"Now that I think about it, the AC does seems to frequently cycle on and off in the house, like on five minutes, off five minutes."

Now there's a clue! The equipment could well be oversized and overcooling the air without enough time to dehumidify it. But it sounds like the ducts definitely need to be insulated.

So, if they did put an oversize system in, how do I correct it?

There is no good solution other than replacing the equipment with the correct size.

You might be a little over sized. Do you know the model numbers of your equipment? I fear you have single stage equipment with one zone calling for cooling and this is causing the short cycling. Most HVAC contractors do a poor job when it comes to zoning especially when it is done with single stage equipment.

Here's what the contract says...

Is the furnace supposed to be a Trane? If so the model number is wrong. Where are you located? For new construction a 100K BTU furnace seems over sized.

I think the single stage condenser is going to have issues when one zone is calling for cooling. I am suspicious why the AC has been advertised as having on a 14 SEER rating. It should be higher with a variable speed furnace. The contract does not say "Trane furnace" so I am also curious as to exactly what has been installed. Check the model numbers on the furnace and condenser the next time you are at the house.

Not a great HVAC plan.

What are the 2 zones? I am guessing a first and second floor above grade?

What you get is a floor calling for cooling and then 4 tons cooling it in 5 minutes short circuiting humidity control. When you zone, you almost always want to do a 2 stage system. That way, you can run at a lower cooling level when only 1 floor is calling. In your setup, you can get into a pattern where 2nd calls for 5 minutes. Shuts down for 5 minutes. Then 1st floor calls for 5 minutes. Then shuts down for 5 minutes. Repeat. Will do a horrible job at humidity control. It is unlikely that you have a more sophisticated system that would prevent this. You witnessing 5 min cycles is somewhat proof of that. My Carrier stats could limit cycle times and I am sure that something can be done with your stats. We have Trane ones that appear to control cycle times even forcing the temp below set point to have a longer cycle.

I don't post this because you have an easy way out. You really don't. It is a cautionary tale for others. The whole humidity thing will improve over time in the house and you may find the end result completely fine.

Staged systems have more problems. Zoned systems have more problems. Simplicity generally favors 2 systems. I am in NC and we are doing a 1.5 ton first and 2 ton second. XR14 like yours. Then doing a minisplit in the master - that scales way down and can remove humidity better with super long run times.

On our vacation house (NC beach) we have 2 XR14's also - about 2 tons each (can't remember).

You can inquire about switching units out. They do it all the time during a build. Ask me how I know....

They accidently put a furnace-a/c unit in when I had speced heat pump. Last house they put 4 tons upstairs when the Man J called for 2. I compromised with 3 tons. It was staged so it was fine.

Also - it is unlikely that you need 4 tons. Those specs look like they were done before house construction. So it was unlikely they were based on a Manual J. They could have built this house exactly before and in the exact same orientation - but I doubt it. The area of west facing windows dominates and so identical houses across the street from each other can have very different a/c loads.

Agree that a zoned system should be at least 2-stage for cooling.

The sizing should have been determined from the construction plans, similar to ManualJ. They would have known the insulation properties as well as the windows, and square footage.

Moisture on a vent is not that strange, anymore that moisture on a glass of cold beverage. Usually, though, I would want the vents insulated to prevent this.

If the house is still under construction, you can talk to the HVAC guy about upgrading to a 2-stage unit. And 2-stage thermostats.

It has been both hot and unusually humid around Baltimore for the past several weeks. Flooding rains and such.

I don't recognize that furnace model. Even omitting the leading 'T' (for Trane?) yields no joy.

"We had the same thing in our basement some time ago. I have no idea what caused it. My GC got the Hvac people out there and they had it fixed in no time. "

If they didn't wrap it in insulation, about the only think thay could have done would be to increase the airflow - that would reduce the temperature differential, apparently enough to stop the condensation.

HVAC is so mysterious... I can't really look at my system and easily tell what I have. Since the furnace is two stage, wouldn't the AC be as well? Would the AC also be variable speed too? The unit outside is a Trane, and I think the indoor unit is as well, they have the same color scheme.

