My vokera linea ag plus domestic hot water cycles hot and cold. I've watched the boiler with hot tap on, the burner fires up, runs for maybe 30 seconds, shuts off for 30 seconds, then fires up, on for 30 secs, shuts down etc etc. No category below for central heating.

Symptoms sound like crossover might be part of problem:
Crossover is when cold water enters hot water line due to faulty plumbing part like check valve, mixing valve, or bad faucet cartridge.
http://waterheatertimer.org/Crossover.html

try check in the water pressure on the boiler should be on 1.5 bar

Yes usually it is indeed the thermostat, or Thermistor, all part of the control circuit. You did not indicate what type of setup you have. I suggest you Google for a thing called a service manual. This will aid in diagnosis, and repair of your system.

Hi - hope someone can help. I have a VOKERA EXCEL 80SP combi boiler and been without hot water/central heating for two days. The pilot light is on, the fan is on, but boiler not triggering once the hot water tap is turned on.
Hopefully someone come to look at it today but hoping for some advice. Anyone else had this problem?
Thank you.

most likely the diverter valve.. when hot tap turned on the movement of the water through the valve operates a diaphragm which pushes a pin/button//switch to tell the boiler to fire up

not an engineer but maybe a heating compant,, it sounds like you have a relay that has gone bad, you have what we call a winter summer system your domestic water comes from a tank inside the boiler, you might want to think about splitting the system to save you some money in the long run, seperate the water heater from the boiler so you only heat up in the winter, right now you are heating the system that heats your house to get 30 or 40 gals of domestic hot water, it would cost you less by seperating them

If there is a hi-limit switch, maybe it's open, reset the switch, and monitor the temperature, if the temp. increases over the set temp. replace stat, and clean lime off from the tank, if does not have a filtering system

there is a zoneing valve thats suppose to be in the line to keep this from happening, might even have a check valve either way which ever one you have is bad and should be changed with a new one

sounds like you have an earth leak or defective pump winding

David,

The fact that you're getting a little burst of domestic hot water suggests that the boiler is capable of circulating hot primary loop water through the heat exchanger, but doesn't necessarily do so when it needs to.

What seems to be lacking is a recognition that domestic hot water is now being drawn off, and that the boiler needs to fire up and divert the primary flow through the heat exchanger.

A good place to start might be the flow switch (usually in the bottom of the boiler, where the cold water supply comes in) or the electrical connections to it.

A.