SEER 38 units - worth it?

Poodle Head Mikey · Jul 2, 2019

There is no technical reason.

Any size coil can be made and compression ratios can be reduced much lower than they have been - so long as refrigerant will still flow in the system.

But design is always about compromise. The makers are not going to (or won't unless forced to) build custom coils for some limited market share. They naturally want to use what they already make.

So it's easy to mate a 2 ton compressor with a 5 ton coil. But as most don't have a larger-than-five ton coil in their product line - the efficiency gains are less as they mate larger and larger compressors with the same 5 ton coil they already have in production.

PHM
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gustavhorna said:
so you say e.g. 12.000 and 18.000 are just same coil with different compressor, so on 18.000 SEER is going down because the coil is not scaled up, right?

there is no physical reason why the 18.000 could not have same SEER rating as the 12.000 does (e.g. some volume to surface ratio effect etc?) ?

coolcoil · Jul 2, 2019

gustavhorna said:
so you say e.g. 12.000 and 18.000 are just same coil with different compressor, so on 18.000 SEER is going down because the coil is not scaled up, right?

there is no physical reason why the 18.000 could not have same SEER rating as the 12.000 does (e.g. some volume to surface ratio effect etc?) ?

And it's not even necessarily a different compressor. Take a look at most of the VRF catalogs. You will see that the outdoor unit compressors for the 6, 8 and sometimes 10 ton units are the same. They just run them at different max speeds. In fact, ten years ago when I was a VRF product manager, if you had the secret dip switch decoder ring, you could change a unit from six tons to ten tons by moving switches on the control board. I don't know if that can still be done by dip switch today, but somebody with the right firmware access could change the unit.

The same goes for indoor units. Typical 7.5 KBtu/h through 15 KBtu/h (approx) will be the same indoor units. A line of indoor units that offers say, 10 different capacities is typically made up of three or four physically unique models. One easy way to tell is to look at the weights of the different models. Again, the capacity can be changed in the field. I was reading the service manual for one manufacturer recently, and buried way in the back were instructions for changing the dip switch settings on an indoor unit.

gustavhorna · Jul 2, 2019

Can I convert a 18000 19 SEER unit into a 9000 38 SEER unit?

coolcoil · Jul 2, 2019

gustavhorna said:
Can I convert a 18000 19 SEER unit into a 9000 38 SEER unit?

Not sure if you are just joking, but in any case I would think not. Reducing the capacity of a given unit will get you a few SEER points, but it's not going to double.

gustavhorna · Jul 2, 2019

hacking cheap 18s into super-efficient 9s would be a very lucrative business, taking into account how expensive are these hyper-efficient units.

if it could be done...

ga-hvac-tech · Jul 3, 2019

gustavhorna said:
hacking cheap 18s into super-efficient 9s would be a very lucrative business, taking into account how expensive are these hyper-efficient units.
if it could be done...

One of the problems with this idea...

Is there would not be a factory warranty... as you 'modified' the unit.

And yes, these units are computer driven... the first time you tried to troubleshoot it... the factory tech guys would figure out what you did, then label you as a bad contractor... and your support would go out the window.

One cannot game the system... and expect the factory to stand behind you...

pecmsg · Jul 4, 2019

gustavhorna said:
Can I convert a 18000 19 SEER unit into a 9000 38 SEER unit?

How do you plan on controlling the SH at the pump?

Poodle Head Mikey · Jul 4, 2019

What are the differences between a 18K 19 SEER unit and a 9K 38 SEER unit exactly?

Can you find out?

PHM
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gustavhorna said:
Can I convert a 18000 19 SEER unit into a 9000 38 SEER unit?

Poodle Head Mikey · Jul 4, 2019

Non-VRF compressors run at 3600 RPM. How much faster than that can VFR compressors be made to run?

And are the performance results linear?

If the capacity is 5 tons at 3600 RPM - is it 10 tons at 7200 RPM? And 2.5 tons at 1800 RPM?

PHM
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coolcoil said:
And it's not even necessarily a different compressor. Take a look at most of the VRF catalogs. You will see that the outdoor unit compressors for the 6, 8 and sometimes 10 ton units are the same. They just run them at different max speeds. In fact, ten years ago when I was a VRF product manager, if you had the secret dip switch decoder ring, you could change a unit from six tons to ten tons by moving switches on the control board. I don't know if that can still be done by dip switch today, but somebody with the right firmware access could change the unit.

The same goes for indoor units. Typical 7.5 KBtu/h through 15 KBtu/h (approx) will be the same indoor units. A line of indoor units that offers say, 10 different capacities is typically made up of three or four physically unique models. One easy way to tell is to look at the weights of the different models. Again, the capacity can be changed in the field. I was reading the service manual for one manufacturer recently, and buried way in the back were instructions for changing the dip switch settings on an indoor unit.

gustavhorna · Jul 4, 2019

if the coils are standard, so basically a 9000 @ 38 seer is getting the coils from a 18000 @ 19 seer, then the issue can only be the compressor, the valve and probably the software.

