Nervous about 2-ton AC for a 3400 sq ft zone in a new house

[teddy bear]

The attachment shows average R.H. at 57%.
Which area is this _ Lower level in WI?

This is SE WI two level home built into a earth bank on one side. The lower level is finished with full exposure on one side and 50% exposure on the two sides. All of the walls are close cell spray foamed. The slab has 4" of ridged foam. The ventilation is ERV, the heat is heated floors and Inv. VS heat pump/elect backup. This data was collect during a draught condition. This problem is much worse during wet cool weather, which we have not had.
The cooling supplies to the lower level are all closed this week. It is better. The fan is "on" low which is better to warmup the lower level. The a/c is doing as much as can be expected with vey little short cycling. The occupants complain about the 3^F overcool.
Fresh air ERV ventilation is limited by a CO2 controller which limits ventilation to a minimum.
We have a dehu on site but did not operate it this week. The next week will have the dehu set at 50%RH and a/c dehu overcool mode off.
All of the testing that we have done in various parts of the green grass climates confirm the point of a/cs being unable to provide <50%RH during wet cool weather and during low/cooling loads with high outdoor dew points plus moisture from occupants.
Regards TB

[teddy bear]

I think what is happening with the oversized systems is airflow is well below 400cfm/ton due to poor ductwork, therefore better dehumidification.

All a/cs should have the air flow reduced to get the 45-48^F coil temperature. It is critical to get the dew point and %RH down to a comfortable level during the longer cooling cycles. Small a/cs have fewer off cycle which results in less coil moisture being evaporated back to the home. Oversized, ideal, and small a/cs are unable to remove moisture from a home when the cycles are short, like <20 mins. starting with a dry coil. All the moisture left on the coil evaporates back into the home during the off cycle. Fan "on" is about 45 mins. Fan "off" is 90 mins.
There are many days when this no cooling load, the outdoor dew point is about the desired inside dew point, and the house is occupied by moisture generating occupants. Then what. Live with the humidity or get a get a good whole house dehu.
From my testing, it appears that the you should have enough cooling capacity to pull a home down from t-stat setup when not occupied, an extra couple occupants at design temp, and for future capacity decline as the system ages. Mainly because an undersized a/c is unable to maintain <50%RH during low/no cooling loads.
Regards TB

[jtrammel]

TB, What VS unit is used in the home? Is it a variable compressor or 2 speed?

[teddy bear]

A gallon vs a pint? That gets MY attention.

TB, could you please tell me more about this?

There are three dehumidifier designs.
1. A/cs are the most basic dehumidifiers. The cool temps determine the amount of sensible cooling verses the latent cooling. High air flow get you the most btus per watt of electricity. Set up for 75^F, 50%RH, 55^F dew point space requires a 45^F coil temp. This is the lowest pints-lbs. per kw. Consider that an a/c runs for 20-30 mins. to load the coil (1 lb. per ton) and then starts to drip to the drain. Moisture left on the coil evaporates back to the home and increases indoor %RH. Efficiency 1-2 pints/kw.
2. Conventional dehumidifiers have a cooling coil followed by by the condensing coil. Larger coils, more efficient compressors/fans determine the efficiency. Results are 1-4 pints per kw.
3.The low grain/high efficiency dehumidifiers have large cooling coils/condensing coils plus a heat exchanger installed between the cooling coil and the condensing coil. The warm wet air entering the dehumidifier passes through one side of the heat exchanger being cooled by the cooled dry air leaving the cooling coil. This is the source of the great boost in efficiency. Results are 5-8 pints per KW. The top Santa Fe/Ultra-Aire models include this technology.
I hope this helps.

Regards TB

[teddy bear]

Yes I want to know more also. Does the A/C calculation include the sensible heat removed as waste or useful work?

The a/c calculation includes the sensible as waste. The senerio is over-cooling for the sake of dehumidification. The idea is to set up the a/c to remove moisture while cooling the space as the first line of dehumidification by maintaining a 45^F coil temp. During long runs the a/c will maintain <50%RH in the space. After the space reaches the desire temp, the a/c shuts off. Use a high efficiency dehumidifier to provide <50%RH while space is cool.
Regards TB

[teddy bear]

TB, What VS unit is used in the home? Is it a variable compressor or 2 speed?

