern HXs have a relatively low heat capacity.

 

[chris watkins]

What percentage of your class would you say were genuinely interested in the work?

Almost all as I am not in an FE college & they are all apprentices.

 

[doitmyself]

During normal continuous operation the external bypass valve should not be passing flow. The regs haven't permitted 'always-on bypassing' for years because it wastes a lot of energy.

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.

An external bypass circuit is not able (or intended) to dissipate 5kW continuously. It needs to have sufficient heat capacity to absorb heat from the HX at a rate of 20kW for the short period between when the zone valves all shut, which shuts down the burner, and when the heat exchanger has cooled sufficiently to avoid damage due to overheating the 'cold' section.

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. But if the burner has been turned off the boiler is no longer producing heat at 20kW. All you have is the residual heat in the pipes, which has to be dissipated somewhere. Do you really need a bypass capable of handling the maximum flow to do this?

 

[fishpond]

Low loss header is the way to go in my opinion

 

[Last Plumber]

Almost all as I am not in an FE college & they are all apprentices.

That's good then. What level are the ones at that you were discussing by-pass settings with? Would they be level 3? It's a long time since I was in college.

 

[chris watkins]

That's good then. What level are the ones at that you were discussing by-pass settings with? Would they be level 3? It's a long time since I was in college.

This is normally done right at the end of their Level 3 as it combines a lot of units they have done through-out (heating, wiring, Un-vented) it is kind of an end test & quite a challenge, how many of us could have wired an S plan aged 20 - 21 years?

I think a lot of people out there, pipe up, switch on and walk away.

I know so, not just from the fed back I get from them but because I go out regularly to thing like 2-3 year old new build flats with HIU, the pumps are set to fixed speed 3, none of the U/F heating coils have had the flow rates set. How much energy is being wasted for want of 10mins commissioning, after all it is a legal requirement.

People in your position play a vital role in putting things in the minds of those learning. Hopefully that makes them ponder over the subject matter and ask questions on.

With luck.
The longer I teach the more I come to think it is not the knowledge I can provide that is the important thing, it is the enquiring mind & the passion for what we do. Of course they have to trust you know what is what & they will find you out quick enough if you don't.

Especially if they ask questions on site and want to know 'why' something happens.

Are may favourite word 'WHY'
They get ask it so many times, don't just tell me the answer, tell me WHY.

 

[Frog67]

I contacted Vaillant support who advised me that the boiler does not require an external bypass as the built in one will (should) handle the excess heat/ flow when any external valves are closed.
I wont pretend to be an expert when it comes to setting bypass's but I'll certainly be paying close attention when (if) i get the install.

Chris Watkins, what a brilliant thing your doing with today's apprentices, i wish i had had that level of training when i was at college in 1986.

 

[chris watkins]

i wish i had had that level of training when i was at college in 1986.

So do I mate, if I had I wouldn't be stuck teaching the little buggers. Only joking.

It is not until we can look back that you can truly appreciate those who taught us for being good or bad.

Thanks for putting up with all my questions Ron H. AKA 28mm Ron (Dent Head)

 

[chris watkins]

I contacted Vaillant support who advised me that the boiler does not require an external bypass as the built in one will (should) handle the excess heat/ flow when any external valves are closed.
I wont pretend to be an expert when it comes to setting bypass's but I'll certainly be paying close attention when (if) i get the install.

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

 

[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