HP Defrost Alternatives

[Tom Campbell]

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.

[hvacvegas]

Yep. It's called an air conditioner, and a gas furnace.

[Swampfox]

Newer Goodmans have an updated board that will quiet the defrost cycle somewhat, mainly the "moose call and jackhammer" portion, it does so by shutting the compressor down when the reversing valve shifts

part # is PCBDM133S

[beenthere]

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

Unless that heat wire was wrapped around the tubes of the coil, it would cost a lot more. And it would also do a very poor job of defrosting.

- 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.

Then when one of those heat wires shorted out to the coil and blows a whole in the coil losing the entire charge. The cost of replacing the coil, if not the whole condenser would far out weigh any savings it might have given.

Not to mention the neat fire it could cause at your outdoor unit, and burn your house down.

[hvacvegas]

I don't think you can put that board is ASZC's. Just ASZ's, since on the ASZC's everything is controlled by that massive board. No contactor, or relays.

[ascj]

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.

Noise is dependent of equipment and install. But the bulk of your post was why do they use a reverse cycle over electric defrost. Reverse cycle defrost is very similar to hot gas defrost. If you ever dealt with refrigeration, you would know hot gas defrost can be 50-70% move efficient. Why? Faster defrost!

[Swampfox]

The 14 SEER and up all have them now

[SoFlaDave]

Not a terrible idea. I think it comes down to a few issues though.

1:to be effective the element would need to be fairly hefty to carry enough current through such a long circuit. Defrost on commercial freezers is done this way and the element is about 1/4" diameter and thats for a very small amount of surface area. Now you have the load from the back up heat AND the defrost heat,which could be fairly comparable.

2: it would be very impractical from a repair standpoint. A bad element thats weaved throughout the coil could mean that the entire coil would need replacing.

I think the best solution comes down to proper noise reduction techniques. Line mufflers and things of that nature which could potentially drive up the cost of the equipment.

[Tom Campbell]

Newer Goodmans have an updated board that will quiet the defrost cycle somewhat, mainly the "moose call and jackhammer" portion, it does so by shutting the compressor down when the reversing valve shifts

part # is PCBDM133S

As HVACVEGAS points out, that option may only be possible with the non-comfortnet units. But based on the thread here: http://hvac-talk.com/vbb/showthread.php?t=709431 a number of members (and my local installer too) seem to feel the 30 second won't really make a difference. Have you had any good experiences where it made a difference? That would be reason enough for me to spend the $ or so to buy the CN thermostat.

(I don't want to dilute this thread with my own situation any more, so please make any replies in http://hvac-talk.com/vbb/showthread.php?t=709431 )

[Tom Campbell]

Noise is dependent of equipment and install. But the bulk of your post was why do they use a reverse cycle over electric defrost. Reverse cycle defrost is very similar to hot gas defrost. If you ever dealt with refrigeration, you would know hot gas defrost can be 50-70% move efficient. Why? Faster defrost!

So the reverse cycle allows the system to draw more heat from the air-handler coil and the return duct air passing over it than could be generated with a resistive coil on the external coil?

I can see that as plausible, because when in defrost, even with the heat strips on, the air coming out the ducts feels colder than the air doing into the returns: eg more than 14Kw's worth of heat is being extracted from the house to warm the external coils. (My coils are 14K obviously)

[ascj]

So the reverse cycle allows the system to draw more heat from the air-handler coil and the return duct air passing over it than could be generated with a resistive coil on the external coil?

I can see that as plausible, because when in defrost, even with the heat strips on, the air coming out the ducts feels colder than the air doing into the returns: eg more than 14Kw's worth of heat is being extracted from the house to warm the external coils. (My coils are 14K obviously)

Why are we using heat pumps vs electric heat? Because of COP. Wouldn't it work backwards for a defrost cycle?

[Tom Campbell]

Not a terrible idea. I think it comes down to a few issues though.

1:to be effective the element would need to be fairly hefty to carry enough current through such a long circuit. Defrost on commercial freezers is done this way and the element is about 1/4" diameter and thats for a very small amount of surface area. Now you have the load from the back up heat AND the defrost heat,which could be fairly comparable.



I agree the load of both indoor and outdoor resistive heating units might be more than a 100 AMP service house could handle. Without the reverse cycle "chilling" effects on the inside coils though, the inside coils would not be a requirement to keep from blowing cold air into the house. During the defrost cycle the inside air handler could simply shut off, or keep running in circulatory mode. In circulatory mode it would be no colder than defrost with simultaneous resistive heat is now anyway. There would need to be some sort of lockout though, so both could not come on at the same time.




2: it would be very impractical from a repair standpoint. A bad element thats weaved throughout the coil could mean that the entire coil would need replacing.



Since aluminum is second only to copper (and other rarer metals) in conductivity, would it be possible to manufacture a 4-panel set of resistive meshes that affix to the outside edge of the aluminum fins - between the housing and the fins (or maybe it's better on the inside fins), with some sort of high-conductivity adhesive? Field replaceable?

I'm guessing the efficiency of such an approach would be even worse - but I have to wonder if any of the manufacturers is even experimenting with ideas like this to see if they can be made effective?




I think the best solution comes down to proper noise reduction techniques. Line mufflers and things of that nature which could potentially drive up the cost of the equipment.

