2 weeks ago installed new grant vortex condensing oil boiler. Great it heats house up very quickly but since I only have a horstmann basic programmer 425 diadem - decided to install a room thermostat to control heating better. Purchased and fitted cm927 but the house warms up very slowly. Thermostat is in hall with target temperature of 18. The last 3 days temperature reads 15 when heating comes on and it is taking 2 1/2 hours to reach 18. Previously without stat this was reached in 30-40 minutes. I am working the stat on manual it was my belief that boiler would stay on still targeted temperature was reached or close to it. Default setting was for a gas boiler but I have changed this to oil burner. Burn time 4 minutes and cycle 3 times an hour. Would be grateful for any advice as to why stat is so slow.