Mountain Ridge Heating and Gas | Advice: No Heat? Strange Noises from your Furnace?

06 Dec Advice: No Heat? Strange Noises from your Furnace?

Modern appliances can be complex. Electric motors, belts, fans, and even electrical components such as transformers and relays make noise when energized. New central heating appliances go through a “start-up sequence” which may include:

Control board does a safety check (limit switches, roll out switches).
Inducer motor comes on.
Pressure switch must close to go to the next step.
Hot surface ignitor begins getting hot / direct spark ignitor begins to spark.
Gas Valve opens.
Furnace Lights.
Flame Sensor senses flame
30-seconds to 1-minute: blower motor comes on.
Once the thermostat is satisfied, the burner shuts off.

Of that startup sequence there are a number of things that will make some sort of “sound”, the first being the inducer fan.

When the fan is new it should be nice and quiet with minimal fan and motor noise. As the “sealed” bearings endure the heat of the exhaust and byproducts of combustion, they eventually start to dry out. Shafts and shims also start to wear and we eventually develop a poor fitment which only compounds the problem, resulting in a rattling and unbalanced sound – the sound of end of life. Once it fails the startup sequence will also fail, and the furnace will not ignite.

Pressure switches will also make tiny clicking noises as they open and close but usually barely audible over the inducer sound.

There will be a click or two when the relay energizes the hot surface ignitor, followed by another click as the gas valve opens. The sound of gas flowing from the manifold, through the burners, and mixing with air for combustion will follow.

After 30 seconds or so the blower motor will typically spin up to distribute the heat through the home. This is usually the next noise that will draw attention. A blower motor can really make some noise if the bearings are failing, or if the blower wheel itself is unbalanced from damage, foreign materials, or even build-up of dust and dirt.

Lastly, the ductwork that distributes air throughout the home can often make banging noises from either the increase or decrease in static pressure within. This is often described as the “oil canning” effect.

Knowing the startup sequence is a great way to narrow down the source of an issue with your central heating appliance.