2 Stage Heat Pump Install Issue

[uga1985]

I just purchased house a month ago that had a new HVAC system installed at the beginning of August and we have been having humidity issues with the unit since we moved in. The unit appears to be grossly oversized since even on days that were around 90 degrees the unit only ran for 8-9 minutes at a time(I live in the Atlanta area and the design temperature appears to be 95 degrees from what I have researched).

I have reached out to the installer of the system and they have had a technician and the install manager come out on 2 separate occasions to see if there is anything they can do and they claim everything is just fine with the system. After researching this issue more in depth I discovered earlier today that I have a two stage heat pump but for some reason the heat pump was installed as a single stage unit. I verified this by going into the install settings on the Edge Thermostat.

I want to contact the installation company to see if they can enable the heat pump to use 2 stages so that our system runs longer to improve efficiency and the removal of humidity from the air. I do not have a variable speed blower so from what I have read on this website it appears that the two stage unit will be of limited value but any increase in cooling times that I could get at this point would be helpful since I do not want to get rid of a brand new heat pump. I know the big challenge in using a two stage heat pump without a variable speed unit is getting a proper CFM for both the high and low stages but is it possible to have the thermostat wired to use different blower speeds for each cooling/heating stage(my furnace appears to have 5 speeds from what I see in the manual)? Any help that you could provide would be greatly appreciated.

Here is the list of my equipment if it helps:
Heat Pump: Carrier 25HCB648
Furnace: Carrier 58PHA090
Evaporator Coil: ADP C60A210C179W
Thermostat: Carrier Edge

[martyinlincoln]

Any good tech could make your furnace blower operate at different speeds for high and low stage.

[Bsmith816]

Changing the settings on the tstat to two stage won't necessarily help you out. It would also have to be wired for two stage. Sounds like you have done your research and understand why your humidity is too high. You really need a two stage furnace variable sped for it to work right. Also I'm not there to see how it's wired but there would be way to lock it into low stage, so the unit would run longer and dehumidify the air better... I can't stand it when people spend good money for equipment and to have it not work properly and the homeowner not to be comfortable. It really ticks me off.

[54regcab]

Looks like you got the all too typical "size it by the 1st stage and use 2nd stage as reserve capacity" installation. I've NEVER seen an installed 2 stage unit under 3tons, most are 4 or 5 tons.

[uga1985]

Thank you everyone for the quick responses. I will be contacting the owners of the installation company tomorrow to give them a chance to make this right. If not, I will find a competent local contractor to correct the improper install. I was wondering if anyone could tell me approximately what the CFM should be in the high and low stage? I am concerned about whether the blower speed was ever set right and I want to make sure when a tech comes out that they take the ESP and get the CFM as close to the recommended settings as possible.


From looking at the install manual located at :http://www.docs.hvacpartners.com/idc...25hcb6-1pd.pdf
It looks like my high CFM should be about 1,420 and in low CFM it should be 1,105.(I pulled this from page 24 and it is based on my furnace model but does not take into account the proper evaparator coil since I do not have a carrier coil). This seems to indicate roughly 350 CFM per ton of cooling. Does this sound like it is in the ballpark or am I misreading this manual? Also in heating mode should the CFM be similar?

[teddy bear]

Thank you
From looking at the install manual located at :http://www.docs.hvacpartners.com/idc...25hcb6-1pd.pdf
It looks like my high CFM should be about 1,420 and in low CFM it should be 1,105.(I pulled this from page 24 and it is based on my furnace model but does not take into account the proper evaparator coil since I do not have a carrier coil). This seems to indicate roughly 350 CFM per ton of cooling. Does this sound like it is in the ballpark or am I misreading this manual? Also in heating mode should the CFM be similar?

