Bypass when using Grundfos Alpha2 pump

[matt1e]

Hi Smokebox and all
well I've just sat and read through this entire thread properly and the Vaillant manual and have noticed that one of the problems is that you don't know the boilers min flow rate

Only Vaillant can advise a satisfactory MINIMAL ACCEPTABLE FLOW IN LITRES PER MINUTE. for thispurpose....... only they don't understand or refuse to specify, and eventually get rather rude to emails until you write to the UK Managing director. but they still don't know. One technician said the boiler flow must never fall below 1220 l/hr - a real jester! and I suppose he is paid!

The minimum flow rate of your boiler is 20.3 L/min, it's in the specification table on page 8 of your manual

also
3.14 Pump specification
A variable duty pump should be set to give a
temperature difference of no greater than 20°C

so now you have
your min flow rate
recomendations regarding variable pumps
tech info on your pump regarding flow rates and recomendations reg auto bypasses
and a couple of diagrams regarding using valves/solenoids etc for overrun bypassing
you should have enough info now surely

Matt

 

[smokebox]

Hi Smokebox and all
well I've just sat and read through this entire thread properly and the Vaillant manual and have noticed that one of the problems is that you don't know the boilers min flow rate

Only Vaillant can advise a satisfactory MINIMAL ACCEPTABLE FLOW IN LITRES PER MINUTE. for thispurpose....... only they don't understand or refuse to specify, and eventually get rather rude to emails until you write to the UK Managing director. but they still don't know. One technician said the boiler flow must never fall below 1220 l/hr - a real jester! and I suppose he is paid!

The minimum flow rate of your boiler is 20.3 L/min, it's in the specification table on page 8 of your manual

also
3.14 Pump specification
A variable duty pump should be set to give a
temperature difference of no greater than 20°C

so now you have
your min flow rate
recomendations regarding variable pumps
tech info on your pump regarding flow rates and recomendations reg auto bypasses
and a couple of diagrams regarding using valves/solenoids etc for overrun bypassing
you should have enough info now surely

Matt

But matt this huge throughput of 1220 litres/ min (20.3 litres/hr) can never be maintained in all situations. It is because of the wrong (impossible) statement in the Vaillant manual, and their refusal to acknowledge this that whole thread was raised by me, as you have spotted. In a way the Alpha2 issue is a separate but but related one

I and Hailsham seem to agree that the flame WILL be out in the situation we are specifically discussing, and what we are really doing is dissipating residual heat only, to prevent extreme local "left over" hot spots - little more than mixing the water abit around the HE !

Agile is fearful of abrupt shut down eg while heating the water cylinder at 30kW (interestingly the software shows that for this sole purpose you can set the output to30kW indeed it is the preset default). This abrupt shut down of course does happen with an ordinary CH Thermostat eg when it has done a heating blast and pauses to asses progress , or of course with timer switch-off in full blast, or manually. BUT again as hailsham says and I have elsewhere, the flame again is extinguished. We do not need 1220 litres/min in this situation any where near. Actually it could damned near be provided with my solenoid valveor matts zone switch proposals. But it doesn't NEED to be, for this purpose.

Furthermore I would argue that a house that has warmed up with all TRV's shut down, but still occasionally taking heating blasts through its remaining lockshield only rad, will not be allowing such huge flow as 1220l/min to be possible unless the pump rises to enormmous output pressure to cope with the much increased resistance compared to of a cold, open TRV'd house.

smokebox

 

[Dan Robinson]

1220 l/min


Don't you mean l/hour?

 

[gaswizzard]

1220 l/min


Don't you mean l/hour?

1800kw?

 

[Agile]

But as soon as the boiler turns off it is no longer generating any heat to bring the water back up to temperature. So the fact that it was running flat out when the boiler turned off is irrelevant. All you are doing is dissipating the residual heat in the water.

Its very obvious that you have not looked closely at the heat being produced in a 30 kW premix boiler.

The insides are glowing red hot and all that latent heat has to be dissipated by the water continuing to flow through the heat exchanger.

The INSIDE of the heat exchanger is at the water temperature of say 70 C whereas the outside of the thin metal is close to the flame temperature of perhaps 500 C. Thats a lot of latent heat in the heat exchanger.

If water flow is stopped dead the inside of the heat exchanger rises to over 130 C and the water flashes to steam. The metal becomes very hot and softens and deforms!

Thats not very good for the boiler!

 

[matt1e]

But matt this huge throughput of 1220 litres/ min (20.3 litres/hr) can never be maintained in all situations.

Its roughly half a pint a second that it needs
and if it needs it, then there's no getting round it, it has to be maintained

Matt

 

[smokebox]

But as soon as the boiler turns off it is no longer generating any heat to bring the water back up to temperature. So the fact that it was running flat out when the boiler turned off is irrelevant. All you are doing is dissipating the residual heat in the water.

Its very obvious that you have not looked closely at the heat being produced in a 30 kW premix boiler.

The insides are glowing red hot and all that latent heat has to be dissipated by the water continuing to flow through the heat exchanger.

The INSIDE of the heat exchanger is at the water temperature of say 70 C whereas the outside of the thin metal is close to the flame temperature of perhaps 500 C. Thats a lot of latent heat in the heat exchanger.

If water flow is stopped dead the inside of the heat exchanger rises to over 130 C and the water flashes to steam. The metal becomes very hot and softens and deforms!

Thats not very good for the boiler!

Agile
I had no idea!
That is fascinating,
and somewhat salutory, to say the least!

Thank you vary very much for this info.

smokebox

 

[smokebox]

But matt this huge throughput of 1220 litres/ min (20.3 litres/hr) can never be maintained in all situations.

