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05-02-2009, 10:34 AM #1New Guest
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- May 2009
Relative humidity settings and system behavior problem? for Houston
I hope some of you might be able to help me.
I have a 2 unit Rheem system with a Honeywell PC8900 Comfort Center thermostat downstairs and a simple basic thermostat upstairs. The home also has a Honeywell whole house ventilation system but I dont know how that works at all!
I read that I should set my relative humidity on the downstairs thermostat - which I did at 50% RH and a temperature of 77%. The compressor and low speed fan have been running for more than 15 hours (the RH reading shows 55%) and the temperature is 67F! Is my system not operating correctly or is the AC running like this to reach the RH of 50%, then will shut down? I have not set this humidity setting before - it was at 60% before I set it to 50%. If it just needs to run to bring down the RH, how long should I expect that to be? Will it all get back to normal when the RH% is met?
Are there any tips in terms of what the optimal RH % and temp settings should be for me living here in Houston?
I would appreciate any insight. I am worried that my system might not be operating correctly. I hope you all can help.
05-04-2009, 09:14 AM #2
IMO you would get a much better professional response, if you would repost this same message in the "RESIDENTIAL A/C" forum. I am a homeowner in your general area and have seen lots of activity on that board, including a number of topics which ideally would be on a (better frequented) IAQ forum. A lot depends on the airtightness of your house, in mine 50% RH seems unattainable without a dehumidifier but in some Houston houses you can do it.
Your reported %RH would be much lower if that same air were simply warmed to a comfortable room temperature. There are some twists to "Relative Humidity" which easily mislead a person. If you can get to where you can express it in units of absolute humidity (e.g. dewpoint) that would support more advanced talking about your problem.
If it were my system behaving as you describe, I definitely would think it was malfunctioning and a service call would be in order. However I am sometimes a hypochondriac when it comes to HVAC things (I prefer to call it "perfectionist"<g>) and occasionally what happens is the service pro tells me there is nothing wrong. I can think of a couple cases where I paid for an hour's labor to educate me that way.
Especially if you are a DIY kind of guy (only a last resort for me), there is a nifty other board which sometimes has good help. But only a few of the posters are as professional as on Hvac-Talk.
Best of luck -- Pstu
05-04-2009, 10:47 AM #3
If the whole house ventilator has no way to dehumidify any outdoor air it brings in, you're just loading your house with excessive moisture by running the thing. Could be why your a/c system is working overtime to dehumidify...not only is the home being overventilated, the outdoor air being brought in is too humid. Tough call since I can't see the equipment or your house from my station.
Before I would cry foul re: the a/c, I'd check out the whole house ventilator. Being a "ventilator" I'm assuming from the start that it brings in outdoor air to ventilate the house. Being you live in Houston, if this outdoor air is not being dried before entering the house, you'll never get ahead of the humidity curve. I would get that figured out first before pointing fingers at the a/c. If you're attaining 67 degrees at 55% RH with your a/c, it's doing a bang-up job given its circumstances."In this house we obey the laws of thermodynamics!"
- Homer Simpson
05-04-2009, 07:23 PM #4
OK, on all the cases where Shophound says something contradictory to what I say, you can assume Shophound has the better knowledge!
I would also like to hear more about this Honeywell ventilation system. Can you provide a model number for it so we can look it up? Is it one of those on this page?
Best wishes -- Pstu
05-04-2009, 09:10 PM #5New Guest
- Join Date
- May 2009
Pstu and Shophound
Thanks for your responses. Really appreciate it. The whole house ventilator is a Honeywell ER150. Appears that it is set to standby mode to be controlled by the Honeywell thermostat (PC 8900 Comfort Center). I cant tell if it ever turns on because short of pushing the VENT button on the thermostat, I don't know if it ever turns on. Should I turn it off and see if things improve (reaching 50% Humidity and 77F without the low speed downstairs air conditioner running all the time)?
When I first found out about setting the humidity level this past weekend, the thermostat setting was at 60%. Getting it to 55% and maintaining the same temperature (77F) did not take too long (about 3 hours). When I then moved the setting to 50% and the same temperature, the system kept running for about 18 hours straight. Once I adjusted the humidity on the thermostat (same temp), the system turned off. Seems like moving from 55% to 50% is where it started acting strange.
