What setting for thermostatic radiator valves when leaving a house at low heat for the winter?

Question

We just bought a new house (new to us, it was built in 1960 and unrenovated) and only have a few days before we leave it for November/December. We are planning to leave the heat on with the thermostat set low (50f/10c), but I'm not exactly sure what to set the thermostatic valves on the radiators to. My first thought is that I should make sure to set them all to the same temperature to ensure even heating throughout the house, however I'm not sure which setting that should be at, or even if maybe I should be setting the valves further from the thermostat to higher settings, since they will lose heat more quickly, and it doesn't help anything if it's 10c in the middle of the house and below freezing for the pipes in the exterior walls.

A few details about the house:

• Three stories though only the first and second floor are heated.
• Built in 1961, concrete construction, totally uninsulated, totally unrenovated.
• Radiant heating from a heating oil furnace.
• We only have about 700 liters of fuel to make it until the end of December when we return.
• Average temperature in the town we live in (French alps) never really dips below freezing until January.

To sum up, what should I set the thermostatic valves at to maintain an even temperature throughout the house with minimum fuel usage?

Answer 1

I'm no expert on this, but there are three stages to frost protection.

The first is if the temperature outside falls to near freezing, or inside below about 12°.
This is time to activate the pump alone, just to keep the water moving. Moving water is a lot less likely to freeze. If the pump isn't separately switchable, then this is not an option & you need to look at stage two.

At about 7° internal temperature you should run the heating sufficiently to prevent itself freezing.

Below that, at about 4°, you need to actually heat the property itself.

If you have one global thermostat & individual room stats on the radiators, then I would set the main to perhaps 7 - 10° & the room stats to 4 - 7°.

This way the system itself will run pre-emptively, but the full cost of running all the rads is kept for only when it gets properly cold. Heat will rise, taking some care for the top floor, but your concern is only really for frost, not comfort. Leaving internal doors open is something you'll have to consider on a case-by case basis… whether the washroom is cold & draughty, with no radiator… but has cold-water piping, etc.
If this looks likely to be an issue, drain the cold system & leave the taps open, sinks unplugged.

As this will otherwise keep the entire property above 4° this may obviate any need to empty the cold water system - assuming it is sufficiently prepared at the house entry point - presumably coming above ground only once inside the building. I cannot imagine anywhere in the Alps this was not considered even in the 60s, but it's wise to check.
Also check for any external plumbing less than half a metre below ground. Anything connected to this should be drained, as 60s construction is unlikely to be proofed against frost to bib taps etc.