Vaillant echo tech plus 418 short cycling on hot water but OK for central heating. Best recommended settings

[Samsg30]

wow.. that's getting way too technical
Attached is the picture of 3port valve and irrespective there's a hot water is on or off, the small lever on the valve can freely move between the two red lines marked on the picture. Could this be the contributing factor for short cycling?
Just to be clear, when I say short cycling, it means, for first few minutes (5-10 mins) boiler seems to be working OK and flow temperature on the boiler gradually increases to 65C Then it just shuts off immediately and turn back on again after let say 2-3 minutes. And then it starts doing short cycling of very quickly increasing the flow temperature to 65 in few seconds, shut off for 2-3 minutes, start again for few seconds, shut off, start, off . ... until the scheduled timer on the controller stops the hot water schedule.

 

[fixitflav]

Could this be the contributing factor for short cycling?

I don't see how. The most likely scenario to give short cycling is when all the flow is going to the cylinder, any other position of the 3-port is better. By its design, the valve can only give HW, CH or both.

And then it starts doing short cycling of very quickly increasing the flow temperature to 65 in few seconds, shut off for 2-3 minutes, start again for few seconds, shut off, start, off . ... until the scheduled timer on the controller stops the hot water schedule.

Is that a problem? Presumably it also stops on satisfying the cylinder stat. Typically how many cycles and how long does it take?
You might try higher control-stat or lower cylinder stat. setting.

 

[Johntheo5]

wow.. that's getting way too technical
Attached is the picture of 3port valve and irrespective there's a hot water is on or off, the small lever on the valve can freely move between the two red lines marked on the picture. Could this be the contributing factor for short cycling?
Just to be clear, when I say short cycling, it means, for first few minutes (5-10 mins) boiler seems to be working OK and flow temperature on the boiler gradually increases to 65C Then it just shuts off immediately and turn back on again after let say 2-3 minutes. And then it starts doing short cycling of very quickly increasing the flow temperature to 65 in few seconds, shut off for 2-3 minutes, start again for few seconds, shut off, start, off . ... until the scheduled timer on the controller stops the hot water schedule.
View attachment 309213

The lever shouldn't be loose like that but the valve just may still be operating OK, have you felt the cylinder coil flow & return piping?,

The boiler should not cutout at 65C IF the target temperature is set to 65C, it should rise to 70C before cut out, try increasing the "dial" RH temp to 70C. if its marked HW, that should certainly allow the cylinder temperature reach 60C to satisfy the cylinder stat before running out of time.

 

[fixitflav]

Newer ones like like the one I have, have electronic settings where you can have different temperatures for the heating and hot water circuits

I suppose that's possible, and would be necessary with weather compensation. As you say, needs appropriate controls. And either/or CH/HW, so not using the functionality of the mid-position valve. And PDHW.

 

[Samsg30]

Thanks guys for the replies. From the conversation above, it looks like there's no obvious reason for it to short cycle that can be quickly diagnosed and fixed as such. Given that boiler, cylinder and heating system would be upgraded next year during renovation / extension, wondering is that OK to leave it short cycling like this when only HW is on. When CH is on, it works OK.. which means even if boiler last for next 18 months or so, I am OK with it.
But indeed comments and information above are very useful

 

[fixitflav]

it looks like there's no obvious reason for it to short cycle

There's a reason for it to cycle, as I said in #12. Whether that's short cycling is a matter of definition. You haven't said whether it causes a problem.

 

[Johntheo5]

There's a reason for it to cycle, as I said in #12. Whether that's short cycling is a matter of definition. You haven't said whether it causes a problem.

Indeed, but running for just a few seconds isn't normal and the cylinder stat is never satisfied.

"Just to be clear, when I say short cycling, it means, for first few minutes (5-10 mins) boiler seems to be working OK and flow temperature on the boiler gradually increases to 65C Then it just shuts off immediately and turn back on again after let say 2-3 minutes. And then it starts doing short cycling of very quickly increasing the flow temperature to 65 in few seconds, shut off for 2-3 minutes, start again for few seconds, shut off, start, off . ... until the scheduled timer on the controller stops the hot water schedule."

 

[fixitflav]

the cylinder stat is never satisfied.

I haven't seen where he said that

 

[Johntheo5]

Yes, Maybe I'm reading too much into the fact that the boiler keeps cycling until the HW schedule times out.

 

[Harry Bloomfield]

But I'm curious why a heat-only boiler has 2 temperature dials. I suppose by dial you mean a rotating knob which adjusts the control-stat setting.

It must be an older 418, with manual control knobs. The more recent ones use digital display and button settings.

I would suggest to the OP, he might be better updating the control system, if it's possible, to the Vaillant system, which uses a thermocouple to feedback actual current water temperature, so the boiler knows how far short of the desired temperature the cylinder is. By knowing the temperature, the boiler instead of switching on/off - short cycling, it can modulate its output precisely to match the demand for heat. The thermocouple, VR65 etc. kit, does away with the on/off stat on the side of the cylinder.

 

[fixitflav]

boiler keeps cycling until the HW schedule times out.

I queried that in #17 but he hasn't confirmed. Obviously if it doesn't reach cylinder stat setpoint there's a problem!

 

[fixitflav]

It must be an older 418, with manual control knobs. The more recent ones use digital display and button settings.

I must admit I've been assuming the boiler fires On/Off (like mine) If it modulates down to something like the cylinder demand there clearly should be much less of a problem. Must look at the operating manual!

 

[Harry Bloomfield]

I must admit I've been assuming the boiler fires On/Off (like mine) If it modulates down to something like the cylinder demand there clearly should be much less of a problem.

It probably does modulate, but modulates based solely upon the return flow temperature. Much better if it modulates based upon knowing the actual temperature of the water in the cylinder, or how close it has got to the desired room temperature when in CH mode.

 

[Samsg30]

I queried that in #17 but he hasn't confirmed. Obviously if it doesn't reach cylinder stat setpoint there's a problem!

Cylinder is set to 55c at the moment but I have tried 50-60c as well. The flow temperature on the boiler is set to 65c.
The thermostat on the cylinder is quite old style so not sure how can I confirm if water temperature is reached to the desired level in the cylinder. But water in tap is definitely hot.

 

[Johntheo5]

Cylinder is set to 55c at the moment but I have tried 50-60c as well. The flow temperature on the boiler is set to 65c.
The thermostat on the cylinder is quite old style so not sure how can I confirm if water temperature is reached to the desired level in the cylinder. But water in tap is definitely hot.

While the boiler is actually firing can you keep turning the stat down, you should hear a click from the stat and the boiler should shut down.