I was given a few options early on, like adding a standalone dehumidifer, which I did, and adding a humidifer, which I did not. Also had the option to upgrade to a 15 seer system, which I did not. Also added an electronic air cleaner.

How can I tell by looking at my system if it is a 2 stage variable AC?

The installers used honeywell lyric T6 Pro thermostats, I wanted them to use 3rd gen nest thermostats, but they refused and threatened to drop the warranty if I used the Nest. I probably will eventually replace them with the Nest thermostat, because of the automation I am using in my house. Can you configure the nest to use two stage and stop the short cycling?

I checked this morning and there was no condensation, but I don't think the AC had been running before I showed up. I'll check later this afternoon to see if the condensation if forming again.

The model number of the condenser (outside equipment) tells you whether it is a single or 2-stage. You have an XR14 condenser. If the furnace is a Trane then the wrong model number is in the contract.

It is possible to use a variable speed 2-stage furnace with a single speed condenser. Since the condenser is a single stage the thermostat has no option to control two stage of cooling.

The dehumidifier is not going a good job of controlling the humidity. You need to start measuring the humidity on each level so you can show the problem to the HVAC contractor.

Hmmm... After abit more thought and reflection, here is some more detail. Like I said before, I have 2 zones, upstairs, and downstairs/basement. What seems to happen(I have not timed this with a stop watch) is I can feel and hear the AC running upstairs for like five or 10 minutes, and then shutoff. But before it shuts off upstairs, the downstairs might kick on and run for five or 10 minutes, overlapping the runtime of the upstairs abit. So the system as a whole is running longer than 5 or 10 minutes at a time, it just seems that the AC is only going to each zone for 5 to 10 minutes at a time. Is this still considered short cycling?

I'm not in the house yet, the outdoor condensing unit was just placed about a week and a half ago. Would it be considered unreasonable at this point to ask for them to replace it with a smaller variable speed unit?

You need to post the indoor and outdoor model numbers. They should be on a plate on the outside of each unit. If you can't find them, you need to contact the builder to tell you what models you have. A 4 ton unit shouldn't be grossly oversized but what you ordered isn't always what you get (depending on what bargains the sub can get his hands on on any given day)! If it is, indeed, a 4 ton unit, then you need to insulate the basement ducts.

So while I was at the house early this evening I timed the cycles on the AC, and it was running for 9 to 10 minutes, off for 9 to 10 minutes, then running again for 9 to 10 minutes. Pretty regularly. Had condensation on the basement supply duct again. It is reasonable to assume that condensation is forming on the ductwork in the walls where I cannot see it?

Oh, and the indoor unit is a trane as well. I did look at the model number on the outside unit, and it was the TTR4048.

You have a Trane XR14 AC. 16 SEER, 47k Btuh, 4 ton, 1-stage.

Furnace is unknown.

9-10 minute cycle time is on the low side but not alarming.

HVAC contractor needs to be informed of the condensation issue and send you a report including action plan.

I hate to say it but HVAC almost 'always' oversize to cover their a^^$ and to put more money in their pockets. I've read numerous times over the years that houses are oversized over 90% of the time. Again, what did the inspector say? I would hire an 'expert' independent to evaluate the situation. Yes, it might cost several hundreds of bucks but could save you thousands down the road.

While I am not a Nest fan, it will probably vary the set point and that will force a long run time which will improve the humidity situation.

Generally dehumidifier are unnecessary in our climate (but I am dragging you down south a bit). Humidity problems are caused by new construction (where a temp stand alone is better), excessive door opening where the fix is behavior, oversized units, excessive plants and cooking in a small space, and excessive infiltration. Most houses have oversized units and excessive infiltration and still don't need a dehumidifier. They are fairly inefficient so better to use a/c.

Your HVAC guy could of course just claim it is the new concrete in the basement and the trades leaving doors open. And he/she has a good chance of being right.

Likely the ducts in the walls don't have enough humidity around them to be a problem.

Need to see a good humidistat's numbers.

I would also play with programming the stat to drop the temp 4 degrees say at 8 in the am for the workers. That should force a longer run time which will do better at dropping humidity.

Is your dehumidifier running a lot?

