AC humidity Issue

ChrisCole

I am in a condo that is using fan coil system. Having an an issue with high humidity. When AC is running RH out of vents is 65% when AC stops RH out of vents is 90%. I am guess not reaching dew point therefore ac turns off air is just running over wet fan coils and blowing high humidity back in. I am running fan on auto, however when AC is not running fan still runs at low speed. But thinking in condo fan would always have to be in for air flow. 

Anyone know if this would likely be issue with fan speed issue or maybe not cold enough water from chiller?

Any help would be greatly appreciate. 

HVACNUT

Is the damper open on the fireplace?
If the fan setting on the thermostat is set to auto, then then fan should be off when the A/C goes off. There might be a time delay, but it should turn off. 
Proper system sizing and insulation of the ductwork and the building envelope will help control humidity. 

ChrisCole

There is no fireplace.  This is a condo. I am not sure if in condos the fan should turn off when ac cycle is done . If it did how would you get air. 

EBEBRATT-Ed

Is this chilled water system or a dx system? I am guessing chilled water.

See if you have a strap on thermometer and check the temp of the supply and return water to the coil. You should be something like 54 return water and 44 supply (or lower) water in and out of the coil. This will give you an average coil temperature of 49 which will give you adequate dehumidification if the equipment is sized right.

Filters clean, coil clean, coil control valve wide open????

An ac coil is always wet when running, you can't measure the humidity of the discharge air it will always be high when dehumidifiying as the air temp is cold. This is relative humidity. Measure the return air or room humidity which should match

pecmsg

Being a condo the supply fan may always run to bring in fresh air.

If it is Chilled water the control valve should modulate matching the heat load.

ChrisCole

Yes it's a chilled water system. Yes filter clean, coil clean, valve is wide open. 

The RH of the room inside is 75%, outside air is 52% RH, when Ac on RH at air at vent is 65% and when air turns off RH out of vents is 90%. 

Could this be caused by the water supply being greater than 44?

Is the dehumidifying capability have to do with supply temp or is it the difference between supply and water out temp or both?

dopey27177

Wild Guess!!
Do you have a damper in the unit to mix outside air and inside air. Some units have have a damper that modulates with a humidistat or maybe the damper is stuck in one position.

Jake

Jamie Hall

Let's go back to square one here. We are dealing with two very different measures of the amount of water in air: relative humidity and dewpoint. The principle behind dehumidification by cooling (there are other methods) is very simple: a stream of air is passed over a coil, where it is cooled. As the air is cooled, it may drop below the dewpoint for its moisture content, and, if so, the excess water will be condensed on the coil and, hopefully, drain off. When the air exits the coil under those conditions, the dewpoint will be the same as, or very close to, the air temperature, and the relative humidity will be at, or very close to, 100%. In order for dehumidification to be accomplished, that air must be reheated. The dewpoint will remain the same (no moisture added or removed) but as the temperature is increased the relative humidity will drop.

The reheat can be accomplished in one of two ways. The most controllable is to simply add heat with a heating coil. This is done in more sophisticated systems, but uses more energy. A simpler, but much less controllable approach, is to mix some uncooled air with the cooled, dried air. It is very likely that this is what is being done in @ChrisCole 's system. This works, but the output air will have a higher dewpoint than the air directly from the coil, but at the same time lower than the ambient air.

What happens when you turn the fan off running air over the coil? The air is no longer moving, and remains at or near the coil temperature -- and at or near 100% relative humidity. This appears to be what is being observed in this instance, and is to be expected. Air with a dewpoint of 44 -- the lowest that coild be provided by the chilled water in this instance -- will, when reheated to 68, have a relative humidity of around 46%. That the relative humidity when the system is running is higher suggests that there is some bypass air, but possibly not that much; the dewpoint quoted with the system running is around 55F and may simply reflect some inefficiency overall in the cooling.

ChrisCole

Yes the reheat is using uncooled air from outside. 

But if the supply water to coils is greater than than 44 could this be the cause of the issue?

Note there is no water in drain pan. I can only see water in drain pan when I shut ac and fan off manually for longer period of time. 

Does water only drain to drain pan, or is there another drain behind fan coils?

Jamie Hall

What is the relative humidity and ambient temperature in the conditioned space with the AC running, and with it off? What is the relative humidity and ambient temperature of the outside air being used for reheat?

As I implied earlier, if you are running 100% recycled air and your coil can drop the exit air temperature to 44F, the relative humidity when the air warms back up (from other ambient heat sources) will be around 43% when the system is running. Addition of any outside air with a higher dewpoint will require a cooler air temperature on exit from the coil. The lowest you can feasibly go with chilled water and 100% recycled air is 26% relative humidity when warmed to room temperature, but that requires a chilled water temperature of 32F, which is difficult to maintain, and no outside air whatever, which leads to very poor indoor air quality.

ChrisCole

The thermostat is set to 21C

Ac conditioned space with AC running is 73% RH with an ambenant room temp of 22C. AC turns off RH 70% and ambenant temp is 21C. 

Info above I did not use the fan coil unit thermostat because the thermostat was installed under a vent and right near fan coil unit. Also when I remove the thermostat there is a hole in wall leading to fan coil unit. I attached a picture. Note fan coil unit  just right of wall in picture.  picture. 

pecmsg

Have you measured the inlet and outlet pipes?

ChrisCole

Unfortunately I can not measure pipes. 
Is it possible if inlet water temp is not lower than 44F this could be cause of moisture issue?

Jamie Hall

I don't think your water temperature has much to do with it. I've tried to explain above -- but the bottom line is that you may not be able to do much better, given the overall situation.

EBEBRATT-Ed

@ChrisCole

Yes lower your water temp will drop the humidity. What temp are you running?

