how to fix valance 835 boiler ecomax?

Error code f.25 and f28 are coming up. When power is switched on the temp rises to max 100degrees and then goes back down (when nothing is in use). When hot water is turned on, the temp again rises to 100degrees (very hot) even when the thermo is set on lowest setting and then the temp drops down to cold and then the boilers ignitions kicks in and temp goes back up (a bit like a yo-yo). Central heating dosnt heat up. Can any one help? sagsta_1 yahoo.co.uk