Dehumidifer was set to 45%, and not running. I lowered it down to 40%, it started up and said something like analyzing air, and the humidity shot up to like 76%. I was like, WTH? I came back a couple of minutes later, and it was at 35%, and had shut off.

So my house has 3ft overhangs, casement windows, multipoint locking doors, minimal west facing windows, and the garage is on the west side of the house, protecting a good bit of the house(maybe 25%) from the west sun. These are all things that should be considered in the sizing of my AC, right?

It's got me thinking too, is the furnace oversized? Is that as bad of a situation?

I do have a 70 pint portable dehumidifer I'm currently using in my current residence, I had thought about leaving it here, but maybe I should bring it with me to help dry out the basement?

So the dehumidifier has a malfunctioning stat - at least it seems that way.

Yeah - if you don't have much west facing windows and you have good overhangs on the south, the 4 ton is likely too much. Yes this should be considered for sizing. You can consider getting an independent Manual J and then asking the HVAC crew to change it out.

An oversized furnace is not generally an efficiency issue but it is a comfort issue. Yes you are wildly oversized. You probably have a 40k range load.

I would bring the portable over to the new house.

The heating and cooling loads of a new construction house needs to be done by a Manual J calculation. So the first thing you should be doing to asking the HVAC contractor to give you a copy of the calculation. I suspect one was not done and he will either give you an excuse why he can't or create a report for you after the fact. We have seen this scenario dozens of times on this forum.

I think your furnace is over sized, but I don't know your location. I also know how much insulation was put into the exterior walls and attic.

To be fair, the humidity in a new house is going to be high due do all the moisture in the construction materials. Once all the door and window opening has stopped, it may take several weeks to get the humidity below 50%. At that point we will be in fall so you won't know if you have a humidity problem until next summer. In the meant time buy some inexpensive humidity gauges and get a copy of the load calculation.

The manual J calculation... What data is needed for this? Just the construction documents? Is this something I might be able to hire someone on the forums to do for a small fee?

The construction drawings showing the sizes of rooms, windows and insulation, along with the orientation of the house and the location.

"Since the furnace is two stage, wouldn't the AC be as well?"

No. two stages of heat does not mean 2 stages of cooling.

"Would the AC also be variable speed too?"

Variable speed here applies to the furnace blower, shared by both heating and cooling. There are such things as variable heat, called 'modulated', and variable AC, called 'invertor', but they are all high end stuff.

"Can you configure the nest to use two stage and stop the short cycling?"

I think the Nest can handle 2-stage equipment, but this has nothing to do with short-cycling. And it won't magically change single stage equipment into multi-stage equipment.

"It is reasonable to assume that condensation is forming on the ductwork in the walls where I cannot see it?"

No, not really. The walls are confined and fairly sealed spaces and aren't likely to 'sweat' like this.

" Would it be considered unreasonable at this point to ask for them to replace it with a smaller variable speed unit?"

You may not need smaller depending on the predicted load. But 2-stage would be a good investment. And it's going to cost you for the upgrade.

"These are all things that should be considered in the sizing of my AC, right?"

These are all things that get rolled into the Manual J calculation for predicted cooling and heating load.

Sheetrock is not a good moisture barrier. Therefore the humidity level inside a wall will be about the same as the interior of the house. If the humidity is high enough even a cold water pipe will have condensation. It is possible to have condensation inside a wall.

So, yesterday(Sunday) afternoon I went by the house and checked the ductwork. No one had been at the house that day, and it had been closed up for about 24 hours. No condensation on the ductwork in the basement. Did not stay long enough to see if the AC was short cycling or not. HVAC company did send me the manual J calculation, here is the first page from it. It does seems to state 3.9 tons of AC, if I am reading it properly?

You've got a couple of issues. Here are two:

Water is an ingredient in concrete; it takes time for concrete to cure during which most of the water will be liberated. Running a dehumidifier in the basement for a year or so after construction is complete should address that.

