Performance of new heat pump system
Greetings experts. I posted previously on this forum about replacing my Lennox 50,000 BTU furnace and 2-ton A/C with a heat pump and received many thoughtful comments on my initial post here (https://www.greenbuildingadvisor.com/question/38marb). I took a lot of recommendations from people like @Akos to calculate my heat load and live with the old equipment for one winter etc. which turned out to be great advice.
The new system is a 2-ton Carrier 38MURA / 40MUAA and it has been working great after I got the contractor to correct some things. The outdoor unit fan motor also needed to be replaced after a couple months as it was rattling (maybe defective from factory). This winter I used a little bit of aux heat but not much (maybe 4 times, 30 minutes each). It has been a milder than average winter.
I recently had a post-audit done and the excellent technician came with the necessary equipment to measure my duct performance (if only HVAC companies did this!) It looks good to me but I am a bit concerned about the “low” TESP.
Wondering if my new system might be undersized ? I mean it is heating and cooling just fine….and by my calculations the operating cost is a bit less than gas so I’m satisfied. But it does run pretty much 24/7.
Thanks
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By "aux" heat required a few times, do you mean the unit's heat strips? I've been kicking the tires a bit on an update to our gas furnace/AC system so your comments are very useful. What climate zone are you in? What was the heat load number you calculated using your furnace/gas consumption?
If the heat pump is running 100% at your lowest design temp, then I'd say you have the sizing pretty close. I'll let the HP experts here chime on in that point though.
Edit..I read through your original thread, and see that you're in Toronto, and had calculated around 24K BTU/h for heat load. It looks like you're still using the Ecobee though for the stat. I have installed about a dozen of the Ecobee stats and recommend them. That said, my home setup uses a communicating stat with a modulating condensing gas furnace (ECM motor) and AC unit. Once you figure out how to access the installer setup, there are a pile of options to access that are simply not available via an Ecobee. I don't mess with too many settings, but do change up the fan circulation CFM seasonally. I'm guessing the communicating stat in your setup would be required to take care of tuning modulation, CFM adjustments etc. as your air handler is using an ECM motor as well. This is one of those cases where I would not recommend a "smart" stat as they are not that smart at all when connected to modulating systems with ECM motors etc. so you can't adjust circulation/heating/cooling CFM, set points etc. Given your interest in diving in for max efficiency, I think you should consider this. The Lennox communicating stat lets you access the installer settings by touching/holding the Lennox icon for 10 seconds or so. I would end with the caveat that changing anything "under the hood" without understanding the impact is not a great idea either :-)
Thanks again for posting your full journey from sorting out an accurate heat load yourself, to commenting on the system install and follow up performance. Many of us I suspect are looking to do the same. I'm on a bit of a fence as the gas furnace is fairly new variable speed, modulating, 98% efficient, ECM motor etc. and the AC outside is in the 18 SEER range. We don't use AC more than a 3-4 weeks a year.
Hey No problem I'm glad you found my posts to be insightful!
It's worth mentioning that I did my original heat load calculation slightly wrong. I used a design temp of -16C (this is what ASHRAE says for Toronto), but our local building code lists -22C. I also had accidentally used a 60F balance point in one spot when it should have been 65F. After correcting these, my new heat load would be 29,000 BTU/h. I'll see how much of a problem this is when/if we have a really cold winter. But at least the size isn't far off if at all, and not too big for cooling. Still maybe a bit big for cooling though, as our cooling loads here are anywhere between 1/2 and 1/3 of our heating loads. Also, I may be able to make up for the error by working on my air sealing/insulation since my basement rim joists have drafts.
If my old equipment hadn't been pushing 30 yrs old and not breaking down I probably would've kept it for a while still. It sounds like you have pretty new equipment. Assuming your gas prices are affordable I would just stick with what you have until it nears end-of-life otherwise it would be pretty wasteful to throw all of that in the landfill (apparently there's no market for used hvac equipment). By then, I'm sure there will be even better heat pump options on the market and maybe thermostats too!
