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Discuss Vaillant S53 compared to other brands in the Central Heating Forum area at

  1. fezster

    fezster Member

    Hi All.

    For those who read my LLH thread, this question is because I'm weighing up whether my problems are just down to it being a Vaillant boiler.

    Quick background - I have 3 heating zones (u/s, d/s and hw). Any individual zone works fine. Multiple zones work fine when running together. The problems occur when the boiler is firing and individual zones open and close, because the return temp to the boiler suddenly cools, and it hits S53.

    If you imagine a typical heating system, zones open and close as the stat calls for heat, so depending on timings, sometimes the boiler can sit in S53 for hours, and the heating runs only luke warm. Other times the rads are at full temp.

    My question is this - is this a Vaillant problem? (specifically the 438). Or do all boilers have this issue? I just cannot believe in the modern day with people having multiple underfloor heating zones etc that this problem does not occur more often and for more people. Unless everyone fits a Low Loss Header when running multiple zones?

    Thanks for reading and appreciate any input.
  2. Last Plumber

    Last Plumber Plumber GSR Top Contributor!!

    I think there is more likely an issue with the system.
    You can't rule out the Boiler obviously as Vaillant Boilers do develop faults like any other.
    I can only speak from my own experience here, I have been installing Vaillant for 15 years ish, Commercial and domestic and I can't say I have ever known one sit in S53 for as long as yours seems to.
    I still think your best bet is to get a Heating Engineer to look at the whole picture.
  3. fezster

    fezster Member

    Thanks. I do have an engineer coming next week with regards to a LLH installation.

    Have you fitted a Vaillant on a heating system with multiple zones? I think the S53 in the scenario I described is inevitable. Please correct me if Im wrong?

    What's frustrating is the amount of time it sits in S53, which seems to be unpredictable. But the ideal aim is for it to not hit S53 in the first place. What would work is if an opening of any zone first turned the boiler off (allowing the return temperature to stabilise between the zone which is hot and the zone which is cold) and then to fire up again so that the delta between flow and temp never exceeds 30 degrees. Maybe a relay arrangement of some sort.
  4. Last Plumber

    Last Plumber Plumber GSR Top Contributor!!

    Yes I install them in all sorts of buildings, all sizes, multiple zoned too.
    I have one in my own home with 3 zones. No UFH (Under floor heating).
    I have not come across your problem. That's why I think it needs looking at.
    Have you checked the flow and return temps at the Boiler ?
    You can check on D40 and D41.
    Have the UFH zones been set up properly?
  5. fezster

    fezster Member

    No UFH heating zones. Only rads - upstairs zone, donwstairs zone, HW zone.

    I've monitored D40 and D41 for individual zones and zones in combination. The delta is always roughly 20 degrees now (I say now, because it took some fine tuning to achieve). It may creep up to 22 degrees, but no more.

    It's *only* when a cold zone (say upstairs heating) opens whilst another zone (downstairs heating) is fully hot. So D40 will be 70, D41 at 50 - then the upstairs zone opens and D41 shoots down to 30. Boiler sees the large differential and goes into S53. It then waits an arbitrary time (must be some logic to it?) before powering to full power again. At which point, the delta is again 20.

    However, what happens far too often is that whilst in S53, the u/s room stat becomes satisfied and closes the zone. When the boiler comes out of S53, it's only the d/s zone open and so it fires back up to 70/50. Then the u/s zone opens again and the cycle repeats. I should note this sequence doesn't happen all the time, but far too often. The result is that the d/s zone always remains luke warm and the stat temp is never satisfied. This may be because during the day (on weekends) we have the d/s quite hot with young children in the house, and leave u/s relatively cool.

    I also suspect (not proven) that the boiler increases the S53 duration every time it occurs, exacerbating the situation.

    Apologies for the long post. Appreciate you reading and taking the time.
  6. Last Plumber

    Last Plumber Plumber GSR Top Contributor!!

    I misunderstood your post. I thought you were saying that You had underfloor heating. Sorry about that.

    Why have you set the temperature differential to 20 degrees ?
    Last edited: Dec 4, 2017
  7. Chalked

    Chalked Plumber GSR

    In my experience, that boiler suffers from poor circulation when the pump is fitted ( as it usually is) on the flow outside of the boiler.
    Vaillant system boilers have the pump fitted to the boiler return, when fitted integrally.
    I would go with a low loss header and fit a 15/60 pump to the return leg.
  8. fezster

    fezster Member

    As I understood it, this is the temperature differential condensing boilers run optimally at. My rads are sized according to this differential. Although getting the differential any less will be quite difficult with my current setup regardless - the flow rates become much larger at 11 degrees.