Basements typically don't have much summer cooling load. Since there isn't an independent zone of control for your basement, it's gets conditioned air supplied to it whenever the 1st floor is calling for it. So, the basement could be sitting at 65 F and the 1st floor calling for air conditioning. Cool air will be introduced into the basement that it is not calling for--which will lower the temperature and increase the relative humidity of the space (think cool and clammy.) If you plan on storing humidity-sensitive stuff in the basement, you'll probably need to run the dehumidifier seasonally from spring through fall. If you ever convert the basement to finished living space, I suggest you consider installing a dehumidifier integrated with your HVAC system.

That makes sense, so it would be a good idea to close off the supply and return vents in the basement. I already have a dehumidifier integrated with my HVAC system. But I guess closing the vents off in the basement might be a bad idea then, because then it would not dehumidify the basement air?

That's good news if you have a dehumidifier integrated with your HVAC system. I suggest you work with your HVAC contractor to set up your system for your specific climate and needs.

Yep, they are have agreed to meet with me in a few days to give the details on what I have. Somethings about this setup is so odd. For example, the fan in the powder room on the first floor will come on all by itself for some reason, and the HVAC guys designed it that way.

This is a summary of the calculated loads. The only input data you can see are the indoor and outdoor design temperatures. All of the input data could have been wrong. Ask for a copy of the input showing the loads for the two zones.

Did you notice the heat load was 40K BTU? Ask why a 100K BTU furnace was installed. That is two sizes too big. The 40K BTU sounds on the low side, but this is new construction and it could be a very tight and well insulated house. If that is the case then a 4 ton AC would seem over sized for the Baltimore airport area.

Overall I am skeptical of this report. Ask the HVAC contractor to explain the calculation to you and how he chose the equipment sizes. Be prepared for a very creative explanation.

Hmmm, after taking a closer look, I do see that on the floorplan drawings(not previously shown) that show all the doors, windows, and registers, that North is wrong. What they show as North what is really West by Northwest. Or in other words, its about 65 degrees off?

Mike - The fan in the bathroom is providing ventilation for your house. The modern way of handling ventilation requirements is to have a fan run at intervals to give fresh air to the house. It is less than ideal but I have to admit that I am leaving that for now to pass code.

What happens is you need 20 cfm ventilation. The bath fan is perhaps 80 cfm so they have a timer to run 25% of the time. Made up numbers. It is a formula based on sq feet and then presumed occupant load which is based on bedroom count. The formula that they are using now is probably too high. This does lead to increased humidity in the house. But sucking on a non commissioned cheap fan is not a reliable way to get fresh air anyway and will tend to suck from bad areas - whether they are crawlspaces, attics or garages. I had a rental townhouse that literally had a flow of air under the garage door when the fan ran. Thankfully we have only electric vehicles.

It is also less than ideal because they use cheap and loud fans and the intermittent running is annoying.

Why build tight and then run a fan? - well a fan gives a stable amount of air not super excessive when there is wind and zero when there is none.

The proper best practice for a basement is a temporary standalone dehumidifier. Your basement needs humidity reduction and not air temperature reduction. And then the need goes down significantly after the first year. You can't design an HVAC system to do that right. (ok you could with a zoned basement on your whole house dehumidifier with a basement humidistat but that is extremely unlikely). Do not close your basement vents. Removing humidity is significantly more important than maintaining proper temperature.

Now I do find that after the first year, comfort and humidity control is well maintained by leaving returns open but closing supplies. Humidity stabilizes much better and faster than temperature. A sucking of the basement air does not cool it much but it gives enough airflow for dehumidification. At least it worked well in my last house - daylight basement. Not first year though.

Trust me - there are far more mistakes in you manual J than your orientation. They are absurd every single time in my experience with multiple companies.

I was told by my HVAC company that I should not be questioning them, that they do close to thousand houses a year, and rest assured, they know what they are doing.

They used cheap plastic vents with plastic screens, or no screens at all. I asked them if I could have a more durable metal vent and screen that might last more than five years, and they said they use this same plastic vent on every house they do.

No Nest thermostats are allowed either, unless you want them to disown you and void your warranty.

Well, we'll see what happens after I have lived there a few weeks. The place has been sealed up for close to a day now while the floor stain dries, and I checked in the basement and while there was some fogging on the duct work, it was not dripping like before. So that might mean the AC isn't doing the short cycle loop. Could the doors being left open cause the system to short cycle for several hours afterwards?

