Humidity Spike After New A/C Unit Installed

[motoguy128]

I'd like to take a step back and ask how the previous systme performed in terms of cooling hte house. You said it was undersized, but you currently keep the upstairs and downstairs warmer than normal indoor design conditions.

I had a 3.5 ton system upstairs before and it struggled to keep it under 55%RH at night all last summer in hot and even more in mild weather. I downsized to a 2 ton and now it's under 50%RH without even going into "dehumidification mode" at all. This morning it was 78F outside and it was still 47%RH upstairs.

Could something be improved on your new system. Possibly. Anytime you change the equipment size, airflow and coil size, you'll change the dynamics of air distribution and humidity removal.

For all we know you ductwork was marginally sized for 2.5 tons and 1000CFM, now at 3 tons and 1200 CFM it's undersized so you have issues with even air distribution, so the downstairs is now getting more cool air.

How can we fix your problem? A competent contractor the really knows ductwork, equipment per formance and building envelope issues need to look at your home. A lot of these issues aer almsot improssible to diagnose and identify through a computer screen.

For example, maybe the new systm is depressurizing the basement more than before and now it's drawing in hot humid outside air through the sill plate, which on most homes unless it's carefully sealed, is very leaky.

[SoFlaDave]

No harm no foul, I apologize if I came across a bit saucy lol. The odor is odd but it may be mastic or some other adhesive off gassing. Some of them smell awful. I wouldn't be too concerned with the slight rise in humidity. If there are any blower off delays they should be disabled as they tend to dump moisture back into the air when the compressor cycles off.

[ThatOneGuy]

Where are you located?

I ask this because you said that it has been 100 degrees is that normal?
If you are in an area where the design temperature is 92 then your unit could be sized 100% correct but it is doing all it can given the load placed upon it.

It's like getting a truck that says 20 MPG but that is designed for certian circumstances. Put a ton of cement in the back and tow a trailer and the truck will still move, but you won't get 20MPG.

Indiana. We're having a record drought, and flirting with records for high temperatures. We've had countless days in the 90's and hit 100 several times.

The old system was undersized, and didn't cool the basement well, but the humidity was lower. I'm beginning to understand from this thread that humidity is relative, meaning a higher humidity reading doesn't mean more moisture.

[ThatOneGuy]

I'd like to take a step back and ask how the previous systme performed in terms of cooling hte house. You said it was undersized, but you currently keep the upstairs and downstairs warmer than normal indoor design conditions.

I had a 3.5 ton system upstairs before and it struggled to keep it under 55%RH at night all last summer in hot and even more in mild weather. I downsized to a 2 ton and now it's under 50%RH without even going into "dehumidification mode" at all. This morning it was 78F outside and it was still 47%RH upstairs.

Could something be improved on your new system. Possibly. Anytime you change the equipment size, airflow and coil size, you'll change the dynamics of air distribution and humidity removal.

For all we know you ductwork was marginally sized for 2.5 tons and 1000CFM, now at 3 tons and 1200 CFM it's undersized so you have issues with even air distribution, so the downstairs is now getting more cool air.

How can we fix your problem? A competent contractor the really knows ductwork, equipment per formance and building envelope issues need to look at your home. A lot of these issues aer almsot improssible to diagnose and identify through a computer screen.

For example, maybe the new systm is depressurizing the basement more than before and now it's drawing in hot humid outside air through the sill plate, which on most homes unless it's carefully sealed, is very leaky.

The previous system performed poorly, in my opinion. The home builder sized everything for 2000 square feet because that was the floor plan. We chose to add a basement to the floor plan. The HVAC company, whether at their own discretion or at the builder's direction, ran the ductwork for the finished basement but made no changes of any kind to the original equipment. The was no load calc, etc.

So, we moved in, and the basement was FREEZING cold. This was with 2 x 4walls, insulation, insulated ceiling, etc. I installed a supplemental heating system so we could utilize the space in the winter. When the original furnace had a part failure, I upgraded to an efficient, expensive Lennox system. The difference was immediate and dramatic and we know longer use the supplemental heat. In fact, now I have a separate cooling zone for the theater room because that room gets so hot in the winter, even when I shut the heater vents to that space.