    Appreciate that tip. Most of the LLH diagrams I've seen show the pump on the return.
  9. tolly

    tolly Plumber GSR

    Whos sized the you need 38kw? What do you have D0 set to in the diagnostics menu?
  10. tolly

    tolly Plumber GSR

    Auto bypass fitted aswell?
  11. fezster

    fezster Member

    The installer recommended it and I did a total house heat loss calc, which came to 34KW (this was 5 years ago). And it said to allow an additional 3KW for the hot water cylinder. D0 is set to 34.

    Yes. It's a Honeywell DU144 I believe. Set according to the charts to 0.1. Without it, it was impossible to achieve a 20 degree delta at the boiler.
  12. doitmyself

    doitmyself Well-Known Member

    A 20C delta is just the recommended maximum; it doesn't matter if it is less. What sort of delta were you getting with the DU144 on a higher setting?

    Let's say that the calculated head is 2 metres, the flow rate is 500 litres/hour, and we have a 15-50 pump. The attached pdf shows what happens in real life: the actual flow rate is 573 litres/hr and the head is 2.63 metres. This is because the working point has to be on one of the fixed curves (in this case speed 1). At 573 litres/hr instead of 500, the temperature differential will be 20x500/573 = 17.45C. So you need to balance for that temperature, not 20C.

    When demand reduces the boiler will automatically reduce output (modulate down). If the pump (assuming constant speed, e.g UPS15-50) has been set to give a 20C differential at 34kW, then at 17kW the differential will be 10C, and 5C at 8.5kW. It's only when the boiler is unable to modulate any lower (approx 6.5kW for the 438), that the ABV needs to come into operation to prevent the differential rising above 20C.

    Attached Files:

    Last edited: Dec 5, 2017
  13. fezster

    fezster Member

    I used the chart in the manual:

    The theory - So at 1462 l/h (34KW) and at 650 mbar (6.5m total resistance) differential pressure, the setting is 0.1. My index circuit is 2.4m downstairs and 1.6m upstairs. The 438 has a hex resistance of 3.5m at 1462 l/h. I've taken worst case of 4.05m at 38KW, hence arrived at a total resistance of 6.5m.

    The reality - anything above 0.1 on the ABV and the delta at the boiler rises. At 0.3 for example, the delta is closer to 26 degrees. At 0.1, it just about manages 20 degrees, usually 22/23.

    The grundfos 25-80 can overcome 6.7m head at 0.4 l/s (1462 l/h):

    Even if just the u/s zone is open, total resistance 4.05+1.6 = 5.65m (far below what the 25-80 is capable of), I still see a delta T of around 22.

    I think I understand what you say about the boiler modulating down but Im not sure how that ties into what I am seeing above. I know the 438 fires at max (or 60%?) output for the first minute or so before modulating down. It's possible the actual index circuit resistance is higher than that which has been calculated. In any case, it varies depending on the zone which is open and the flow through the boiler (given the biggest head loss is in the boiler).
  14. doitmyself

    doitmyself Well-Known Member

    You are using the ABV to compensate for too low a flow rate. If the delta is 26C for an output of 34kW, the flow rate is 18.77 litres/min, whereas it should be 24.4 litres/min to give a delta of 20C. You're just feeding hot water straight back to the boiler, which won't help efficiency.

    You said, in another topic, that you are running at 75C flow, 55C return. The output of a rad is not constant, it varies with flow, return and room temperatures. Assuming a 20C room temperature, the total output of your rads at 75C/55C is 38.8kW. But your boiler is only giving of 34kW, so the the output from the rads can only be 34kW. As the flow temperature is fixed, the return temp automatically reduces to balance the rad output to the boiler output. This is achieved when the return is about 49C, which gives a delta of 26C. Where have we seen that before?

    Lowering the flow temperature to 71C, with a 20C delta, will reduce the rad output to about 34kW, matching the boiler output.

    Have you confirmed this by observation? The fan speed is the parameter to monitor as it controls the boiler output; d.33 is the target, d.34 the actual. If you checked this with the boiler output (d.0) set to different values, you would soon see if changing d.0 has any effect during the first minute.
  15. fezster

    fezster Member

    So how do you achieve this? I'm assuming you don't mean just reduce the dial on the front of the boiler - the boiler obviously ramps up the heat from 30 to 75 flow, so simply limiting it's max temperature can't be the answer, as the 26 degree delta persists throughout.

    Sorry, how did you work this out? Or did you mean the output required

    In fact, the rads are oversized intentionally and total quoted output is close to 46KW.

    I'm not necessarily disagreeing with you, but do want to understand why you think this is the case. As per my previous post, the ABV has been set exactly as the MIs instruct (i.e. total head loss around 6.5m, setting of 0.1). The purpose of the ABV is to maintain minimum flow rate through the boiler as valves close. So either:

    1. It's possible the head loss calc is greater than calculated. In which case the ABV setting would have to be even lower.

    2. The pump is not delivering the head the manual indicates - maybe it needs to be on the return, as suggested by someone else?
    Last edited: Dec 6, 2017
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