I got my HVAC system & water heater installed 2 years ago. The furnace is a 2.5 ton two-stage 95% efficiency and my AC is a 2 ton 15 seer. Sometimes I feel that my AC turns on for long enough to lower the temp but not long enough to get the humidity. I had my house air sealed and insulated. My air leakage is about 1.1 ach and my attic insulation is now R-60. I obviously live in a climate with a lot of humidity so I want the dehumidification of my AC more often than I want the temp lowered. It has NO PROBLEM getting the temp down. I could turn the thermostat down to 70 and leave it there and it could handle it. I would be freezing, however.

Is this a valid concern? Regarding my furnace, can a two-stage be a little oversized and still be ok? Is there anything I can do about it?

When you say your air leakage is about 1.1 ach, that would be relatively high for a natural air leakage (NL), or relatively low for an air changes per hour measured at 50 Pa (ACH50). Which is it?

Your symptoms sound like your airconditioner is oversized.

Without knowing more information , and the results of a heat load calculation, yes it is possible that you are oversized.If humidity is the real problem and the humidity in the space is not from an internal source then a small dehumidifier may be an inexpensive solution. Another possibility is that your furnace blower cfm is not matched well with your a/c system.In order for a a/c system to remove humidity it must operate long enough to remove the latent load.

Blower cfm is a catch 22 and needs to be carefully balanced to the requirements. In drier climates such as here in Utah dehumidification is less of a consideration - such that many find an evaporative cooler acceptable – adding humidity to the dry air. A high air flow rate over the coils gives a higher EER for cooling but the air needs to reach dew point to do any significant dehumidification. If you are moving a large volume of air over a limited surface area it never reaches dew point. In designing a system the usual solution is to oversize the coil giving it more surface area so that the same CFM reaches a lower air flow rates over the fins of the coil. If you slow it down too much then you end up with frost on the coils - not good. I would suggest you have a good local HVACR tech check it out. If you have either a variable speed or multi speed blower it shoucl be a fairly easy fix.

[quote]
Posted By Lee Dodge on 08 Apr 2012 11:02 PM
When you say your air leakage is about 1.1 ach, that would be relatively high for a natural air leakage (NL), or relatively low for an air changes per hour measured at 50 Pa (ACH50). Which is it?

Your symptoms sound like your airconditioner is oversized.
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I just had the house retested after air sealing the attic and rim joists and putting new basement windows in. I believe the blower door said my leakage was 933 cfm at 50 pascals which my auditor said amounts to about 1.1 air change per hour.

Posted By strategery on 09 Apr 2012 03:40 AM
[quote]
Posted By Lee Dodge on 08 Apr 2012 11:02 PM
When you say your air leakage is about 1.1 ach, that would be relatively high for a natural air leakage (NL), or relatively low for an air changes per hour measured at 50 Pa (ACH50). Which is it?

Your symptoms sound like your airconditioner is oversized.
[/quote]

I just had the house retested after air sealing the attic and rim joists and putting new basement windows in. I believe the blower door said my leakage was 933 cfm at 50 pascals which my auditor said amounts to about 1.1 air change per hour.

Yes, that's 1.1ACH @ 50 pascals.

But the natural infiltration will be on the order of 5% of that, or ~0.06 natural. The actual average natural leakage depends a lot on the height of the house and the exposure to wind, etc. it could be as high as 0.15 or a low as 0.02- it just depends. But whatever it is, it's pretty tight.

Given the size & tightness of the house I would have thought that a 1.5 ton mini-split would likely handle the loads (except maybe at design heating temp if you're in a location with zero design temps.) The 2.5 ton furnace would heat my house (more than 3x that size) @ +10F, and the air conditioning would too.  Mini-splits are modulating systems with a 3:1 or greater turn-down ratio, so something sized for the anticipated peak load (be it AC or heat) at the design condition would have long, nearly continuous on-cycles rather than freeze/thaw/freeze/thaw with minimal dehumidifcation. (Some have "dehumidify" modes to subdue the stickies without much sensible-cooling.)

Where is this house located? (zip code or city)

I'm in Des Moines Iowa. I would have rather had a few zones of mini splits with maybe a small strip or two of electric baseboard heat. Or even a small heat pump to hand the ac and smaller heating demands, but I'm 2 years and 5 thousand dollars too late.

