Keith,
I've been following this thread and I have an opinion, even though I
don't have a greenhouse.
Since you already have your heater system set up for day and night
temperatures, I think you should aim for a comfortable 10 degree (f)
difference between day and night. If you want to keep the night
temperatures no lower than 59f, then set the day temp for 69f. Now, I
don't think it would be necessary, or even desirable to keep the
temperature that warm all day (unless it is up there due to sunshine). I
would set that timer to be on from 11 AM to 3 PM.
I would either do away with the timer for the 59 degree setting or, if
it is easier, set it for the 20 hours of the day that remain.
Hopefully, you will have many days where greenhouse warms beyond 69
degrees before 11 AM and the higher temperature thermostat will not even
have to kick in.

Steve (wishing for zone 8 but more like 3)

kenty ;-) wrote:
I have a electric heater run through a thermostat controller & probe which
is very accurate,I have two of this set-up one for day & one for night,both
on timers so one set-up takes over from the other,it is automatically done.I
just have to initially set the parameters I want,the night is set to 59f/15c
and day has been at 71f/22c,but today was really sunny in the after noon so
the temp got in the 80`s.After input I have received off you guys I am going
to lower the day temp to 66f/19c?What do you think.I live in Robin Hood
country,that being Nottingham,England,UK.It can get quite cold here(zone
8)but really not that bad!My greenhouse is only a measly 10x8,but we all
have to start somewhere.