Boiler feeding thermalstore repeatedly fires 1 min on 2 mins off

[Brix]

I have a thermal store cylinder heater by a Potterton Suprima 100 (recently serviced) which provides hot water and UFH. I have noticed in the morning (as the boiler is in a cupboard off my bedroom) that the boiler fires up for perhaps 50-60 seconds, then off for about 2 minutes, then back on for a similarly short period, and carries on on and off.. I checked and there was no demand from the UFH at the time. I do have a secondary water loop which runs through a towel rad but the Thermalstore is about 280L at 75-80C so it presumably shouldn't be struggling with feeding that circuit (pumped at low speed)?

The cylinder stat is an orange coloured Aqualux (TC100AN possibly) set at 75C. From what I have seen online, I think the differential on the thermostat might be 6C. The stat on the boiler itself is set to Max.

Any thoughts what might be causing it to behave like this?

 

[SimonG]

Pump on it's way out, circulation issue through boiler.

 

[Brix]

Cheers. When there is full demand it stays on no problem. Could that still be the case?

 

[John.g]

Is the boiler modulating down in this period?.
You could reduce the cylinder setpoint to 70C for a while and see if any improvement although a thermal store if directly heated by the boiler should require a decent amount of firing to increase its temperature by 5/6C depending on the location of its temperature probe.

 

[SimonG]

Dont do gas, but if it's working full tilt could be a boiler issue.

 

[Brix]

Is the boiler modulating down in this period?.
You could reduce the cylinder setpoint to 70C for a while and see if any improvement although a thermal store if directly heated by the boiler should require a decent amount of firing to increase its temperature by 5/6C depending on the location of its temperature probe.

Hi. I'm not quite sure how to tell if the boiler is modulating down but the burner turns off and then the fan stops a few seconds later. After a couple of minutes the fan comes on for about 15 seconds before firing back up.

The temperature probe is fairly close to the bottom of the cylinder. About 30cm above the boiler return.

 

[John.g]

You should/might hear the fan speed falling if the boiler is modulating within this 1 minute period, the cylinder temperature might show up something as well, if the boiler setpoint is say 80C then the boiler should start modulating down at this (boiler) temperature and shut down the burner at 85C if it continues to rise at the boiler minimum output. If everything is working normally then you might see a max cylinder temperature (after burner shut down) of ~ 80C. The 2 minute boiler off period is probably the anti rapid cycle time. If this problem started around the time of your service then maybe time to get it looked at again.

Have you a link to show your make/model of store?.

 

[John.g]

Don't know if you have solved your problem but came across (I think) your very interesting thermalstore below.
Re your problem: Its possible that with little or no demand that the 70 deg water is replaced very rapidly with hotter water at 75/76C even after one minute, as the boiler circ pump should have moved 15 to 20 litres from the store bottom to the top in this time, its harder to explain the re firing 2 minutes later but possibly due to where the towel rail secondary loop is returning the water to. Less likely but also possible is a drip from the hot water system as its primary coil is right in the bottom of the store and will very quickly cool the water adjacent to the stat.

My Thermalstore - is it Vented / Unvented? | UK Plumbers Forums | Plumbers Forums

Find My Thermalstore - is it Vented / Unvented? Advice and Help. How-to My Thermalstore - is it Vented / Unvented? in the UK Plumbers Forums advice boards on Plumbers Talk | Plumbing Advice | Plumbing Forum - Professional and DIY Plumbers and Bathroom Fitters Forum. My Thermalstore - is it...

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[Brix]

Don't know if you have solved your problem but came across (I think) your very interesting thermalstore below.
Re your problem: Its possible that with little or no demand that the 70 deg water is replaced very rapidly with hotter water at 75/76C even after one minute, as the boiler circ pump should have moved 15 to 20 litres from the store bottom to the top in this time, its harder to explain the re firing 2 minutes later but possibly due to where the towel rail secondary loop is returning the water to. Less likely but also possible is a drip from the hot water system as its primary coil is right in the bottom of the store and will very quickly cool the water adjacent to the stat.

My Thermalstore - is it Vented / Unvented? | UK Plumbers Forums | Plumbers Forums

Find My Thermalstore - is it Vented / Unvented? Advice and Help. How-to My Thermalstore - is it Vented / Unvented? in the UK Plumbers Forums advice boards on Plumbers Talk | Plumbing Advice | Plumbing Forum - Professional and DIY Plumbers and Bathroom Fitters Forum. My Thermalstore - is it...

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Ha - that's mine!

Interesting idea. Someone suggested reducing the stat to 70C and so far seems good but have not been keeping a close eye on it. Not sure what effect that will have on performance though.

Thanks for your thoughts on this!