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Frog67

Gas Engineer
Messages
81
Evening all i have a technical question i hope you can help me with.
I've been asked to quote to fit a Vaillant 630 system boiler on an S plan with 3 seperate zones, 2 x CH and 1 x HW

The system comprises of 14 rads and a 42 x 18 indirect cylinder
8 rads on ground floor (1st zone)
6 rads on first floor (2nd zone)
Total rads output = 24kw
The 630 has a built in bypass but should i still fit an external one?.
I'm concerned about an additional bypass causing circulation / cycling problems if the built in one opens as'well as an external one.
Thanks
 
Evening all i have a technical question i hope you can help me with.
I've been asked to quote to fit a Vaillant 630 system boiler on an S plan with 3 seperate zones, 2 x CH and 1 x HW

The system comprises of 14 rads and a 42 x 18 indirect cylinder
8 rads on ground floor (1st zone)
6 rads on first floor (2nd zone)
Total rads output = 24kw
The 630 has a built in bypass but should i still fit an external one?.
I'm concerned about an additional bypass causing circulation / cycling problems if the built in one opens as'well as an external one.
Thanks

I definitely would do yes.
 
I just deleted that message Chris. I was writing something different.
Do me a favour and get rid of it so I can rewrite it will you.
 
Sorry Chris, I'm not ignoring your question. I got distracted and then Question Time started.
I'm busy today but I will come back to you soon.
 
Not a problem, no rush.
Just interested in what you all think & how you go about the issue of minimum flow rates through boilers & to ensure boiler controlled pumps are not damaged by pumping against a closed head.
 
Not a problem, no rush.
Just interested in what you all think & how you go about the issue of minimum flow rates through boilers & to ensure boiler controlled pumps are not damaged by pumping against a closed head.

I will start by saying that the best way in my opinion is to install a low loss header or similar. I realise that in a typical domestic situation that might be viewed as overkill or unnecessary and I am sure that the extra costs involved for time and materials would be too much in most cases.

I have found in the past that built in bypass valves can be inadequate. In some circumstances the flow rate through the bypass has not been enough to prevent overheating particularly when they are the only bypass on an S plan system with spring return valves.
I have also found their lack of bore can raise noise levels beyond the acceptable. That is purely down to the pump (fixed speed) pumping against too high a resistance as you are no doubt aware. I realise that the new pumps are 'clever', however, from experience with variable speed pumps they can be a nuisance. Sometimes they need to be set on 100%.

In order to avoid possible problems and allow the Boiler and pump to work comfortably, (in the absence of a Low loss header or plate etc), I would install a larger automatic bypass externally with a decent pipe run, as that is probably the next best thing.

Until the new Boiler is up and running with its new 'clever' pump, you cannot always predict the performance. It is easy and cheap to install an external bypass that gives as much control as you will need, whilst you're piping things up.

Setting an external auto bypass where there is also an inbuilt bypass is a matter for the Engineer on site to observe what is required. They are both sprung valves and will work together to some degree.

I am talking here of my own thoughts and experiences over the years. No doubt others will think differently.
I hope I answered your question well enough?
If not, let me know.

Do you teach at a college or were you talking about an apprentice who worked for you?
 
Not a problem, no rush.
Just interested in what you all think & how you go about the issue of minimum flow rates through boilers & to ensure boiler controlled pumps are not damaged by pumping against a closed head.

In the sort of systems being discussed, the external bypass is not there to protect the pump, which is the purpose of the internal bypass. It is there to allow a reasonable volume of water to continue to circulate during the over-run period when the demand valves are shut but the HX is still hot. This is why the MIs specify a minimum distance between the bypass and the boiler, to ensure there's sufficient heat capacity to keep the HX within its limits.

There is an obvious mismatch between a smart 'constant head' pump and a pressure-activated bypass valve. If the pump worked perfectly and the pipe-impedances were sufficiently low, i.e. the pressure was really constant, the bypass would either never open or always be open.

In practice the pumps aren't perfect (and the impedance of the pipework is not zero) but one still needs to set the bypass very carefully for it to operate in the correct manner at the right time.

Traditional methods of setting the bypass (using the pump characteristic-curves) tend to assume the pump is a constant-displacement rate type, which means they aren't appropriate for constant-pressure smart pumps.
 
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Do you teach at a college or were you talking about an apprentice who worked for you?
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.
 
Just interested in what you all think & how you go about the issue of minimum flow rates through boilers & to ensure boiler controlled pumps are not damaged by pumping against a head.
If I may be permitted to give a layman's view on this, I think there is a lot of confusion over "minimum" flow rates.

Before modulating burners came along a boiler's output was fixed. So it was putting out the same amount of heat irrespective of how many radiators were turned on. A bypass was necessary to maintain the flow through the heat exchanger to prevent it overheating.

If the boiler has a modulating burner it will automatically reduce the output as radiators are turned off. If the same flow is maintained through the boiler, the temperature differential reduces - a 20°C differential at 20kW output becomes a 5°C differential at 5kW output. But if the heat exchanger is OK with a 20°C differential at 20kW, surely it will be OK with the same differential at 5kW?

If the differential is reduced, because the flow rate is constant, the return temperature could rise above the dew point so condensing will not occur, which is not what is wanted. Feeding hot water back to the return via a bypass to maintain the flow will also raise the return temperature.

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.
 
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?
 
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.
 
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?
 
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.
 
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.
 
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)
 
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.
 
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.
 
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.
 
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.
 
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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.
 
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.
 

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