Based on my discussion with geo installers, as well as this community, it seems that there is a general consensus that stage 2 heat is "bad." I believe this is because the COP number is worse in stage 2 than it is in stage 1, giving people the impression that it is a less efficient heating stage. I had one installer I talked to actually size the unit to avoid all stage 2 operation, he was all about the higher stage 1 COP number. There are posts on this forum about folks trying to keep their thermostats from entering stage 2 as well. However, if the energy monitoring on my WF 5 Series is correct, I would have to say that stage 2 is actually better than stage 1 in terms of efficiency. Let me give an example: Stage 1 on WF 4-ton: Fan: 39w Pumps: 472w Compressor: ~1739w (varies up and down continuously) Total: ~2250w Stage 2 on WF 4-ton: Fan: 39w Pumps: 472w Compressor: ~1939w (varies up and down continuously) Total: ~2450w So it seems stage 2 consumes about 200w more than stage 1. This is a difference of about 9% total wattage. Given that stage 2 actually produces about 30% more heat than stage 1, isn't stage 2 actually more efficient when the overall operation of the furnace (pumps and fan in addition to compressor) is considered? This all of course assumes my energy reporting is accurate.
This reminds me of an article I came across the other day. A study of three GSHP installations in Wisconsin, Connecticut, and Virginia measures high-stage COP to be equal or greater than the low-stage COP in all cases. In fact, field measured COP values were on average 27% worse than the published specification for low-stage, and 13% worse for high-stage.
To quote Joe Hardin " congratulations, you just reinvented the ineffecient single stage unit ". Installers who design a 2 stage unit to only run in 1st stage, missed out on the basics of design somewhere along the line. Will these same installers only run the new variable speed compressors on low? Eric
Please all keep in mind here that stage 1 operations are rated at 41F entering water temperature (EWT), while stage stage is rated at 32F. this is all in the lab. In the field, EWT doe not change going from stage 1 to stage 2. Secondly, your overhead (for example pumping power) remains the same, so it is proportionally higher in 1st stage than in second stage. The study you are citing are perfect examples of very poorly designed and installed systems, where we have commented on here before. As I always say, geo can be heaven and geo can be hell. While heatpump COP is advertised at lets say 5.0 in first stage, it can be 2.0 if designed and installed incorrectly.
Yeah. To me, that standard of measurement is not very meaningful. I wonder why it was defined that way? When comparing the models used in various bids for my own house, I looked up COP (using the engineering datasheets for a given model) at different operating points of CFM, GPM, and EWT that seemed more "realistic" to me for the 5 ton system we'd be installing. For example, here's my table for COPs at various EWTs for the Hydron HXT060: [TABLE="class: grid, width: 500"] [TR] [TD]EWT[/TD] [TD]Low (10 gpm, 1300 CFM)[/TD] [TD]High (15gpm 2100 CFM)[/TD] [/TR] [TR] [TD]30[/TD] [TD]3.99[/TD] [TD]4.08[/TD] [/TR] [TR] [TD]40[/TD] [TD]4.53[/TD] [TD]4.49[/TD] [/TR] [TR] [TD]50[/TD] [TD]5.04[/TD] [TD]4.82[/TD] [/TR] [/TABLE] To find the COP at each point, I took the gross BTU output (air + desuperheater) and divided by power consumption (kwh, converted to BTU). Actual real world values would be lower by some constant multiplier of course, but for comparing the machinery I liked using these values better than the ISO standard values. Basically, just about every 2-stage model I looked at using these points didn't show much difference in efficiency between low and high stage. (As you note, if pump speed were to be held constant between stages, the low stage gets additionally penalized slightly). Anyway, my takeaway was that the staging would probably affect comfort and durability (avoiding short cycling) more than efficiency.
It must be terrible difficult to design a system for temperatures in the summer @ 95 Degrees and winter @ -15 degrees. We had 3 days with morning temps below zero and daytime highs in single digits within the last two weeks. My electric resistance ran like it was designed to during this time. I dont think that my house went below 69 degrees that entire cold spell. That was worth every single penny I paid to install the unit and run the unit each month.
Doc, in the study cited, what exactly is "designed and installed incorrectly"? I read the cited study and I don't see where installation would vary the results. How about my system? It's a 4 ton Waterfurnace with a dual 26-99 Grundfos loop pump. As far as I know the pump is single speed, and uses the same energy in either first or second stage. Is my system designed or installed incorrectly? What are your criteria for correct design and installation?
