Dehumidifying without a cooling load
If you live in a heating-dominant climate typically the below-ground temperature is below room temperature. This means that basements require little cooling and can often stay cooler than the rest of the house. But cooler temperatures mean that dew points that are no problem in the rest of the house are problematic in the basement.
With the rising popularity of mini-splits the problem is worse, because there isn’t as much air circulation with the rest of the house as there would be with a conventional ducted system.
Stand-alone dehumidifiers aren’t a good solution, they tend to be noisy and balky and not very durable.
I’m wondering why there aren’t systems that are designed just to dehumidify that are integrated with the HVAC. This would require cooling the air and then warming it. Some minisplits have a de-superheater that allows you to capture waste heat for hot water. You could attach that to a hydronic coil in the air handler. Run the air over the cooling coil to extract the moisture, then over the heating coil to get it back to room temperature. Control the cooling coil with a humidistat and the heating coil with a thermostat.
Thoughts?
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There are such systems, but I’ve only seen them used in commercial applications. The system is known as “reheat”, and it works backwards from how you describe — it heats the air prior to the air entering the cooling coil, which allows for better dehumidification efficiency. This is the same way a dehumidifier works, but a dehumidifier works using its own waste heat for the heating while a reheat system in an air conditioner typically uses an outside source of energy (usually electricity, but sometimes hot water or steam).
A dehumidifier is a small air conditioner with the condenser coil ahead of the evaporator, so that the air coming in is heated by the condenser and then cooled by the evaporator, which allows for reuse of the incoming thermal energy (a little like recycling).
Bill
I don't understand how a heat-then-cool system could work. Heating air does nothing to its dew point. To reduce the dew point of the air you have to cool it down to the desired dew point. Then you need to warm it back up to room temperature.
The cooling coil is still cold enough to do the work. The commercial systems I work with in datacenters and telecom facilities have the heating element (regardless of the energy source used) ahead of the cooling coil. I’ve never seen one done any other way.
My old broken GE dehumidifier from my basement was the same way, heat then cool. I haven taken apart my new unit that replaced it. Perhaps the thinking is to make the unit exhaust air closer to the intake temperature.
Bill
Take a look at an Aprilaire whole home dehumidifier. It is exactly what you are describing. :)
And yes, Bill has it backwards on this one. The cooling coil cools the air to remove moisture, then reheats it with the waste heat.
No it isn't, because it's a whole other compressor and fan. I'm talking about using the HVAC that's already there but plumbing it so it works like a stand-alone dehumidifier.
I know some mini-splits have a "dehumidify" mode but it's just cooling with a low fan speed.
Ah, my apologies. I did not read close enough the first time. You are talking about something like the Lennox Humiditrol. See attached here for a technical explanation of how it works: https://www.lennoxpros.com/docs/Technical/210430.pdf
The reason you can't do it yourself with refrigerant coils is because you would have liquid refrigerant heading back to your compressor, which would destroy it. The Humiditrol uses a couple diverter valves, temperature sensing and variable fan speeds to accomplish the task without that issue. Not really a DIY thing.
Now your hydronic plan might work but the cost of a desuperheater and another coil and increased energy to push air through second coil etc. would probably be more than just getting an add-on whole house dehumidifier.
There are three-pipe heat recovery mini-split systems where the compressor doesn't reverse to switch between heating and cooling, the flow of coolant does. With them you can have two heads on the same compressor and one heating and the other cooling.
Mhm, as far as I know nothing like that exists for typical residential construction. It is a commercial system. And doesn't really solve your problem, unless you are installing a second coil for reheat that would have a separate refrigerant line from the compressor.
Like I said above, the Humiditrol by Lennox is exactly what you are asking for, something that uses the hot gas byproduct of cooling to reheat the dehumidified air and prevent overcooling, using the DX system.
Yupster:
Is five tons small enough to be residential? I'm thinking the LG Multi V S with heat recovery. LG calls it "residential/light commercial." To get the heat recovery option you have to buy the five ton model.
https://lghvac.com/residential-light-commercial/product-type/?productTypeId=a2x44000003XR0O&iscommercial=false
This is a three pipe system where there is both a condenser and an evaporator at the compressor, each with a separate fan. That's why it has the distinctive two-fan look. Any head can call for heating or cooling at the same time.
Manual J for my house is about three tons for both heating and cooling, but if I also used it for hot water it might justify the size.
There are multi-pipe mini-split systems that allow different zones to heat and cool simultaneously from the same compressor. What I'm talking about would be a head that has two coils, a heating and a cooling. Cooling is controlled by a humidistat, heating by a thermostat. That would be simpler than the hydronic method I first suggested.
Are you talking during heating season or during cooling season?
During heating season the OA conditions generally require humidification to get to the comfort range. Looking at the conditions chart for MA Oct and May are about the closest to Ground temp dewpoint. Winter months are generally too low absolute humidity for the ground temp dewpoint to be a concern.
June-Sep are a concern for sure, but that's generally when people are cooling already, and dehumidifying their basements already.
As far as efficiency goes a transfer fan to mix cool and dry air and warm and moist air to produce the desired conditions seems more efficient and less complex than either a dedicated piece of equipment, or something combined with existing cooling/heating coils.
It would make sense to have a different airflow option through a condenser to change the sensible heat ratio to something more towards latent cooling instead of sensible cooling.
Where I live (Central Oregon) we deal with low humidity basically all the time. Have been using all types of humidification systems to maintain conditions where I work.
In much of the Northeast stand alone dehumidifiers are common. Winters are cold, summers are not that hot but humid. Air conditioning -- based on a thermostat -- is not sufficient to give humidity control.
Once you get down to basement floor level the soil temperature is going to be close to constant year-round, at the year-round average outdoor temperature. In New England that is typically in the low 50's. There's not much call for air conditioning because there is essentially negative cooling load. If the dew point of the air is above the surface temperature, condensation forms. Where this is really a problem is sweating pipes, since the water enters at the soil temperature. So you need to keep the dew point in the high 40's, without further cooling the basement.
The daiken quaternity units have an dehum mode where the indoor coil is modified by a solenoid valve during dehum. I’ve only read about it though.
My understanding is that this is just cooling with a slower air flow.
Your understanding is how most mini-splits work in "DRY" mode.
But that is NOT correct for the Quaternity series, which uses a proprietary split coil in the wall head to adjust the sensible heat ratio. A Quaternity can indeed dehumidify to a humidity setpoint without sensible cooling. The sensible cooling and RH setpoints are independently settable.
Thanks Dana, that's along the lines of what I'm looking for. It basically runs like a dehumidifier, with one coil acting as the condenser and the other as the evaporator. I assume it also needs a proprietary compressor outside with a way of bypassing the condenser.
While I agree that most stand-alone dehumidifiers are not heavy-duty appliances, have you looked at these units: https://www.thermastor.com/? They are built to last, and pretty efficient.
The Thermastor units can also be provided in a ducted configuration that allows integration with the primary air handler. This works well in finished basements with dedicated HVAC systems. I have specified them for a number of projects. The ducted units can be installed in crawlspaces with a simple uninsulated plastic duct system supplying dry air to one end of the space, with the unit drawing the return air directly at the other end. They do seem rugged, at least for the 5-10 year timeframe, with some maintenance. Crawlspaces are tough locations.