Its roughly half a pint a second that it needs
and if it needs it, then there's no getting round it, it has to be maintained

Matt

Matt
Together with Agile's point above, and yours now, I am being forced to regroup my thoughts rather drastically, and concede there is more to this cooling down after flame extinction than I realized!!!!

I think your unloading (at high volume flow) into a standing zone is the best heat conserver, meets the min flow specs,and will also stop wasteful loss of condensing in ordinary running.

Agile's information stunned me I admit!

smokebox

 

[D_Hailsham]

The minimum flow rate of your boiler is 20.3 L/min, it's in the specification table on page 8 of your manual.

also
3.14 Pump specification
A variable duty pump should be set to give a temperature difference of no greater than 20°C

We know what the manual says; but does that apply in all circumstances, particularly while the boiler is in pump overrun?

If you use the familiar equation [litres/min = (60 x kW) / ( 4.18 x Temp diff)] on the OP's boiler you get (60 x28.2) / (4.18 x 20) = 20.24 litres/min.

Let's assume that this flow rate has to be maintained under all circumstances. So when the boiler is running at min output (5.3kW) the flow rate is still 20.24 lpm. In this case the temperature differential will be only 4°C!!. If you want to maintain a 20°C differential at minimum output the flow has to reduce to 3.8 lpm.

recommendations regarding variable pumps

They said variable duty pumps, i.e the traditional Selectric types, not variable speed pumps such as the Alpha.

I see the point Agile is making about high temperatures within the heat exchanger, but still think that you have to take into account the fact that the boiler is no longer generating any heat when the pump is in overrun.

Has anyone noticed how few manufacturers provide minimum flow information?

 

[Agile]

I see the point Agile is making about high temperatures within the heat exchanger, but still think that you have to take into account the fact that the boiler is no longer generating any heat when the pump is in overrun.

You may see the point but you are not apparently taking it on board!

Immediately combustion ceases the inside of the combustion chamber is at about 500 C and glowing whitish red hot! It takes about 5-10 seconds for most of the glow to dissapear. Glowing only occurs at a few hunded degrees. During this time AND for a while afterwards this heat has to be removed by the water flow!

I know that as you are not gas registered you will not be able to look inside the combustion chamber on those boilers which provide a viewing window.

If you could then I would suggest that you would find it frightening to look closely at the viewing window when I turn the boiler on. I could almost place a bet you will recoil from the sight ( and the radiated heat ! ).

Does anyone have a video to show him? Its not as impressive as looking yourself though! Until someone finds a better video try this one although there is a combustion fault! http://www.youtube.com/watch?v=gf0-EJL07i4

Using an auto bypass to generate the maximum flow rate will be most effective at removing the heat quickly which is the objective. The amount is only equivalent to perhaps 20 secs of boiler burning so hardly worth trying to save.

Tony

 

[D_Hailsham]

Immediately combustion ceases the inside of the combustion chamber is at about 500 C and glowing whitish red hot! It takes about 5-10 seconds for most of the glow to disappear. Glowing only occurs at a few hundred degrees. During this time AND for a while afterwards this heat has to be removed by the water flow!

But some current boilers do not have pump overrun. So how does the heat exchanger get cooled down?

 

[smokebox]

I see the point Agile is making about high temperatures within the heat exchanger, but still think that you have to take into account the fact that the boiler is no longer generating any heat when the pump is in overrun.

You may see the point but you are not apparently taking it on board!

Immediately combustion ceases the inside of the combustion chamber is at about 500 C and glowing whitish red hot! It takes about 5-10 seconds for most of the glow to dissapear. Glowing only occurs at a few hunded degrees. During this time AND for a while afterwards this heat has to be removed by the water flow!

I know that as you are not gas registered you will not be able to look inside the combustion chamber on those boilers which provide a viewing window.

If you could then I would suggest that you would find it frightening to look closely at the viewing window when I turn the boiler on. I could almost place a bet you will recoil from the sight ( and the radiated heat ! ).

Does anyone have a video to show him? Its not as impressive as looking yourself though! Until someone finds a better video try this one although there is a combustion fault! http://www.youtube.com/watch?v=gf0-EJL07i4

Using an auto bypass to generate the maximum flow rate will be most effective at removing the heat quickly which is the objective. The amount is only equivalent to perhaps 20 secs of boiler burning so hardly worth trying to save.

Tony

Agile
As always your field experience is invigorating! Thanks.
Your solution to revert to auobypass, while using Auto-mode Alpha2, however gets me right back to page 1!. Illogical, out of line with Grundfos's pdf s that MJN uploaded for me, and in ordinary running, loosing condensing function of boiler (its whole basis for economy!, which is exactly what worries the Grundfos writer of that pdf, and which of course applies to all Alpha2 modes using PP -proportional pressure- adaption techniques.)

Thus I am going to unload this red hot HE immediately into a zone, or by separate solenoid operated bypass, and set up a low standing bypass of 2 litres/min for any emergency unexpected boiler shut down's ,really for extra protection. Inevitably this will waste some condensing potential, but not much I hope.

Hailsham has an interesting point about not all modern boilers having pump overrun.
Does anyone understand that?

smokebox

 

[smokebox]

Agile
a fascinating glimpse of a 'burn' occurring - thanks
smokebox

 

[smokebox]

But matt this huge throughput of 1220 litres/ min (20.3 litres/hr) can never be maintained in all situations.

Its roughly half a pint a second that it needs
and if it needs it, then there's no getting round it, it has to be maintained

Matt

Matt Isn't it actually closer to 1220/60 per sec = about 20 litres/sec = about 4 gallons per sec!
smokebox
seems excesscive but with Agile's further information maybe not!!

 

[Agile]

But some current boilers do not have pump overrun. So how does the heat exchanger get cooled down?

Name a boiler model and we will comment on it !