Going with the ecobee was a compromise for sure but it seems to have been the right choice based on how everything is working. If the manufacturer's communicating thermostat was better, I would definitely consider it. See my comment #7 below for more detail.
If anything it's perfectly sized or oversized. Stage 1 almost all the time and stage 2 mostly during the cold snap? That's exactly what you want. Only 2 hours of resistance heat is great. A hard thing for people to grasp is that more run-hours DOES NOT equal more energy used.
Are you using a setback?
*Noticed that Carrier 38MURA is variable speed. How does the thermostat decide what is 1st vs 2nd stage? It likely has hundreds of speeds.
There is no such thing as too low of TESP. Because the unit was replacing a much larger furnace your ducting is larger so you get overall much less restriction. This is all plus as lower TESP means lower blower power for the same CFM thus less energy consumption.
If you really want to start tweaking your system for best efficiency, with such low TESP, you can crank up the CFM of the unit a bit in heating mode. This generally gets you a couple of points of COP improvement at the cost of slightly colder supply temperature. You don't want to do this in cooling season though as it reduces humidity removal which you want in our hot and humid summers.
Looking at the thermostat log, looks like the system is running great.
Thanks @Akos! You were instrumental in helping me get to this point. Good to hear that my low TESP is an plus, and not an issue.
I did look into the CFM settings and it looks like there is one notch above what it's set to, but it's unclear if this 'max' CFM would be used during normal operation, or only when aux heat comes on. I'm getting pretty warm air from the vents when the compressor is running at full power (as high as 42C / 107F). However, since things seem to be working well and I am concerned about humidity in the summer, I probably won't tinker with it.
Regarding humidity removal, would you recommend connecting the "Dry" terminal on the air handler to my ecobee accessory terminal? I currently have the Humidifier connected which is nice for winter, but I could just as well use the Humidifier's own humidistat.
I do not see in this post where tkzz says they are using an Ecobee thermostat. or can you tell from the graph?`
If true I agree it is a mistake the factory communicating thermostat is the only thermostat that will allow the full speed range of the compressor. My Rheem is almost an entertainment center the amount of data I can see about my system is amazing. It displays the high and low pressures and 6 temp sensors. The super heat is a very interesting number. It lets me lock out the strip heaters when the outdoor temp is above the number, I set of 6°F. If your system has the same setting you need to find the lowest number possible without falling below your set point.
Am I wrong when in thinking the Ecobee can only command the compressor to run at full speed a fixed low speed and off? You paid thousands more to get variable speed compressor seems silly to accept a thermostat that prevents you from enjoy the benefits you paid so much to get.
Seems to me the only real features the Ecobee has is remote control and occupancy sensors. With time the Ecobee learns you habits and builds a set back schedule. I think set back schedules are counter productive on HP systems and factory thermostat will give you set back & remote control if you want it.
@walta100, yes, Ecobee based on the graph. That Rheem data looks pretty comprehensive. I have to figure the installer menu is even more fun!
Communication/features issues aside, @tkzz , I really like https://beestat.io/ which you can connect to your Ecobee for more detailed data. That's assuming you keep using it :-)
Agreed - beestat fills a real gap as the ecobee-provided charts are pretty crummy!
Ecobee yes. But it's not true that the compressor runs at full speed. The air handler unit has a communicating interface with the outdoor unit. So while the ecobee can only ask for stage 1 (y1 terminal) or stage 2 (y2 terminal), the air handler and outdoor unit have sensors and built-in logic to modulate the compressor. In other words, the ecobee does not control the compressor directly. The system knows when the weather is mild and will not blast the compressor. The indoor blower fan also changes speed thanks to the connection of the two Y terminals.
To me, this was a good compromise as the factory thermostat is reportedly awful and buggy, and lacking several features such as wifi and remote sensors which were important to my wife and I.