Vaillant 438 with 3 zones, frequent S53

[fezster]

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.

The split is right at the boiler, where the zone valves are located.

 

[fezster]

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..

I bought my boiler in Dec 2012. Any way of checking the pcb version ?

 

[fezster]

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.

S53 occurs on first fire. The boiler zooms up to about 55 flow temperature, and the return temperature is barely above 20, and doesnt rise even a single degree until at least a few minutes after the boiler has started firing. Therefore, I doubt even the most powerful pump would be able to push the water round fast enough to keep up with the boiler, which says to me the boiler is simply overpowered for that single zone (hence the range rating has fixed it for now). But GasGuru says this may mean the boiler is underpowered during the winter. Sounds like I could benefit from the new software if it stops the boiler from powering up too fast and / or lets it modulate down faster.

 

[Agile]

If Alec or any of the Vaillant engineers are able to answer my questions then we would know!

It seems to be a closely guarded secret!

 

[ALEC1]

Best thing is for you to ring Vaillant direct..I understood that it had the same part number as the original board..it just had to work itself into the system..that was 6 months ago...

Can some one who knows Vaillant intimately say so

 

[fezster]

Updated diagrams:

View media item 62243 View media item 62242

 

[Steelmasons]

I doubt even the most powerful pump would be able to push the water round fast enough to keep up with the boiler,.

 

[Agile]

S53 occurs on first fire. The boiler zooms up to about 55 flow temperature, and the return temperature is barely above 20, and doesnt rise even a single degree until at least a few minutes after the boiler has started firing. Therefore, I doubt even the most powerful pump would be able to push the water round fast enough to keep up with the boiler, which says to me the boiler is simply overpowered for that single zone (hence the range rating has fixed it for now).

I don't think that any of this is new to you!

One of the ( annoying ) features of this model is that it fires ( and stays ) at about 70% of the max power for the first fifty seconds!

At first sight that sounds as if it will help it to heat up the system quickly. Well it does but then causes it to potentially go off just because the return is not increasing fast enough! ( But you can bleed a little flow back to the return. However, this reduces the maximum power you can remove from the boiler ).

When the system is already hot it causes the flow to reach the set value very quickly and go off to await the anti cycling delay.

Some annoying quirks! Somewhat less annoying if you have a properly matched pump!

Tony

 

[fezster]

S53 occurs on first fire. The boiler zooms up to about 55 flow temperature, and the return temperature is barely above 20, and doesnt rise even a single degree until at least a few minutes after the boiler has started firing. Therefore, I doubt even the most powerful pump would be able to push the water round fast enough to keep up with the boiler, which says to me the boiler is simply overpowered for that single zone (hence the range rating has fixed it for now).

Some annoying quirks! Somewhat less annoying if you have a properly matched pump!

Tony

Im not sure if im not explaining myself properly, or if im missing something fundamental. I'll try once more though:

I concede my pump may be underpowered for upstairs. But regardless, the following still troubles me, and I cant see how a bigger pump would solve this. If I reduce the boiler output to match the upstairs rads (18KW), I dont hit S53. If I increase the boiler output for both u/s and d/s rads (38KW), and both zones are open, I dont hit S53. This suggests to me I have adequate flow (though, maybe not optimal).

However, if I set the boiler to anything above 18KW, and only have the u/s zone open, I hit S53, because of the reasons I described above. I don't see how a bigger pump can solve this. Please set me right if Ive missed something.

 

[Agile]

Whenever you write something, I am getting the impression that you don't realise that with LESS radiators in circuit you get LESS flow!

Conversely, more radiators MORE flow! MORE flow means a LOWER diff temperature!

LESS flow means a higher diff temp and a higher likelihood of an S53 !

Tony

 

[matt1e]

Whenever you write something, I am getting the impression that you don't realise that with LESS radiators in circuit you get LESS flow!

Conversely, more radiators MORE flow! MORE flow means a LOWER diff temperature!

