Multi-Stage Dual Compressor vs 2 speed compressor.

[Illusion]

Looking at the Trane high end units Trane XL16i Model 4TWX6024B and Trane XL20i Model 4TWZ0024A.

Are the first stages of compressor operation the same BTUs but different energy consumptions. I have read that the two speed compressors operate at about 70% in first stage, while the dual compressor models operate at about 50% in first stage. Is this an energy usage statement or a BTU output statement? Is the BTU output close and the difference is an efficiency issue with varying the output of a single compressor?

Asked another way: Is a dual compressor model more efficient in its first stage compared to a 2 stage single compressor due to the motor being more efficiently matched to its load?

Does anybody have data on the BTU/Kw per stage for these units available?

And finially, I have only been able to find dual compressor models available from American Air/Trane. Is there another manufacturer with dual compressor models?

[DanW13]

Might want to read this tread, it might answer some of your questions:
http://hvac-talk.com/vbb/showthread.php?t=338312

[Illusion]

Thanks for the suggestion, but nope that did not help. I actually authored that other thread, what I am asking here is something different.

[Illusion]

I have seen talk about one of the Carrier/Bryant models that have a 50/100 capacity split. So I guess I would like to modifiy my original thread a bit by saying, I am looking for a 50/100 split more than a dual compressor model.

And on that note, do the dual compressor models stop compressing for a bit while they switch compressors? Do the single compressors need to stop before staging up?

[allan38]

The two compressor systems, the first compressor is rated at a lower BTU.
The blower will operate at a slower speed with the first stage compressor.

The two speed compressor (if it is a copeland 2 speed scroll compressor) has a solenoid to unload the compressor so it moves less refrigerant. It uses less electricity in the unloaded state. The blower also operates at a lower speed on 1st stage.

The two compressor system is electrically and mechanically more complex than the two stage copeland scroll.

In my opinion the two speed bristol reciprocating compressor used on many of the 50/100 Carrier/Bryant systems is more vulnerable to mechanical failure than the two speed copeland scroll compressor used on many 75/100 systems.

[Illusion]

Allan38,

I understand that the two speed bristol reciprocating compressor used on many of the 50/100 Carrier/Bryant systems is more vulnerable to mechanical failure than the two speed copeland scroll compressor used on many 75/100 systems, but I am zoning the heck out of this system. One of my zones is quite small, and I really want that 50/100 capacity split.

It is looking to me like if I want that 50/100 split I either need the Carrier mid line 2 stage system or the Trane high end two compressor system. I will accept mechanical complexity in exchange for that 50/100 split in my application.

Anyone know of another option I should be looking into?

[mchild]

Allan38,

I understand that the two speed bristol reciprocating compressor used on many of the 50/100 Carrier/Bryant systems is more vulnerable to mechanical failure than the two speed copeland scroll compressor used on many 75/100 systems, but I am zoning the heck out of this system. One of my zones is quite small, and I really want that 50/100 capacity split.

It is looking to me like if I want that 50/100 split I either need the Carrier mid line 2 stage system or the Trane high end two compressor system. I will accept mechanical complexity in exchange for that 50/100 split in my application.

Anyone know of another option I should be looking into?

Might want to consider inverter driven compressors where the compressor is completely variable in output from about 25% of nominal capacity to over 100% (how much depends on manufacturer). This allows for the system to vary the compressor output to match the actual gain/loss of the structure and even provide cooling/heating when the load is very light. That is especially good for dehumidification in warm but not hot weather.

[thrashme]

Ditto with mchild. Many brands use inverter technology...Daikin is a wonderful piece of equipment. Low amp, "soft starts" and variable capacities based on loads. Rheem prestige also makes dual compressor condensing unit.

[Illusion]

Thanks for the suggestions. I unfortunatly have to disqualify the very cool inverter based units as they all require their special thermostats to control.

Using my existing thermostats is a requirement of this upgrade as I have hundreds of programs written that adjust the temperature via RF to said thermostats so I will have to stick with 2 stage units that allow conventional non-communicating thermostat wiring. I will be using the HZ432 Zone controller from Honeywell so the unit needs to be looking for the classic O/B C G R Y1 Y2 W (or whatever for aux/emergency) and I would also like BK as that zone controller will activate that feature when only one zone is calling which is pretty cool.

I looked at the Rheem info. The info avail to consumers seems to indicate a single 2 speed scroll, and the performance tables show the usual 80% BTU rating in first stage, so maybe I was looking at the wrong units or they have stuff that is not viewable by the general public.