I dont say this would be a commercial project to sell hacked AC to end customers, but just as a hack-a-day project, why not? Not sure about you, but I love to play with technology.

I'm just asking myself, what would I get if I put the compressor of a 9000 BTU unit into its 18000 btu sister from the same product line (also replacing the board). Would that give anything?

coolcoil · Jul 4, 2019

gustavhorna said:
if the coils are standard, so basically a 9000 @ 38 seer is getting the coils from a 18000 @ 19 seer, then the issue can only be the compressor, the valve and probably the software.

I dont say this would be a commercial project to sell hacked AC to end customers, but just as a hack-a-day project, why not? Not sure about you, but I love to play with technology.

I'm just asking myself, what would I get if I put the compressor of a 9000 BTU unit into its 18000 btu sister from the same product line (also replacing the board). Would that give anything?

I don't think anybody said that a 38-SEER 9-KBtu/h unit is necessarily using the coils from a 19-SEER 18-KBtu/h unit - it was just an illustration of the idea. There are other things that have a very large effect too, such as the efficiency of the evap and condenser fans.

The other thing to keep in mind is that a SEER rating is based only on tests run at three outdoor air conditions:

95 degrees, compressor and indoor fan at full speed
82 degrees, compressor and indoor fan at full speed
82 degrees, compressor and indoor fan at low speed

Indoor air is 80 dry bulb / 67 wet bulb for all tests.

Manufacturers optimize their units operate the best in these conditions. If you use the units in only these conditions, a 38-SEER unit will be 2X the efficiency of the 19-SEER. For other conditions, the difference is highly likely to be different. To be sure, the 38-SEER unit probably will be much more efficient in any other condition, just not necessarily 2X.

If you've got nothing better to do on the 4th of July, you can download AHRI 210/240 here. It's free and shows all the rating conditions and calculations. Though, it's not easy reading.

http://www.ahrinet.org/App_Content/ahri/files/STANDARDS/AHRI/AHRI_Standard_210-240_2017_add1.pdf

gustavhorna · Jul 4, 2019

coolcoil said:
I don't think anybody said that a 38-SEER 9-KBtu/h unit is necessarily using the coils from a 19-SEER 18-KBtu/h unit - it was just an illustration of the idea. There are other things that have a very large effect too, such as the efficiency of the evap and condenser fans.

I know! It was just an example how it could be.

My question was just practical - could I hack a more efficient unit taking the hardware of a 18000 BTU and "downsizing it" to 9000 BTU? It is not for commercial purposes, just call it hack a day project. I'm also not claiming I'm trying to get it certified under the standard testing conditions. Just as a private project for my garage

Could I hack the 18000 unit to operate like a 9000 unit but with higher efficiency? Isn't that an interesting question?

coolcoil · Jul 4, 2019

Poodle Head Mikey said:
Non-VRF compressors run at 3600 RPM. How much faster than that can VFR compressors be made to run?

Actually, I don't know the typical speed range for VRF. We have always referred to the speed based on the Hz of the AC feed. That is running at 120 Hz is twice the speed of 60 Hz. I believe that they can go up to 7200 RPM but I am not sure. I do know that for a given capacity, the variable speed compressors are physically smaller than the fixed-speed compressor with which they are often paired, so they have to be spinning faster.

That said, colleagues of mine are doing VRF testing right now, and I know they are recording the compressor speed, so I will ask them what range they are seeing. This is a test for one size of one brand, so it wouldn't necessarily tell you the range across which all VRF compressors run.

Poodle Head Mikey said:
And are the performance results linear?

If the capacity is 5 tons at 3600 RPM - is it 10 tons at 7200 RPM? And 2.5 tons at 1800 RPM?

PHM
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I am out at the ragged edge of my expertise here, so maybe a compressor engineer will jump in to correct me, but I am fairly certain that what I am going to say is accurate:

If the suction and discharge pressures were the same at all conditions, the performance of the compressor would be pretty close to linear, just like a water pump. However, in a real system, this would not be the case. Typically as speed goes up, discharge pressure goes up and suction pressure goes down. Since gas, unlike water, is compressible the volume moved per revolution will go down as the speed goes up, so it won't be linear.

I will use the idea of a piston in a recip to illustrate. A scroll would follow the same principal.

If you have a water recip pump, each stroke of the piston will sweep the same volume, so if you double the strokes you will double the volume of water moved.

With a compressor, as suction pressure drops, each stroke of the piston moves fewer molecules of refrigerant. The volume of the piston is the same, but with the lower suction pressure there are fewer molecules in the cylinder when the piston is at the bottom of its stroke.

