Are there benefits or serious drawbacks to using a variable speed air handler with a single speed compressor?
We’re restoring a 19th century house in very humid Gulf Coastal climate, similar to New Orleans. We askled HVAC company for variable speed heat pump, and got a quote on a variable speed air handler matched (by the manufacturer) to what appears to be a single stage 14 seer outside unit. I appreciate that there are small benefits to variable speed for heating loads, but don’t give a flip about that in our climate. Have been reading as much as I can about interaction between variable blowers and outside units (single stage or otherwise), but remain confused about the actual benefits in this situation.
Additional details: House leaks like sieve — we can’t strip down to studs but will wrap where possible. Dense plaster walls and ceiling. Will insulate ceiling to R30. Crawl space semi-enclosed, floor uninsulated (will be hard to change that). Lots of windows, but thanks to porches, little or no direct sunlight on east or west windows. Duct work entirely within conditioned space on a short run (less than 25 feet) through central hall. 1750 sq feet. Would probably go with 3 ton on variable speed, and 2.5 ton if fixed.
And one last note: I somehow violated a right of entry to GBA and thus my name doesn’t show up automatically, but thank you for your previous responses to my questions.
Bill Finch
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Replies
With a single stage compressor and a variable speed air handler it has to be set up so that the air handler can run slow enough to handle the latent cooling loads (moisture- does it ever get sticky on the Gulf Coast? :-) ), but not so slow that it ices up. It's more satisfactory if it's at least a 2 stage compressor, if not fully variable.
Running the air handler at higher speeds delivers a higher SEER, but lousier latent load managment. Most AC units installed out there are grossly oversized, often to the point of not having a high enough duty cycle to truly handle the latent loads. The first and most critical thing to get right is the actual sizing- without it the rest of it (multi-stage, variable speed etc) is just the frosting on a fairly punky cake not worth eating. See:
https://www.greenbuildingadvisor.com/articles/dept/building-science/manual-j-doesn-t-tell-you-equipment-capacity
https://www.energyvanguard.com/blog/55157/Why-an-Oversized-Air-Conditioner-Is-a-Bad-Idea
https://www.energyvanguard.com/blog/air-conditioner-sizing-rules-of-thumb-must-die
The bottom line is that a competent third party (and not the average HVAC contractor) should be calculating the loads, and specifying the equipment.
Bill,
Here is a link to an article that explains how you can change your screen name from "User-6970120" to "Bill Finch."
How the GBA Site Displays Readers’ Names.
Thanks for the responses -- as for the website, by the way, I've actually tried to update profile a couple of times, but I had to sneak around to do it: There is no way to click on to the "welcome" button, and even though I've updated profile, it doesn't show up in the welcome line...checking here to see if it at last pops up on the Q and A. Almost as tricky as sizing an HVAC...
Thanks, Bennett. That's getting to the heart of the issue. Glad I don't have to teach a class on it...but it makes me feel better about that as an option.
Getting the right size unit is very much a challenge, further complicated (at least in my mind) by recent studies indicating that in some situations variable speed blowers are more efficient (even when ductwork is in conditioned space) when heat pumps are somewhat oversized...for reasons no one seems fully ready to explain. Oh, and let's not even talk about the counterintuitive humidity control of fixed vs variable speed revealed in those same studies...But yes, it would be great to have a disinterested and better trained third party, though you recognize outside of Seattle and larger metro areas, it might be hard to find someone locally...I'll take the search far and wide if I need to, but there is value in having someone who's had long experience applying calculations in real world local conditions...the hvac contractor is defacto the engineer in residence for much of the world...hope we can find ways to improve their knowledge and reliability, as well
First, I'm no HVAC expert, just an engineer that's done a lot of thinking and reading about residential systems, particularly the ones in the houses I've owned.
What Dana Said: get your load calcs and HVAC design done by an independent consultant. The closer your system is in size to your actual load, the more your AC runs and the better it will pull moisture out of the air.
A couple benefits of a variable speed air handler:
An ECM blower (DC drive, much more efficient than Permanent Split Capacitor motors at part load and somewhat more efficient at full load. (there are multi-speed ECM air handlers as well)
It will ramp up and down for quiet starts and stops.
The ECM might pull something like 100W at low, fan-only speed, making timed circulation and/or supply-side fresh air ventilation reasonable in cost
The VS AHs I've been able to get data sheets on have a "dehumidify" control terminal that will allow the use of a humidistat to run the fan @ ~80% of full speed when the humidity is high. This function can also be used when zoning to reduce the airflow for single-zone calls.
If I were choosing, I'd opt for a single stage heat pump (or AC) compressor. Granted the two stage and variable systems offer potential comfort advantages, but they are expensive, complex, and have to be sized correctly so that they actually run at full speed near design loads and thus slower at part loads. Note too that the two stage units only drop to something like 60% of full load 3/2, 2/1.5 ton, etc.
@ Bill - I'm not in a metro area either. I'm looking at future HVAC plans for a remodel and thinking I'll use a remote service (a number have been mentioned on this forum) if I can't find a good one locally. A competent ME with experience in the domain is probably "good enough", but I'd rather use a service that does it every day.
I'd love a link / titles. I've not seen the studies you mention. It might inform the decision to replace or keep my current, oversized unit when we remodel and also be an interesting read for a number of frequent posters here. Without having read the articles, my conjecture is that the systems might be oversized for sensible capacity, but run at a lower cfm as Dana noted to get a better latent capacity.
Bennett