Vaillant 438 with 3 zones, frequent S53

[Agile]

Regardless of everything thats been said about the first 50 seconds being at full pelt, i can assure you that at 18kw in d0,the boiler definitely maintains a lower flow temperature when firing up than at 26kw. No s53 since changing to 18kw.

I still think that you are not fully assimilating what goes on in your boiler!

The d0 setting is the maximum POWER that the boiler will run at after THE FIXED first 50 seconds.

During the first 50 seconds the boiler runs at 70% of the BOILER'S maximum output! In your case that's 70% of 38 kW !

During the first 50 secs the boiler will turn off the burner if it reaches the set flow temperature!

They may expect that doing this will help a system to heat up faster. Its probably a good idea if the boiler is not seriously oversized or the water content of the system is not too high.

Tony

 

[ALEC1]

By passes on older system primaries were to avoid condensation...I can't see any reason to put them on a condensing boiler primaries..

I thought their main function was to maintain a minimum flow rate through the boiler. A by-pass may be useful on a system with TRVs fitted on most radiators, but the Building Regs (ISTR) require them to be ABVs, rather than fixed by-pass valves.

I was really referring to installs with a low loss header...

Building regs have a parochial understanding of boiler and heating technology, I wouldn't pay much attention. Study what vaillant and viessman produce instead!!

 

[matt1e]

Regardless of everything thats been said about the first 50 seconds being at full pelt, i can assure you that at 18kw in d0,the boiler definitely maintains a lower flow temperature when firing up than at 26kw. No s53 since changing to 18kw.

I still think that you are not fully assimilating what goes on in your boiler!

The d0 setting is the maximum POWER that the boiler will run at after THE FIXED first 50 seconds.

During the first 50 seconds the boiler runs at 70% of the BOILER'S maximum output! In your case that's 70% of 38 kW !
Tony

Can't see it Tony, surely if it does indeed run at 70% for the first 50 seconds then it's 70% of the value set in d.0 not the unranged Max of 38kw
otherwise range rating it down to 18kw (just under 50% of max) wouldn't make a difference to the frequency of s53 errors, but it does

Matt

 

[matt1e]

By passes on older system primaries were to avoid condensation...I can't see any reason to put them on a condensing boiler primaries..

I thought their main function was to maintain a minimum flow rate through the boiler. A by-pass may be useful on a system with TRVs fitted on most radiators, but the Building Regs (ISTR) require them to be ABVs, rather than fixed by-pass valves.

bypass's are used for a variety of reasons Onetap
two of which alec and yourself have pointed out, you are both correct

Matt

 

[Onetap]

bypass's are used for a variety of reasons Onetap
two of which alec and yourself have pointed out, you are both correct

Matt

You don't say!

 

[fezster]

Sorry, Ive been caught up in other things past few days.

I'm not sure where to go from here. D_Hailsham is being kind enough to look at my circuit pictures and hopefully work out the head required / pump required. A lot of information to assimilate from this thread, but Im thinking solution will either be:

1) Fit a single pump to the entire system, probably a light commercial. Cost ~ £400 including labour. Not convinced this will solve all of my problems. I.e. will i be able to range rate my boiler to max output (38KW) to accomodate all zones open in winter, but not get S53's with a single zone open in summer ?

2) Fit a LLH. I dont completely understand how this works, but it looks like a magic solution to eliminate the S53 problem, whilst providing adequate flow to all zones, as and when they require it. Cost ? Can anybody give an approximate ?

 

[scoobydo123]

I still think that you are not fully assimilating what goes on in your boiler!

The d0 setting is the maximum POWER that the boiler will run at after THE FIXED first 50 seconds.

During the first 50 seconds the boiler runs at 70% of the BOILER'S maximum output! In your case that's 70% of 38 kW !

During the first 50 secs the boiler will still modulate DOWN if it reaches the set flow temperature!

They may expect that doing this will help a system to heat up faster. Its probably a good idea if the boiler is not seriously oversized or the water content of the system is not too high.

Tony

Tony you a wrong on the above the boiler on standard software WILL NOT modulate down during the first 50 seconds this is part of many peoples problems, both on VRC and standard on off controls. However as you know the S.53 is a different problem. Have you actually stood in front of one of tese for long enough and monitored the behaviour on subsequent fires especially this time of year when heating a cylinder ?