At the top of the stroke, there is always a little bit of gas that cannot get out, just like you can't get the very last bit of toothpaste from the tube. When the discharge pressure is higher, there are more molecules of gas left in that space, so when the piston moves back to the bottom they will take up some of the space that would have gone to new vapor moving into the cylinder.

So, in this case doubling speed would not result in a doubling of the number of molecules moved.

gustavhorna · Jul 4, 2019

well what you write about the compressor is roughly correct, however mini splits should be controlled by some kind of closed loop circuit sensing the air temperature. So no matter what the volume moved per stroke is, the circuit would regulate the speed of the compressor until some target temperature of the returning air is met. I doubt there is a hard coded curve of how to drive the compressor, it must be driven by a temperature sensing circuit.

coolcoil · Jul 4, 2019

gustavhorna said:
I know! It was just an example how it could be.

My question was just practical - could I hack a more efficient unit taking the hardware of a 18000 BTU and "downsizing it" to 9000 BTU? It is not for commercial purposes, just call it hack a day project. I'm also not claiming I'm trying to get it certified under the standard testing conditions. Just as a private project for my garage Could I hack the 18000 unit to operate like a 9000 unit but with higher efficiency? Isn't that an interesting question?

In that case, you could. Ideally, you would reset the evaporator temps higher, condenser temps lower, slow down the condenser and evap fans, and/or slow the speed of the compressor. All of that would require access to the firmware or knowledge of the dip switch settings. I'm not sure how much you could accomplish without those.

coolcoil · Jul 4, 2019

gustavhorna said:
well what you write about the compressor is roughly correct, however mini splits should be controlled by some kind of closed loop circuit sensing the air temperature. So no matter what the volume moved per stroke is, the circuit would regulate the speed of the compressor until some target temperature of the returning air is met. I doubt there is a hard coded curve of how to drive the compressor, it must be driven by a temperature sensing circuit.

That is correct. Though I am not sure how mini-splits are controlled, I do know that most VRF units are controlled by the suction pressure. If the suction pressure goes below set point, the compressor(s) speeds up. If the pressure goes above set point, the compressor slows. In the early days, the pressure was a fixed value. Nowadays, sophisticated VRF systems reset the suction pressure set point based on outdoor air temperature. LG advertises that it resets on both outdoor temperature and humidity.

The evap temperature curve can even be reset in the field in many cases. If you look in the service manuals of most manufacturers there is a section that tells you (or more likely the service tech) what the codes are for various ways to run the unit. For example, if you are in a dry climate or humidity control is handled by a different system you will save energy by resetting the evap temps higher.

That said, it's not always so simple. We worked with one system where neither the service tech nor the distributor had any idea what we were talking about when we brought the subject up. Even after we pointed out the info in the service manual we never got the temps reset. Apparently that particular brand has chosen to discourage changes in the field even though their systems allow it.

gustavhorna · Jul 4, 2019

coolcoil said:
That said, it's not always so simple. We worked with one system where neither the service tech nor the distributor had any idea what we were talking about when we brought the subject up.

coz only the engineering team has an idea what is going on inside the unit. The techs only install it having some rough knowledge and the distributor... better forget that one.

coolcoil · Jul 5, 2019

coolcoil said:
That is correct. Though I am not sure how mini-splits are controlled, I do know that most VRF units are controlled by the suction pressure. If the suction pressure goes belowabove set point, the compressor(s) speeds up. If the pressure goes above below set point, the compressor slows. In the early days, the pressure was a fixed value. Nowadays, sophisticated VRF systems reset the suction pressure set point based on outdoor air temperature. LG advertises that it resets on both outdoor temperature and humidity.

The evap temperature curve can even be reset in the field in many cases. If you look in the service manuals of most manufacturers there is a section that tells you (or more likely the service tech) what the codes are for various ways to run the unit. For example, if you are in a dry climate or humidity control is handled by a different system you will save energy by resetting the evap temps higher.

That said, it's not always so simple. We worked with one system where neither the service tech nor the distributor had any idea what we were talking about when we brought the subject up. Even after we pointed out the info in the service manual we never got the temps reset. Apparently that particular brand has chosen to discourage changes in the field even though their systems allow it.

I just realized I had reversed how VRF controls to suction pressure. High suction is the signal to the compressor to slow down, because the indoor units are closing down their EXV's. Low suction means the EXV's are opening and more refrigerant is needed.

SEER 38 units - worth it?

Poodle Head Mikey

coolcoil

gustavhorna

coolcoil

gustavhorna

ga-hvac-tech

pecmsg

Poodle Head Mikey

Poodle Head Mikey

gustavhorna

coolcoil

gustavhorna

coolcoil

gustavhorna

coolcoil

coolcoil

gustavhorna

coolcoil

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