Boiler short cycling on hot water

[dilalio]

If you increase the boiler stat to max, does the period of short cycling reduce?

Can you prove, via other thermometers that the return pipe and cylinder (at stat location) are reaching the actual temps the system deems them to be reaching?

There is obviously conflict between the boiler overheat stat and the cylinder stat being satisfied, with the ability of the coil to transfer heat to the stored water at the fulcrum!
My physics ain't brilliant but I would expect there is a formula which involves a curve, that describes the diminishing heat transfer rate between the two, over time as the differential reduces!
Defective or worn components, depending on the their age, would also be a factor.
What temperature is your hot water as it comes out of a tap?

 

[dilalio]

You could "circumnavigate" this issue, in the short term, by setting your cylinder stat to 65 degrees and adjusting the programmer to only come on for @45 mins.
This would need some playing around with to get just right and would obviously be affected by any draw off of hot water during the heating period.

 

[bhm1712]

If you increase the boiler stat to max, does the period of short cycling reduce?

Can you prove, via other thermometers that the return pipe and cylinder (at stat location) are reaching the actual temps the system deems them to be reaching?

There is obviously conflict between the boiler overheat stat and the cylinder stat being satisfied, with the ability of the coil to transfer heat to the stored water at the fulcrum!
My physics ain't brilliant but I would expect there is a formula which involves a curve, that describes the diminishing heat transfer rate between the two, over time as the differential reduces!
Defective or worn components, depending on the their age, would also be a factor.
What temperature is your hot water as it comes out of a tap?

In answer to these:

No not really, it still Short cycles roughly the same amount with the boiler stat on maximum.

Yes, I have several sets of clamp on thermometers and hand held PT100 contact probes which confirm the flow and return temps.

 

[dilalio]

And the temp of water from hot tap?

AFAICT: it's your cylinder coil furred up or overheat stat in boiler.

 

[Agile]

I you have a digital K-Type thermometer than that's exactly correct.

Using that you should be able to measure the turn off temperature of the cylinder stat and its hysteresis.

It all sounds about right to me.

It would be slightly better to do the final heating of the cylinder at the same time as the heating is on.

I typically time the HW to start 30 min before the CH.

Tony

 

[SuperSix6]

Did you ever resolve this?

I have some similar behaviour with a 330+ boiler which is short cycling without any demand for HW/CH. It boosts flow temp from around 55 degrees for 20-30 seconds before shutting off. As temperature falls again it then repeats, pump doesn't run unless 2 ports engage so it's creating air and eventually dry faults.

I've confirmed that it's not the PCB, 2 Port, Controls or Thermistors.

There is definitely something sending a heat-demand.

 

[SuperSix6]

In answer to these:

No not really, it still Short cycles roughly the same amount with the boiler stat on maximum.

Yes, I have several sets of clamp on thermometers and hand held PT100 contact probes which confirm the flow and return temps.

Did you ever resolve this?

I have some similar behaviour with a 330+ boiler which is short cycling without any demand for HW/CH. It boosts flow temp from around 55 degrees for 20-30 seconds before shutting off. As temperature falls again it then repeats, pump doesn't run unless 2 ports engage so it's creating air and eventually dry faults.

I've confirmed that it's not the PCB, 2 Port, Controls or Thermistors.

There is definitely something sending a heat-demand confirmed in the diagnostic menu.

When boiler is set to 'off' there is no autonomous behaviour or cycling.