Comprehensive Heat Pump Noise Reduction
The topic is a recurring one, so much so that I'm surprised there isn't a sticky thread on it. There are bits and pieces of wisdom and anecdotal advice scattered throughout the forum, but nothing that pulls it together. Perhaps (wishful thinking?) I can draw out a comprehensive set of recommendations and/or explanations as to what causes Heatpump defrost noises and what you can reasonably ask your contractor to do about them.
Let's try to keep this thread just to what causes and can be done about air-to-air heat pump defrost noise and not digress into whether people should use ground source or other non air-to-air HP solutions. Assume the person reading this has a heat pump, can't reasonably replace it right now, and is looking to understand whether the situation is hopeless or whether they should pursue their contractor to help find some relief.
Based on what I've seen other people ask, many scroll compressor heat pumps make a lot of noise when they go into the defrost cycle that removes ice buildup. Some do it based on a clock cycle, others have a smarter demand based algorithm that only defrosts when actually needed. But nearly all have the noises to one degree or another.
These noises are different than normal operational noises (Thrum, thrum of the compressor or fan noises). The defrost noises fall into three categories:
1: The Flush - sounds like a toilet flushing or a valve opening - usually not too loud.
2: The Moose Call - One long, low pitched mooing sound, or for my system, something like a semi-truck slamming on its brakes and skidding on a wet surface.
3: The Jackhammer - This may go on for 2 to 20 seconds after the Moose call. It may taper off until it goes away. For my system it sounds like the 20 gallon compressor I use to run my air tools.
(There is often also a second flush when the defrost is over, and a bit of a groan as the heat pump starts back into heat mode, but it is generally pretty quite and few people seem to complain about it.)
So the first question for the HVAC experts on this site: What causes each of these sounds?
Yes, I know each situation is different and your mileage may vary, but what is most common?
I've seen all sorts of causes given:
- Refrigerant backs up into the compressor diluting the oil in the compressor.
- The reversing valve gets stuck and pressurized refrigerant pounds in the line until it opens up.
- The reversing valve gets caught halfway and jackhammers open and closed.
(note - to me a lot of the sounds seem like sheet metal vibration set off by something in the unit, but generally I can't see where it would be coming from)
If I understood what was causing each I could better discuss them with my contractor.
And the second question: What can be done about each?
More specifically, what can I ask my contractor about having them investigate and install?
It is interesting that some high end brands like the Lennox X-21 seem to go through cycles without a peep - what must be different about them (they all use the same scroll compressors don't they?) and can any of those features be added aftermarket to someone's existing heatpump?
So far I have seen the following solutions recommended, but I have little idea about what each really addresses:
Compressor noise blanket: OK, so I guess I understand that one
Crankcase heater: What does this do and why would it help?
Muffler: Again - what does it do an why would it help?
Replace the reversing valve (are some better than others?)
Defrost Delay: Some compressors have a delay between shutdown and defrost to give the reverse valve time to equalize?
I know these are all questions that have been running through my mind for the last few months since the weather turned cool, and I'd be willing to bet dollars to donuts that a lot of other visitors are trying to figure out the same things. It's hard to get any kind of feel for which direction to go on the topic with so many bits and pieces in threads here and there - I hope putting it all in one thread will be helpful to others too.