The unit in this home is a variable speed inverter compressor. I was impressed with it's ability to keep running thus avoiding the "off" cycle with the 3^F over cool limit. The system did better than single speed with VS fans or two speed systems. However, without any cooling load, the end result is the same, high humidity with a colder than desired indoor temperatures. The up charge on this system was enough to add a small whole house dehu.
Regards TB

[motoguy128]

Keep in mind, that we have run the numbers and per pint of water removed, a conventional split 15+ SEER systme is still evne more efficient for what water it does remove. But TB is right, in that durign low load conditions, they are unable to effectively remove enough water and you often overcool the space to dehumidy unless you have a home with higher than normal heat gain (lots of windows). The best set-up uses a combination of a WH dehumidifier with ventilation, and a HE AC system. The AC system remvoes what it can and even uses dehumidify on demand, and the WH unit pick up the slack. In winter a tight home, when it's below 45F outdoors, you can use outdoor air primarily to dehumidify the space.

I'll install one in my home after I upgrade the downstairs HVAC system, then upgrade the electrical upstairs so I can actually power a WHD. My furnace doesn't even have a dedicated circuit. It's almost a must have items for good IAQ in a spray foam house.

[tedkidd]

jt - pretty sure it's a Greenspeed.

3.The low grain/high efficiency dehumidifiers have large cooling coils/condensing coils plus a heat exchanger installed between the cooling coil and the condensing coil. The warm wet air entering the dehumidifier passes through one side of the heat exchanger being cooled by the cooled dry air leaving the cooling coil. This is the source of the great boost in efficiency. Results are 5-8 pints per KW. The top Santa Fe/Ultra-Aire models include this technology.
I hope this helps.

Regards TB

Links? Diagrams?

I have a high end zoned, communicating hybrid system going in this fall. Thinking whd might be in order, particularly at that cost per gallon. So time to start digging.

Thanks!

[teddy bear]

jt - pretty sure it's a Greenspeed.



Links? Diagrams?

I have a high end zoned, communicating hybrid system going in this fall. Thinking whd might be in order, particularly at that cost per gallon. So time to start digging.

Thanks!

http://ultra-aire.com/pdf/Ultra-Aire...05H_Manual.pdf
This is the install manual.
Is this what you had in mind? On the last page, there is a line drawing showing all parts including the air to air heat exchanger.
Do you need clarification on how the heat exchanger concept works?
Regards TB

[tedkidd]

Hey tb, thanks. I think I get it, except the air to air he. I see there is a fresh air option, I guess you'd connect the erv their? Is that what you were referring to?

[CraziFuzzy]

Hey tb, thanks. I think I get it, except the air to air he. I see there is a fresh air option, I guess you'd connect the erv their? Is that what you were referring to?

The picture doesn't really show it that well. It has nothing to do with the outside air intake. Effectively, here's the air-flow path:

Intake -> HEX -> Evaporator -> HEX -> Condenser -> Outlet

So the air is first pre-colled in the HEX, then passes through the evap, further cooling it below dew-point, condensing out the moisture, then it passes back through the HEX, cooling the incoming air, then through the condenser, removing the heat from the refrigeration system, then the dry air leaves through the outlet, warmer, but MUCH dryer than it entered the inlet. In essence, it converts a lot of the latent heat to sensible heat, plus some extra sensible heat from the electrical draw of the compressor.

[duckman06]

Trane makes a 2.5 Ton two stage XL20i

I have been a trane dealer for 8 years and have never seen/heard of a (2.5) ton XL20i? I am not saying you are wrong! What is the model number? I want to check it out.

[teddy bear]

The picture doesn't really show it that well. It has nothing to do with the outside air intake. Effectively, here's the air-flow path:

Intake -> HEX -> Evaporator -> HEX -> Condenser -> Outlet

So the air is first pre-colled in the HEX, then passes through the evap, further cooling it below dew-point, condensing out the moisture, then it passes back through the HEX, cooling the incoming air, then through the condenser, removing the heat from the refrigeration system, then the dry air leaves through the outlet, warmer, but MUCH dryer than it entered the inlet. In essence, it converts a lot of the latent heat to sensible heat, plus some extra sensible heat from the electrical draw of the compressor.

Right on. The free cooling can double the amount of moisture removed the compressor verses conventional cooling.
The heat is 1050 btus per lbs. of moisture removed + 3414 btus per KW used = btus added to the air. The high efficiency of the deh means less heat added to the space. Keep in mind the heat from the dehu only occurs when the properly set up a/c is cycling on/off. So dehu does not add to the cooling unless reheat is needed to extend a/c dehumidification.
Regards TB