2 stage compressor vs dehumidifier

[whatname]

I am in the market for a new ac system for a hot humid climate and I am getting conflicting information regarding the use of a whole house dehumidifier vs a 2 stage compressor system. I am looking for alternative opinions to help me make my decision.

One of the problems I am having in my house is during mild temperatures the interior humidity gets a bit too high. Outside air will be 60-70% RH at 65-75 F, inside will get 60%-80% RH but its too cold for cooling and too warm for heating. In this situation I am used to running a portable dehumidifier to drop the RH.

Initially I thought that I would buy a whole house dehumidifier and a new heat pump. However I have had more than one company try to sell me a 2-stage compressor and multi speed fan with the idea that the system would run off a humidistat and forego the dehumidifier.

Some questions that I cannot get a straight answer to is that if the system runs to control humidity would the system have to overcool the house? Does this mean at 68-72F I have to accept colder air blowing out of the vents? Also what is the overriding control, a set temperature or a set humidity?

 

[Stuff]

Yes, it will be cooling so only effective in your example if you like 50º temps inside. Note there used to be some systems that would turn on the electric heat strips at the same time to make up for this but not many in their right mind do it today. Most salesmen only know the basics of the product they are selling and they don't sell dehumidifiers so don't want to lose your sale. Keep on calling around and you should find someone who knows something.

A two stage a/c system runs longer on 1st stage to remove more humidity than if on high. It helps when it is hot outside and the system is too big to run long enough to get the humidity down.

 

[Sponsor]

 

[Reach4]

Not what you are asking, but maybe the vapor barrier and air leak situation could be improved.

At first I was thinking that a vapor barrier paint on the stucco might help, but searching shows that could be a problem if there is a vapor barrier farther in. I am now thinking that a blower door test, with resulting caulking etc, could be the best way to go. https://www.energy.gov/energysaver/blower-door-tests

In hot climates, the vapor barrier is on the outside. In cold climates, it is on the inside.

 

[Dana]

A whole house dehumidifier in most houses would be using a sledgehammer to swat flies, and maybe yours too. A dehumdifier converts a latent cooling load into a sensible cooling load. That's fine if your house is on the cool and clammy side, but will heat up the house more than you like if it's on the slightly warm & sticky side, forcing a higher sensible cooling load.

Right-sizing the AC would be a good start- most equipment out there is more than 2x oversized, so even at low speed it's oversized for the 1% load. A Manual-J using AGGRESSIVE assumptions on shading factors, R-values, air leakage is the right place to start. Using online freebies like LoadCalc or CoolCalc is good enough for sizing the equipment if you're aggressive enough (eg assume air-leakage and duct leakage = 0 , even if you know it's not.)

A right-sized 2-stager supplemented with a 70 pint room dehumidifier for those cool but damp days is usually enough for most normal-sized houses, installing the room dehumidifier near a return register.

Don't let an HVAC installation pro run the Manual-J for you. Nineteen out of twenty Manual-J reports performed by HVAC pros are worth less than the paper they are printed on. If you really want to do it right, hire an engineer or RESNET rater to run the Manual J/D/S on the system, somebody who makes their living & reputation on the accuracy of their numbers rather than on installing & maintaining equipment.

In terms of just how much AC you need, this graphic was compiled by the consultants at Energy Vanguard (in Decatur GA) plotting house size against the square feet per ton of cooling from Manual-J calculations they did for clients. You can see that it's really all over the place, but only the crummiest houses were anywhere near the "ton per 600 feet" or other common rules of thumb used in the trades:

In almost all homes in Florida a heat pump sized for the cooling load will more than cover the space heating loads even at near-record low temps, but if it looks marginal in your Manual-J you can always add some "Hail Mary" heat strip back up in the new system.

The amount of latent load from vapor diffusion is negligible. Air leakage is what you're after- a square inch of air leak moves as much moisture as an entire house's wall area of vapor diffusion through latex painted wallboard.

With multiples the up front cost can be an expensive way to cool and heat the whole house, but just one 3/4 Daikin Quaternity ductless mini-split it's possible to shift the sensible heat ratio (SHR) to being nearly 100% latent load, which make it behave like a dehumidifer that cools just a tiny bit. This mini-split series has a proprietary split coil and valving on the indoor head that uses part of the coil as a condenser for re-heat even when the rest of the coil is operating as an evaporator. With the Quaternity there are separate temperature and humidity setpoints, and it changes the SHR dynamically to not overcool or over dry while it's normal cooling mode. So even when the sensible cooling loads are very low it can still do a lot of latent cooling without frosting you out. For shoulder-season cooling it will cover the load on it's own- you can turn the central air off, and it'll do a better job of keeping the humidity in check. (It won't dehumidify in heating mode though.)

 

[WorthFlorida]

Living in Florida for thirty years I know what you mean about high indoor humidity in the cooler months. I have a two stage heat and one stage cool heat pump. The heat source is heat pump plus electric heat for the second stage. I use a WiFi Honeywell thermostat that reads the humidity but does not and cannot control the system based on humidity. When you set up and register the thermostat, on the Honeywell web site with your login, there are email alert options. I have mine set to 55% humidity max when I get an email. This past winter my home was empty for four weeks and when I received the email I remotely lower the temperature for maybe an hour or two, from 77 to 72. When you program the override you can set the time when to end the temperature change.

When I’m home and feel the humidity is high, I’ll run the ac for 15 minutes or so and at times lower the set temperature to 75 from 77. Two stage cool with humidity control is a nice feature but the cost does go up and you need to watch the humidity setting. That is if you lower the setting too low to try to dry out the air the first stage cool could constantly run most of the time. What you may notice is the air from the registers can get extremely cold and you’ll start to get a headache. My son’s in laws have this feature and one time while their house we all were feeling it. Just like your temperature setting it will take a while to get the setting just right for comfort. Also, thermostats with humidity control for the manufacture of the heat pump you’re looking at may be a proprietary thermostat. You want to ask about that and having a WiFi thermostat is the best way to go.

 

[Dana]

When I’m home and feel the humidity is high, I’ll run the ac for 15 minutes or so and at times lower the set temperature to 75 from 77. Two stage cool with humidity control is a nice feature but the cost does go up and you need to watch the humidity setting. That is if you lower the setting too low to try to dry out the air the first stage cool could constantly run most of the time. What you may notice is the air from the registers can get extremely cold and you’ll start to get a headache. My son’s in laws have this feature and one time while their house we all were feeling it. Just like your temperature setting it will take a while to get the setting just right for comfort. Also, thermostats with humidity control for the manufacture of the heat pump you’re looking at may be a proprietary thermostat. You want to ask about that and having a WiFi thermostat is the best way to go.

That's what's nice about the Daikin Quaternity mini-splits: With part of the evaporator coil operating as a condensing coil providing reheat it dehumidifies without the very low exit-air chill (at very low sensible cooling when that's what is called for.)

I wish they had a full split system with the split coil / dynamic SHR feature, but they don't.

 

[whatname]

Thanks all for the replies. I think i found my answer. The two stage quits running after -5 deg drop even if humidity is not met. I dont think i can adjust for high humidity by running the heat to get from 70% to 50% when indoor temps are 65F. A five degree increase in temp probably only adjust for 5% RH or so. Looks like i will have to go with a separate dehumidifier and a basic system.

mini-split is really not an option. Manual J shows 3 ton is right sized. Summer dehumidification is not a problem even with my old system