Your flow center is not as smart as the unit causing a lowering of efficiency in first stage as pumping cost remains the same. As to the broader question of what is correct design and installation, there are many factors, including soil type, electric cost, heating or cooling dominated climate etc. The biggest folly I hear repeatedly however is that it is advantageous to oversize to stay in first stage and if you heard it here it wasn't from a pro. j
Independent U.S. study measuring CoP - GreenBuildingTalk - GreenBuildingTalk - Green Building Forums on Insulating Concrete Forms (ICF), Structural Insulated Panels (SIP), Radiant Heating, Geothermal Heat Pumps, Solar Power, Green Construction Projec Here is a link about some discussion points.
I design the dual-capacity (5 tons and 6 tons) to run one pump - ie, ground loop pump - during first stage heating or cooling. Then second stage heating or cooling both pumps can come on.
Don't make the assumption that you're necessarily getting 30% more out with second stage, and therefore second stage is more efficient, without doing some measuring. FWIW, I've measured the COP of my 3 ton system as well as I'm able to. Like yours, my system uses roughly 200 more watts to run in second stage. COP comes out roughly the same between 1st and second stage, though 1st stage is more often a little better. The bottom line is, I get a number. I like the number because it's over 4. It's not necessarily a correct number. It's a pain in the butt to try to measure, cause you're chasing a moving target (and there's lots of room for operator error). I figured that if I make some measurements monthly, or so, over the season, I'll see a trend and I might know in advance if there's a problem in the future, before it gets too bad. So I get a number I like. I get an electric bill I like even more. I don't care that it goes into second stage, when it needs to, because it's very efficient there. It goes into second stage cause it needs to make some more heat. I don't even care when it goes into 3rd stage, when it needs to, cause it's really cold outside then, and it's pretty darned efficient when it's cycling the aux on 15 minutes out of every couple of hours to help out. And my house stays where I set the tstat...win win
The efficiency of 2 stage is to not use full compressor capacity (and amps) when it is not required. That is why the COP is measured at 2 different temperatures.
Joe, enlighten me here. Why is the COP measured at different temps (41F 1st stage, 32F 2nd stage)? How in the real world does the EWT change by 9 degrees when going from 1st to 2nd and vice versa?
My observation in my first winter of operation. I started heating season with EWT around 60*. I am deep into heating season and seeing a pretty steady range around 30*. I have only used second stage heat in the last month or so when it probably did average about 32*. Through the course of heating season, starting with the warmer water and now into the upper 20's lower 30's, it seems in my case, to be probably a right ballpark average of what temps each stage sees when looked at an average for the whole season.
EWT is certainly warmer in the beginning of the heating season, but trust me, it will be much colder at the end of the heating season when it starts to run again more in 1st stage due to warmer weather. In real life, the EWT does not change when the heatpump goes from 1st stage to 2nd stage, actually the leaving water temp (LWT) does drop due to the heatpump taking more heat out of the water. Thus I would question whether the heatpump indeed runs that much less efficient going from 1st to 2nd stage. Engineer once pointed me to the performance tables which use constant EWTs, which show much lesser difference between 1st and second stage than the normal ratings suggest. However, raising the water temp in the lab from 32F to 41F will significantly increase COP, I would guess around 10%. Thus a significant portion of the the higher rated COP in 1st stage is due to the different test condition. In the field, the "overhead" is practically the same (unless you use variable speed pumps), so proportionally to the output the power for loop circulation etc decreases in 2nd stage. In terms of overall system COP I cannot measure any difference between 1st and 2nd stage on our monitored systems.
It didn't suggest EWT changed with stage selected, I said the efficiency of 2 stage is to use less when less is required. My suspicion would be the big pay off is in the off dominant season for most folks. i.e. cooling in MI or heating in FL. The big payoff would not be during peak heating season in MI when you have a 30* EWT (so why test it there?). It is inexplicable to me how they (manufacturers and testing agencies) pick EWTs to test at, if design low EWT is 30 then why pick 32 to test at? I think I was among the last on board with 2 stage due to poor cost v benefit in my mind. I think the dehumidifying argument is way over blown outside of Florida. When comparing the two back in the day I usually found op cost difference between 1 and 2 stage equipment to be less than $75/yr in my AO and most of that savings was during cooling season in MI.