[Irascible]

Just following the convo Illusion, I notice that allan mentioned the complexity of the Trane two compressor system to which you replied with a comment about the two-speed Bristol compressor system. I wouldn’t call a two-speed compressor particularly complex.

As for reliability, there is the assumption that a scroll is inherently more reliable than a recip. It probably is, but then recips have been chugging along reliably from long before I was born. I’ve got a few two-speed Lennox systems from the early eighties on an account of mine. They’re still running like champs.

I’m not the multi-stage expert. However, I believe both systems will have a short delay between stages.

Early this year I went on a hunt for 50/100 staging. I live in a dry climate, so the 70/100 staging is in my mind a pretty horrible idea – even if I just have two zones. Just as you found, Trane and Carrier were the only 50/100 options I could find.

[docholiday]

If you have a dual compressor system where the compressors are equal, there is a slight load carried by the 1st stage that gives you somewhere around 55% capacity. The energy usage is also higher than half since you are running the blower and the Outdoor fan with 1st stage and only slightly increasing them in wattage when the second compressor kicks in.

[Illusion]

Just following the convo Illusion, I notice that allan mentioned the complexity of the Trane two compressor system to which you replied with a comment about the two-speed Bristol compressor system. I wouldn’t call a two-speed compressor particularly complex.

Irascible,

Thank you for taking the time to reply to my post. It is a particular honor for me to have you comment on my question as I am a huge fan of yours! I have read nearly everything on your website, even the stuff that has major formatting problems. I have been so interested in what you had to say that I copied text off your website with the formatting issues and pasted it into a text editor to be able to read it. I greatly respect your way of thinking and running your company. I am a sole proprietor as well and I am also a one man band in my chosen field. I also strive to be highly technically competent and not just do what works or makes me the most $$, but what is best for the customer. I am sorry that you are not in the Dallas, TX area or I would already have my AC guy for this upgrade.

Now as for mechanical complexity. It is my understanding that the Carrier 50/100 split system uses a compressor that reverses direction in order to stage. It has two pistons, one of which is engaged in forward rotation and both pistons are engaged in reverse rotation. I do no know how this is accomplished, but certainly this is likely more mechanical complex than a 2 speed scroll that simply unloads the compressor via a solenoid.

I am not however excluding this unit from my selection process based on this. In fact, I think this unit is the leading contender for my application. I have a piston based compressor right now that I have been very happy with for 15 years and I am not afraid to put a two capacity version in service here. But I do think it qualifies as an increase in mechanical complexity.

[Irascible]

Thanks for the good word Illusion. I have to admit that I didn't know of the negative feelings towards Bristol until I did a search prompted by your post. My own experience with the TS product has been very good. As I mentioned, they're very old. They're located at a church/school and have been used almost daily since they were installed. plain spoken (http://hvac-talk.com/vbb/showthread.php?t=161433) and jrbenny (http://hvac-talk.com/vbb/showthread.php?t=161433) agree in their own way.

I'm was going to PM you about the formatting problem. It seems you can't receive PM's. :confused:

[Silver06]

This may be a dumb idea, but what about a dual unit configuration, using two separate condensing units (with scrolls if you like) and just have two independent refrigerant circuits with two separate evaporators. I believe commercial units like rooftop units ususually stage like this. Although getting static pressures and proper air flow would probably be an engineering nightmare.

[gary_g]

I'm was going to PM you about the formatting problem. It seems you can't receive PM's. :confused:

Must be a "pro" member to get PM's.

Take care.

[Irascible]

Ah. Thanks.

[Cold Feet]

Using my existing thermostats is a requirement of this upgrade as I have hundreds of programs written that adjust the temperature via RF to said thermostats so I will have to stick with 2 stage units that allow conventional non-communicating thermostat wiring.

Many inverter units have BACnet and/or Lonworks interfaces. You'd need to rewrite your software, but automatic control is possible.

[Illusion]

This may be a dumb idea, but what about a dual unit configuration, using two separate condensing units (with scrolls if you like) and just have two independent refrigerant circuits with two separate evaporators. I believe commercial units like rooftop units ususually stage like this. Although getting static pressures and proper air flow would probably be an engineering nightmare.

Damn! That is a fun idea. I could make the stages whatever I wanted then. I could have a 1 ton low stage and a 2 ton high stage and then I could run both stages for 3 ton. I just need to tell the AC contractors when they come to sell me the new units with cool air cleaners that I want their techs to put two coils in my air handler, but I still need that full warranty. I am worried about finding someone who understands zone control wiring. I cannot imagine asking someone to think so far out of the box as to have two condensers.... and heat pumps no less. Wow, that would be a disaster.