I was trying to get to a 78F - 79F / 50% humidity level, thinking it might save some electricity and create a more comfortable home vs. the 60% and 74F it has been running before I found out about the humidity settings. THe home is about 6 years old and is supposed to be an energy star home.
Thank you for helping me try to figure it out. If my best course of action is to have someone come out and take a look, I will do that. Will need to find someone i the Houston area that is good at spending a little time to help me figure out what the best means of running the system should be.
05-05-2009, 12:14 AM #6
Here is a pdf for your model Honeywell whole house ventilator, which is an energy recovery ventilator, aka ERV:
Reading that manual, one can not help but notice the tilt toward winter time operation of the machine. Not one word about how it should perform in summer, particularly in a humid climate. Regarding that topic, you might find this link interesting:
In the meantime I'd try turning the ERV off and then seeing how your a/c system performs at the set points you established, 77 degrees at 50% RH, and see if the system still runs like crazy to get RH down to 50. It really should not. What may be going on (again, tough to see from here) is that your ERV is not set up properly, and excessive moisture is getting into what otherwise might be a tight house, since it is supposedly built to Energy Star standards.
I would only do this as a temporary measure - experimental. If your house is pretty airtight and is relying on the ERV to keep enough fresh air ventilating the home, leaving the ERV off too long may make your indoor air become rather stale and yucky.
Personally, for humid climates, I heavily favor NO outdoor air entering a house unless it is first filtered and dehumidified (when outdoor dew points are higher than desired indoor dew point). A ventilating dehumidifier is one approach; fresh air intake on the a/c another. The article I gave a link to above stresses how important it is to install and set up an ERV correctly for summertime humid climate operation. And that's the problem...our HVAC and IAQ industries have a SERIOUS problem understanding how to properly install and set up higher grade equipment of whatever variety. It is our biggest hamstring, which can only be mitigated by stepped up training efforts combined with more consistent enforcement of installation standards from building departments and industry professionals."In this house we obey the laws of thermodynamics!"
- Homer Simpson
05-05-2009, 12:33 AM #7
Further thoughts after reading the pdf file for your ERV. Specs say it has a 28% moisture transfer capacity. If this is true, let's run some Houston type numbers to see if this ERV, if it was set up to optimize a moisture transfer rate of 28%, could ventilate your home without increasing its humidity load.
I'm gonna pick a rather abysmal outdoor summer day condition for Houston, say 90 degrees dry bulb at 74 wet bulb. That's a relative humidity of 48%, a dew point of 68 degrees, and grains of moisture per pound of dry air (gr/lb) of 104. Indoor target conditions are 77 degrees dry bulb, 65.5 wet bulb, which yields a relative humidity of 50%, a dew point of 60, and a gr/lb of 78.
Now, IF I'm on the right track, 28% of 104 grains is around 29, so 104-29 = 75 grains, which is 3 grains below your indoor target conditions. Looks like set up as such, your ERV should not add humidity to your indoor air at those conditions. Since an ERV has to be installed and balanced correctly to deliver near its design abilities, there's a lot of "ifs" popping up all over."In this house we obey the laws of thermodynamics!"
- Homer Simpson
05-05-2009, 07:43 AM #8
maybe its 28% of the difference between the two air streamsThe way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.
05-05-2009, 11:38 AM #9
Summer day Houston conditions, a 2nd option:
90F dry bulb
76F wet bulb
54 %RH, 71 dewpoint, 116 grains/lb
I really advocate the higher dewpoint, in fact it is 72 now and that is closer to typical.
I am using this psychrometric calculator which I hope is a good one (200 ft above sea level):
Indoor target conditions:
77F dry bulb
64F wet bulb, 57F dewpoint, 70 grains/lb
Absolute humidity delta = 46 grains/lb
Humidity removed from input stream = 0.26 * 46 = 12 grains/lb
Intake air about 104 grains/lb which suggests a real latent load on the AC due to ventilation.
I wonder if Carnak and Shophound would critique the idea of a direct fresh air vent to the AC return for most of the year, instead of that ERV.