You are on the right track. Your tech will not be able to measure air flow accurately but there is simpler way. First, determine the %RH you are looking for. Typically most like 45-50%RH for comfort and control of mold/dust mites. If you are cooling to 75^F, 45%RH is a 52^F dew point. Moisture from infiltrating fresh air and moisture from the occupanats must be removed by the cooiling coil of the a/c. Your cooling coil needs to be 5-8^F colder than the dew point desired.
Measuring the air flow is near impossible. Measuring your coil temperature is much simpler. A tech with a set of gauges can determined your coil temperature while cooling at the typical indoor/outdoor temps. Slowing air flow lowers the coil temp while increasing air flow raises it. Typical indoor/outdoor temps are critical to accurate adjustment of the fan speed to get the desire coil temps.
In addition to the correct air flow, consider that it takes 20-30 mins. continuous cooling for the cooling coil/pan to load with moisture to the point that moisture goes down the drain. Coils hold about 1 lb. of moisture per ton of capacity. At the end of the cooling cycle, the moisture on the coil/pan, slowly re-evaporates back into the air in the home. It is important to cool as long as possible in each cycle. The dead band of the t-stat should be 2-3^F to extend cycle time. Also using t-stat setup during unoccupied times will lengthen the cooling cycle time in pull down, which improves the moisture removal. To minimize the re-evaporation, avoid fan "on" mode when cooling loads are light.
All of these things will improve moisture removal. But there are limitations with perfectly sized and setup a/cs. During cool wet days and evenings, there is not enough cooling load to remove the moisture from the occupants and infiltrating/ventilating fresh air. You need 1-4 lbs. of dehumidification per hour to maintain <50%RH when the outdoor dewpoints are +55^F. Supplemental dehumidification is the real solution to have ideal humidity control when there is low cooling loads. There are several whole house dehumidifiers like Ultra-Aire dehu that are specifically designed to compliment a properly setup a/c oversized or ideally sized. With this equipement, the whole house is maitained at 50%RH even with the a/c off a minimal energy use.
Regards TB

[Bsmith816]

General rule is 400 per ton. So on your high stage 4 ton u would be looking around 1600 Cfm. Of course just by setting it by the book doesn't mean you will achieve the proper airflow. That will only work with properly sized ductwork. In which most houses never have

[uga1985]

The install company came back and set the unit up as a two stage unit. When I asked about the proper blower motor settings, the tech told me that the furnace is matched to the outdoor unit so there is no need to mess with the blower settings. After he left, I took a look at the settings and it appears that he left the fan speed at the lowest setting. From what I have read in the furnace install manual this would be around 200-275 CFM(depending on ESP) and the first stage cooling runs at the fan speed. Is this going to cause serious harm to my system? Also, does anyone know any good techs in Atlanta? I have lost all faith in the install company and want to find someone else to take a good look at the unit.

[teddy bear]

The install company came back and set the unit up as a two stage unit. When I asked about the proper blower motor settings, the tech told me that the furnace is matched to the outdoor unit so there is no need to mess with the blower settings. After he left, I took a look at the settings and it appears that he left the fan speed at the lowest setting. From what I have read in the furnace install manual this would be around 200-275 CFM(depending on ESP) and the first stage cooling runs at the fan speed. Is this going to cause serious harm to my system? Also, does anyone know any good techs in Atlanta? I have lost all faith in the install company and want to find someone else to take a good look at the unit.

You need enough air flow to provide good air circulation throughout the home and not freeze, yet allow the cooling coil to be cold enough to remove enough moisture to maintain <50%RH during moderate cooling loads.
This is a fine line that requires fine tuning.
During days of low/no cooling loads, +55^F outdoor dew points, and moisture generating occupants, several lbs. of dehumidification per hour is required to maintain <50%RH in the home.
Regards TB

[hvacrmedic]

The install company came back and set the unit up as a two stage unit. When I asked about the proper blower motor settings, the tech told me that the furnace is matched to the outdoor unit so there is no need to mess with the blower settings. After he left, I took a look at the settings and it appears that he left the fan speed at the lowest setting. From what I have read in the furnace install manual this would be around 200-275 CFM(depending on ESP) and the first stage cooling runs at the fan speed. Is this going to cause serious harm to my system? Also, does anyone know any good techs in Atlanta? I have lost all faith in the install company and want to find someone else to take a good look at the unit.

The low cooling blower speed chosen will depend upon how much dehumidication is required. In general the lower the blower speed the better the dehumidification. As long as the evaporator coil isn't icing up, supply vents aren't sweating, and the metering device on the evaporator coil is a TXV no harm should come to the equipment. Higher blower speeds will improve efficiency and lower blower speeds will improve comfort while decreasing efficiency. The blower heating speed is more critical, as far as potential for damage to the system (to the hx and flue in particular), and must be adjusted to provide the correct temp rise. If this wasn't done, then definitely get someone back out to properly set this system up.

If humidity is an issue, you'll also want to defeat the blower off delay in cooling. The installation manual explains how to do this.