Bypass when using Grundfos Alpha2 pump

I have recently had a Vaillant ecoTEC plus 428 fitted in my house. The pump is the Grundfos Alpha2, which costs twice as much (approx) as an ordinary 'Three Fixed Speed' type of pump. However I specified it because it is much more energy efficient (of electricity obviously !) and is "grade A " efficient. I gather the recent regulations actually require a new installation to have a grade A pump anyway.

The purpose of the bypass is largely to maintain a minimum flow through the boiler to prevent heat exchanger damage and hot spots in heat exchanger (the inside surface of the heat exchanger may be at temp, at least in patches, of over 1000*C) eg as all the TRVs gradually shut down and importantly when all demand is shut down with closure of all zones. In my case a 28kW flame has been shut down just at the moment the water flow ceases. Hence the boiler putting the pump into a preset overrun period, and the bypass coming into play.

Have I got this understanding correct?

If so I will proceed to my problems with the Bypass correct type and setting.

What do you expert people say - so far?

smokebox said:

I gather the recent regulations actually require a new installation to have a grade A pump anyway.

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Not true. There is an EU directive that all circulators placed on the market after Jan 2013 must be Grade A. This will eventually rise to grade A*. But at the moment all pumps are acceptable.

Have I got this understanding [about the purpose of a bypass] correct?

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Essentially, yes.

But the boiler will not be providing 28 kW when the TRVs are shutting down.

Indeed if its still firing at all it will be on minimum power.

Why do you believe that you needed a 28 kW boiler?

Whats the sum of all the individual radiator heat outputs?

Whats the whole house heat loss?

Tony

Hailsham and Agile thank you for responses.

I am very sorry I had to stop the thread just after starting and it petered out.
[illness]

So we don't need yet grade A or A* yet! Even in "part L".and/or CHeSS guidelines? I confess to hearing differently but can't say where.

Agile
"But the boiler will not be providing 28 kW when the TRVs are shutting down.

Indeed if its still firing at all it will be on minimum power.

Why do you believe that you needed a 28 kW boiler?

Whats the sum of all the individual radiator heat outputs?

Whats the whole house heat loss?"


The Zone room thermostats sometimes shut off long before all or any TRV shut downs, eg often soon after start of a new heating epoch. Yes I do mean wall stats - not boiler cycling. Even if they are partially limiting the output, the flame temp will still be the same if it is burning at all.

The roomstats are elderly but electronic "self teaching" digital prorammers - 2 zones for CH, thus 2 programmers. basically up and down stairs. They work ok and try to keep cycling to minimum and approach end target temps gently with little overshoot, and hold steady temps. [Landis & Gyr]

The house is large (7 bed 3 bath 3 shower)and Radiators at delta 50 emit 24kW. DHW also 2 large cylinders. Set up is with Vaillant ecoTEC plus 428 and is open vent, open feeder/header. (large water consuming family!) low mains pressure certain times. The calculated heat loss with 20 *C differential inside to outside is 11kW [very very good insulation] without DHW allowance.
The boiler was actually specified by a gas safe heating engineer upon eyeballing the house.
After that I applied my brain to the matter! but too late.
Actually, although excessive, I don't mind too much as we did get days of AVERAGE -5*C last winter1! I suspect heat loss of whole house is very considerable in -5 strong N wind, (top of hill) and I am pretty sure I will cover this but without too much to spare when heating DHW on a cold winter am! I also like it on the warm side! There is also a loose plan to heat garage as model engineering workshop, though this would involve going underground with flow and return!
So that's the picture.

I am not a heating engineer, but like plotting technical stuff in my retirement!
smokebox

You should have one radiator, usually in a hall or bathroom, on lockshield valves and no thermostat. You do not then need a bypass. The pump overrun will dissipate the residual heat through the open rad. I'm not familiar with your controls, but weather compensation will effectively relieve most of these symptoms anyway. The need for a bypass is an admission of failure in system design. Boiler manufacturers demand them to protect themselves against numpty installers!

Thanks mysteryman.

mysteryman said:

You should have one radiator, usually in a hall or bathroom, on lockshield valves and no thermostat. You do not then need a bypass. The pump overrun will dissipate the residual heat through the open rad. I'm not familiar with your controls, but weather compensation will effectively relieve most of these symptoms anyway. The need for a bypass is an admission of failure in system design. Boiler manufacturers demand them to protect themselves against numpty installers!

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I am not so sure about your solution, though it would certainly prevent kettling and bangs etc.
Are you not saying introduce a large or at least significant permanent bypass in effect ? This however will substantially heat the return temp and significantly reduce efficiency of condensation. This return temp elevation is important, the more so when TRVs are nearing closure and a bigger proportion of hot flow will be bypassed directly back to return.
At return of
15*c efficiency might be 96%
30*C efficiency might be 92%
55*C efficiency might be 87%
75*C efficiency might be 84%

12% difference if system heating to 70 or 75*C.

