Oversized DX Equipment causing High Humidity
Oversized DX Equipment causing High Humidity
(OP)
I am looking at a project that was not my design but is now mine to resolve.
Without having access to the early design decisions, it appears that the rooftop units are about 2 x as big as necessary. When the thermostat calls for cooling, it is satisfied pretty quickly, thus the compressor does not run all that long, so there is not a whole lot of dehumidification going on. Additionally, this is on the coast, so all the time the compressors are NOT running, the system is dumping a fairly humid mixture of outside air and un-cooled return air.
As I said, these are DX units, with 2 stage electric heat. To my advantage, they are 2 compressor units, so 2 stages of cooling.
Now in the good old days, I would have used a chilled water system with hot water reheat, cooled all the air down to 55F then reheated for comfort, thus yielding the sweetest, dry-est system in town. But not in these energy conscious days.
To obtain a dry space, I COULD replicate the good old days, driving the air cold then using those honking big EHCs to reheat. Not a good plan.
I COULD re-sheave, driving the cfm low, and the air cold to have the compressor run much more of the time...except...I believe I am going to be told by the manufacturer that the minimum acceptable cfm for the unit won't be low enough to match load/cfm/humidity to give a comfortable space and not damage the compressor. It will be better, but better probably won't be good enough.
So do you long-time DX guys have a magic bullet to shoot and put this high humidity problem down?
Without having access to the early design decisions, it appears that the rooftop units are about 2 x as big as necessary. When the thermostat calls for cooling, it is satisfied pretty quickly, thus the compressor does not run all that long, so there is not a whole lot of dehumidification going on. Additionally, this is on the coast, so all the time the compressors are NOT running, the system is dumping a fairly humid mixture of outside air and un-cooled return air.
As I said, these are DX units, with 2 stage electric heat. To my advantage, they are 2 compressor units, so 2 stages of cooling.
Now in the good old days, I would have used a chilled water system with hot water reheat, cooled all the air down to 55F then reheated for comfort, thus yielding the sweetest, dry-est system in town. But not in these energy conscious days.
To obtain a dry space, I COULD replicate the good old days, driving the air cold then using those honking big EHCs to reheat. Not a good plan.
I COULD re-sheave, driving the cfm low, and the air cold to have the compressor run much more of the time...except...I believe I am going to be told by the manufacturer that the minimum acceptable cfm for the unit won't be low enough to match load/cfm/humidity to give a comfortable space and not damage the compressor. It will be better, but better probably won't be good enough.
So do you long-time DX guys have a magic bullet to shoot and put this high humidity problem down?





RE: Oversized DX Equipment causing High Humidity
With both stages cycled off, and that steady fan drawing in outside air, I think you are doomed.
Slowing the fan removes more moisture, but not solving the real problem, humidity from outside air getting pumped into the place when the compressors are off.
Add a dehumidifier if there is no way of ventilating more intermittently, or put in smaller equipment so at least one stage runs steady.
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Oversized DX Equipment causing High Humidity
Replace evaporator coils with ones with low fpi.
Hot-gas injection at evap entry.
Hot-gas bypass at compressor.
RE: Oversized DX Equipment causing High Humidity
RE: Oversized DX Equipment causing High Humidity
RE: Oversized DX Equipment causing High Humidity
I would do two things: 1) disconnect the second stage compressor and 2)Put the fan on on auto ..to come on when the compressor comes on.
RE: Oversized DX Equipment causing High Humidity
from Copeland
RE: Oversized DX Equipment causing High Humidity
RE: Oversized DX Equipment causing High Humidity
Does anyone have a slick way to pipe up the hot gas reheat?
RE: Oversized DX Equipment causing High Humidity
RE: Oversized DX Equipment causing High Humidity
RE: Oversized DX Equipment causing High Humidity
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Oversized DX Equipment causing High Humidity
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Oversized DX Equipment causing High Humidity
RE: Oversized DX Equipment causing High Humidity
RE: Oversized DX Equipment causing High Humidity
RE: Oversized DX Equipment causing High Humidity
If you have free/cheap steam, you can also try to dehumidify the air with a dessicant system.
I've used a system from a dutch company, and it works quite nice. Before they were called "Kathabar" but 2 years ago they were bought. I don't remember the new name now. Try to Google it.
RE: Oversized DX Equipment causing High Humidity
I got from the manufscturer that if I "lock out" the 2nd compressor I can cut airflow to 200 CFM per ton on the operating compressor.
So I am thinking: Use the T-stat to enable the compressor, and the leaving air sensor to drive the air to 55F or colder. Using a VFD run down to this minimum of 200 CFM per ton. If the temperature gets too far out of setpoint, ramp up the air and turn on the 2nd compressor. But based on historical data, I don't think that will happen...maybe ever.
Additionally, carefully reviewing the OA schedule, I am able to reduce OA by maybe 60% and still comfortable meet code.
RE: Oversized DX Equipment causing High Humidity
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Oversized DX Equipment causing High Humidity
Right now all the OA is temporarily blocked off until we get this resolved.
RE: Oversized DX Equipment causing High Humidity
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Oversized DX Equipment causing High Humidity
RE: Oversized DX Equipment causing High Humidity
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Oversized DX Equipment causing High Humidity
http://www.heatpipe.com/
Are the DX coils split-face? Can one compressor then be locked out and the associated coil face be blocked off. Then reduce air flow.