I posted a question here recently about thermostats with humidity control, since we had speced out one for a client but instead received one without separate humidity control.
I called the A/C subcontractor today to ask them to please replace the client's thermostat with one that had independent humidity control. His response was that he had never seen a residential heat pump with a true variable speed air handler. He said that they have to set the dip switches for a certain fan speed, but that after that, it doesn't vary the speed.
My understanding was that his description was correct for lower or middle end units but that a true variable speed air handler would start our very slowly and ramp up speed over time to help pull more humidity from the home. If you had a thermostat with separate humidity control and it was calling for a reduction in humidity, but not temperature, that it would kick the unit on with a speed just high enough to prevent freezing the coil in order to pull out the greatest possible moisture. The A/C subcontractors point was that having separate humidity control on the thermostat wouldn't really accomplish anything beyond what would be accomplished by turning down the temperature.
So what is the reality of variable speed air handlers in this situation? Am I wrong in my understanding? If they don't dynamically changes the speed, I am curious what their advantage is.
FWIW, this is on a Lennox 18.6 SEER heat pump. Sorry that I don't have the model info with me here.
Air handlers with ECM motors are about as close to "true" variable speed as you're gonna get with resi equipment. ECM motors (if their set up correctly) will vary their speed to a point to help compensate for poor duct design, but that isn't their designated purpose.
Correct
Yes and no.... it will kick the system on, but it will be in a "normal" mode with the blower ramp up, etc. Depending on how you program the stat, to drop the humidity, it may lower the temp as much as 3 degrees. In my area the target indoor design temp/RH is 75degrees/50%RH. I have no idea where you are, or what your design temps are so I can't do a lot for you on that. FWIW, some systems are set up so that the heat strips will energize while in cooling mode for reheat to help with humidity removal.
Ummmm......no....he missed the mark on that one.
The ECM motor has several advantages...one is being more energy efficient to run, another is that they will ramp up to speed over a period of time, another is they will (to a point) compensate for improper ductwork design and air flow problems.
Its not just the heat pump, but the particular hair handler it is coupled with that will make the difference. Without actually *seeing* what you have and how its installed, my best recomendation is to get the Honeywell VisionPRO 8000 stat (TH-8321U1006)
FWIW, if you have humidity issues because of an oversized system, thats a whole different problem.
Lets see... for starters, it has built in humidity controls, will support most any configuration with 3 heat and 2 cool capabilities, its easy to program, it doesn't have to be on the wall and connected to program it, it has auto change-over, it has intellegent recovery, and a whole lot of other features. The IAQ version only requires 3 wires to connect the control head on the wall to the control module that you mount on the side of the furnace or air handler. anything else??
Yes, you forgot to mention it offers an intermittant fan function that operates the air handler blower motor for approx 40% of the time in a 24 hr period.
Luckily, its touch screen is under the minimum size stated in California's new law regarding hazardous waste disposal of LCD screens.
Are you saying that most higher end residential A/C equipment uses ECM motors?
We are in Central Florida. We usually use Lakeland, Florida as the closest well-known city, when using software such as Energy Gauge, which I use for determining eligibility for contractor federal tax credits on new construction.
I'm glad to see that humidity is controllable with the variable speed air handler, beyond what I was being told.
In this case, the Air Handler is a Lennox CBX-32-MV-048-2306 and the Condensor is a Lennox XP19-036-230-02 with an HSPF of 9.3 and SEER of 18.6. You see that this is a 4-ton air handler and a 3-ton condensor unit. It is my understanding that by "mismatching" the two that you can effectively increase the efficiency. Is that correct?
After talking with the A/C subcontractor yesterday (when he said that he had not seen a true variable speed air handler in a residential unit and that the only variableness was via dip switches) I called the Lennox support department about this. A guy named Ron there confirmed that, when calling for humidity reduction but not cooling reduction, that the thermostat sends a DS signal to the air handler to slow it down. I asked him if other thermostats, like the Honeywell TH-8321, sent the same DS signal to the air handler in the same scenario. He replied that he knew that Honeywell was working on that, but that he didn't think that their products (or any others presumably) were sending that signal, but were rather just running the unit longer to bring down humidity at the expense of lower temperature. Do you know if the Honeywell TH-8321 does indeed send the same DS signal to the air handler that the Lennox Signature Stat does? I suspect that you know that true answer to this question.
This house has 2581 square feet and has Low-E double pane windows and R-45 fiberglass in the attic. The local power company, Progress Energy, conducted a blower door test and the technician said it was the tightest house he'd ever tested. Before construction, the A/C subcontractor recommended installing a 5-ton unit! We instead used the services of
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here in Florida to help size the ductwork and equipment properly, since we wanted to "get it right". I hope that we did. Since we have a 4-ton air handler and a 3-ton condensor, I don't know if is is more accurate to say that we installed a 3-ton 3.5-ton or 4-ton system here but we tried to do our homework to get it sized correctly.
One other related issue that I'd like to clarify. . . The A/C subcontractor said that they don't usually change the factory default settings on the DIP switches. Ron at Lennox said that it is best to meaure the temperature differential across the coil and adjust the fan speed DIP switches until you acheive a TD of 18 - 22 degrees. Since the default factory setting is apparently at the highest speed, it would seem that, to the extent that you could lower it to the higher end of the 18 - 22 degree TD range, you would be helping the humidity issue there as well.
If you want to control humidity, you sure as hell don't over size the evaporator! That's going in the WRONG direction.
After reading the rest of your post, it's unfortunate, but your HVAC contractor is out in left field. I would suggest that you find a competent, reliable tech that is familiar with your Lennox equipment and have them go over the complete system and set it up properly and locate any other problems that may not have been diagnosed.
I would completely agree that it should not be the building contractors job to educate the HVAC guys on their own trade, but I've seen an unfortunate amount of ignorance out there. The old 500 SF per ton rule still seems to dominate around here. That is why I question everything and look for consensus on forums such as this.
I for one would love to see David post the ARI number for the mismatched system......without such ARI certification number his system is probably 10 seer at best.
increase in efficiency by having condenser of 4 ton and air handler of 3 ton NOT in MILLION years! you contractor maybe good janitor if that. not only will you have poor efficiency but you RH in house will be unbearable I am not AC person but you better believe what I just told you. Tony
OK, I finally got the ARI certification reference number for this system and it is: 551503. The ARI directory shows the pair with a SEER of 18.6. I do not know the lineset size, however, as I am not at the site location.
One other interesting point to note. regarding the ARI certification of this unit. As installed with a 3 ton condensor and 4 ton air handler (ARI reference # 551503) the unit is rated at 18.6 SEER. I looked for and found the certification number for the same 3 ton condensor matched with the same model air handler, except at 3 tons instead of 4 (ARI reference # 551667) and the SEER rating for that matched set is only 17.75. Still good and efficient, but less than the 18.6 obtained by oversizing the air handler. As long as you slowed the fan down on the four ton unit to match the condensor, it seems like it would even do better at lowering humidity. Am I thinking straight on that one?
Thats probably a very good idea Jake, however I would imagine on a brand new install the equipment manufacturer would probably void the warranty on his equipment. Of course if nobody tells him....
Your Sub-contractor is a friggin idiot and you should get rid of him, before he screws anything up. I would definitely brag to anyone you know about his incompetence. But I am sure he has great prices!!!
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