For example we had a space we had to maintain sort of a clean room. We had to maintain 70-72 temp and 50% rh in the room. The chiller was running 54 return and 44 supply. Wouldn't work

To get the RH down the entire coil must be below the dew point which in our case was about 50 degrees if I recall. Our coil only 1/2 of it was below the dew point. Lowering the temp dropped the RH immediately. We had to run 38/48, 40/50 almost did it but not quite

ChrisCole

So the issue may be from mixing of outside air with conditioned air?  Could changing how much outside air is mixed help?

Jamie Hall

ChrisCole said:

So the issue may be from mixing of outside air with conditioned air?  Could changing how much outside air is mixed help?

Yes -- but be very careful about indoor air quality.

pecmsg

That “System” was engineered. Do not make changes without the building owners permission. 

Indoor air quality and Fire Protection laws are strict. 

ChrisCole

I assume the outside air is mixed with inside using actuator and a vent flap. Where are these usually located?  On the outside vent? Or within the coil unit?

Also just trying to find possible root causes. I would not make any changes without building owners and system engineers permission. 

Jamie Hall

Dehumidification isn't rocket science, @ChrisCole . There are two ways to remove water vapour from air. If you really want to get it dry -- very dry -- you use a desiccant. Two problems. Desiccants in any quantity aren't cheap, and the other is they have limited capacity. Hence, limited use.

The other way, which works splendidly, is to cool the air below its dewpoint. Down, in fact, to the temperature corresponding to the desired dewpoint which, in turn, determines the relative humidity of the resulting air in combination with the final air temperature. This condenses the excess moisture. You then reheat the air to the desired final temperature and there you are.

The arithmetic becomes interesting when you are dehumidifying some of the air and then mixing it with non-dehumidified air, such as Then you need to convert the dewpoint of the two air streams into grams per cubic metre, and add them up to get the grams per cubic metre of the mixed air and convert that back to dewpoint or relative humidity. Not hard, just messy.

And yes the amount of outside air brought in is usually controlled with a vent flap somewhere, or sometimes by an exhaust unit or combined exhaust and heat exchanger. You want at least a certain minimum level of outside air per hour to maintain air quality -- usually we use a figure of two to four air changes per hour for most residential or office type usages. Some other usages -- medical offices or dusty manufacturing, for instance, may use 100% outside air, with no recirculation at all, and may (often do!) have much higher air change per hour figures.

ChrisCole

So if I am understanding correctly.  To reach dew point the two temperature would be fan coil temperature and the air temperature (recycled air or mixture of recycled air and outside air) blowing over fan coils. 

How is the fan coil temp changed if there is a set temp from chiller via inlet pipe to fan coil unit?  Also how does the outside RH a factor in creating dew point?

Jamie Hall

If the chiller temperature is fixed, which it probably is in your condo, you can raise the fan coil temperature -- which you don't want -- but you can't lower it. Sorry.

The outside relative humidity factors in in this way. Air can hold a certain number of grams per cubic metre of water vapour in it, depending on temperature. The higher the temperature, the more water. Relative humidity is simply the fraction of the water vapour in the air vs. the maximum amount the air can hold at that temperature. Changing the temperature of the air will change the relative humidity, but not that actual mass of water vapour so, for example, if you raise the air temperature the relative humidity drops.

The dewpoint of an air mass is the temperature at which the actual mass of water vapour in the air equals the maximum amount that can be held at that temperature. Any decrease in temperature below that will result in some of that water condensing out -- in a fan coil, on the coils. Outside, as fog or clouds.

Now as I noted above, your fan coil will take the air passing over it and lower its temperature to the average temperature of the coil. If that temperature is below the dewpoint of the incoming air, some water will be removed and the exhaust air will be saturated, or nearly so -- 100% relative humidity. That really means that it contains the maximum mass of water vapour that it can possibly contain at that temperature. The outside air will not, in general, be at its dewpoint, so one can take its relative humidity and from that and the temperature determine the mass of air per cubic metre which it contains. Now if you mix the two air streams, you will have a total flow of the combined flow, of course, and you will also have a total mass of water of the combined masses, and a temperature related to the relative volumes and initial temperatures. If you take that combined mass of water vapour and the combined volume of air, and compare it to the maximum amount of water vapour at that temperature, you can calculate the resulting relative humidity.

mattmia2

Jamie Hall said:

If the chiller temperature is fixed, which it probably is in your condo, you can raise the fan coil temperature -- which you don't want -- but you can't lower it. Sorry.

Couldn't you reduce the coil temp somewhat by reducing airflow through the coil so the rise in temp is less so you are condensing more moisture out of less air? There would be some reduction in capacity, but maybe you still have enough capacity with that reduced capacity.

Or does it balance out to the same amount of moisture removed just from more or less air so it ends up being the same rh when blended with the fixed amount of outdoor air?

Jamie Hall

mattmia2 said:

Jamie Hall said:

If the chiller temperature is fixed, which it probably is in your condo, you can raise the fan coil temperature -- which you don't want -- but you can't lower it. Sorry.

Couldn't you reduce the coil temp somewhat by reducing airflow through the coil so the rise in temp is less so you are condensing more moisture out of less air? There would be some reduction in capacity, but maybe you still have enough capacity with that reduced capacity.

Or does it balance out to the same amount of moisture removed just from more or less air so it ends up being the same rh when blended with the fixed amount of outdoor air?

You'd have to run the numbers on it... you would get a slightly lower overall temperature of the coil with reduced flow, so a slightly lower output dewpoint, but I rather suspect that at the end of the day the overall dewpoint -- and hence relative humidity after blending -- would be pretty close to the same.