Boiler stat issue?

[vala]

I’ve got a Worcester external oil combi which feeds a buffer vessel which in turn feeds my UFH systems.
On my buffer vessel I have 2 thermostats which monitor the temp of the vessel and this info is displayed on a Grant GSX1 controller. The control is currently set for an on temp of 45° and an off temp of 75°.
When there is a call for heat from the UFH, if the buffer temp is lower then 45° a relay instructs the boiler to fire.
However today I noticed the boiler was firing every 45-60mins and the vessel temp was only dropping to around 63°.
After a quick chat with Worcester technical it would appear this was because the boiler was set to 75° and was reacting to the stat inside the boiler housing on the flow pipe.

Anyone got any ideas how I can get the boiler to react to firing up based on what this controller is reading from the vessel?

 

[vala]

Can anyone help?

 

[oilhead]

As it is a combi, it will also be firing to maintain the heatstore for DHW. This is what oil combi's do. You could always lower your boiler stat, but would need to lower your hot water stat correspondently, so poor hot water

 

[vala]

@oilhead
Yes it does have a heat store however the controls for this is off. The house is unoccupied and when there is someone here it's to work, which doesn't require hot water.
So I don't think it's the heat store it's firing up for, especially every 45-60mm mins.
I do believe the boiler stat is taking priority over the stats on the buffer vessel as, when the boiler stat was set to 75°, and the buffer stat 'off' temp was 75° it never made it according to the controller. It hit 69°.
Then now with the boiler stat at 50°, it hasn't fired the boiler and the temp in the buffer is down to 39-40°, yet the 'on' temp is 45° providing there is a call for heat, which there is.
My thought is the flow temp is till around 50°, unsure what the hysteresis is on these, hence why it hasn't fired up. And even if it did it wouldn't be on for long.
So I'm wondering if I can bypass that flow temp, there's still a max safety sensor and have the boiler firing as per the temps at the buffer only when there's a call for heat???

 

[oilhead]

There is obviously a live feed that is bypassing the buffer controls. As the ambient temp is quite low at the moment, could it the frost stat that is producing this? Without seeing a full wiring diagram of the system including the buffer it is not possible to follow it further. These boiler control stats are fairly coarse and may have up to 5-6oC differential.

 

[vala]

@oilhead
My thought was that the boiler stat was taking priority over the buffer stats.
And so they would turn the boiler on or off depending on if they are satisfied providing there is a call for heat.
But how to bypass the internal boiler stat and rely on the buffer stats is an unknown.
There is still a max safety sensor with this boiler.

My first thought when I noticed the boiler firing up often was just that. The internal frost protection was kicking in.
However since setting the boiler stat to 50° the boiler hasn't fired so I'm ruling that out. In fact the buffer dropped below its 'on' temp of 45°, there is a call for heat, and the boiler still didn't fire to bring up the temp in the buffer.

 

[oilhead]

Without seeing a full system wiring diagram, I can offer nothing further.

 

[vala]

@oilhead

 

[oilhead]

Sorry, but doesn't make a lot of sense to me. Would need to see the connections into the boiler control box with the switched live connections identified along with any permanent live connections.

 

[vala]

@oilhead
That is the electrical schematics for the S Plan for this system.
Wunda is the UFH controller,
HS is the hub switch which allows the UFH controller and thermostats to connect to the internet,
TDC is the temperature difference controller that monitors the 2 thermostats in the buffer and when the temp is outside of the temps set, send a a signal to the relay to then switch on the boiler - providing there is a call for heat from the Wunda UFH,
Relay is the relay that sends a switch live to the boiler,
and B is the boiler.

so on that a plan,
1 is the lives
2 is the neutrals
3 is the earths
4 is switch live when TDC is outside of temps set
5 is switch live when there is a call for heat from UFH controller,
10 is switch live to turn on boiler when there is a call for heat AND when the temp in the boiler is below the 'on' set point.

 

[oilhead]

As I see that, there are 2 permanent live feeds from the spur. The is the brown one connected to the same terminal on the boiler as the switched live?