But seriously, I really like the thinking here. The real problem would be implementation.


Many inverter units have BACnet and/or Lonworks interfaces. You'd need to rewrite your software, but automatic control is possible.

I appreciate the thought, but no, that is not going to be happening in my case. I have decided that using my existing T-Stats is a requirement of the project. I would rather sacrifice staging flexibility than deal with new control integration.

[Irascible]

The real problem would be implementation.Few could design it. Quite a few could install it if they had plans in front of them. You don't need a local guy to design it, so you've got an entire country and perhaps world (via the internet) with which to find a designer.

As fun as that sounds, that was not an implication that I want the challenge (at this time). :)

[thrashme]

A quote from Rheem's spec sheets
"039, 048 & 60 nominal capacities
— Dual Drive scroll compressors –
Tandem
— Operates at 50% capacity on first
compressor in non peak conditions to
improve efficiency and comfort
(humidity control)
— Compressor “A” operated by control
board “A” Compressor “B” operated
by control board “B”." you can check out the specs at
http://globalimageserver.com/fetchDocument.aspx?id=8c185efa-0448-4cbf-8552-b0d406cc3b6f
on page 2 of 20

[Illusion]

A quote from Rheem's spec sheets
"039, 048 & 60 nominal capacities
— Dual Drive scroll compressors –
Tandem
— Operates at 50% capacity on first
compressor in non peak conditions to
improve efficiency and comfort
(humidity control)
— Compressor “A” operated by control
board “A” Compressor “B” operated
by control board “B”." you can check out the specs at
http://globalimageserver.com/fetchDocument.aspx?id=8c185efa-0448-4cbf-8552-b0d406cc3b6f
on page 2 of 20

Thanks for that! How very interesting. I was, and still am, unable to navigate to any page on the Rheem site that gets me to that info. I see the product line you refer to in the AC only models, but in the Heat Pump section the top of the line units stop at the 16 seer single compressor RPRL-JEC Series. So thanks for the PDF link, cause even with scouring around I still cannot find that. I am looking at the smaller units so no Dual Compressor for me but that is okay as the literature specifically says the single scroll in the RASL- JEC Series 024 and 036 is 50% at low stage. This may be even better! I did not know there were any single scrolls that went down to 50%. Again, thanks for the post as I would not have been able to find out about this on my own.

Also, Tandem? Does this mean that in high stage both compressors are operating? If so, I think this is the only unit I have read about that operates that way. All the others I have looked into switch to a full load compressor for stage 2 and then back to the small compressor for low stage.

[dash]

Allan38,

I understand that the two speed bristol reciprocating compressor used on many of the 50/100 Carrier/Bryant systems is more vulnerable to mechanical failure than the two speed copeland scroll compressor used on many 75/100 systems, but I am zoning the heck out of this system. One of my zones is quite small, and I really want that 50/100 capacity split.

It is looking to me like if I want that 50/100 split I either need the Carrier mid line 2 stage system or the Trane high end two compressor system. I will accept mechanical complexity in exchange for that 50/100 split in my application.

Anyone know of another option I should be looking into?

Infinity zoning ,with properly sized ducts,wiil be fine with the Copeland Compressor.

Yes you might have a minor issue,if a small zone is in a setback ,and you want to change the temperture in a short time,but otherwise no issues. go for it,and get the tax credit,plus higher SEER!

[thrashme]

Illusion, as far as the "Tandem" is concerned, I have not laid my eyes on one of these units, I just called my Rheem Distributor and he said...comp A is stage 1 and compressor A & B is stage 2. I asked if the were piped in parallel ( common suction & common discharge lines)or in series (like a compound compressor). I also asked if one comp failed if both had to be changed or if they could be changed independently of each other. He could not answer either question but would find out and get back to me.

[thrashme]

Thanks for that! How very interesting. I was, and still am, unable to navigate to any page on the Rheem site that gets me to that info. I see the product line you refer to in the AC only models, but in the Heat Pump section the top of the line units stop at the 16 seer single compressor RPRL-JEC Series. So thanks for the PDF link, cause even with scouring around I still cannot find that. I am looking at the smaller units so no Dual Compressor for me but that is okay as the literature specifically says the single scroll in the RASL- JEC Series 024 and 036 is 50% at low stage. This may be even better! I did not know there were any single scrolls that went down to 50%. Again, thanks for the post as I would not have been able to find out about this on my own.

Also, Tandem? Does this mean that in high stage both compressors are operating? If so, I think this is the only unit I have read about that operates that way. All the others I have looked into switch to a full load compressor for stage 2 and then back to the small compressor for low stage.