 

[Agile]

Yes, I agree the boiler does not modulate down during the first 50 seconds of firing. I have edited to correct what I wrote before.

If the set flow temp is reached then it just turns off the burner!

That is one of the reasons why adequate flow rate through the boiler is so important.

Tony

 

[D_Hailsham]

Here are the results of the Index Calculation.

I have assumed: flow = 75°C; return = 55°C. (20°C differential)

The index rad is on the ground floor, but not the one suggested by the OP. It is the top right corner. The loss is 2.12m. The worst rad on the first floor is only 1.09m.

If an 11C differential is used (75°C/64°C), the head losses are 8.53m (gnd) and 3.74m (1st). The high losses are due to incorrect pipe sizes for an 11°C differential so you get a max velocity of over 2m/sec in the pipe from pump to boiler.

The losses above do not include the loss through the motorized valve (0.4m) or the heat exchanger (4.05m). If these are taken into account the total loss is 6.57m.

The flow rate for 38kW at 20°C differential is 1.633M³/hr.

The Grundfos recommended pump is the Magna 25/100 @ £1200

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• D_Hailsham
• 12 Jun 2013
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• D_Hailsham
• 12 Jun 2013
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[matt1e]

The total loss is 6.57m.
The flow rate for 38kW at 20°C differential is 1.633M³/hr.

The Grundfos recommended pump is the Magna 25/100 @ £1200

Full marks for taking the time to work the head loss out Dave, but I would have Thought the Magna 25/80

Matt

 

[fezster]

Thanks very much for taking the time to do this.

A couple of points which are interesting:

The worst rad on the first floor is only 1.09m.

This would mean the first floor rad requires 5.54m head. Looking at:

http://www.anchorpumps.com/media/downloads/75/Grundfos Light Commercial Circulators.pdf


It doesnt look like the 25-55 provides more than 4.25m head at 1.6m^3/s. So the u/s definitely requires either a bigger pump, or a secondary pump. One thing I would say is that my pump is a Grundfos 26-50, which doesnt look to be identical to the 25-55. I'll try and post a picture of the front, as I believe it may be more powerful than the 25-55.

View media item 62496

The Grundfos recommended pump is the Magna 25/100 @ £1200

Am I right in saying a Grundfos 25-80 would suffice, as a single pump for the whole system ? Looking at the pdf, it seems it would be ok (and definitely be a cheaper solution).

Edit:

At 26KW, flow rate is 1114 l/h, and the boiler head resistance is under 2m. This means total head loss is just under 4.5m, at which the 25-55 would be sufficient.

 

[fezster]

I did attempt to do the head loss calculation for upstairs, and have come up with a figure of 0.8m for the index circuit upstairs. This was using a 20 degree temperature differential. Based on this, it would seem my single pump is sufficient for anything less than 30kw output on the 438 (factoring in head loss at the boiler).

So my u/s calc wasnt that far wrong (!)

 

[fezster]

This would mean the first floor rad requires 5.54m head.

Correction, after thinking this through more clearly. At 26KW, the u/s circuit has a resistance of less than 4m, which means the S53 is not a flow problem. Someone correct me if Ive missed something ?

 

[D_Hailsham]

Full marks for taking the time to work the head loss out Dave, but I would have Thought the Magna 25/80

Grundfos Webcaps seems to have 'moods'!

Yesterday they suggested the 25/100; today their first choice is the Magna1 32/100 and the 25/80 isn't even on the list, so I added it in as an option.

It does the job OK. The only problem, according to Grundfos, is that it will cost more to run than the 32/100. Capital cost (Anchor Pumps: £354+VAT for Magna 1 25/80; £427+VAT for Magna 1 32/100) does not seem to come into the equation!

Even if you had a low loss header, you would still need a Magna sized pump on the primary circuit due to the high head (4.05m) and flow rate (1.633m³/hr) required by the boiler.

 

[matt1e]

Am I right in saying a Grundfos 25-80 would suffice, as a single pump for the whole system ? Looking at the pdf, it seems it would be ok (and definitely be a cheaper solution).

I would say so, hence my earlier post, I don't know where Dave got the 25/100 recommendation from (enquiry,looking at pump curves or software) but I have just been on the grundfos website and their webcaps select tool even suggests a magma 25/80
They are a decent pump by the way
DAB evotron range are also a good pump
The evotron 80 would also suit your needs and will be much cheaper than the grundfos

Matt