Vaillant 438 with 3 zones, frequent S53

[Andygasman2010]

Sounds like like your system needs a LLH, as already suggested.

You could have a header with a 15/60 as the shunt pump (between the boiler and header)

Then a single Magna 25/80 powering the whole system on the other side of the header (no other pumps)

Or you could have smaller pumps, one for each circuit.

Depends how much money you want to spend!

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• Andygasman2010
• 5 Jun 2013
• 2

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• Andygasman2010
• 6 Jun 2013
• 1

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• Andygasman2010
• 6 Jun 2013
• 1

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• Andygasman2010
• 6 Jun 2013
• 1

 

[Gasguru]

...The heat loss calc was 36Kw, as said.

....The radiators total to 44Kw The split is 26kw d/s, and 18 kw u/s.

...I, too, am confused how 26kw of radiators can get fully hot when the boiler is range rated at 18kw

So the heat loss is 36 Kw and therefore the boiler will need to run at maximum output to cope when it snows outside....so forget range rating it down as a bodge fix.

The radiators add up to 44Kw. Radiator outputs qouted in catalogues are based on the connection configuration AND the differential temperature between the average radiator temperature and the room temperature. The cataloque should list an adjustment factor to adjust the radiator output dependent on operating conditions. Anyway, the heat required from the upstairs and downstairs circuits will be 36 Kw as you calculated NOT the figure of the radiators output (adjusted or not). However, the fact that the radiators appear to be oversized will alllow the larger temperature drop to be used (where the average radiator temperature will be lower).

Of course the downstairs circuit will heat up properly...you have 2 pumps in series to cope with the boiler and d/s circuit head loss, the weather is warm outside and therefore the circuit dissapates less heat, you've lowered the Kw output and as you have calculated the head loss is much lower.

You have the initial solution...either a single large pump (not just picked at random but the correct head capacity at the required flowrate), either a couple of pumps in series or a low loss header with multiple pumps (the most expensive option and a custom wiring job).

No amount of software will get the heat from the boiler. However, if you have the older boards then maybe the latest software is more tolerant.

 

[fezster]

Thanks. All of what you've said makes a lot of sense.

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).

Putting that aside for a second, let's just assume I did upgrade the pump and the flow was sufficient. Am I still not going to have the problem of my boiler pumping out too much heat for a single zone ? As I said earlier, my d/s has sufficient flow, yet if I leave the boiler at max power output (i.e. 38kw), it'll work fine with all zones open, but the minute only the d/s zone opens, it will S53. This suggests to me that, in this scenario, the S53 is unrelated to the amount of flow ?

 

[Agile]

The S53 is triggered by a too wide temperature differential ( as indicated by d40 and d41)

The diff temp is directly related to the flow!

More flow then a LOWER diff temp.

So your S53 is caused by too LITTLE flow!

There will also be another problem of too little power loss to the house and when the required power to reach the set flow temperature is reached and the boiler cannot modulate any lower then it will cycle off !

That's inefficient and a direct result of an oversized boiler ( for the zone in use ).

You keep asking about a way to dynamically alter the boiler power output ( d0 ). There is no way anyone seems to know of doing that.

The Vaillant Ebus controls do apparently do this but only as they think fit for any particular set of operating conditions. If you were particular clever with IT and RS422 communication protocols then you could probably design something to do this. But as far as I know its not a listed Tupperware project at this time.

Tony

 

[D_Hailsham]

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.

You can't have separate index circuits for upstairs and downstairs. The index circuit calculation takes into account the losses in both.

A loss of 0.8m sounds wrong for 11 rads. I estimate it would be at least 4.5m.

 

[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.

You can't have separate index circuits for upstairs and downstairs. The index circuit calculation takes into account the losses in both.

A loss of 0.8m sounds wrong for 11 rads. I estimate it would be at least 4.5m.

This does not include the head loss at the boiler.

My u/s and d/s are completely separate pipe runs. They tee off before zone valves, after the pump. And in my case, Im trying to understand whether I have insufficient flow u/s, with the other zones closed.

The S53 is triggered by a too wide temperature differential ( as indicated by d40 and d41)

The diff temp is directly related to the flow!

More flow then a LOWER diff temp.

So your S53 is caused by too LITTLE flow!

