I've searched the forum and can't find anyone else posing this question, so here goes:
Toilet Flush
Moose Call
Jackhammer
After a couple months of frustration with the noises inherent in the defrost cycle of my Goodman/Amana scroll compressor HeatPump, I got to thinking about why it works the way it does. It's not just Goodman - from all I've read, all the scroll compressor systems defrost the same way. Regardless of whether it is a smart on-demand system (like many more expensive systems use) or a basic time-based schedule, it still works the same - trigger the reversing valve and send the system into a psuedo Air Conditioner mode with the compressor fan blades turned off (it's this process that creates all the noise).
And to offset the fact that it is now effectively sending cold refrigerant to the indoor air handler, the electric coils kick in until the defrost cycle is off. Nice idea - using the inherent heat-transfer capabilities of a heat-pump system to warm the outdoor coils until any ice melts off them. And of course heat pumps are efficient so it must be the most efficient way to warm up those exterior coils, right?
But if you think about it, that heat is coming from the house itself - the return ducts are supplying that heat. In fact, since the coils in the air handler are now cold, the resistive electric heat system has to be engaged to offset that cold (5K to 15K depending on the installation). So during defrost time you are engaging the most-expensive heat-source (resistive electric heat) to blow air through the house that may or may not actually be colder than the ambient indoor temp.
Why oh why aren't the external coils manufactured with a resistive heat element built in? An electric heating wire could be bonded to the tubing as it is extruded during manufacture, before the aluminum fins are affixed. When a defrost is called for, the compressor and air handler would shut down, and the resistive wire would be engaged heating the external tubing directly (and quietly). It would probably also shorten the defrost cycle since restive heat comes on almost instantaneously and could melt any frost more quickly, resulting in less time the system is not in operation.
Now, it might be necessary to involve the reversing valve in some way to be sure the heat-created pressures in the external coil can be equalized, and I'm sure there are other caveats that would have to be addressed. I'd imagine that it was these issues, if anything that have kept manufacturers from pursuing this in the past.
But it can't be the electricity use that justifies the present design. A resistive heating wire on the exterior coils can't require much more electricity than running the resistive electric coils in the air handler - and since the compressor and fans aren't running it should actually take less electricity to complete a defrost cycle. Heck, even the heat created by the exterior resistive wire wouldn't be lost - much of it would be recovered when the compressor comes back on to start generating heat again. And since it might be practical to run defrost more often (multiple short defrosts instead of one long defrost every 60 or 90 minutes), the HP would be operating without frost blockage (and at higher efficiency) most of the time.
There's also the support aspect of it. How many service calls have you had to make (hopefully reimbursed by the manufacturer) to address defrost-originated issues? Reversing valve failures, oil contamination due to failed reversing valves, or even fully failing compressors due to the excessive shuddering shaking and squealing of the enclosures and fittings?
And let's not forget service calls for noise. Hard feelings from customers when you have to tell them "Get used to it, there's nothing I can do." Lost customers. Lost customers who speak poorly of you to their friends.
The systems you HVAC guys install and service are way more complicated than my seat-of-the-pants physics knowledge can fully understand. But I have to wonder if there aren't some simple parts of the systems that manufacturers haven't tackled because they're too focused on improving the efficiency of the complicated parts (DC motors, etc).
Maybe going back to something so simple as resistive wire on the external unit is beneath them. Or maybe there really is proof that it uses significantly more electricity. Or maybe the issues surrounding the pressures and lubrication etc are too difficult to manage (although with modern sensors, actuators and systems logic I find that hard to believe). Or maybe they don't realize how disruptive the current approach is.
But coming from a homeowner who is tired of waking up every morning at 4:30ish do the sounds of:
Toilet Flush
Moose Call
Jackhammer
(In that order)
I have to believe there is a relatively low cost way to design a system that isn't so disruptive.
Toilet Flush
Moose Call
Jackhammer
After a couple months of frustration with the noises inherent in the defrost cycle of my Goodman/Amana scroll compressor HeatPump, I got to thinking about why it works the way it does. It's not just Goodman - from all I've read, all the scroll compressor systems defrost the same way. Regardless of whether it is a smart on-demand system (like many more expensive systems use) or a basic time-based schedule, it still works the same - trigger the reversing valve and send the system into a psuedo Air Conditioner mode with the compressor fan blades turned off (it's this process that creates all the noise).
And to offset the fact that it is now effectively sending cold refrigerant to the indoor air handler, the electric coils kick in until the defrost cycle is off. Nice idea - using the inherent heat-transfer capabilities of a heat-pump system to warm the outdoor coils until any ice melts off them. And of course heat pumps are efficient so it must be the most efficient way to warm up those exterior coils, right?
But if you think about it, that heat is coming from the house itself - the return ducts are supplying that heat. In fact, since the coils in the air handler are now cold, the resistive electric heat system has to be engaged to offset that cold (5K to 15K depending on the installation). So during defrost time you are engaging the most-expensive heat-source (resistive electric heat) to blow air through the house that may or may not actually be colder than the ambient indoor temp.
Why oh why aren't the external coils manufactured with a resistive heat element built in? An electric heating wire could be bonded to the tubing as it is extruded during manufacture, before the aluminum fins are affixed. When a defrost is called for, the compressor and air handler would shut down, and the resistive wire would be engaged heating the external tubing directly (and quietly). It would probably also shorten the defrost cycle since restive heat comes on almost instantaneously and could melt any frost more quickly, resulting in less time the system is not in operation.
Now, it might be necessary to involve the reversing valve in some way to be sure the heat-created pressures in the external coil can be equalized, and I'm sure there are other caveats that would have to be addressed. I'd imagine that it was these issues, if anything that have kept manufacturers from pursuing this in the past.
But it can't be the electricity use that justifies the present design. A resistive heating wire on the exterior coils can't require much more electricity than running the resistive electric coils in the air handler - and since the compressor and fans aren't running it should actually take less electricity to complete a defrost cycle. Heck, even the heat created by the exterior resistive wire wouldn't be lost - much of it would be recovered when the compressor comes back on to start generating heat again. And since it might be practical to run defrost more often (multiple short defrosts instead of one long defrost every 60 or 90 minutes), the HP would be operating without frost blockage (and at higher efficiency) most of the time.
There's also the support aspect of it. How many service calls have you had to make (hopefully reimbursed by the manufacturer) to address defrost-originated issues? Reversing valve failures, oil contamination due to failed reversing valves, or even fully failing compressors due to the excessive shuddering shaking and squealing of the enclosures and fittings?
And let's not forget service calls for noise. Hard feelings from customers when you have to tell them "Get used to it, there's nothing I can do." Lost customers. Lost customers who speak poorly of you to their friends.
The systems you HVAC guys install and service are way more complicated than my seat-of-the-pants physics knowledge can fully understand. But I have to wonder if there aren't some simple parts of the systems that manufacturers haven't tackled because they're too focused on improving the efficiency of the complicated parts (DC motors, etc).
Maybe going back to something so simple as resistive wire on the external unit is beneath them. Or maybe there really is proof that it uses significantly more electricity. Or maybe the issues surrounding the pressures and lubrication etc are too difficult to manage (although with modern sensors, actuators and systems logic I find that hard to believe). Or maybe they don't realize how disruptive the current approach is.
But coming from a homeowner who is tired of waking up every morning at 4:30ish do the sounds of:
Toilet Flush
Moose Call
Jackhammer
(In that order)
I have to believe there is a relatively low cost way to design a system that isn't so disruptive.