That could be a lot of gas over the winter. What we want to achieve is the minimum safe bypass flow volume when pump is in over run for a few minutes after flame shut down. When zones are open, (and one rad inside each zone should always be on "open flow" albeit controllable by lockshield), there is no issue. It is to prevent damage when zone valves close. If you close your bypass (not recommended for long!) you may get some really large bangs and explosions undoubtedly capable of damaging the thin walled modern tubes of the heat exchanger. This happened to me (qualified numpty gas safe registered fitter issue. Hadn't a clue about Alpha2 pumps let alone ordinary auto pressure settable bypasses!).

These are approximate but realistic figures, rather old BRE figures (CAN ANYONE HELP and tell me how to insert a PDF file references in these posts, or point me at instructions. BBCode is new and tricky for me. Also I am in trouble even with URLs . I am a beginner at this game! I really want to do that to properly discuss the pump's function and relationship to auto bypass. I understand that it is de rigueur to have an auto-bypass now in new installations.

I am in trouble explaining the issues without PDFs!
smokebox

http://net.grundfos.com/doc/webnet/poweredby/gb/alpha2_explorer.html

I think I have inserted a link successfully to the Alpha2 explanatory web pages!

In the page called Explorer a rough depiction of pump characteristics can be seen. In Autoadapt mode the pump is working in the grey triangle region. The output line here is represented by the green line, though the electronics adapt the position of this line depending on conditions. The line will stay within the grey triangle area.

You can see that in Auto adapt the highest pressure it will achieve is about 3 m head for this pump, and will then be delivering about 0.8 m ^3 /hr, though I think , depending exactly what the electronics have remembered of recent, and past, behaviour, it might be as low as 0.2 or even nearly 0m^/hr. curiously the pump could put out about 1.5m^3/hr at a lower pressure of 2m head. This is the essence of the pumps self adjusting capabilities!
you can play about with the settings using the grey arrow button, and adjust radiaor demand with their dials, watching the white blob move about on the chart, with its associated performance curve or "area" shown.

Above the picture is a blue band and on the left "Pumps" where the cursor hover will reveal further information options to choose. Rather a clever web page I thought, and worth some study and thought.

CAN ANYONE TELL ME how to link to a PDF file to this blog. I have had some very useful ones emailed to me from Grundfos which people interested would find very good.

smokebox

The need for a bypass is an admission of failure in system design. Boiler manufacturers demand them to protect themselves against numpty installers!

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An auto by-pass in a lot of ufh systems with p&s arrangements fulfills an important role and a rad with a couple of lockshields fitted falls well short of that role.

In reply to smokebox and norcon:

Having a radiator on lockshields includes that rad as a bypass. This ensures that the return from the bypass is at a low temperature. The same applies to underfloor systems, it is best to have a loop permanently open. There is an easy way to get the lowest return temp - fit the system with weather compensation.[/quote]

http://www.grundfos.pt/web/homept.n...d003071c2/$FILE/Grundfosliterature-815904.pdf

I think this links to a useful Grundfos PDF on Alpha2 pumps

smokebox

To mysteryman

Weather compensation is planned, and I plan a discussion of how it will interact with the auto adapt pump behaviour in a future thread.

I believe however it has little to do with the protection of the heat exchanger in the pump over run period after flame and zone shut down.

That is truly the real function of a bypass. Properly set a pressure sensitive auto bypass [now required by part L] will only really allow bypass in a correctly working system at this phase of boiler operation.

When the CH is running correctly little or no bypass should be heating up the return - very counterproductive to condensation on which up to 12% of boiler efficiency can depend.!

If you want a towel rail constantly running, outside the control of the zone valves, mysteryman's suggestion is viable, though not everyone wants 200watts trickling out all the time!

smokebox

norcon

Norcon said:

The need for a bypass is an admission of failure in system design. Boiler manufacturers demand them to protect themselves against numpty installers!

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An auto by-pass in a lot of ufh systems with p&s arrangements fulfills an important role and a rad with a couple of lockshields fitted falls well short of that role.

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Agreed especially if the rad sugested by others is under zone valve control.

smokebox

Am I right about part L requrement for "auto-bypass? Do "Gas Safe" know about this? Do decent fitters agree?

smokebox

Edit post script. What does Lord Hailsham say to this?

What if this 'bypass' rad is turned off, or removed by someone who doesn't know in the future?

The bypass is mostly fitted these days.

Also stops the need for a rad blasting out heat when maybe when it's not needed, wasting heat.

xr4x4x

xr4x4 said:

What if this 'bypass' rad is turned off, or removed by someone who doesn't know in the future?

The bypass is mostly fitted these days.

Also stops the need for a rad blasting out heat when maybe when it's not needed, wasting heat.

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I am pleased proper bypassing is being done, because I think it is "illegal" not to in new installations.

smokebox