Im unsure about this. Whilst what you say seems logical on the face of it, have a look at the points I made above which seem not to support this:

The d/s zone has plenty of flow with the 2 pumps in series (over 8m head). Every rad heats up absolutely fine and very quickly. However, if I reduce the number of rads in the zone by turning some of them off, I will still get an S53 if my boiler is not range rated correctly. If it was purely down to flow, then a single rad on a 37kw boiler would quickly reach temp and the boiler would then switch off. But instead, what actually happens is the boiler raises the flow temp too fast, hits S53, and the rad is left cold until the return temp is raised sufficiently. Admittedly, the more flow there is, the faster the return temp will rise, but it's not going to rise instantaneously, and in either case, will never rise quick enough to meet the excessive output of the boiler before the boiler hits S53.

 

[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.

You can't have separate index circuits for upstairs and downstairs. The index circuit calculation takes into account the losses in both.

A loss of 0.8m sounds wrong for 11 rads. I estimate it would be at least 4.5m.

This does not include the head loss at the boiler.

My u/s and d/s are completely separate pipe runs. They tee off before zone valves, after the pump. And in my case, Im trying to understand whether I have insufficient flow u/s, with the other zones closed.

Sorry, Im going to caveat this with Im using the pdf I posted earlier as a manual for calculating the index circuit and it's quite possible ive done it wrong.

 

[D_Hailsham]

A loss of 0.8m sounds wrong for 11 rads. I estimate it would be at least 4.5m.

This does not include the head loss at the boiler.

Neither does my estimate.

It's quite possible I've done it wrong.

I wasn't going to say it but: Yes, you probably have done it wrong.

 

[fezster]

I dont want to take liberties, but if anyone does have the time (like I certainly seem to have !), Id be grateful if you could look at this diagram of my upstairs circuit and calculate the head. My belief is that the index circuit is the 2.5kw radiator.

View media item 62184

 

[fezster]

Similarly for downstairs:

View media item 62188

 

[Onetap]

You keep asking about a way to dynamically alter the boiler power output ( d0 ). There is no way anyone seems to know of doing that.

A 0-10V signal from a controller to a VR34 (if they still offer those) in the boiler, as for Opentherm boilers. It doesn't work with any other Ebus control devices, so the controller needs to do everything else.
Only way I know.

 

[Onetap]

I dont want to take liberties, but if anyone does have the time.......

You don't seem to show where the upstairs and downstairs split. The flow and headlosses through the boiler would be calculated on the total flow rate. I won't be calculating this, so don't wait for my answer.

 

[scoobydo123]

Key perhaps to this does the s.53 only happen after the boiler has cycled? Can you shut the zones down when on the first fire and see if an s.53 occurs before a cycle this will indicate if that circuit is capable of taking the chosen d.0 setting? Have you adjusted pump overrun and the anti-cycle as suggested this may well help if it is only apparent if the boiler has cycled.

For Agile the PCB can be provided on request (I finally got one as I had a service plan direct with them). I had suffered years of far from optimum boiler behaviour with various visits blaming sludge etc etc but eventually got resolved with the revised software. Not sure if a new 4xx will automatically be provided with the revised board. The engineer that fitted it was not even aware that such a thing existed. I got mine by direct request to product support manager. Havent spent much time in front of the boiler since the fix by it looks like during subsequent fires the boiler is happy to overshoot the value of your target flow temp and once the first 50 seconds has passed and it looks a the value of d.0 it then modulates down to sensible levels. My was overshooting within 7 or so seconds under some conditions (HW demand only) and then micro-fired for ages. I no longer have an issue

 

[ALEC1]

It's interesting that Vaillant never blamed my pump sizing, system cleanliness, or hydraulic layout or me when they visited an install of a 428.

The new PCB resolved the issues from day 1, same pump, same heat exchanger and same system and I never had to install a header..

pcbs determine the boilers tolerance, the new ones ramp up progressively and stop when the return starts to rise I think. So on one zone you will never get any where near the maximum output..

 

[Agile]

What is the part number of the modified PCB? Presumably its just a swap for the old one?

Are all 4xx series boiler now supplied with the modified PCB?

If so then from what date ( serial number? ) have 400 series boilers been supplied with the modified PCB?

How can you visually identify this modified PCB?

Tony