Poor Humidity Removal

[khc95]

Other than fan speed and sizing of unit is there any other factors to affect moisture removal. Unit has no problem cooling conditioned space, but temerature needs to be cranked down low to acheive a "comfortable" humidity. Cooling is supplied via a roof top, all ductwork in the conditioned space. Unit has run now for three years without this issue and nothing has changed within the envolope that I am aware of. What am I missing.
Thanks

[beenthere]

May need a clean and tune.

[teddy bear]

Originally Posted by khc95

Other than fan speed and sizing of unit is there any other factors to affect moisture removal. Unit has no problem cooling conditioned space, but temerature needs to be cranked down low to acheive a "comfortable" humidity. Cooling is supplied via a roof top, all ductwork in the conditioned space. Unit has run now for three years without this issue and nothing has changed within the envolope that I am aware of. What am I missing.
Thanks

Tha amount of fresh air entering the building, the outdoor dew point of the fresh air, and the number of occupants/activities are other issues. Don't forget the why the a/c is function-refrigerant charge and settings.
Regards TB

[Airmechanical]

Originally Posted by teddy bear

Tha amount of fresh air entering the building, the outdoor dew point of the fresh air, and the number of occupants/activities are other issues. Don't forget the why the a/c is function-refrigerant charge and settings.
Regards TB

Teddy are you implying he does not need a dehumidifier

i am very proud of you

we love ya man



.

[teddy bear]

Originally Posted by Airmechanical

Teddy are you implying he does not need a dehumidifier

i am very proud of you

we love ya man
.

Not so fast. khc95 did not ask if he needed a dehumidifier, only the factors effecting humidity levels. In the final senirio, he will need a dehumidifier to maintain <50%RH during low/no cooling load conditions.

The amount of dehumidification depends on all of the above. When the outside dew point is above the inside desired dew point, an properly set-up a/c with a considerable cooling with maintain reasonable humidity levels. But as the cooling load subsides during the shoulder season, the %RH will rise to the point where a dehumidifier in need to maintain low humidity.
I get worried when you guys think I a reasonable guy.
Regards TB

[khc95]

The amount of dehumidification depends on all of the above. When the outside dew point is above the inside desired dew point, an properly set-up a/c with a considerable cooling with maintain reasonable humidity levels. But as the cooling load subsides during the shoulder season, the %RH will rise to the point where a dehumidifier in need to maintain low humidity.

I could see that but this is in the middle of the summer with 95 to 100 temperatures. Evap and filters are clean with an 18 degree temperature drop ?

[teddy bear]

Originally Posted by khc95

The amount of dehumidification depends on all of the above. When the outside dew point is above the inside desired dew point, an properly set-up a/c with a considerable cooling with maintain reasonable humidity levels. But as the cooling load subsides during the shoulder season, the %RH will rise to the point where a dehumidifier in need to maintain low humidity.

I could see that but this is in the middle of the summer with 95 to 100 temperatures. Evap and filters are clean with an 18 degree temperature drop ?

It is very clear, if you want to lower %RH by 10%, lower the indoor dew point by 3^F, by lowering the coil temperature by 3^F. This will give you 21^F drop. This will work as long as the a/c operates enough to remove enough moisture. On cool damp days when the a/c operates less, the indoor %RH rises above the dehumidistats set point, dehumidifier will remove the excess moisture without overcooling the space. Slick!
Or if you are an a/c guy, you keep lowering the coil temp until the ducts sweat of the coil freezes. The last thing you want to do is add a lowely dehumidifier. It shows you are not a man!
Regards TB
PS All dehus are created equal. Some use <50% of the energy and last 2X as long.

[khc95]

Hey Teddy

I can appreciate what your saying but as for the last three years the rooftop has done the job on it's own and now suddenly for no apparant reason it is not. I am going to slow the fan down to increase moisture removal, but I am must be missing the real issue that caused this to happen. Input has been appreciated.

[Kevin O'Neill]

1. Outdoor humidity is especially high this year.

2. If you can drop the indoor temperature to 70 degrees to control humidity, likely the unit is oversized.

3. Measure air flow. Use a hood. Or use static pressure and blower charts. Or use an annemometer and do a traverse. But measure air flow, don't guess. Should be 350 to 400 CFM per ton.

4. Have your tech clean the indoor coil.

5. Have your tech check refrigerant levels with super heat and sub cool, not just pressures.

6. Make sure you do not have outside air dampers stuck open.

7. Install a ducted dehumidifier.

It could be other things, but I would start there. What is the building used for? That could give us clues.

[martyinlincoln]

I've seen txv's fail to maintain a good superheat and not dehumidify well. Matter of fact had to replace mine two days ago because the humidity was running 65-68% with a 73 degree setpoint. Superheat went from 27 to 12 and the condensation flowed like a faucet. No idea if your rooftop has one or not but the tech will figure it out quick enough.

[Kevin O'Neill]

Originally Posted by martyinlincoln

I've seen txv's fail to maintain a good superheat and not dehumidify well. Matter of fact had to replace mine two days ago because the humidity was running 65-68% with a 73 degree setpoint. Superheat went from 27 to 12 and the condensation flowed like a faucet. No idea if your rooftop has one or not but the tech will should figure it out quick enough.

Unfortunately, not all techs are good enough. But a good tech will find that problem.