Furnace Tries To Start Then Shuts Down – What To Do?

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A furnace keeps your home heated by warming air and distributing it through the ductwork. So, it can be very frustrating if the furnace starts and then shuts down in the dead of winter. But how can you resolve short cycling? We studied the topic, and these are our findings.

If your furnace starts and then shuts down, take the following measures to rectify that.

  • Clean or replace the air filter
  • Inspect the thermostat
  • Check for air leaks
  • Call an HVAC technician

Read on to discover why your furnace short-cycles and how to address that. We will also discuss maintenance tips for the furnace and whether furnace size matters.

Heating Central Gas Furnace Issue. Technician Trying To Fix the Problem with the Residential Heating Equipment, Furnace Tries To Start Then Shuts Down - What To Do?

Why does my furnace keep short cycling?

A person changing an clean air filter on a high efficiency furnace

The furnace is fitted with a thermostat that detects changes in indoor temperatures. When these temperatures fall below the pre-determined level, the furnace comes on to warm your home and then turns off.

The furnace should cycle three to eight times per hour while running for 7 to 20 minutes per cycle. But attributes such as the condition and age of your furnace, outdoor temperatures, and your home's characteristics and insulation influence this frequency. 

If a furnace short-cycles, it keeps switching on and off without reaching the desired temperature. Short cycling leaves your family cold and uncomfortable. It also poses the risk of causing irreparable damage to your furnace. For this reason, take measures to resolve the issue as soon as possible.

Causes of short cycling

Restricted airflow

The furnace draws air from home, heats it, and then cycles it back to the house. When air circulation in the furnace is constrained, hot air gets trapped inside the furnace. The hot air can cause the unit to overheat, resulting in frequent cycles where the furnace starts and stops.

Blocked exhaust vents, dirty air filters, or clogged interior vents are often the leading causes of heat back up in the furnace. 

Poor insulation

Although all homes lose some degree of the generated heat, most of the heated air is quickly lost when the house is poorly insulated, causing the furnace to play catch up. Heat loss results in the furnace frequently starting and stopping.

When the windows are left open or are poorly insulated, they create an avenue for the hot air to escape. Room temperatures then fall below the pre-determined level, triggering the furnace to start running. 

Defective Thermostat

The thermostat is critical in the furnace cycle in that it determines when the furnace starts or stops running based on the indoor temperature levels. If it is defective, it can alert the furnace to shut off prematurely.

Also, positioning the thermostat near a source of heat or in the line of direct sunlight can cause it to trigger false furnace shut down actions.

Dirty flame sensor

The flame sensor regulates gas flow into the furnace and automatically shuts off the gas valve if the furnace does not light up. This can result in the furnace starting and shutting down repeatedly. 

How to resolve short cycling

Clean or replace the air filters

New clean filter is being inserted into a home furnace

A clean filter ensures that your furnace receives enough air to run efficiently. Also, when the exhaust and interior vents are clear of objects, air flows freely. Proper air circulation keeps the unit from overheating, preventing short cycling.

Change your filters every 30 to 90 days during the heating season to keep your unit in optimal functioning condition.

 Adequately insulate your home

Minimize air leaks from your home by caulking around window frames and properly insulating crawl spaces and attics. Insulation keeps the heated air indoors, enabling your furnace to maintain the desired temperature level in your home without repeated cycling.

Also, ensure that you close your windows when running the furnace to reduce heat loss.

Probe the thermostat

Begin by checking if your thermostat is strategically positioned away from direct sunlight or other heat sources. When you place the thermostat in a place with the slightest temperature fluctuations, it accurately detects your home's temperature levels and triggers the correct furnace response.

You can also change the thermostat batteries and observe whether short-cycling is resolved. 

Call an HVAC technician

It would be best to ask an HVAC professional to check your thermostat's wiring if you suspect it is faulty. If the thermostat is in optimal working condition, the professional can rule it out and investigate other potential causes for short cycling.

It is also advisable to ask a technical expert to inspect and clean your furnace's flame sensor. Examining the flame sensor involves working with dangerous flammable gases and electricity. When the gases or electricity are mishandled, you may hurt yourself.

Why does furnace size matter?

A home high energy efficient furnace in a basement

Furnaces use a lot of energy when they start running and achieve optimum efficiency when indoor temperatures reach a steady, consistent temperature. Notably, the furnace shuts down when a stable temperature is reached.

