Why is my rooftop heater so freaking loud? (Phoenix, Scottsdale: house, heat pump)

[Chrisk-K]

I moved to Scottsdale in August and the rooftop HVAC was new to me. The A/C made a typical compressor noise when cooling, but it wasn't too bad. The rooftop heater makes a much louder compressor-type noise (it sounds like a jet taking off). My place doesn't use gas, so the heater must use electricity. How is it possible for a rooftop heater powered by electricity to make such a loud noise?

[Zippyman]

Originally Posted by Chrisk-K

I moved to Scottsdale in August and the rooftop HVAC was new to me. The A/C made a typical compressor noise when cooling, but it wasn't too bad. The rooftop heater makes a much louder compressor-type noise (it sounds like a jet taking off). My place doesn't use gas, so the heater must use electricity. How is it possible for a rooftop heater powered by electricity to make such a loud noise?

The rooftop “heater” you’re talking about is most likely a “heat pump”.. in the summer, it’s your air-conditioning, and in cold weather, it literally reverses operation & sends heat from the outside air into the house. Same compressor, same coils, the function and refrigerant flow just reverses when you switch to “heating” mode..

Periodically, especially on freezing days, the outdoor coil will build up frost & ice, which causes it to lose efficiency.. the fix for that is that the unit to “reverse” operation in mid cycle.. the compressor will stop heating inside the house and start cooling.. which causes any frost on the outdoor coil to melt. Then it’ll “reverse” again, and resume heating the home. The reverse cycle is typically just a couple of minutes..

The sound “like a jet taking off” is actually very normal when a heat pump reverses. Inexpensive heat pumps (like Goodman units) don’t actually check for frost on the coils before reversing, they just do it based on a timer setting.. once an hour, once every half hour, etc. more expensive brands have more sophisticated sensors, and don’t reverse as often in cold weather.

In Phoenix, where the coldest temperatures are rarely much below freezing, heat pumps don’t typically build up much ice anyway.. (I’ve never seen one build more than a thin layer of frost).. if you were using a heat-pump in the Midwest, you could easily build up inches of ice..

In any case, to add further to the mix - there are two primary types of heat-pump compressors, the “Bristol” compressor and the “scroll” compressor. The Bristol compressor is renowned for screaming like a banshee when it reverses.. The only way I can describe the Bristol reversing is kinda like a metal screwdriver hitting a metal fan at high speed.. The scroll has a less-pronounced, but still noisy reversing noise.

You’ll have neighbors who never hear the reversing from their units, and they’re probably not lying - more expensive units come with a foam compressor “blanket” to reduce noise, the scroll compressor, which doesn’t make as loud of a noise & typically have the more expensive sensors for frost, which means reversing happens less often, if ever.

There’s no fix that’s worthwhile.. I’d be quite reluctant to toss a perfectly functional unit over a normal sound, but when it’s time to replace it, if the sound bothers you, look for a brand that’s more premium, like carrier or trane, ask specifically about compressor type & whether it has a “blanket”, and also if the defrost is set by timer or sensors.. expect to pay a couple grand more than an equivalent Goodman unit, just for that noise benefit..

one more wrinkle is the factory home-builders seem to enjoy putting the condenser/compressor under a bedroom window for convenience (for them).. shifting a condenser/compressor even ten feet further along the house (next to a garage, vs next to a bedroom) is usually not very expensive or difficult at replacement time & makes a huge difference for anyone trying to sleep in a bedroom with a condenser/compressor just outside (winter or summer).

[Chrisk-K]

Thanks a lot!

[Pink Jazz]

Rooftop HVAC units are kind of a throwback to a swamp cooler days, since they were basically a drop-in replacement for swamp coolers which were mostly on rooftops. It is only more recent that split systems are becoming more common. Heat pumps have been popular in new construction homes the 1990s and 2000s, and are still the preferred choice for replacement systems, with some HVAC contractors installing them exclusively. However, since the early 2010s, gas furnaces are making a surprising comeback in new construction homes here in the Phoenix area, since many homeowners value comfort in those occasional days we get those freezes.