Dew Point Control

[rknauer]

I have a residential application in Houston where the humidity is oppressive during the warm months (April - November). Dew points are almost always in excess of 70F, and 75F is typical during the daytime.

I find I have to manually adjust my conventional thermostat throughout the day to get the best balance between temperature and humidity. As the evening approaches and the temperature outside drops, I have to raise the setpoint or otherwise it is too cold. But the next morning I have to lower the setpoint to lower the humidity.

I have an electronic thermostat which has provisions for 4 temperature setpoints during the day. I have experimented with different values in an attempt to hit the right combination. But I am not able to find the right combination because the humidity varies from day to day at any given time. I am shooting at a moving target.

What I need is a dual setpoint thermostat, one channel for temperature and the other for humidity. If the temp gets too high, the temperature channel turns on the A/C to cool things off. If the humidity gets too high, the other channel turns the A/C on to dry things out. If both the temperature and the humidity are in the comfort zone, then the thermostat turns the A/C unit off.

Is there such a thermostat, priced for consumer use?

Thanks,

Bob Knauer

 

[ChasBean1]

5 tons for 1,825 square feet is way oversized and is the cause of your humidity problem. You DO NOT want the unit to cycle on and off during design summer conditions. The compressor(s) should run continuously to avoid the humidity issues you mention. I have an 1,870 ft2 house (albeit in New England) that I keep comfortable during the summer with a 10,000 BTU window unit at one end of the house and an 8,000 BTU unit at the other end. They only cool to 78-82° when it's ~90 and humid out, but it lowers the dew making it quite comfortable. You have a higher sun load there, but what percentage of outside air does the unit take in? How is the wall construction? I would not exceed 2-3 tons for your application.

 

[rknauer]

1. I can't afford to run the compressor 24x7.

2. The unit performs adequately when the computers are turned off. It cycles even in the hotest weather. But turn those 4 computers on and I have to raise the setpoint several degrees to get it to cycle at all.

3. New England summers are nothing like Gulf Coast summers. Our dew points run 75-80F during the day and 70-75F at night for 6 months of the year. Our summer temperatures in my yard run highs of 105-110F. Sunshine is 14 hours thru clear skies.

4. There is no way that 2 window units would even begin to make this house comfortable when it's 105F outside with a 75F dew point - and 4 computers running inside.

5. Indeed we do have a much higher sun load here, being only 7 degrees latitude from the Tropic of Cancer. When our summers are really hot, like this one, it's because the skies are completely clear of any obstructions, and we get a heavy load of humitity from the nearby Gulf of Mexico. It's like walking into a sauna to go outside. It takes an hour to get comfortable when you come inside from outdoors, and that's from an air conditioned car.

6. The unit takes in no outside air. It is 100% recirculated air.

7. Wall is brick verneer and wallboard with insulation.

6. The HVAC contractors around here have been designing cooling plants for nearly a century and contractors would not let them overspecify because of the excess cost. I believe this unit, which was the one put in the house when it was built, is the correct specification assuming there are not 4 computers running on the hotest days of summer.

 

[lilliput1]

Typical indoor design condition is 75°F 50% RH. The dewpoint at this point from the psychrometric chart is about 55°F. This means the cooling coil has to be below this temperature so it can dehumidify - condense the moisture in the air. Thus when you set your thermostat above 75°F, you are preventing the cooling coil from doing any dehumidification.

 

[rknauer]

I am not qualified to debate your figures. All I know is that when the computers are not running and I set the setpoint to 79-80F, the house is comfortable. The unit has to be removing humidity because the air feels comfortable.

If I shut the unit off for a while the humidity builds up to a level that feels muggy. Then I turn the unit on and in a while the air feels comfortable even though the temperature has not changed all that much.

That happens when we open the house on a cool night and have to close it the next day because of excess humidity. The temperature remains about the same but the air dries out considerably.

The inlet temperature is about 80F and the discharge temperature is about 60F, maybe a bit warmer. That's about a 20F drop which is typical. That means the evaporator coils are running at least 60F or lower.

 

[lilliput1]

I suggest you read up on psychrometrics and the psychrometric chart. Dr. Carrier came up with this chart. It is a great aid in understanding air processes.

 

[rknauer]

Thank you for the recommendation. But I have so much else to read up on that I will defer to the experts on matters such as this.

 

[ChasBean1]

rk, referenced to your first 9/30 post:

1. You can't afford it because it's a 5-ton unit. It's too big to run all the time. If you can afford to run a 5-ton unit 70% of the time, you can afford to run a 2-ton unit 100% of the time.

2. Stop trying to get it to cycle! It's too big. The cycling of this unit is your problem. Especially during hot days. The compressor should run all the time and not cycle, while keeping it just comforable inside.

3. See your #6. The unit is 100% recirculated air, which means it can be even smaller in your climate because at equilibrium, outiside air conditions will have minimal effect.

4. For kicks, kill the central unit and buy two, 120 v, 1-ton window units and run them when it's too hot outside. Do this for 28 days and if you're not happy, return them on the 29th day to get your money back.

5. I'm not sure. I have no reply to this.

6. How many of your neighbors have these problems?

 

[rknauer]

My neighbors do not have 4 computers running all the time.

As I said, when the computers are off, everything works as it should. It's only when the computers are running that I have a problem.

