Vaillant 438 with 3 zones, frequent S53

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I have a 3 zone sealed heating system which comprises of an upstairs, downstairs and hw zones. The u/s zone has 11 rads and the d/s has 13 rads. The HW zone has a Megaflo indirect 250l cylinder, and until recently had a 1.3KW radiator attached too (more info on this below). The main pump is a Grundfos UPS 26-50r, upgraded from a 15/60 to try to improve flow. The d/s zone also has an additional 15/60 pump. The boiler is a Vaillant 438 and has been range rated to 26KW.

The d/s zone seems to work absolutely fine since adding the second pump (previously had problems with radiators not warming up). It heats up nicely within 20 mins either independently, or with the other zones open too.

The problems I have are as follows:

1) The HW zone when calling for heat on it's own works for a long period of time, but eventually starts short cycling to get fully up to the stat temp. I suspect this problem is due to my removing the additional radiator on the HW zone, which was recently done. I intend to add it back in. My question is, given the power requirement for all three zones, is there a solution to range rate dynamically when only the HW zone is calling for heat ? I understand a VR65 can independently control the HW flow temperature, but can it also control the output power to lower it ? Eg. If I range rate the boiler to 18KW, then the short cycling disappears. Am I right in saying the extra radiator should also alleviate this problem once again ?

2) The u/s zone seems to have a flow problem. If I have only the u/s zone calling for heat, I can see the flow / return delta is quite large. The return temperature seems to take an absolute age to increase. This frequently causes an S53. What else can I do to try and resolve this - would a second pump on the u/s circuit help ? Do I need a larger pump than the 26-50 ? What's strange to me is that all of my d/s radiators drop from u/s, so the "index circuit" is my furthest d/s radiator. But maybe the second pump is what is helping here. Tbh, I didnt really notice a substantial difference when I upgraded my main pump from a 15/60 to the 26-50r.

I've upgraded a significant portion of the u/s pipes to 22mm, reducing to 15mm just before the radiators. I've got an ABV, but this only kicks in if all circuits are off. Should this be kicking in sooner perhaps to aid the return temperature, or is this masking the problem instead of solving it ?

All in all, the system is heating up ok in the mornings, though I suspect this may be because HW and u/s come on simultaneously, perhaps aiding each other given they have the opposite problems (one's return temp is too high, the other's is too low).

Any guidance on where I should go next in terms of trying to get this working as it should - i.e. each zone able to independently function. Or is this an impossibility ?

Thanks.
 
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A low loss header will allow all the zones to be run independently, a problem you might have is finding a heating guy with experience installing them.
 
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didnt really notice a substantial difference when I upgraded my main pump from a 15/60 to the 26-50r.

.

Well you wouldn't , 26/50 is around 5m head , 15/60 is 6 m head , hex resistance on the 38 is around 3.5 m head which leaves around 1.5 metre for pipework losses/resistance.

Either fit a LLH as suggested or replace 26/50 with a 25/80.
 
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Do you think the 25/80 would also negate the need for the second pump on the d/s zone ?

LLH does sound ideal but also sounds like it's adding more complication to an already complicated system (!). The pump sounds like the easier fix to try. 25-80's are expensive though! Seems like Ive been had on the 26-50 then.
 
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You said YOU upgraded the 15/60 to a 26/50 , so how have you been 'had'? , maybe take some profesional advice before doing anything else , good heating guys are hard to come by in this trade , you pays yer money.
 
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A llh will simplify your installation, the boiler will be much happier and the seperate pumps will allow each zone to work independently, do you know what the heat loss of your house is ?
 
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steelmasons, it's a figure of speech. I dont have the confidence to change it myself.

picasso, heat loss for entire house according to idhee whole house calculator is about 34kw. Cant remember if this included the HW requirement or not. How much should I expect to pay for a LLH + installation ? Is the install itself straightforward (i.e. will any heat engineer local to me do) or do I need to hunt around for someone more specialist ?

I honestly wish I could simply pay somebody to come in and sort this out, but the fact is that finding a heating engineer who goes into even a fraction of the technical depth that users on these forums seem to, is almost impossible. Ive spoken to around 7 gas safe engineers (some from vaillant, others from british gas, and also independents) and all have their own theories on what to try and most of their opinions are based on gut feels and experience, rather than hard facts. Not saying this may not work, but Ive already shelled out too much money to keep "trying" ideas out. I'd like to get a definitive solution (or as close as) before spending more than I already have done.
 
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Incidentally, the engineer who fitted the 26-50 for me also had a 25-100 or 25-120 (sorry, cant remember which). It was a second hand unit that he brought along for experimentation. It was in pretty bad condition, though functioning. If anything, it performed worse than the 26-50, though this may have been a result of it not functioning properly.

In laymans terms, could you explain at a high level the difference between the following models:

26-50
25-80
25-100
25-120

Ive spent a lot of time looking at grundfos spec sheets and reading posts about flow rates and calculating heads, but to be honest am still not clear. Having some knowledge would help me to distinguish between an engineer who has some idea of what he's doing compared to one who is just guessing.
 
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In laymans terms, could you explain at a high level the difference between the following models:

26-50
25-80
25-100
25-120

15/60 = 150mm (end to end)/6 meter head
26/50 = 260mm/5m head (guess it's a 25/55?)
25/80 = 250mm/8 m head
25/100 = 250mm/10 m head
etc etc.
 
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The first two figures represent the diameter of the inlet ports ie 15mm or 25mm, the second figures are the amount of pressure loss in metres head the pump can overcome, all fittings and pipes have a pressure loss which can be totalled and the correct pump chosen from the pump graphs.
 
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A different approach here.

What you seem to have is a thermostat controlled ON/OFF system which runs at a pre-set flow temperature, say 70 degC (I'm assuming a lot, you'll have to confirm this).

The TRVs control the room temperatures, throttle the flow (as they should), boiler fault code for low flow comes up regularly under part load conditions.

So, have you considered weather compensation (VR65 & VRC 430, ISTR)?

The difference is that the boiler runs continuously (no more ON/OFF) at a variable temperature, which resets according to the outside air temperature (and so, in theory, matches the heat losses from the building). The TRVs no longer control the room temperatures (because the boiler flow temperature modulates to keep a representative room comfortable), but are merely trimming the flow rates to the warmest rooms. Flow is not throttled nearly as much, the boiler fault code for low flow does not appear and they all lived happily ever after.

I'm not sure the VR65 thing can do two heating zones, but haven't looked it up.

I'm a great fan of LLHs, but I'm not sure this is the best application for one. Ebus wiring to/from the boiler VR65/VRC430 (470?) is the major problem.
 
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the heat exchanger taking amost all the head from the 15/50 and then there is 24 rads, I dont think w/c is going to overcome undersized pump/s in this setup.
 
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This boiler is a bit conflictive...


http://vaillantcyclingproblem.blogspot.co.uk/


where I have had issues the new printed ciruit board which handles lower thermal loads has made the boiler behave as it should.

Note though my experience is only with çompensation controls...not On-off controls
 
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