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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?

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  • Is the system capable of switching the pump on separately to the burner? This would change the approach slightly.
    – Tetsujin
    Oct 29, 2022 at 11:47
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    This isn't an answer, but one thing you really should do to winterize the house is turn off the water and open all the faucets. Even better is to blow out the pipes using a compressor. Also, of course, turn off the water heater. Without knowing more about your system, I can't give better advice, here in the states 45F is considered a good temp. Oct 29, 2022 at 13:14
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    Somewhat unclear how this system is set up - normally thermostatic valves are used on a system where the loop is always running, and the radiator is locally controlled by the valve - having a central thermostat and thermostatic valves is odd, and has potential control issues. Balancing valves would be more typical for a central thermostat setup. Put non-toxic antifreeze in the heating loop and drain traps & drain/blow out the plumbing supply lines...
    – Ecnerwal
    Oct 29, 2022 at 13:22
  • Thanks for the comments. @Tetsujin I don't think that the system is capable of switching on the pump separately from the burner, though I have only been in the house for a few days, and my only evidence for this is that I haven't seen the pump running when the radiators didn't feel warm.
    – alvion
    Oct 29, 2022 at 15:48
  • I did some reading and some reasoning. I set all the valves to "1" which some research suggests would keep the valves open when the temperature at that radiator is in the 12-15 degrees range, and I set the thermostat to 12 degrees. I believe that in this configurations, the valves should ALL always be open when the pump kicks in, since it's difficult for me to imagine a scenario where the center of the house would be 12 and one of the radiators is at 15. I'm now gone from the house and won't know until I get back in January how things worked out :).
    – alvion
    Oct 29, 2022 at 15:50

1 Answer 1

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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.

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