We always say that 1st stage is more efficient than 2nd, but is there actually any evidence for it? 1st stage is rated at 41F EWT versus 2nd stage at 32F EWT. That difference in EWT itself explains the difference in COP. Now in the real world the loop temp remains the same, with the LWT dropping down further when the 2nd stage licks in. I don't pick up a difference between 1st and 2nd stage COP with the WEL. Does anyone know more about it?

Have you compared WEL with mfg extended specification tables? WF and CM publish tables showing COP and kW and capacity for a wide range of water temperatures and flows?

To verify, your WEL setup would have to measure system power and the change in enthalpy on the air side. Measuring air flow is even more difficult, but it might be reasonable to assume the configured CFM is correct - supposedly the ECM blowers are accurate to 5-10% if ducting holds TESP within limits. It gets even trickier, basically impossible, if there is a bypass damper.

Fair question, but:
Are we looking at this correctly? Using less amps if you need less heat IS more efficient right?

I sold a single stage unit as recently as last fall and have never shied away from them. If not for tax credits, they might be the minority of my sales.

Per my WF envision two stage specifications catalog.
50 degree EWT on a NDV038, three ton, dual stage with variable speed blower
Low Speed - 8 GPM - 1050 CFM's - 5.11 COP
High Speed - 9 GPM - 1250 CFM's - 4.69 COP
Single Stage w/ECM - 9 GPM - 1250 CFM's - 4.78 COP

How about a 4 ton dual stage at 30 EWT with lower flow rates
Low Speed - 8 GPM - 1350 CFM's - 3.72 COP
High Speed - 9 GPM - 1550 CFM's - 3.68 COP
Single Stage w/ECM - 9 GPM - 1550 CFM's - 3.71

While first stage is slightly more efficient it is the comfort that I sell of it, It is a lower blower speed so less air noise and longer run times so better air cleaning capabilities as well as more even temperatures.

Posted By SkyHeating on 02 Mar 2012 11:17 AM


How about a 4 ton dual stage at 30 EWT with lower flow rates
Low Speed - 8 GPM - 1350 CFM's - 3.72 COP
High Speed - 9 GPM - 1550 CFM's - 3.68 COP
Single Stage w/ECM - 9 GPM - 1550 CFM's - 3.71

While first stage is slightly more efficient it is the comfort that I sell of it, It is a lower blower speed so less air noise and longer run times so better air cleaning capabilities as well as more even temperatures.

Comfort: No question. But given the same EWT the COPs above are within 1 %. It is probably just the minor difference in the fan speed. But the overall COP appears to be the same whether the unit runs in 1st or 2nd stage. We have watts meter or amp sensors on the units. So yes, the amp drawn goes up, but also the LWT goes down (increasing the delta T on the source side) and the blower speed increases, giving everything a higher capacity. But this all happens linear. The overall COP does not change. So the difference in overall COP in the ratings appears to be solely a factor of the different EWTs used, and not a difference in the efficiencies between the stages.

Zoning, dehumidification, and quiet operation combine to make dual stage our deafult choice down here.

Posted By engineer on 02 Mar 2012 10:20 PM
Zoning, dehumidification, and quiet operation combine to make dual stage our deafult choice down here.

Same up here. The question was if 1st and 2nd stage run with the same efficiency.

Posted By docjenser on 02 Mar 2012 10:49 PM 
... The question was if 1st and 2nd stage run with the same efficiency.

I'll look to see if I have enough data to make a statistically meaninful analysis.  Not sure I do, though, with this year's Winter being so mild down here.

Down here (Dallas) heating is an afterthought.  Thus I'm not sure I've got any data from my WEL system (WEL0043) that is reflective of the running in 2nd stage.  Heck, it's been so mild of a Winter here that lowest EWT this Winter season was approx. 48° F, and I'm already up to 56° at the moment.  (Last season EWT got down to 38°F.)

If I do have some 2nd stage data, further, I'm not sure it's enough.  I've learned that a COP or EER analysis has to be a statistical analysis over a period of time, i.e. a month time period. There are too many variables that come in to play to make any conclusions based on a single time point COP calculation.

I am with you, you need to have a data logged. May be you can see if the AC has the same COP/EER as well. I would predict so.

I had to ammend my prior post. Using less amps when you need less heat IS more efficient. That does not mean a greater COP.