Ductwork for Whole-House Dehumidifier
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I am building a 6000 sq ft house in Alabama. Downstairs is about 4000 of those sq ft. On one of the two A/C units downstairs, we are installing a whole house dehumidifier. My installer will install however I want but he typically pulls air from the return trunk, and returns into the supply trunk bypassing the air handler. Is this the best way to do it?
I’ve read some prefer to have a dedicated vent in the house to pull in air to WHD and then return to the supply trunk after the air hander. Thanks.
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Richard. If construction is not finished, what makes you think you need a dehumidifier? If your house and basement are well constructed and the HVAC system is correctly sized, you may not need dehumidification.
Also curious about your floor plan. How are you managing 2000 square feet above grade with a 4000 square foot basement?
Are you sure you even the whole house dehumidifier?
In a tight house with a right-sized (or better yet, right -sized and modulating) air conditioner(s) there is rarely a need for a whole house dehumidifier, unless you insist on very high ventilation rates.
Oversizing the AC such that it runs at a low duty cycle is where poor latent load management usually creeps in, prompting the whole house dehumidifier band-aid. The exceptions would be houses designed for very low sensible loads, such as a PassiveHouse, etc.
In Alabama, we have high humidity. Often in fall, winter and early spring, we have no need for heating or cooling so HVAC doesn't run unless I set thermostat into the 60s but then we all get cold. Hence, I want a dehumidifier. Steve, we will have 2 units downstairs - 5 tons total for downstairs. Plan is to put WHD on the 3 ton unit.
Back to my original question - how should it be ducted and why?
Richard. I live in Georgia and use two tons to heat and cool 3,200 square feet. Your HVAC sounds massively oversized if your home is being built with proper air sealing and insulation. But it's your house.
You need to install the dehumidifier according to the manufacturers design guidelines to preserve your warranty.
Does anyone have a substantive answer to my question? I'm sure there are several ways to duct a WHD that won't void a warranty. What I'm asking is for the preferred way to duct a WHD and why?
https://buildingscience.com/documents/information-sheets/information-sheet-supplemental-humidity-control
Richard,
1. There is a good chance that your cooling system is oversized. This can lead to comfort problems. An oversized air conditioner will do a worse job of dehumidification, not a better job of dehumidification, compared to a right-sized unit.
2. Most homes don't need whole-house dehumidification unless (a) the HVAC contractors did a poor job of equipment sizing, or (b) it's a very high-performance house that needs very little cooling so that the air conditioner runs rarely.
3. If you want to buy a whole house dehumidifier, follow the installation instructions provided by the manufacturer. The illustration below comes from the installation instructions for the Therma-Stor Ultra-Aire.
-- Martin Holladay
.
I was previously concerned about units being oversized. However, I've been to the installer's office, looked at his inputs on the Manual J, and seen the output results. Everything looked correct (SHG, facing W, window sizes, overhangs, etc). Only thing he did was round up on tonnage. For instance, upstairs Man J calls for 2.1 tons. Since the units I'm looking at only come in whole tonnage, installer says to go up to 3 tons.
Hi Richard, what did you end up doing? I'm in Houston in a similar situation to you, and wanted some advice on how you installed and whether you are satisfied with the performance of that design.
If you haven't yet installed the dehumidifier, keep in mind that unless you but a split system it will create a lot of excess heat that you will have to offset by using the AC. If the house is tight and well insulated, you will probably be better off not installing the dehumidifier and just turning the AC down a couple of degrees in the shoulder seasons when it is cool and humid. This comes from personal experience. I had a dehumidifier installed and ended up taking it out because of the excess heat it produced.
I am indebted to a fellow GBA reader -- I wish I could remember who -- who pointed out that a dehumidifier is a space heater, one with a pretty high COP. It takes all of the heat from the electricity it consumes, plus all of the latent heat from the water it removes, and dumps that into the interior. As a space heater it has a COP of about 3.
So if you're cooling, running a space heater doesn't sound like something you'd want to do. There are however two edge cases where a dehumidifier makes sense. The first is when it's cool and damp, and you want both heating and dehumidification. The second is when you want cooling, but your AC is not capable of delivering enough dehumidification -- the sensible heat ratio is too high. In that case the dehumidifier creates more sensible load and less latent load, but at the cost of more energy usage.
It's more efficient to get the SHR as low as possible on your AC, which means configuring it for the coldest coil possible and long runtimes -- both of which mean low airflows. And I agree that sometimes dehumidifiers are prescribed as an alternative to proper system design. But that might not be enough. What makes this hard is that the standard design tools don't take any of this into account -- Manual J cares about peak loads. There's no place to put into the model, "in the spring and fall we get lots of rainy days in the 60's with 99% humidity," which is the kind of weather you will need a dehumidifier for.
Richard
Here in Virginia, I definitely have a problem with dehumidification. My house has a a blower-door test of 0.55 ACH@50 and a HERS rating of 46, which makes it a modestly high-performance house. We have 3000 ft2 with one ton of AC, which is far too much.
I have been running a portable dehumidifier nearly constantly in the summer, and am considering adding on a Daikin Quaternity mini-split, to be used primarily for dehumidification.
I wish I had done better planning for dehumidification. My suggestion is to take the problem of dehumidification seriously, and if you choose not to install a dehum initially, at least have a contingency plan for adding it later.