Sorry, but I missed the part about a HP, the HP does NOT use two compressors. Good luck on finding your system.

[Illusion]

Okay, I am getting ready to pull the trigger on this new system. The Rheem looked great to me until I found out that the 50/100 capacity split only applied to the AC only units. The heat pumps are still closer to 75/100 and use a single compressor.

So as it stands now, if I want 50/100 capacity split I am going with high end Trane/American Standard with dual scroll compressors or Carrier/Bryant mid line with the single two [speed] stage Bristol Piston compressor.

Any other contenders now that a few months have passed? Any new options out there I should be considering that offer 50/100 capacity splits in a heat pump?

[beenthere]

No t that i know of.

But. Neither the Scroll nor Bristol TS is 2 speed.They are 2 stage. But they don't stage their RPM.

Inverter systems change speed.

Nothing wrong with the Bristol TS compressor.

[Illusion]

Edited,

"Stage" is what I meant. Thanks for the note...

[thrashme]

Illusion, you may also consider a Nordyne made piece of equipment distributed under Maytag, Westinghouse, Frigidaire and a few others. They have an inverter driven compressor that is variable speed and varies the capacity from 25% to 118%. Hp @ 23.5 SEER on a dual fuel fully modulating variable speed furnace and 24.5 SEER with a fully modulating VS furnace. It is marketed under the IQ Drive.
Here is a link to Nordyne
http://www.nordyne.com/web/ResLand.aspx
Here is Maytag's IQ Drive line HP
http://www.maytaghvac.com/MTHP.asp

[Illusion]

thrashme,

Thanks for the suggestion, but I require a conventionally controlled unit. I already have my t-stats and zone controller of choice installed. Using these t-stats and the existing zone controller is a requirement of the install.

While I am very enamored with the inverter units, I am unwilling to give up my control scheme. I love the increase in the number of stages, and ability to match load with capacity, but I have just worked too hard on the other part of the system to give it up. My t-stat settings are controlled by over 150 interdependent programs and I have worked many many hours tweaking the conditions of those programs to get where I am.

So I guess it is time to start getting bids on the American/Trane top of the line or the Carrier/Bryant mid line.

[CynicX]

Interesting thread but are we even helping you out? Seems like questions are being answered with more questions.

So not exactly starting over. What exactly are you trying to accomplish at this point?

You just want a high efficiency heat pump that can be operated from a normal switching thermostat?

I think you can pretty much use anything. Carrier and Bryant high end models require there specific control for a thermostat. I think you can step down to there 16 models and use your thermostat. However I think the best thing about the units is the thermostats control of the system.

Any model Lennox is fine. There 2 stage systems use a single 2 stage copeland scroll. With your thermostat wiring throughout.

I really have no experience with the residential multiple compressor systems Like the Trane. But I would question its expense vs the money it can save. Also seems like there is a big additional point of failure, esp when it gets older (which compressor is going to die first?). I dont know much about the communicating Rheems either, or whether you HAVE to use there control over a regular thermostat.

Also you have to properly match the inside unit to get any efficiency out of the unit at all. Basically you could install the latest and greatest 20+ seer system to an air handler and get no where near that efficiency mark...

[Illusion]

Thanks everyone.

I went with the 2 Ton Trane XL20i. So far really happy. Surprisingly, I received no bids on the Carrier/Bryant option. 75% of the contractors bid the Trane System with the other 25% bidding out the American Standard equivalent.


System is controlled by a Honeywell HZ432 zone controller controlled by 3 thermostats (3 zones) controlled by my home automation manager computer. Aux heat call from the zone controller goes through an external relay controlled by the same home automation manager that decides whether or not to allow aux heat based on a variety of conditions.
I then added 3 relays that are controlled externally via the home automation manager. The relays energize O, Y1, G but not BK. All my dampers are normally open so I get whole house de-humidification when those relays are energized. Fan starts at 160 CFM for 1m goes to 256 CFM for 7.5m and then settles on 320 CFM for duration of the call. All in 1 stage compressor.

Attached is the incredible 10 fan speeds I am getting from this non-communicating set-up.

[chuckcrj]

Nice Work!

I love all the speed options with the Trane/Am Std product. 80-90% of the forced air equipment I sell has VS and have gotten very good feedback from my customers in regard to noise and humidity control.

I would like more details on how you set up your home automation for my own house. If you have the time my email is in my profile.

Thanks.

[Illusion]

Way complicated. Short answer here:

http://forum.universal-devices.com/viewtopic.php?p=22258#22258