 

[DouginMB]

Why do you keep mentioning the computers?! The computers are irrelevent. You state above that the computers account for 2000W. Does everyone in your family have their own personal server? A typical server has a plug rating of 400W. A typical PC was a rating of about 150W. The actual heat gain is only about half of this, and that's if someone is using it. If it's in energy saving mode, the heat loss is negligible. So, even if everyone is using their computer at once, you're only talking about about 2,000 Btuh. Your 5 ton unit is capable of cooling about 40,000 Btuh at the indoor conditions you describe. That 5% from the computers has almost NO effect on the space conditions. It sounds to me that when everyone is home and using their computers, there is more space load from added people, lights, TV's etc. This would add more load and might cause the A/C system to operate longer and improve the condition of the space. And then you're associating this with the computers alone.

 

[rknauer]

You ask why we keep the computers running. Good question. I try to get my wife and children to shut them off when not in use but they bitch that they have to restart them. As far as energy saving mode, I agree that they should not be putting out so much heatr. But the fact is they are.

Here is how it stacks up in reality. On a very hot day, when the temperature in my back yard is 100-105F, and there is not a cloud in the sky - and the computers are all on - the A/C unit will not shut off if I set it at 79F. It will cycle, however, if I set it to 82F.

Now I shut all the computers off and set the A/C to 79F. After a while the house cools off a bit and the A/C unit starts cycling. I have done this enough times that I know it is a real effect. Despite the theoretical calculations, the real heat load created by those 4 computers is enough to disturb the behavior of my A/C unit on a very hot day. On a "cooler" day (under 95F outside with cloud cover) I do not have this problem - the A/C unit can deal with the fact that the computers are on.

It is widely appreciated in Houston that you cannot cook on a stove or in an oven when the temperature is over 95F outside. That's why Texans are famous for their BBQ. People are literally forced outside to cook. You can read about it in Robb Walsh's book "Legends of Texas BBQ Cookbook", which is the most definitive source on Texas BBQ.

I have no idea why 95F is a magic number, but I can tell you that the phenomenon I describe is very real. As a scientist I ought to know how to conduct an experiment, so believe me when I tell you that to the best of my ability, I have verified this effect empirically on several occasions.

Perhaps you HVAC gurus can explain why 95F is the beginning of this problem. Just remember that the dew point is tgypically around 75F. Maybe when the temperature outside gets above 95F the dew point rises to a point where the A/C can't deal with it.

Who knows.

 

[lilliput1]

Your unit got enough capacity. Just leave it on at 75°F and let it pull down the stored heat and you will be comfortable. Spend money & enjoy life with your wife & kids! Heat is stored in the building & the AC unit has to bring this down first. This stored heat acts like a thermal flywheel so there is a lag effect that will affect your observation. Also note that the capacity of the unit at 105°F outdoors is reduced to about 96% of its capacity at 95°F outdoors. Turning your unit on & off can use more energy than leaving it continuously on because of stored heat building up & the AC always having to pull it down.

 

[rknauer]

Of course - that's what it is. When it gets very hot outside the condenser is less efficient.

The condenser unit sits in the backyard facing the south so it gets a steady dose of sunlight all day long. Would it be worth the expense to shade it from the direct sunlight?

As far as leaving the unit on 75F being less expensive than cycling it at 82F, that seems to contradict common sense. Since the unit is working at full capacity in terms of the electrical load at either of those setpoints, it would seem that having it cycle would use less electricity than having it on all the time.

Regarding the quality of life, I must remind you that my electric bills are in excess of $300, of which $250 is due to A/C costs - and that's with an approximate 75-25 duty cycle during the day. If I let the unit run continuously then I would expect the bill to jump another $100 - and that means the quality of life is affected in some other area.

 

[DouginMB]

I don't know what your power rates are in Houston, but $300.mo. seems awfully stiff. But I agree with lilliput: keep your t'stat at 75F. If you keep cycling the unit on-and-off, you will end up buying a new compressor real soon. And that will cost you about $1,000.

Could the electric heat be running concurrently with the cooling? That would make your power bill go through the roof and minimialize your cooling. Have you checked the ductwork for leaks?

 

[rknauer]

Texas has its own power grid - no blackouts for us. Because the state is growing so rapidly, electric power generation capacity must increase rapidly - and that costs money. We would rather pay now than pay later - like California. So our electric bills, which are regulated by the Public Utilities Commission, are reflective of the actual costs of electric generation, including new construction. And they include the cost of fuel adjustment, which is over $100 on my bill. This is the first year this has been so steep, because of very high natural gas prices. But if the natural gas producers don't make enough, they will cap the wells (which kills the well) and then we will be in trouble later when we need that gas.

The furnace is not running concurrently. It is a gas furnace and I would know it is running.

I now believe my problem is caused by two things happening at the same time: 1) When the temperature gets very hot in my yard, the condenser efficiency falls off which reduces the cooling capacity of the unit inside; 2) When this happens, the extra heat load from 4 computers strains the already marginal cooling capability. If the temperature outside is not as hot, the unit can remove the extra heat from the computers. Alternatively if it is very hot outside, then turning off the computers stops the extra heat flow in which case the unit can handle the house load even with impaired condencer operation.

I would like to know the answer to my earlier question. The condenser sits in the back yard exposed to direct sunlight for 14 hours a day. Would it be worth the cost and effort to put up some kind of sunlight barrier to keep the direct sunlight from beating down on the condenser? I estimate I could put something attractive up for under $50. There is nothing I can do about the 105F temperature in the air, but at least I can lower the heat flow into the condenser from direct sunlight.