If the furnace is undersized, it will run consistently to compensate for its small capacity when heating your home. The small furnace may sometimes fail to heat indoor air to the desired level. Moreover, when the furnace is overworking, it can break down frequently.

If the furnace is oversize, it will short-cycle and turn off before it attains a steady temperature. Installing an oversize furnace wastes energy and reduces its useful life due to increased wear and tear from the frequent short cycling. You also have some hot and cold patches in the home.

Right size furnace

The size of a furnace is determined by how much heat it can generate in an hour, measured in British Thermal Units [BTUs]. The higher the BTU rating, the more heat a furnace can produce.

When you install the right furnace size in your home, it gradually warms the air and is energy-efficient. The furnace ensures that all the rooms in your home retain a stable temperature. In addition, the unit serves you longer because it has sufficient capacity to warm your home without straining.

It is also advisable to consider the furnace efficiency rating as this indicates how effective the furnace is in converting air to heat. Most furnaces have an 80% efficiency rating which means that the heat output is 80% of its input.

Some more expensive models have a 93% energy efficiency rating, with the most costly models nearing 97% efficiency. Take note that the unit's efficiency reduces over time.

Calculating the right furnace size

Factors such as your home's size and the zone where you live determine what furnace size is appropriate for your heating needs.

1. Determine your home's square footage.

Calculate the area of each room, then add all the areas to get the total square footage. Consider the shape of the room when calculating its area. For rectangular rooms, multiply the length by the width. If you have triangular rooms, multiply the length and width, then divide the product by two.

If some rooms are unusually shaped, partition the room into smaller sections and calculate the area of each section. You can then sum up all the areas to get the room's total square footage.

2. Determine your climatic zone

Typically, the distance from the equator influences how many BTUs you require to heat a square foot in your home. The more the distance from the equator, the more units you will need. But geographical factors such as ocean currents and altitude cause a variance in the BTU requirements at a given latitude.

Homes in climate zone 1 require 30-35 BTU per square foot, while those in climate zone 2 need 35-40 BTU per square foot. The heating requirements for zones 3, 4, and 5 are 40-45, 45-50, and 50-60 respectively. 

It is recommended that you use the lower of these numbers if your house is well insulated. Insulation provides resistance to heat flow, thus minimizing the quantity of heated air that escapes. A lower BTU would reduce your heating costs while maintaining the furnace's efficiency.

3. Estimate the furnace size

To estimate a close enough furnace size, multiply your home's total square footage with the recommended BTU requirements per square foot. For instance, if you determine that you live in an 1800 Sq. Ft home in climate zone 2, you would require a furnace with 63000 [1800*35 = 63000] BTU.

What maintenance does a furnace require?

Soft Focus to Filter of Air handing Unit, Technician checking a Pre-filter of air handling unit for replacement a new filter - HVAC maintenance

Regular maintenance keeps your furnace running efficiently, thus reducing downtime and conserving energy. Servicing your furnace also enables you to catch and address defects before they escalate.

Clean the heating system vents

Clean the vents at the start of the heating season to keep them accessible. Use a vacuum cleaner to keep the vents and the surrounding area free of debris.

Clean the blower motor

Because the blower is located next to the air filter, it gathers dust and debris that penetrate through the air filter. When the blower motor is kept clean, it can sufficiently power the fan, providing air movement for the furnace.

Conduct a home energy audit

Protect your unit from overworking by reducing your home's heating load. Conducting a home energy audit helps you identify the significant ways heat escapes from your home. The heating professional can then help you determine what improvements you need to make to reduce heat loss.

Final thoughts

Boiler and pipes of the heating system of a house, cover removed for furnace reapirs

Carrying out some TLC on your furnace, such as changing the filters or strategically positioning the thermostat, can restore the efficiency of your furnace, reducing constant cycling. At other times, say when the flame sensor is defective, you may need to consult an HVAC expert to resolve the issue for you.

It is also advisable to install the right furnace size since size also affects how many times your furnace cycles. When choosing the furnace, consider both energy efficiency and BTU rating.

You may also be curious about these topics:

Does A Furnace Room Need A Louvered Vented Door?

How Far Should A Thermostat Be From Return Air Or Furnace?

This Post Has One Comment

  1. Erwin Osborne

    “The most common causes of a furnace that won’t start are either the thermostat or dirty filters. Causes for short cycling and how to resolve this issue are given in the article.

    Calling professional maintenance for any furnace issue will help in the proper repair of the system, Thank you for sharing the article.”

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