Gas Furnace is Making Too Much Noise – The FixModulating Gas Furnaces . Gas furnace manufacturers have for many years been working on modulating furnaces to make furnaces more efficient. Before modulating furnaces, the highest rated (in efficiency) were two-stage gas furnaces. Then came the modulating furnaces. Efficiency ranges started increasing to 98% and fractionally more than 98% for some modulating gas furnaces. Always check the AHRI website for certified efficiency of any HVAC product you purchase. What makes this furnace different than the other types of gas furnaces you can purchase?

Modulating gas furnaces are also condensing gas furnaces. All natural gas and propane gas has a small percentage of moisture in the gas. When the combustion process takes place the moisture in the gas turns into steam. A condensing gas furnace has two heat exchangers. In the secondary heat exchanger any residual heat is absorbed into the exchanger and the air passing around the exchanger. This maximizes the amount of heat used by the furnace. It also makes it far more efficient than a non-condensing gas furnace. That moisture is then drained down the same drain the air conditioner uses. The moisture produced by the evaporator coil.

Modulating Gas Furnaces

A modulating furnace will operate incrementally in a range from 40% all the way up to 100%. The increments for some manufacturers is 1% which offers a good demand based furnace. This range depends on demand. Therefore, for the modulating furnace to operate properly the controls need to be set up properly. In many cases, the controls including the thermostat are proprietary. You will need the manufacturer of the furnace to provide the thermostat. Other gas furnaces such as the two-stage gas furnace only offers two stages. Consequently, they stage from 50% and 100% depending on the manufacturer and furnace set up.

Modulating Gas Furnaces - condensing gas furnace
Some manufacturers will program their modulating furnace to operate on a conventional single-stage or two-stage thermostat. In that case, rather than getting the furnace to modulate incrementally, the furnace will stage. It will stage to certain percentages based on the type of thermostat used. A single-stage thermostat will offer less staging than a two-stage thermostat. In either case, it allows you to have a more efficient furnace. More efficient than just a single-stage or even a two-stage furnace.


The two-stage gas furnace is more efficient than the single stage gas furnace which only operates at 100%. The cost also varies from furnace to furnace with the modulating gas furnace costing more than the other two. In many cases a lot more so it is important to compare the cost. The cost of the furnace plus operating expenses and figure out the payback or ROI for buying efficient. Living in the house for a long period of time? Then it is better in most cases to purchase the most efficient.

However, if you sell the home a new efficient HVAC system will add value to the home. It will definitely be a plus for the buyer and possibly help you sell faster for a better price.

Use our handy calculator to see how many BTU's your natural gas appliances are producing:


When you purchase expensive equipment it is important that a certified HVAC contractor install the equipment. The higher end HVAC equipment usually comes with bells and whistles. The final start-up is important so that you get the proper rated efficiency out of the equipment. Additionally, I always recommend that you purchase the extended warranty that includes labor. The parts are expensive and the troubleshooting can often take longer because these systems are complex.

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Modulating Furnaces - Gas Furnace Purchase Options