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Roughly what is the minimum temperature to set a house heating thermostat controller to in order to keep pipes from freezing? Assume no other considerations, e.g. assume no one is living inside.

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  • How uniformly does the house heat? – ThreePhaseEel Dec 29 '19 at 15:33
  • pipes are mostly on the first floor and basement. The heating system can heat the places that have pipes uniformly, though less so second and third floors. – Tim Dec 29 '19 at 15:39
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    "mostly " does not matter, The important factor is where is the coldest pipe ; likely somewhere in an exterior wall. – blacksmith37 Dec 29 '19 at 16:23
  • @blacksmith37 what are an exterior wall and an interior wall? – Tim Dec 29 '19 at 17:52
  • An interior wall is a wall that is NOT exposed to the outside world on any side. An exterior wall is a wall that IS exposed even a little, to the outside world. Usually walls with windows, doors, and if you took it down, you'd see the world around you or at least get cold/warm air from the external environment. – noybman Dec 29 '19 at 18:56
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This depends on the particular house - both how the plumbing is laid out (are water pipes located in exterior walls, or not?) and the relationship between heat delivery and plumbing.

It may also depend on the heating system - i.e. hot water baseboards often have pipes that run in poorly insulated spaces at the edges of the house, and can themselves freeze if the house is too cool for too long.

Typical recommendations range from 45 to 55 F

One solution to avoid spending much on heating for a non-occupied house is to drain the plumbing and put non-toxic antifreeze in the traps so that freezing won't bother the pipes.

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  • Thanks. Water freezes at 32F. Why the recommended range is 10~20F higher? – Tim Dec 29 '19 at 15:48
  • @StackExchangeforAll -- as the thermostat measures the temperature at its own location, so you need some margin so you don't get bit by temperature gradients (unless you have a house that's insulated/air-sealed to like PassivHaus standards, that is...) – ThreePhaseEel Dec 29 '19 at 15:57
  • That is ... what? – Tim Dec 29 '19 at 16:01
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    Thermostats are typically located on a central interior wall. If you have any pipes that are closer to the exterior (or, worse, actually IN an exterior wall, which smart builders in cold climates avoid) they may be significantly colder than the location of the thermostat. Your house is unlikely to be "insulated and air-sealed to PassivHaus standards" without you being aware of it - and the cost of heating it if it were would be very low (traded off by paying a lot to insulate it very well in the first place.) – Ecnerwal Dec 29 '19 at 16:53
  • not to mention, 45-55 is a guideline. If the weather where you live is -30 outside, then 55 isn't going to cut it in all homes. Why? Because you have a box of cereal with a weak heat gun on one side, and a freezing ice ray on the other, with a water pipe in the middle... Who wins? Like in superhero movies, you need the heat to overpower the cold. – noybman Dec 29 '19 at 18:13
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I don't believe anyone can provide a solid numerical answer to this question. We can all provide guidelines, advice, and considerations.

There are far too many variables to consider to give this one mathmatical answer. I have heard of hundreds of cases of peoples pipes freezing when they have their heat set in the 60's. (not sure I have heard of it happening when heat is set in the 70's; with exception of actual draft issues and other conditions promoting the freeeze; but it has probably happened!!!).

One major factor you cannot control, is the external temperature. When temps get to sub zero, all bets are off. Pipes will even freeze in the ground outside of the house where it isn't heated anyways. (this has happened to us).

A recommendation is to:

  1. Ensure all pipes are properly insulated
  2. Do #1 with pipe wraps or wall insulation, also make sure there is no exposure to colder metal conducting surfaces
  3. Ensure there are no airgaps anywhere. this means ensuring all exterior wall drillings and holes are sealed
  4. Ensure all exterior door and window framing is caulked and sealed
  5. Ensure all windows and doors are sealed shut, check for any drafts
  6. Run water at a TRICKLE, seriously, a trickle is all that is needed, and do this at fixtures where pipes are run on exterior walls or wind facing walls.
  7. Keep your heat on
  8. Consider installing a smart thermostat, even better, install a smart system/sensor that detects or knows the external temperatures
  9. On #8, if you have remote control, you can remotely increase the heat at needed times, and lower it when the temp is not in the freezing range. A smart system with a sensor is a better way to go.

Keeping the temp @ 10-15 degrees higher than freezing doesn't give you much leeway for the battle between cold and heat both working on opposite sides of a wall. Especially when the outside temp will drop below freezing, removing margin. If the house has good insulation, and pipes are well protected (or exposed to internal environment), then 10-15 above freezing might cut it, but why do this all of the time?