I may start another thread on this aspect - I've seen many posts about line mufflers, crankcase heaters, etc. etc. I'd really like to understand where each comes in to play (and I'd bet there are other interested homeowners who wish they could ask intelligent questions about them to their contractors as well). But I'll save that question for a dedicated thread.

[Tom Campbell]

Why are we using heat pumps vs electric heat? Because of COP. Wouldn't it work backwards for a defrost cycle?

Heat pumps COP comes from the fact that we are extracting heat from the outside air, not that we are creating more heat from a Kw of electricity.

The heat used to defrost the external coils is coming from the pre-heated air inside the house. The heat taken from the house will have to be put back in to the house some way.

[ascj]

Heat pumps COP comes from the fact that we are extracting heat from the outside air, not that we are creating more heat from a Kw of electricity.

The heat used to defrost the external coils is coming from the pre-heated air inside the house. The heat taken from the house will have to be put back in to the house some way.

I agree with you.

I'm no engineer, so I try to think simple. COP is high in heat mode, because you add the compressors work to the added heat gain to the space. That would also, work in adding heat while reversing the flow to defrost a coil....no?

[SoFlaDave]

If i recall, heat of compression adds about 20% of heat output.

[mgenius33]

If you want to stick with a heat pump and don't want to have the inefficiency of resistive heat or annoyance of defrost cycles, simply drill some wells and install a geo thermal system....

Problem solved!

[Tom Campbell]

If you want to stick with a heat pump and don't want to have the inefficiency of resistive heat or annoyance of defrost cycles, simply drill some wells and install a geo thermal system....

Problem solved!

For people who can, I highly recommend it. I don't want to take this thread out on that tangent, but if I could have gone ground source I would have - that's what the company I brought in does, and that's what I was willing to pay for - but the geography of my lot just wouldn't allow it.

I also would have gone Natural Gas - but I'm in the boonies.

I considered Fuel Oil but didn't want to build a chimney

I considered going High Efficiency LP, but the local prices and buying / renting a new tank seemed unreasonably high.

So air/air heat pump with resistive coil backup we went. Now I'm just looking to understand why it's weaknesses are what they are.

[SJProwler]

I had a new gas furnace and heat pump installed on the 21st of Dec. I force the heat pump to run down to 20 deg to take advantage of off peak electric rates of $.034 per kw. I'm not used to hearing a heat pump run and it sits right outside my back door. I have yet to hear it defrost. I actually was beginning to wonder if it ever did and finally caught it going in to defrost only because the compressor shut off and 30 seconds later the condensing unit fan shut off and the compressor kicked back on. Very quietly, no louder than when the unit was running normally. I have NEVER heard it make a noise inside my house. The system is a Lennox SLP98 furnace and XP21 heat pump. Both almost silent, so they can be made quieter.

[Tom Campbell]

I had a new gas furnace and heat pump installed on the 21st of Dec. I force the heat pump to run down to 20 deg to take advantage of off peak electric rates of $.034 per kw. I'm not used to hearing a heat pump run and it sits right outside my back door. I have yet to hear it defrost. I actually was beginning to wonder if it ever did and finally caught it going in to defrost only because the compressor shut off and 30 seconds later the condensing unit fan shut off and the compressor kicked back on. Very quietly, no louder than when the unit was running normally. I have NEVER heard it make a noise inside my house. The system is a Lennox SLP98 furnace and XP21 heat pump. Both almost silent, so they can be made quieter.
Very good to know. Thanks!

[SoFlaDave]

Not a huge fan of Lennox equipment in general but the X-21 series is DEADLY silent.

[mgenius33]

You can drill verticle wells, they don't have to be looped horizontally. We typically have 1 loop per ton approx. 200' to 250' deep. Unless you live on granite.
The fact remains, the manufacturers are not going to install a heat coil in the condenser section. The added cost on manufacturing without benifit of gained efficiency, and added risk of expensive warranteed repairs, are just a few reasons this is not feasible.
However, I do believe your thinking is spot on. Manf. are so caught up in the rat race of SEER points that they rarely think outside the box.
If your main complaint is the noise, you bought the wrong unit. There are many manf. that are very quiet. Maybe, try a compressor blanket, and a defrost board change. Some defrost boards will shut down for 30 sec to balance, then reverse for a smoother, more quiet transition.

[Tom Campbell]

You can drill verticle wells, they don't have to be looped horizontally. We typically have 1 loop per ton approx. 200' to 250' deep. Unless you live on granite.
The fact remains, the manufacturers are not going to install a heat coil in the condenser section. The added cost on manufacturing without benifit of gained efficiency, and added risk of expensive warranteed repairs, are just a few reasons this is not feasible.
However, I do believe your thinking is spot on. Manf. are so caught up in the rat race of SEER points that they rarely think outside the box.
If your main complaint is the noise, you bought the wrong unit. There are many manf. that are very quiet. Maybe, try a compressor blanket, and a defrost board change. Some defrost boards will shut down for 30 sec to balance, then reverse for a smoother, more quiet transition.

Boulders prevent getting any kind of drilling rig in to any kind of drillable location.
As for equipment choices, hindsight is 20-20. It was installed about 8 months ago, so I would think it's unreasonable to ask my contractor to swap out just because I didn't know how loud it would be (or is this something that contractors sometimes can do?). I'm guessing sound mitigation is the best I can work towards right now.

[mgenius33]

I guess you're literally between a rock and a hard place!:CU:
Seriously though, try the compressor sound blanket and timed off defrost board. At least you'll sleep better...