Sensible and Latent Load

kellyknitsJuly 9, 2012

New here! What size system does a 2.1 sensible and latent load equate to?

Looking at a manual J done on my home 2 years ago - 1 year after the installation of what I believe is a 3 ton system (has 36 in the unit number for the air handler). Have been dealing with humidity issues since the summer after the install.


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You need to post the full model number of the indoor and outdoor unit.

The outdoor unit is the limiting factor for determining the capacity of the system. It is possible that the air handler is sized larger than the OD unit. While this will increase SEER, it also reduces latent capacity of the system.

Is that 2.1 TONS? It should have been broken down as sensble and latent heat gains.

Your location is an important fact as well as what you typically set your thermostat to. This plays a part in proper sizing of an a/c unit.


    Bookmark   July 9, 2012 at 3:40PM
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The Johnson Control variable speed air handler's model # is (z)AVL36C3XH21H. The York heat pump model # is ZHL8B024f1C, so looks like it is a 2 ton system.

Total Heating required including ventilation air: 33,635Btuh 33.635 MBH
Total Sensible Gain 22,430 Btuh 89%
Total Latent Gain 2801 Btuh 11%
Total cooling required including ventilation air: 25,231Btuh 2.1 tons based on sensible and latent

Here's the Manual J - my corrections are in pencil (sq footage, direction house faces, elevation)

I'm in the Mid-Atlantic - Shepherdstown, WV.
With my prior system which was a 40 year old system in the attic- I never had humidity issues. Now I have to keep the temp on 70F to keep the humidity just below 60%. I let it get up to 74F recently and the humidity jumped to 67%.

Thanks for your help, V!

    Bookmark   July 9, 2012 at 7:06PM
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Oh, when the sq footage is done should it be done per room and added together? If so, my corrected calculation would still be a little high as I measured inside overal dimensions. Thinking they must have used outside measurements...

    Bookmark   July 9, 2012 at 7:10PM
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The printout has your front door the opposite of correct.
I don't see any info on window load.
When we run loads we look at all the window area, and skylights, and what type of glass and shading is on the windows.
All of this may mean the load is not calculated correctly.

You do not appear to have a three ton cooling system, based on the outdoor unit info.

    Bookmark   July 9, 2012 at 10:32PM
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The confusing part is that you need 3 tons for heating or is there another heating system?

If there isn't, then you really should have a 3 ton heat pump.

From my understanding, newer units can have a problem with humidity removal that an older system didn't. Now - I don't know about your particular unit but higher Seer units will run the fan after the condenser is off to continue to cool but this does return more humidity to the air.

Humidity issues are usually - short run times, too high fan speed (yours should be adjustable), leaving fan on all the time, excessive air infiltration (no new reason for this), and plugged condensate line. There might be others but this is a good place to start.

So what are your run times like? Can't hear? - then this afternoon, turn the stat down 5 degrees and measure how long it runs for. It should be at least an hour - if it isn't then you are probably oversized. This is my total shot in the dark estimation but your system really shouldn't be able to drop the temp that fast - I'm assuming your high today is about 80. If it is too cool today, then wait a few days for return of some heat.

But I bet this is a fan speed issue....

    Bookmark   July 10, 2012 at 7:06AM
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There is no other source of heat. Do you know how the 3 tons for heating is calculated? Wondering if something else was put in incorrectly.

Two years ago I contacted a regional Technical Services guy from Thos. Summerville about the humidity issue and he sent pretty specific instructions for me to give to the contractor about setting it on the "humid" profile and slowing the fan speed. Supposedly that was changed at that time and did see some improvement. Prior to that humidity levels had been mostly in the upper 60-mid 70% range. The contractor tried to tell me at one point that this was normal.... At that time he suggested I leave the fan running all of the time - humidity shot up to 88%! Ended up with mold on the wall behind my bed.

By chance I checked the run time yesterday evening. It ran for 14 minutes than shutoff for 45 minutes. Would replacing the 3 ton air handler with a 2 ton air handler be a possible solution? Would that increase the latent capacity? I really don't like having to have the thermastat set at 70F all the time, just to keep humidity levels under 60%.

I think it has to be oversized. I've spent so much money on stuff that was suppose to help (some at the suggestion of the contractor) - new windows, added more installation in the attic, extended a vent and return to an enclosed porch, regraded around the house, installed a solar attic fan and roof vent, took down some trees...

Any suggestions on what I should do now? The contractor wants to take out the solar attic fan now and install two electric attic fans.... I really have no idea what I'm doing, but decided to take temp and humidity readings in the attic throughout a couple of days and the humidity in the attic has been as low 35% but the humidity in the house is unchanged, so I don't think electric attic fans are the answer.

I really appreciate the help! This has gone on way too long and I only have so much free time to pursue an education in HVAC- it's a bit confusing, lol!

    Bookmark   July 10, 2012 at 8:01AM
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Something else- in case this matters- when the system kicks on the lights in the house flicker. Contractor said it's not a problem and only lasts a couple of seconds....

Does any one else have this happen?

    Bookmark   July 10, 2012 at 8:49AM
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When the unit turns on, does the fan run at a slow speed for a while, then begin to speed up?

Can you see if there is condensate draining from the blower unit?

If you added insulation and changed the windows after this ManualJ was done, then it is no longer valid.

