Carrier Greenspeed: real world performance?
As my propane furnace celebrates its 22nd birthday, I am plotting its destruction – err, replacement. I’ve been modeling various alternatives (mini splits, GSHP, even solar thermal “thermal battery”), and I’ve run into a problem – the Carrier Greenspeed absolutely dominates thanks to the, uh, “optimistic” information on their engineering data sheets.
I’m not taking any other data sheets at face value, so it’s not really a fair comparison. But I generally have a good idea of how the competition is misleading (e.g. the GSHP numbers assume 0″ SP on the blower and 0′ head on the pump). I don’t know how traditional split systems game their ratings, and I haven’t been able to find any real world data to calibrate against.
Has anyone seen real world numbers for the Greenspeed they can share? Or at least know reasonable adjustments for the data sheet values? I’m most interested in 0-30f performance.
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Tim,
Your comments are the first that I have seen that imply that Carrier's claims for its air-source heat pump are inflated.
My guess is that your cynicism is probably unjustified -- but I'm interested in hearing whether any GBA readers have any relevant experience with this appliance.
The mild weather performance on the data sheet is on par with top of the line ductless mini splits (COP of 4.5 to 5.0). That number is supposed to include the cost of the forced air blower, so if it really is achieving that level of performance I would be extremely impressed.
I would expect it to be as good as minisplits. It's got the same technological innovations as they do. We've gotten used to mini-splits having much better performance than conventional systems, but that's mostly a result of them being the first systems available with high-efficiency variable speed compressors and fans, not because of the mini-split configuration. And the advantages of size could make up for the disadvantages of fan power needed for the duct system.
That doesn't really answer Tim's question, but my point is that the good performance numbers could be for real.
The main concern is that you are dependent on the duct system. The performance can be undermined if it leaks heat, leaks air, or has high resistance to airflow, and thus requires more fan power. What you think about your duct system could be a key factor in the decision.
If it's so ridiculously oversized that it's cycling on/off rather than modulating during milder weather it'll take an efficiency-performance hit. It's modulation range isn't infinite- don't have a spec on it in front of me but I believe it's ~ 3:1 turn down, maybe only ~2.5:1 for some compressor & air handler combinations. So to have it cover the load at 0F and not be cycling on/off at 35F means you have to pay close attention to the capacity vs. load since it's more sensitive to oversizing issues than a heat pump with a bigger modulation range.
Since you have a heating history on the place with the fossil-burner, it's pretty easy arithmetic to calculate an upper bound on the heat load. Some wintertime propane fill-ups with the exact dates and quantities, and the zip code for weather data, and the furnace's nameplate BTU-in and DOE-BTU out would be enough. It get's screwed up if you say, took a 2 week trip to Belize and left the thermostats at 55F, or you've been running a wood stove as auxilliary heat though.
When you come up with a reasonable estimate of the heat load it's time to sharpen the pen a bit.
There's a handy on-line capacity tool to play with. Click on the Heating Capacities tab, and enter what you think your 99% outside design temp is, then play around with the different Greenspeed compressors and air handler units to find the best fit.
http://www.tools.carrier.com/greenspeed/
I've already got a pretty good handle on my load; I've got circuit level monitoring of my electricity consumption so it's easy to correlate my furnace's duty cycle against outdoor temperatures (I lose about 500 BTU/h/f, or 42.5k BTU/h at a -20f design temp).
You're right that its modulation is pretty limited (I hadn't paid much attention to that previously), indeed only around 2.5:1. With the 3 ton, I'd be modulating above 40f and below 15f, so about 60% of my annual heat load would be handled without modulation, 90% total.