LESS flow means a higher diff temp and a higher likelihood of an S53 !

Tony

well it's a bit of a balancing act really
In the case of constant flow zoned circuits
With less rads in circuit you get more flow resistance
The overall system flow-rate decreases slightly but when a zone closes there is also more available head and higher differential pressure so you will get higher flow-rates through any open zone

Conversely, more radiators, higher system flowrate but Lower flow-rate through each zone


Rather than going down the LLH or buffer tank route the cheapest option for the op IMO would be to swap out the 2 ports he no doubt has fitted and fit 3ports in a diverting/fixed bypass arrangement

A 0-10V signal from a controller to a VR34 (if they still offer those) in the boiler

Or as Onetaps post above,get a suitable controller and utilise the 0-10v interface if possible

Matt

 

[matt1e]

Im not sure if im not explaining myself properly, or if im missing something fundamental. I'll try once more though:

I concede my pump may be underpowered for upstairs. But regardless, the following still troubles me, and I cant see how a bigger pump would solve this. If I reduce the boiler output to match the upstairs rads (18KW), I dont hit S53. If I increase the boiler output for both u/s and d/s rads (38KW), and both zones are open, I dont hit S53. This suggests to me I have adequate flow (though, maybe not optimal).

However, if I set the boiler to anything above 18KW, and only have the u/s zone open, I hit S53, because of the reasons I described above. I don't see how a bigger pump can solve this. Please set me right if Ive missed something.

you have missed the fact that you may not have adequate flow through your upstairs circuit
simply put
downstairs circuit = adequate flow to suit the higher output
upstairs circuit = inadequate flow to suit the higher output

downstairs only on = adequate flow so boiler happy
upstairs and downstairs on = adequate flow (courtesy of downstairs circuit) so boiler happy
upstairs only on = inadequate flow so boiler spits out dummy

are the two heating loads matched btw as just had a quick scan through?
better still I will go and recap before I start talking carp


Matt

 

[fezster]

downstairs only on = adequate flow so boiler happy

This is my point though! At 38KW (or even 30KW), I still get S53 on the downstairs. Yet I have 2 pumps, and therefore plenty of head. Because it's such a big system, I just cant see how the flow temp can rise quickly enough, given the boiler gets to 50 degrees+ within 20 - 30 seconds. The return temp is barely above 20 on a cold system.

are the two heating loads matched btw as just had a quick scan through?

Matt

u/s is 18KW and d/s is 26KW. Radiators have been deliberately oversized. Total of 24 odd radiators.

 

[fezster]

LESS flow means a higher diff temp and a higher likelihood of an S53 !

Tony

Tony, just answer me on this one point, please.

My system, with all radiators in use, needs 38KW (heat loss of house is 37KW, rads add up to 44KW). Assuming we're agreed on that, how do you explain my previous post that d/s only gives me S53, even though I have plenty of head with 2 pumps in series ?

As before, I cant see how from a cold system (and a very large system at that) the return temp can possibly rise quick enough to keep up with the boiler flow temperature rising to 50+ within the first 30 seconds. Surely the boiler isnt designed to hit an S53 and then kick back into life after cycling and the return temp slowly creeping up.

 

[matt1e]

downstairs only on = adequate flow so boiler happy

This is my point though! At 38KW (or even 30KW), I still get S53 on the downstairs. Yet I have 2 pumps, and therefore plenty of head. Because it's such a big system, I just cant see how the flow temp can rise quickly enough, given the boiler gets to 50 degrees+ within 20 - 30 seconds. The return temp is barely above 20 on a cold system.
.

do you get S53 when set to 26kw though?
I take it the pumps are in series?
that gives you more head same flow rate
in parallel will give you same head ,more flow rate

you need to have adequate head and flow-rate, just because you have two pumps it doesn't mean that you have the correct flow-rate andhead

your system is large for a domestic application it needs a better design spec
The boiler is shutting down because it is protecting itself from thermal shock
do you have a differential bypass fitted before the zone valves?


Matt