New A/C

joe amadio

I had to replace my 25 year old air conditioning system this year. We replaced both the condenser and air handler. 3 1/2 tons. The overall feel of the house is warmer and stickier than before. The house is an old colonial and the whole house is 1 zone. Aprox 2000 square feet. Modern windows and doors and all walls where stripped of plaster insulated and sheetrocked 25 years ago. With the old system we would set the thermostat at about 68 or 69 degrees at night to sleep. the new system has to be set at 65 in order to sleep but when the stat is satisfied it feels warm and sticky, Even when running its sticky. Hydrometer ranges between 65 and 75 % humidity on the 1st floor to 70/80% on the 2nd. I never measured the humidity with the old unit because I never felt the need. I am confident that the installer has the coolant levels and all the other settings done correctly. The unit had to pass inspection by the local utility in order to qualify for a rebate. Is the humidity too high? Why isn't the new unit capable of maintaining the humidity at a comfortable level? Is humidity my problem ? What can be done?

Tinman

3.5 tons for a relatively tight, 2000 sf home sounds way oversized to me and would account for your discomfort. Did your contractor do a heat gain load calculation to size your new system?

ChrisJ

What CFM is the blower set for and are there any ramping options?

What equipment is the new system?


How long was the house unconditioned for and how long has the new system been running?

EBEBRATT-Ed

3 1/2 ton requires a total air flow of 1400-1575 cfm of air flow. (400-450cfm/ton)

It will remove humidity better with air flow on the low end.
Did you replace a 3 1/2 ton with a 3 1/2 ton??

It's possible but unlikely your moving too much air.

Take an air temperature check of the air in the ductwork across the coil return air and supply air and see what you get. Should be about 20 deg with the unit running.

My first guess would be an oversized unit if the charge is correct and other parameters are ok

ChrisJ

If the old unit was the same size why would that suddenly be the problem?

Tinman

Maybe the old system, like many old systems, wasn’t working very efficiently. Dirty coils, dirty blower wheel, etc.?

unclejohn

Does the unit shut off by the thermostat or does it run all the time?

Ironman

How do you know the installer has it charged and setup correctly? It may be, but unless you can give us the numbers, we can't tell.

Please get your contractor to give as much of the following info as possible:
1. Outdoor ambient temp.
2. Indoor dry bulb and wet bulb temps after the system stabilized.
3. Suction and discharge pressures.
4. Sub cooling and superheat numbers.
5. Cfm of indoor blower.

John Mills_5

Old systems dehumidified better and got to full dehumidification faster. So it is even more critical these days not to oversize. Shorter run cycles means less humidity removed. Also many mfrs are calling for 350 CFM per ton to help with latent capacity. Rheem variable speed default to that and if you want, you can add 10% more by flipping a DIP switch. I think Trane does too on the S9V2. Finally you do not want to run the blower continuously so be sure you are on AUTO fan setting.

ChrisJ

John Mills said:

Old systems dehumidified better and got to full dehumidification faster. So it is even more critical these days not to oversize. Shorter run cycles means less humidity removed. Also many mfrs are calling for 350 CFM per ton to help with latent capacity. Rheem variable speed default to that and if you want, you can add 10% more by flipping a DIP switch. I think Trane does too on the S9V2. Finally you do not want to run the blower continuously so be sure you are on AUTO fan setting.

I've found this to be extremely true in my own home.
In fact, even using Honeywell's "Circulate" mode raises the indoor humidity even though many claim it won't because the evaporator drips dry first.

Slow blower ramping on startup, short run after shutdown also both help.

GBart

Yeah but running the fan constantly is always better, but I only do it when I'm home, think occupied mode for a commercial building.

ChrisJ

> @GBart said:
> Yeah but running the fan constantly is always better, but I only do it when I'm home, think occupied mode for a commercial building.

How so?

SuperJ

Check out this post, his blog is well respected in the Building Science space:
https://www.energyvanguard.com/blog/76674/This-Thermostat-Setting-Can-Cost-You-Money-and-Make-You-Sick

Constant fan will re-evaporate enough moisture off the coil to noticeable raise the humidity in a home. I only run constant fan in my home when my system isn't naturally cycling (basically shoulder seasons).

If everything else checks, it sounds like oversizing to me. Some stats like the Ecobee have some settings to extend cooling runtime to cool a bit below setpoint under high humidity conditions to try and pull some more moisture out. This won't be a silver bullet but might help a bit.

GBart

ChrisJ said:

> @GBart said:

> Yeah but running the fan constantly is always better, but I only do it when I'm home, think occupied mode for a commercial building.



How so?

Ever heard of stratification?

Commercial buildings run the fans during occupied times to evenly distribute the temperature and prevent stratification, it also cleans the air more by running it through the filter(s), it lengthens the life of the motor by cutting down on 24 or more starts a day, it cuts down or prevents hot and cold spots.

The downside is usually only when the ductwork is in unconditioned space and not properly insulated or sealed and yeah you will use more electricity, the better systems can run the fan on low when there is no call and the best systems are variable speed that ramp the blower down but not off.

Some people claim that if the fans keeps running it will take the dew off the coil and send it into the house and raise humidity, bull, that would happen if your velocity is too high and it would also send water into your ducts.

Running the fan during occupied times also helps with minor balancing problems, when was the last time you saw anyone air balance a home?

SuperJ

I don't disagree, that there a ton of good reasons to run your fan constantly and it's basically unavoidable in commercial buildings with high occupancies and ventilation requirements, but under exceptionally high dewpoint conditions that might only happen for a couple days each year, you may want to cycle the fan at home. It's not all roses all the time.

The humidity is coming from moisture evaporating off the coil, which is greatly multiplied if air is moving, it's not coming from droplets being carried down the ductwork (unless you have a sizing problem leading to excessive air velocities). High velocity is a totally separate problem than evaporating moisture sitting on the coil by flowing air over the coil.

