Preparing Your Home for Negative Temperatures

When extreme cold weather hits New Hampshire (sustained temperatures at zero or below), it is important to prepare your home, your heating system, and your plumbing. Cold that deep pushes even well-maintained equipment to its limit, but most no-heat calls and frozen pipe emergencies during cold snaps are preventable with a few simple steps in advance.

Use the checklist below to prepare your home for negative temperatures.

Heating system preparation

Run your primary heating system (boiler, furnace, or central heat pump). Even if you also use a wood stove, pellet stove, or mini splits, the central system protects pipes throughout the home that the room-level heat sources cannot reach.
Avoid using mini splits or pellet stoves as the sole heat source during sustained sub-zero stretches. Without periodically cycling the boiler or central furnace, pipes in basements, exterior walls, garages, and crawlspaces are at risk of freezing.
Antifreeze the boiler if you plan to leave it off during extended cold weather and the home will be unoccupied. Have your plumber confirm the antifreeze concentration and replace it every few years.
Ensure heating equipment has been serviced annually. Equipment that has been running well all season is far less likely to fail during a cold snap than equipment that has been marginal.
Avoid large thermostat setbacks, especially with heat pumps. Heat pumps run most efficiently at steady setpoints and pay an efficiency penalty for large recoveries.

Realistic expectations

Most NH homes are designed by code to maintain comfortable indoor temperatures down to roughly 0°F outdoor design temperature. Below 0°F, even a properly sized and maintained heating system may fall behind a few degrees. This is normal. Monitor the indoor temperature and adjust expectations during the worst stretches:

Indoor temperatures may drift down by a few degrees
The system will run continuously or near-continuously
Recovery from setbacks will take longer than usual

Generator cautions

If you may need to run the heating system on a generator during a power outage:

Standby generators with proper voltage regulation can run modern heat pumps and high-efficiency furnaces safely. Most modern Generac and Kohler standby units include good voltage regulation.
Avoid powering heat pumps or high-efficiency furnaces from portable generators with unstable voltage output. We have seen equipment damage caused by generators with poor voltage regulation. If you must use a portable generator, run the heating system only briefly and consider running it on backup heat (electric resistance or fossil-fuel furnace) rather than the heat pump.
Verify with the generator manufacturer that the unit's voltage regulation is suitable for sensitive HVAC equipment before relying on it.

Vent and intake care

Clear all air intakes and exhaust vents on furnaces, boilers, water heaters, ERVs/HRVs, dryers, and pellet stoves. Snow drifts and ice buildup can block these and cause shutdowns or safety issues.
For high-efficiency equipment with PVC sidewall venting, check after each storm.
Keep heat pump outdoor units clear with 18 to 24 inches of clearance on all sides. Brush snow gently; do not chip ice.

Plumbing preparation

Shut off outdoor water supplies at indoor shut-off valves if you have them, and drain outdoor faucets and sillcocks.
Open cabinet doors under sinks on exterior walls to let warm air reach the plumbing.
Let cold-vulnerable faucets drip slowly during the worst overnight cold. Moving water is harder to freeze.
Know where your main water shut-off is in case a pipe bursts.

Safety preparation

Test smoke and CO detectors. Replace batteries before the heating season if you have not.
Install CO detectors on every level of the home and within 10 to 15 feet of every sleeping area.
If you smell gas or hear a hissing sound near a propane regulator or gas appliance, leave the home immediately and call 911 from outside.

If the heat goes out

Open faucets slightly on exterior-wall fixtures to keep water moving (slow drips are enough). Flowing water takes much longer to freeze than standing water.
Close blinds and shades to slow heat loss through windows.
Minimize door entries and exits to retain indoor heat.
Concentrate in one room with the most insulation, fewest exterior walls, and any available heat source.
If the outage will extend more than a few hours, consider shutting off the main water supply and draining the lines to prevent burst pipes when the heat returns.
Call A.J. LeBlanc Heating at 603-623-0412 for 24/7 emergency service.

Schedule a winter readiness check

If you have not had your heating system serviced in over a year, or you want a plumber to walk through vulnerable plumbing before the next cold snap, contact A.J. LeBlanc Heating. Serving New Hampshire families since 1928.

Frequently Asked Questions

What is the lowest outdoor temperature my NH heating system was designed for?

Most NH homes are designed by code to maintain comfortable indoor temperatures down to roughly 0°F outdoor design temperature. Below that, even properly sized systems may fall behind a few degrees.

Can I run my heat pump on a generator?

Standby generators with proper voltage regulation can run heat pumps safely. Portable generators with unstable voltage output have been known to damage heat pumps and high-efficiency furnaces. Verify with the generator manufacturer before relying on it.

Should I shut off my boiler during extreme cold?

No. The boiler protects pipes throughout the home, including areas no other heat source reaches. Keep it running, set the thermostat to a steady temperature, and avoid large setbacks during sub-zero stretches.

What is the lowest safe indoor temperature in winter?

60°F. Below that, frozen pipe risk increases dramatically. Avoid setting the thermostat below 60°F even when away during cold weather.

What should I do if my heat goes out during a cold snap?

Open faucets to a slow trickle on exterior-wall fixtures, close blinds and shades, concentrate in one well-insulated room, and call for emergency HVAC service. If the outage will be extended, consider shutting off the main water supply and draining lines.