As mentioned before I have a branch of the heating system that is
slightly at risk of freezing - UFH and an air blower in the conservatory.

I was considering Sentinel X500 which is propylene glycol based, but
thought people might be interested in some pros and cons I've discovered
during research:

Pros:

1) Obviously, reduction in freezing point. Protection to -6C at 20%
dilution.


Cons:

1) Not cheap - would cost £100 to add 20% to a moderately sized house
system;

2) Read reports that it can result in leaks due to lower surface tension
(valve stems, any joint that has a tiny flaw that otherwise holds water);

3) Worcester Bosch on the 42CDi Nat Gas, tell me I can only dose to a
maximum of 20%. I asked because I noticed that auto air bleed bottle
vents specify = 30% glycol.

4) Sentinel specify a minimum dosing of 20% as it contains the
inhibitors too;

5) Question of toxicity if the DHW heat exchanger ever leaks.



So given how little protection (-6C) I can get, I'm cancelling that idea
and getting some modes made (when I connect the UFH and add the auto
bleed vents) to add local isolation inside the house and extra drain
cocks to make it easy to dry out that part of the system with a wet+dry
vacuum.


I'll be doing some detailed measurements this winter on how cold the
pipework gets relative to outside (it is adjacent to the house wall or
under screed. Former, together with wind protection, does buy an
envelope of protection and the screed will slow the rate of cooling of
buried pipes.


So what I expect is that most of the time, when heating is running, it
will be fine.

If we happen to go away for 1-2 weeks in January/February, a quick local
drain down might be in order.