ChrisJ

> @GBart said:
> > @GBart said:
>
> > Yeah but running the fan constantly is always better, but I only do it when I'm home, think occupied mode for a commercial building.
>
>
>
> How so?
>
> Ever heard of stratification?
>
> Commercial buildings run the fans during occupied times to evenly distribute the temperature and prevent stratification, it also cleans the air more by running it through the filter(s), it lengthens the life of the motor by cutting down on 24 or more starts a day, it cuts down or prevents hot and cold spots.
>
> The downside is usually only when the ductwork is in unconditioned space and not properly insulated or sealed and yeah you will use more electricity, the better systems can run the fan on low when there is no call and the best systems are variable speed that ramp the blower down but not off.
>
> Some people claim that if the fans keeps running it will take the dew off the coil and send it into the house and raise humidity, bull, that would happen if your velocity is too high and it would also send water into your ducts.
>
> Running the fan during occupied times also helps with minor balancing problems, when was the last time you saw anyone air balance a home?

The velocity of my constant fan is very low. 400 CFM or so in a 3 ton AHU.

It raises the indoor humidity a lot. A good 10% or so.

GBart

Got any documentation to prove that? Your humidity could be going up on the off cycle through infiltration. Not from your system.

I have a Unico system, when I turn it on I might be @65%, after a while I'm down to 50% or so and when my system cycles on and off my humidity does not shoot up 10%, in fact I don't ever recall seeing any home or building that did that between cycles unless the outdoor humidity was high and there were holes in the walls.

ChrisJ

> @GBart said:
> Got any documentation to prove that? Your humidity could be going up on the off cycle through infiltration. Not from your system.
>
> I have a Unico system, when I turn it on I might be @65%, after a while I'm down to 50% or so and when my system cycles on and off my humidity does not shoot up 10%, in fact I don't ever recall seeing any home or building that did that between cycles unless the outdoor humidity was high and there were holes in the walls.

Documentation?

I've observed it many times. It doesn't do it with the fan on auto. My ductwork is sealed, literally. As in it would pass a pressure test. The fan isn't increasing infiltration.



If you have a hundred wet fins and you pull drier warmer air through them you'll end up drying them.


For years Hunter sold room humidifiers that used a metal filter structure that was very similar to an evaporator that it drizzled water down while blowing air through it rather than a wick and they worked very well.


That said, @GBart my house stays around 46-48%. It slows the blower up at 50%.

KC_Jones

GBart said:

Some people claim that if the fans keeps running it will take the dew off the coil and send it into the house and raise humidity, bull, that would happen if your velocity is too high and it would also send water into your ducts.

So what you are saying is water doesn't evaporate into the air stream raising the humidity, somehow it defies the laws of physics.

That's an interesting phenomenon.

Tinman

If its very hot and humid outside and your AC off cycles are relatively short, the evaporator coil won't have much time to cool off. Why not advantage of that cool coil? Not all systems are new enough to have a fan time delay on the off cycle.

pecmsg

Commercial buildings run the fan constantly because of all the bodies per Sq Ft, ever changing heat loads / losses!
A properly sized home unit should run most of the time. All at design conditions giving enough air movement to prevent "Stratification"

ChrisJ

Stephen Minnich said:

If its very hot and humid outside and your AC off cycles are relatively short, the evaporator coil won't have much time to cool off. Why not advantage of that cool coil? Not all systems are new enough to have a fan time delay on the off cycle.

Hi Stephen,

In my house I'd rather no off delay at all if the firmware in the ECM controller allowed it.

Sadly I'm limited to 1 minute at 100%, or a shorter one but only if I also do a really long 8 minute ramp on startup which seems aimed mainly at those without a dehumificiation control.

While I understand even though the pump isn't running there's still refrigerant boiling for a bit, I'd rather not let the coil warm up any more than it has to while the fan is running.


I've tried both continuous run, as well as the Prestige's "Circulate" option which runs the fan occasionally while the system is off. I guess the theory is the evaporator has some time to drip dry a little more before the fan runs, and I guess it saves on electric.

Both caused my indoor humidity to go up quite a bit. Continuous run of course did more.

If the OP is having humidity issues, by all means his blower needs to be on AUTO. He also needs to lower his CFM as much as possible (325-350 CFM per ton).

ratio

I'm curoius about this. It seems to me that the only water that can reenter the airstream is the water that is actually sitting on the coil, and some/all of the water that is in the pan. If the condenser's been below the dewpoint for a while, there should have been some water leaving the envelope. Unless the on time isn't long enough to let the water start to drip off the condenser, it should slowly remove water, assuming the influx isn't too great.

ChrisJ

> @ratio said:
> I'm curoius about this. It seems to me that the only water that can reenter the airstream is the water that is actually sitting on the coil, and some/all of the water that is in the pan. If the condenser's been below the dewpoint for a while, there should have been some water leaving the envelope. Unless the on time isn't long enough to let the water start to drip off the condenser, it should slowly remove water, assuming the influx isn't too great.

I'd say you're accurate.
But how much water can lay on a high efficiency evaporator? The one I have is the size of a Buick.

I suppose it also depends on the duty cycle of the system. The more it runs the less this will effect everything.

The times I notice an issue is early in the morning when it's cool out and my setpoint goes up 2 degrees so the system stays off for a long time.

Under those conditions my RH climbs a lot if I let the fan run.

ratio

@ChrisJ, are you bring in in outside air whenever the fan is running?

ChrisJ

> @ratio said:
> @ChrisJ, are you bring in in outside air whenever the fan is running?

Not that I'm aware of.

ratio

No fresh air intake? Kind of rules that out then. Back to the drawing board I guess.