In article ,
"Haymish" writes:
This will be our last winter in the house and money is tight so we don't
want to go to the expense of fitting a room thermostat if possible although
it is a good idea.

I set the boiler temperature to minimum and the 3 radiators to the 'blue
snowflake' setting aboy 4-5 hours ago. Now I have a few more questions:

1. The boiler seems to be going crazy switching on and off every few
minutes, is this normal for my settings?

Yes -- you need a room stat (or frost stat) to prevent the boiler
firing up when there's no requirement for heat from the radiators.
Otherwise, the boiler will just be keeping the bypass loop up to
temperature, which is going to result in frequent switching on and
off. (For this reason, boilers fitted nowadays _have_ to include
some mechanism such as a room stat to switch them off when there's
no call for heat, to avoid wasting this energy.)

You might find it cheaper to get a room/frost stat fitted, than
paying for this energy usage and possibly a replacement gas valve
or igniter, or whatever part of the boiler wears out faster as a
consequence.

Oh, and I'm assuming the boiler does have a bypass. If not, then
you should have one radiator with no TRV or other means of closing
off.

2. Despite the radiators being set to 'blue snowflake' setting 4-5 hours
ago. I just checked and two of the radiators are so hot you would not want
to touch them for long and one of them is luke warm, is this right? I would
have thought they would get just warm enough to stave of freezing pipes
rather than a heat up really hot / cool down cycle. Could someone confirm
what should happen? (present conditions are very cold in the rooms).

What are the temperatures in those rooms?

Yes, I would expect the radiators to be hot at the top, as they
will have the full temperature boiler water there. Unless your
boiler is a condensing boiler, the minimum flow temperature is
probably something like 70C. However, if the TRV has reduced the
flow to a minimum, you'll find the radiators get cold as you run
your hand down to the bottom, because the flow in the radiators
is tiny. This is how the TRV reduces the heat output from radiators.

3. I bled all the radiators, one of the radiators had a lot of air in it but
I expect they have not been bled for a long, long time. Am I ok to do this
as I assume this is a closed system.

4. The pressure is about .75 bar, is about right for the above mentioned
settings.

Pressure would typically be about 1 bar when cold (and it goes up
when the system gets hot, but probably not by much in such a small
system). The .75 bar will drop when the radiators are cold, and
there's a risk it might drop below the point where the boiler will
operate (assuming your boiler has a pressure sensor switch).

--
Andrew Gabriel