Best wishes -- Pstu
05-05-2009, 11:48 AM #10
When it comes to an ERV in Houston, "I'm from Missouri" so to speak. We share the same climate but our houses surely don't share the same construction. You are probably more likely West Houston than East?
Hope this helps -- Pstu
05-05-2009, 04:38 PM #11
Originally Posted by pstu
I would want a damper on the fresh air intake going to the air handler return plenum. That way the fresh air intake is not an open conduit to outdoors whenever the indoor a/c or furnace blower is off. The damper could either be set to open only when the blower runs, or open when indoor humidity levels are below a certain setting (I lean toward preferring the latter).
For winter time operation, perhaps not so much in Houston, but more so in my area, thought would be necessary regarding modulating fresh air intake to avoid overdrying the home with cold, arid, outdoor air. A tight house would help in this sense in that interior moisture generation could be sufficient (depending on household use and activity) that occasional run times of the furnace/air handler does not bring in too much dry air through the intake. A low limit on the humidity setting could also be set up, so if the house began to dry out too much, the damper on the fresh air intake closes. When moisture levels rose above a certain point, it would then open again.
It is important to keep interior moisture levels in check in winter to avoid driving too much vapor into the wall cavities when the fresh air intake is open. From a new construction or retrofit perspective, covering exterior sheathing with insulating foam board can reduce the risk of moisture condensating on the interior of the exterior sheathing material that faces the stud cavities. You also get the added benefit of reduced thermal bridging where the stud faces are in contact with the exterior sheathing. The overall R value of the wall assembly increases, while reducing the risk of condensation occurring within the cavities."In this house we obey the laws of thermodynamics!"
- Homer Simpson
05-10-2009, 10:23 AM #12New Guest
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- May 2009
Shophound and PSTU
Thanks again for your help here. i did some experimenting and i think I have made progress.
I tried turning off the Honeywell ERV to see if that would help improve humidity. While doing that I did see one of the ducts leading into the unit had disconnected. I repaired it and reset the PC8900 thermostat to 50% relative humidity and 77F. After running for more than 8 hours without stopping I was still not able to reach the desired humidity and temp setting.
I did another experiment. I had a hygrometer in a guitar case and I took it out and started using that to measure the humidity instead of the thermostat. Setting the thermostat humidity to 55% and 77F, the hygrometer from the guitar case was reading humidity levels as low as 48% and 77F after about four or five hours. Now after running the thermostat at 55% humidity setting and 77 - 78 F, my hygrometer is reading about 52%-56%. The system is cycling on low speed more than I remember it doing in the past, but the temperature and "feel" in the house is comfortable. I have the ERV running again - don't know how often it actually runs.
Maybe the humidity sensor on the thermostat is not calibrated? Perhaps that is why the system kept running so long when I set it to 50% Humidity - it was actually trying to achieve something in the mid 40s for humidity - which I suspect would be taxing the system in hot humid Houston.
So setting the thermostat at 55% and 77F seems to work ok - and the actual humidity according to the hygrometer is a few percentiles less. I am at 78F with 55% RH and outdoor humidity right now is 78%. Does this sound like proper system conditions?
Finally, should I see energy savings running the system this way compared to before where the humidity setting was 60% and the temperatures were between 74 - 77? Since the system is running on low speed more often am I still ahead in terms of energy savings?
Thanks again for your help.
05-10-2009, 10:50 AM #13
>>The compressor and low speed fan have been running for more than 15 hours (the RH reading shows 55%) and the temperature is 67F
While your RH is not down so much, your ABSOLUTE humidity has well overshot your goal. Using this calculator,
that same air at 67F and 55% RH will be 39% RH when it warms back up to 77F. Absolute humidity can be expressed in several different units, a most convenient one is dewpoint -- your dewpoint is a bit over 50F at both temperatures. That is reassuring in that it tells you your AC is removing humidity. I am telling you this with emphasis because it has been most helpful to me in understanding humidity issues, hopefully it will be the same for you. After fixing your ERV dis-connect problem, you may have found your biggest problem. Glad to hear you report improved comfort.
Best wishes -- Pstu