Consider a smart system, and consider programming it to the 50's if you are in a climate that drops 10 degrees below freezing. If you try to push your luck to save a buck, you're likely going to spend more in repairs.

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  • If the weather forecast reports temperature above water freezing degree, is it safe to turn off the house heating system? – Tim Dec 29 '19 at 18:43
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    How often have you found the weather forecast to be accurate? No, it is not safe. You would be removing ALL margin I just got done warning that you need. – noybman Dec 29 '19 at 18:48
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If you're so keen to reduce heating costs, apply heat tape then insulation to all your piping that is anywhere near a cold wall, and then lower your thermostat. The cheap tape is readily available at any hardware store, and is stupid; it just heats the pipe the same at all temps. The good tape is available at industrial supply and is self-regulating, meaning it'll save electricity when not needed, and goes full-bore when in need.

Of course, if you have a power outage, the heat tape will quit.

I myself would install an Empire style furnace with a little mains-powered relay. When power is on (and the heat tape is working), the furnace uses one thermostat set at 40F. When power fails, it uses the other thermostat, set to 75F.

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  • Just a thought, but if the goal is reduce expense, maybe run the water at said pipes. I guess it depends on the OPs utility costs. Electricity in some areas is astronomically expensive compared to gas/fuels, wood/pellets, and others. – noybman Dec 29 '19 at 18:19
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    Yes, leaving the taps on just a dribble can help, but also keep in mind the drains. We had a neighbor in a mobile home that left the taps on, only to come home and find half the house flooded because the sink drain had frozen solid. – spuck Dec 29 '19 at 19:12
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Many points have already been mentioned, here is some supplement:

  1. Emptying metal pipes for a longer time period is a way to destroy them. A much faster corrosion from inside may start, since the residual water can easily react with the exposed metal and the steady delivering of oxygene in the air inside the pipes. The water inside a closed heating system has a very low oxygene level, since after some time all the oxygene is used up thus stopping more corrosion.

  2. A known problem with most thermostat valves of radiators are stuck control plungers, especially if not moved for a long time during summer. Thus one or more rooms may be cut off from the circulating heating water.

  3. Anti-freeze fluid (f.e. as used in cars) may damage the sealings and materials used in domestic heating systems - written statements of the manufacturers (all components) may help. The correct mixture must be maintained all the time, with every topping-up. A low level of anti-freezing may cause more corrosion then normal water. Therefore a return to normal water is very difficult, since many flushing cycles are needed to get all fluid out of the system resp. to clean the system.

  4. Circulating/moving water is less likely to freeze.

  5. Exposed pipes should be considered, f.e. outside taps for gardening.

  6. Hot domestic water pipes are more difficult to protect from freezing since no circulation will take place. Running it on a very low rate may not protect dead zones (which should be anyway avoided for hygienic reasons). Some protection against freezing siphons will be to drop a spoon of food oil into the sinks. But still running water taps can be a risk.

  7. The heating system may fail due to (dependent on type)

    • general failure
    • dirt in the oil tanks or pipes
    • unusual cold oil (needs stronger vacuum to get sucked into the burner -> oil pump failing)
    • dirty safety sensors
    • circuit breakers, lightning strokes
    • birds nest in/on the chimney
    • circulation pump failing
    • etc.
  8. Some manufacturers of central heating systems keep the temperature at 10 degree Celsius (heating water) when in anti-freezing mode.

  9. The circulation pump should be used for some minutes every week even in summer in order to avoid damage to the bearings.

  10. The earth normally stores the summer's heat for some months, in Central Europe the soil in 1-2m depth sees the temperature minimum only in March, temperatures below zero may start in November/December.

  11. Another problem is moisture if a house is on "stand-by" with only minimum heating. The coldest parts and/or wand paper, clothing, leather, books, plastics are the first materials to take damage. A ventilation (timer controlled, ca. 1 hour per day before sun rise) avoids high moisture levels.

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Metallic pipes shouldn't go under 2°, usually the thermostat safety set-point is 6° to compensate for bad heat distribution. Anyway leaving a tap open (say 0,5 L/min) would help because water takes time to freeze having a little flow will keep it moving into pipes at a speed high enough not to let it enough time to freeze.

If you plan to leave your home for a long time would be more convenient to add antifreeze fluid (suggested by your boiler maker) to your radiators' circuit and draining all domestic water piping so even if temperatures plummet well under 0° on the inside, you'll have no issues.

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