Vaillant 630 bypass question

[chris watkins]

edit request

 

[king of pipes]

I always fit em . Kop

 

[Last Plumber]

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.

 

[Last Plumber]

edit request

Thank you Chris.

 

[Last Plumber]

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.

 

[chris watkins]

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.

 

[Last Plumber]

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?

 

[Chuck]

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.

 

[chris watkins]

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.

 

[doitmyself]

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.