S-Plan with no Bypass

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SgtFoley

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Hi
Great forum btw!
I have a Vaillant EcoTech 418, S-Plan system, 2 zones (HW unvented cylinder + CH), installed 1 year ago. Standard 3 speed Grundfos pump.

The system seems to work, but boiler periodically (say once a month) cuts out with error F72... "Flow and/or return NTC fault".
I've now found out the system does not have a bypass loop/valve (I'll get one installed asap).

Q1:
Without a bypass valve, the pump over-run didn't seem to cause any obvious problems. I mean... should the pump not rattle and shake if it's pumping with nowhere to go? Or will it just overheat but keep running?

Q2:
Without a bypass, should I not see the temperature on the display of the boiler spike up every time demand stops (i.e. during the overrun)?
According to the display, the temperature stays high, but doesn't increase (of course if may spike sometimes when I'm not looking; I've never witnessed the F72 error happen in real time; it only happened after CH and HW have been on at the same time... maybe the extra power that requires can cause the overheating?).

Q3:
What kind of damage could take place from running the system with no bypass for a whole winter? I guess my pump didn't love the experience...
But could I have damaged the boiler?

Many thanks
SgtFoley
 
Is the pump powered by the boiler and provides over run?

Do you know if the boiler is the later type with an alloy heat exchanger?

Or the earlier type with a spiral stainless steel heat exchanger?

Most pumps will happily run with no flow but will tend to overheat after a while as the water cools the pump.

Your F72 occurs "after CH and HW" have been used. But what has caused these demands to cease?

Properly timed the HW demand would cease before the end of the timed period because the temperature in the cylinder has been reached.

Tony
 
Tony, thank you for the quick reply.

I believe the pump is powered by the boiler; it definitely runs for a few minutes after demand has stopped.
I will check and get back regarding the exchanger type.

>> Your F72 occurs "after CH and HW" have been used. But what has caused these demands to cease?

The HW runs for just 20 minutes in the morning and 40 in the afternoon. Cylinder is 180L, so I doubt it will reach temperature in the morning. Is this an issue?

Just to clarify, I cannot say that the F72 happens AFTER the demand has ceased (I tried to force the error but failed).

The history: we called Vaillant Service because of the F72 faults; they noted the lack of bypass and refused to look into the problem because of that (fair enough I guess).
So, the lack of bypass may have nothing to do with the F72.

Before I learned I needed a bypass valve (i.e. yesterday!) I conjectured the fault might be due to CH starting up while HW was running... cold water from CH zone entering the boiler and fooling the boiler into thinking something is wrong (the description for F72 is "Flow Return NTC temperature difference too great"). But I guess the lack of bypass is a more likely candidate?

thank you again
 
[QUOTE="SgtFoley, post: 3630070,

The HW runs for just 20 minutes in the morning and 40 in the afternoon. Cylinder is 180L, so I doubt it will reach temperature in the morning. Is this an issue?

thank you again[/QUOTE]


Do you mean that is how you have timed it?

Or the cylinder demand is satisfied in that time.

I don't know why you would want to cut short the reheat times.

The way that normal people use it is to set it for sufficient time to ensure it has reached the set temperature.


Tony
 
Yes, that's how I have timed it.
>> I don't know why you would want to cut short the reheat times.
To save energy... I don't need a full tank in the morning.

I guess I don't understand why you are asking... is there a drawback to stopping short of a fully heated cylinder?
 
SgtFoley said:
Hi

I have a Vaillant EcoTech 418, S-Plan system, 2 zones (HW unvented cylinder + CH), installed 1 year ago. Standard 3 speed Grundfos pump.

I've now found out the system does not have a bypass loop/valve (I'll get one installed asap).

Many thanks

SgtFoley
Click to expand...




It is an installation error not to fit an auto bypass.

Was it not professionally installed? You should be contacting the installer to fit a bypass at no extra cost to you.

You are lucky Vaillant did not charge you £90 for their visit.
 
Are your zone valves honeywell (silver in colour on a brass body) or some other make ?
 
Ian: yes, honeywell, silver box, with manual lever to force open.

Tony: yes, "professional" job. I think I can get him to install the bypass for free...
The question was more "has it damaged my boiler"... but I'll need to check regarding exchanger type.
 
The reason I ask is because on the noneywell you can maunualy operate the lever that you describe without the valve firing the boiler and pump , what you could try is when the boiler is ready to go off for both heating and HW then manually open the valve and click it open and see if this stops your boiler locking out, what you are essentially doing is making a by pass, obviously not a permanent fix but if the boiler is still locking out with a valve manually open then you have more than one problem, you are more than likely going to have to get a by-pass installed anyway regardless
 
Thank you Ian

The problem is the F72 happens relatively rarely. i.e. once every 3 or 4 weeks of otherwise normal operation
I have tried a few things but failed to force the error; so I don't even know when it happens (i.e. when demand ceases or when a new zone opens, etc).
I just know it happens when (or just after) both zones are on.
 
SgtFoley said:
Yes, that's how I have timed it.

>> I don't know why you would want to cut short the reheat times.

To save energy... I don't need a full tank in the morning.

I guess I don't understand why you are asking... is there a drawback to stopping short of a fully heated cylinder?
Click to expand...

Doing the normal setting of giving enough time to reheat means the boiler will have modulated back and will be turned off by the thermostat whilst at a low power output.

Cutting boiler suddenly when on full power is likely to create problems when no ABV.

But there is no significant saving to be made anyway. The water has to be heated some time.

The daily heat loss from a cylinder is minimal.

Are the pipes around the cylinder insulated?

Tony
 
Last edited:
Pipes are insulated but the cylinder is in an un-heated garage.

I hadn't thought about the benefit of the boiler modulating...
So a possibility is that F72 happens only when both HW and CH coincidentally stop at roughly the same time, not allowing the boiler to modulate down naturally.
I guess this could explain why the fault is relatively rare.

I'll have a go at testing that!
thank you
 
Related question...

I noticed yesterday (while I was trying to force the F72 fault) that the CH and HW temperature controls on the boiler are wired incorrectly: the CH control knob controls the HW flow temperature and vice-versa.

Could this cause some problems? Does the boiler behave differently for CH and HW or would it operate/modulate in the same way?

thanks again!
 
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