The fact that they chose 10F as the winter temperature seems low to me, even for Shepardstown.

    Bookmark   July 10, 2012 at 3:04PM
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Just got home from work - It's 85F outside, I have the temp set at 75F (indoor dry bulb temp from the Manual J), the humidity in the house is 60%. The system kicks on for 7 minutes and the humidity actually increases to 61%. Would this be considered short cycling?

Weedmeister, I thought 10F for winter seemed low as well, but was able to look up design temps for the area and it looks like it's correct as far as the data out there...although in by weather standards I don't think it is correct.

Is there a quickie Manual J program online I could do?

    Bookmark   July 11, 2012 at 5:30PM
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CJ Mechanical of North jersey llc.

lowering the fan speed to see if this helps

    Bookmark   July 11, 2012 at 10:19PM
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You can get a copy of HVAC Calc for $49 for a one time use to do a Manual J calculation for your house. I have read good reviews about it.

I would consider a 7 minute run time to be short cycling.

The lights could be flickering because of poor wiring connections, or your condenser is pulling a large amount of current on start up. It may a be sign your compressor is beginning to fail.

Here is a link that might be useful: HVAC calc

    Bookmark   July 11, 2012 at 10:53PM
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When you ran the system for 7 minutes, what was the indoor temperature when it started and when it finished? That is, what was the indoor temp when you set the thermostat to 75 and what was it when the unit shut off 7 minutes later?

    Bookmark   July 12, 2012 at 3:41PM
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Thanks for the link to the HVAC software!

Supposedly the contractor set the fan on the lowest setting (humidity setting)per recommendations from regional rep.

Weedmeister, I had raised the thermostat from 70 to 75 when I left for work. When the lights flickered and the fan came on the thermostat still said 75 and when the fan cut off the thermostat was still at 75. One thing I am noticing after 2 evenings of jumping up every time the fan clicks on so I can record the temp/humidity and the amount of time that it runs, is there seem to be two cycles. During the first cycle which lasts all but the final minute, the humidity level doesn't change (except once going UP 1%), at this point the lights flickr and I hear a clicking noise from the thermostat - the fan runs another minute and the humidity level may or may not drop 1%.

When I lowered the thermostat from 75 to 73 degrees, the first cycle lasted 6 minutes, but the second cycle lasted 12 minutes this time and the fan speed obviously sped up (didn't have to strain so much to hear it)and reduced the humidity from 59 to 55%.

I'm surmising from this the 1st cycle is the sensible load to reduce the temperature and the 2nd cycle is the latent load to reduce humidity. Is that correct? If the fan speed is already at the lowest it can go, can it be assumed just from this that the system is oversized? Or is there anything else that should be checked.

Will monitor the system again on Sunday when it's suppose to be back in the 90's...ugh..

Thanks again for any and all input!

    Bookmark   July 14, 2012 at 8:02PM
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The airhandler looks to be a Luxaire which is the same thing as a York. If you provide me with the serial number, ill see exactly what unit it is... and that will let me see the circuit board inside. The newer Yorks have a dip switch that sets your comfort level ...depending on what part of the county you live Normal, Humid, Dry & so forth. Each setting changes the way the fan ramps up, the duration at each speed and so on. If you have that particular unit, we can make sure the settings are set correctly.

Hope this helps

    Bookmark   July 15, 2012 at 12:51PM
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Supposedly it is set on humid...but that's according to the contractor who I'm not sure realized there was such a thing until I contacted the Regional Thos Somerville rep.

I have 3 numbers for the system - not sure which is the Serial #

Thanks for your help!

    Bookmark   July 15, 2012 at 7:19PM
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Here is the installers copy of the page for setting the humidity level. You might pull off the air handler door and check to see what yours is set on. The circuit board is in the upper right hand corner of the air handler once the door is off. Get a flashlight & look somewhere in the middle to middle bottom of the circuit board. You should see in fine print "Delay" next to 8 pins, 2 pins per row, stacked vertically, each row labeled A,B,C & D. Just check to see what yours is on but dont touch anything. If the black jumper is attached to the top 2 horozonal pins, then your comfort setting is the "A" profile.... if it's attached to the 2nd horozonal pins, then it's in the "B" profile and so on. Then look at the picture i have included in this post and you will be able to read exactly how your fan is ramping up, how long it stays at a certain speed and at what % of it's overall max rating it's running.

Hope this helps

    Bookmark   July 15, 2012 at 9:38PM
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Thank you! My system is in the attic and unfortunately I'm dealing with Lyme disease so I'll have to find someone willing to go up there!

Reading the thermostat manual again this evening and checking against the thermostat...the compressor stage 2 is set to only cycle 1 time per hour. I changed it to 3 which is the recommended option and matches the compressor stage 1 cycles per hour. (the choice was to have it cycle 1-6 times per hour) Any clue what results that may have? Also found an Indoor Dehumidification Setting - it was set to 0 for NO indoor dehumidication control. Changed it to 1 for dehumidification droop control. I don't know if mine has a humidity sensor- or can I assume it does since the humidity level shows on the screen?

If anyone understands what any of that means, please let me know! Will be curious to see the effect tomorrow.

Thanks again!

    Bookmark   July 15, 2012 at 10:17PM
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