Design temp for Des Moines is -20C/-4F, yup ya coulda done it that way.

A gas furnace that's even 3-4x oversized doesn't suffer much of an efficiency loss due to the low thermal mass. If there's a way to hard-wire it to the low-fire stage (there usually is) it'll run more efficiently, but it'll take a bit longer to recover from overnight setbacks. (A guy who works in my office has a 2-stage in his condo that's similarly oversized, and modified his to run only at low-fire.)

On the air-conditioning end, I wonder if there's a place to set up a 3/4 ton Mr. Slim (http://ecomfort.com/products/mitsubishi-msza09namuza09a-mr-slim-wall-mounted-single-zone-heat-pump--9000-btu/1658 ) or Fujitsu 9RLS (http://ecomfort.com/products/fujitsu-9rls-halcyon-wall-mounted-mini-split-single-zone-heat-pump-26-seer--9000btu/1345 )or similar pint-sized mini-split ( and maybe do a DIY install?) Even if it can't handle the AC design condition (though it might), it'll run almost constantly at low speed on hotter days, knocking back the latent loads without chilling you out the way the 2-ton central HVAC does.

Thanks for the information. I might call the place that installed this for me and ask if they have some advice. Maybe I can get their help in hard-wiring it to the low-fire setting.

Interesting about the minisplits. I will look into them.

Now that you have ducts a minisplit makes less sense. Their are truly variable ASHPs that work with ducted systems.
I would say your systems are easily oversized but difficult to correct for the original installer.
1.5 ton is about as small as a conventional split AC goes and you likely don't have 1 ton of load. Worse they cost about the same as a 2 ton so with the "bigger is better" mentality many conventional HVAC guys will put in the 2 ton instead.
A central dehumi or reduced fan speed may help you without adding terrific expense. You might also look into a central dehumi which will still cost less than a mini-split or ducted ASHP. Installation is an easy DIY if you find one on-line.

From a cost vs. efficiency point of view, even the volume of the house, you can probably just set up a 70pint room dehumidifier in the basement draining into a sump or drain to deal with the latent loads. No, it's not as efficient as a whole house dehumidifier or a mini-split, but it could take decades to make up the cost difference between a $200 portable any of the bigger-deal retrofits mention on the utility savings.

Des Moines has roughly twice the latent load @ 55% RH as my location (31 grains per lb vs. 14 grains per lb). My house has more than 2x your volume and isn't nearly as tight, and a 70pint room dehumidifier draining to a sump keeps up with the latent loads just fine (and has higher inherent efficiency than the smallest room dehumidifiers). My central-air runs fewer than 20 hours/year, (primarily due to shading factors limiting solar gain & a modest cooling design temp), so the dehumidifier in the basement is THE critical part of conditioning the interior air in summer (and uses several times the annual power of the central AC to boot.) Even if you have more sun and a higher cooling design temp than I do, you also have 3x the R value in the roof, so you probably won't have much of a sensible load except during heat waves, and during those weeks the 2-ton won't present any comfort issues.

In a tight house if you keep the coolest spot in the house under 60% RH, the rest of the house will stay drier than that, and you won't have to freeze just to bring the humidity down. This is a standard problem in super-insulated houses, since the predominant cooling loads are mostly from interior heat & humidity sources. During the cooler drier shoulder seasons that load can often be met by ventilating (opening up windows, if need be), but during the summer that doesn't work so well, particularly in humid climates.

last comment on this home was without reading the above about inairation (incomingairventilation- Hah!)

like any pool house:
at the right moments, fresh air can be likely very acceptable,
but I will stand with a 3/4 ton a/c for most sunny days, to be drier.

In a living-room tucked beside furnishings
Paul, an Engineer from NASA (customer who keeps me up on fuel cells)
5 years now: 1300 BTUh console in (W:AIR GT- NO LOOP)
to heat his swimming pool with a pool coil fitted OEM for Loop Coil, and a desuperheater...
43" wide x 13 " off wall, two 10" wisper quiet axial fans www.Hydro-Temp.com

He just opens the windows when the house gets too cold in the acceptable ambient conditions.
When pool recirculates off T's...
Then pulls the hoses back into his garage after Pool season;
also has the roll of slinky'd SOLAR coils in back yard seasonally-moved