Anti Cycling mode on hot water only

[mrshr]

Hi All,
I have a Vaillant ecoTEC 837 system boiler.

When heating the cylinder (unvented Megaflow on S plan), I find that it heats up for about 35 mins, and then the boiler enters anticycling mode for a few mins, before pump shuts off.
About 3 mins later, boiler fires up again and runs for about 4 mins, and then enters anti cycling mode again.
This keeps repeating until time clock time expires.

The cylinder stat never gets to its temp i.e. the motorised valve remains open whist the boiler is in anti cycling mode.

Doesn't do it on the heating side of things, only hot water.

Any ideas there I should start?

TIA.

 

[picasso]

What temp have you set the boiler and cylinder stats at ? do you mean 637 ?

 

[mrshr]

Sorry, yes 637.

Boiler temp 62 deg.
Cylinder stat at about 60 deg.

I've tried tweaking both but no difference.

 

[picasso]

The boiler is running too low, turn the boiler stat up to 70c and see if that sorts the problem.

 

[mrshr]

70??
That's near maximum.

I don't think I've ever had it up that high before. Will give it a try.


But I thought you're meant to turn the boiler down during the warmer months of the year?

 

[mrshr]

Turned boiler up to 70 deg.
Still does the same.

 

[picasso]

A 37kw boiler is a lot of power, how big is your house and have you ever had a heatloss calculation done on it ?

 

[mrshr]

It's a big 5 bed house so went for a biggest vailliant do.
We had the system installed about 3 years ago, not sure if any heat loss calcs were done. Who would do that, the heating company?

I have the hot water timer to come on for an hour in the morning, but at this time of year the heating doesn't come on with it, so the boiler is only heating the cylinder. And that's when I notice this anti cycling thing going on.

 

[mrshr]

Any idea what I should be checking for?

Or, what can I expect an engineer to do if they come and inspect?

 

[picasso]

The boiler has a minimum output of about 6.5kw so chances are the boiler cant get rid of the heat its producing as the cylinder nears temp and shuts down, thats why you dont notice it when the heating running.

 

[mrshr]

I think I see what you mean.
So when the heating is on, the cyclinder stat probably reaches temp and shuts off the HW valve?

But when the heating is off, the boiler goes into anti cycling mode because the return temp is too close to the flow temp? Consequently preventing the cylinder stat reaching temp?

Have I got that right? And if so, is it a fault or normal?

Is there something that can be done to stop the boiler bouncing on and off like this during the warmer months of the year (i.e. when heating is off).

 

[David937]

70??
That's near maximum.

I don't think I've ever had it up that high before. Will give it a try.


But I thought you're meant to turn the boiler down during the warmer months of the year?

To heat the cylinder to 60c, you need the flow temperature around 10c higher than that, hence 70c. This may well be too high for heating at this time of the year but you're pretty limited to this unless you have Vaillant controls, which can use two different temperatures for heating and hot water.

 

[mrshr]

I see what you mean.

My boiler does have two temp dials on it (1 for water, 1 for heating), but I believe the one for hot water doesn't actually do anything as I have a system boiler not a combi. (i.e. vaillant use a generic controller for all their boilers).

So 2 questions:

If I crank the boiler up to 70 deg, is there a way to stop the boiler shutting off before the cylinder stat temp is reached?

What are the options for controlling the hw and ch temps independently?
Weather compensator?

 

[David937]

37kW is likely to be way too high even for a property your size. You may find that the total output of the radiators is less than 37kW - quick way to check would be to allow 1.4kW per rad. Anyway, you could try to reduce parameter d.0 to a lower figure like 24kW which might help.

The Vaillant controls use their own cylinder which has a dry pocket for a temperature sensor rather than a cylinder stat, I'm not sure if this is something that can be done to your Megaflow.

 

[Dan Robinson]

Ah, the Spinal tap method of boiler specification.


Try setting D.0 to 20 and see if that helps. As said, when the cylinder is getting up to temp the boiler cycles due to poor heat dissipation.

Also, turn the cylinder stat down a few degrees.