We still struggled with cooling. The basement was always stuffy, oddly, as one would think it would be cooler than the ground level. However, the Santa Fe solved the humidity problem immediately. I highly recommend it, as the cheap $200 dehumidifiers didn't make a dent in our 2000 square foot basement.

So, when the 2.5 ton original unit failed recently, instead of repairing it I chose to upgrade. The 3 estimates ranged in recommendations from 3 ton to 3.5 to 4 ton. All did load calcs, though they obviously observed the same home and reached wildly different conclusions. I went with the 3 ton contractor, and that unit kicks butt. The basement is now actually cooler than the ground level, and we would be very happy if not for the humidity increase and the musty odor that occurred literally within a day of the new system going in.

The air comes out much more forcefully now. It is my understanding that there is now a variable speed blowing action that was not part of the original configuration.

I don't think the new unit is oversized because it has been running consistently (we hit 100 yesterday, though today was only in the 90's). With the old system, when I held my hand over the basement vents with the a/c on there was very weak air flow. Now, it is on for extended periods, which I understand is a good thing because cycling on and off repeatedly isn't ideal, and it circulates cool air instead of giving us the anemic flow we had previously.

[Shophound]

You need to find the dew point temperature of the air in your basement. After that, if you have carpet anywhere on the basement floor, I would peel a section back and measure the temperature of the floor surface below the carpet. If it's colder than the dew point temperature you measured, you could be experiencing high relative humidity levels right where the carpet is, even if you see lower RH levels out in the room.

I'm recommending dew point for several reasons...if it is above a certain number, your air conditioning system is not removing enough moisture to keep musty odors from showing up. It also gives you a tool to find out where micro-climates in your basement may arise favorable for mold development.

Air conditioning systems that are slightly overcharged can cool adequately but lag in dehumidification. This is especially so if the system uses a fixed metering device vs. an expansion valve.

[hvacmike85]

You need to find the dew point temperature of the air in your basement. After that, if you have carpet anywhere on the basement floor, I would peel a section back and measure the temperature of the floor surface below the carpet. If it's colder than the dew point temperature you measured, you could be experiencing high relative humidity levels right where the carpet is, even if you see lower RH levels out in the room.

I'm recommending dew point for several reasons...if it is above a certain number, your air conditioning system is not removing enough moisture to keep musty odors from showing up. It also gives you a tool to find out where micro-climates in your basement may arise favorable for mold development.

Air conditioning systems that are slightly overcharged can cool adequately but lag in dehumidification. This is especially so if the system uses a fixed metering device vs. an expansion valve.

How do you find the dew point of a givin space???

[motoguy128]

How do you find the dew point of a givin space???

What do you mean? You use a hygrometer. In most cases a $20 temp & humidity gauge from walmart will work. Not the most accurate, but precision/repeatable enough for most purposes.

[hvacmike85]

What do you mean? You use a hygrometer. In most cases a $20 temp & humidity gauge from walmart will work. Not the most accurate, but precision/repeatable enough for most purposes.

I don't have anything like that, didn't know I needed it for this trade I have every other air measureing device, i was just curious I have never needed to know dew point, so its a tool for just dew point then?

[tipsrfine]

The old system had a PSC blower motor and the new system has a ECM variable speed motor? The new system blows much more air than the old one? I'm with Motorguy as to the likelihood that the new systems blower motor is increasing infiltration/exfilitration. A stronger blower motor would increase pressure imbalances in the home, if the return & supply system is not ballanced, and especially if there is duct leakage in duct work that is connected to the outside of the homes envelope, i.e., attics. Of course this is dependant on how much moisture the outside air has. If the outside air has high moisture content, and is being introduced into the home faster than the a/c can remove it, then it is a place to look into.

[Shophound]

How do you find the dew point of a givin space???

Fieldpiece just came out with a digital temperature and humidity stick that not only measures temp and humidity but can convert same to wet bulb and dew point. It is not very expensive. I have one and like it.

Dew point is a great thing to measure. If you go on a call with a running a/c that is underperforming, measure dew point inside the conditioned space. If it is above 55 degrees, even if the dry bulb temperature is, say 75 or below, the system is not temoving enough moisture from the air.