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I'm in Seattle and considering to replace my old single stage gas furnace (88k BTU output) with either these two options.

  1. Heat pump + gas furnace hybrid
  2. AC + gas furnace

The primary concern is about the noise from the return air vent as it's fairly loud already. Some of your may suggest a larger return vent and properly sized ducts, which I completely agree with, but just for the sake of discussion and for my understanding of the workings of the heat pump, let's assume no changes to the existing duct and return vents, I wonder if moving to option 1 would make the noise (only consider the return air vent noise) worse as the heat pump will be running longer period of time?

Let's use an example from the past data from my Nest: for a 6-hour window at about 37F outside and 68F indoor, the old gas furnace was on for 50% of the time. If I had a 3T cold climate heat pump (looking at Bryant 38MURA High Heat currently) producing ~40k BTU (I was looking at this data sheet https://www.shareddocs.com/hvac/docs/1009/Public/0A/38MURA-01PD.pdf, page 56 assuming lowest CFM at 37F, it's 38596 BTU. Let me know if I used the wrong data, but for this hypothetically run time calculation purpose, it doesn't really matter and let's still assume the heat pump I got can produce 40k BTU at that temperature), does that mean it will be on at 100% power for the entire 6 hours?

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  • Heat pumps/mini splits should work the same basic way as any other heat/cooling systems. On/off as needed. Most of today's heat pumps should work from 0F to -20F, so extra heat source might not be needed, but should have a backup plan for power outages. Generator or a heat source not needing electricity.
    – crip659
    Mar 13 at 19:20
  • Variable speed blower?
    – Huesmann
    Mar 14 at 12:49

3 Answers 3

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Which type of heat pump are you looking at?

With my minisplits, the loudest part is periodically running the condensate pump (which many/most installations don't need), and an occasional low "gasp" when it opens and closes valves at the start of a heating or cooling cycle. Other than that, the main moving part is inside the house are a wide squirrel-cage fan to move air over the heat exchange fins and occasionally moving the direction vanes, both of which are very quiet.

Admittedly the outside unit is as noisy as a good-sized air conditioner. That's still not bad, and mostly inaudible inside the house.

If you are using a heat pump to drive an existing forced-air system, in the other hand, I suspect it won't change the sound level much; it'll just run more efficiently. And a single mechanism will handle both heating and cooling the air.

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Noise at the air return is related to the blower, not the heat fuel source. Blower noise, in turn, is related to the CFM capacity of the blower. A system with a smaller heating or cooling capacity would be equipped with a lower-capacity and quieter blower.

As a first-order approximation, yes, if you cut the heating capacity in half then the duty cycle will increase by about double. Noise would be "worse" by the measure of operating time, though like I said, it would be somewhat quieter.

There are other issues at play, however. A duct system requires a certain amount of CFM and static pressure in order to push air through it at an appropriate velocity and achieve good mixing of the air in a room. It could be a mistake to down-size 50% without consideration of how the duct system will perform with that change.

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  • To be fair, low velocity in an airway is not generally an issue in a residential environment Mar 14 at 1:49
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My heat pump is outside the house. It connects to the heat exchanger over the gas furnace by pipes containing something like the water/anti-freeze mixture used in a car radiator (note: probably not the same as a car radiator, only similar).

The heat pump does produce noise when operating, just like central A/C would, but it produces that noise outside the house. It will not make your ductwork fan noise inside the house louder. Inside it's just liquid pumped around.

My fans are actually quieter with the heat pump, as they run more slowly. The gas furnace is also lower powered, not requiring as much fan noise. It has a higher setting (second stage), which it normally doesn't need to use. The higher setting is no louder than the old furnace.

The heat pump runs for longer periods of time because it does not move as much heat per minute. If you replace the furnace at the same time as you add the heat pump, it will come with a new, quieter fan that can run slower than the old fan (at least while the heat pump is operating).

I actually use less electricity with heat pump (electric) plus the new fan than with the old gas furnace and fan. I.e. the fan is enough more efficient to power the heat pump. My experience has been that the fan is considerably quieter to the point that I can feel the heat before I hear the fan.

You would have to be much more sensitive to white noise than I am to find this to be worse. Yes, the fan runs longer, but it is much quieter with the heat pump than with the old furnace.

Note: this is heavily weighted by my personal experience. I was replacing a thirty year old furnace with furnace plus heat pump (and I did it in summer for cooling purposes). I obviously know nothing about your setup nor am I familiar with the new furnace/heat pump that you plan on adding. But my personal experience was that both noise and electrical efficiency improved. I find it very unlikely that you will get worse noise even if your old furnace was already more efficient than my old furnace.

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