How can I disable the powder room fan from automatically running? It is right next to the garage, so I could see it sucking in fumes from there. Not good because I will be using stuff in the garage that needs a well ventilated area.

In the planning staged I told the HVAC company I was having a tight house built, and wanted an ERV or HRV, and they nixed it. Would have much preferred that to the garbage of the bathroom fan running all the time.

I am really sorry - that is not the right sub for your project. Did the GC pick them? HVAC needs have changed a lot with tighter homes, as David points out. It's a shame that too many have not changed along with that.

Yeah, it was the company that the GC uses as his "go to".

If you've built a tight house, then it should have a whole-house ventilation system (although it may not be code mandated in your area.)

If your bath vent fan is cycling on/off, there's a possibility that there is an air supply duct from the exterior which supplies outside air to the return side of your air handler. The duct may have a damper which opens when your bath vent fan is running or it may be open all the time. It's not a very good whole house ventilation system, but it's better than nothing.

I'd start with a blower door test to determine just how tight your home is, then work with a local professional who can size and specify an HRV or ERV (depending on your climate) to maintain acceptable indoor air quality. ASHRAE standard 62 is a useful guide.

Mike - know that all HVAC companies I have dealt with are the same. Arrogant and uninformed. It is beyond frustrating.

Know that the vast majority of houses have similar shortcomings.

I asked my builder what he does about the bath fans - he says they usually upgrade to a quieter fan. Doesn't help the supply problem. There are passive air intake vents that can be installed after the fact.

See if you can get the blower door tests. Depending on the results, you can just change out the switch on the fan to one that doesn't turn on intermittently and go without active ventilation.

I can tell you I have researched ERV's up and down. I just don't think they are worth it in our climate. Solar panels would have better payback and less maintenance.

Most general contractors know very little about HVAC and they are going to hire the lowest cost subcontractor they can find. The HVAC subcontractor has given a fixed price quote to the GC, he does not want to be bothered with your questions. It is a situation that has been repeated many times on this forum.

Have you taken any humidity measurements? You will need them if you want to start a discussion with the general contractor.

I assume the bathroom fan is wired to the light switch. If that is the case you can unplug the fan power connection in the ceiling. Otherwise you will have to wire a second switch to control the fan.

If you hired a general contractor, your contract is with them, and you should be conferring with them regarding questions or concerns about any aspect of your project. The exception would be in the event you have contracted directly with others to perform some portion of the work.

HVAC systems need to be designed and specified. This is best done in the design phase as construction details (e.g., type and thickness of insulation; type and performance of windows and doors, home's orientation, etc.) affect the HVAC. You might, for example, find it more economic (and comfortable) to add insulation and reduce the size of your heating and air conditioning system, or to opt for better performing windows and doors to reduce heating and cooling requirements. Once the home is under construction, it's more difficult to make those choices.

If you hired a design professional, I submit that the design of the HVAC system should have been included in the scope of their work unless specifically excluded for some darn good reason. If they don't have the expertise needed, they can hire a third party, but it needs to get done as part of a comprehensive design effort--not construction.

@Charles Ross Homes - mike_home hit it on the head, my GC builds houses, and he relies on the expertise of his preferred HVAC company to do their job well. So I started out asking the GC questions, and he was just passing them along to the HVAC company, and finally I started talking to them directly.

Yeah, I wish I had known some things up front. I knew how things were done in my current house, and just thought that's how they were all done.

For example, in my current townhouse, I have no ductwork in the attic. Its run between the floors, because I have the engineered floor trusses. In the new house they are wood I beams, and I didn't realize that until I saw them put in place. A few weeks later I realized my HVAC ducts would pass thru the unconditioned attic. Really would have rather had them inside in the conditioned area. Guess there's always next time.

So I've been living in there a few days now, and made an interesting discovery when I tried to add my thermostats to my Wifi network. As part of that process, it listed some info on the stats, and it said they were set to three cycles per hour, which matches up with the 10 minute run times I was seeing. But then it also said I had single stage heating and cooling, which I know I have two stage heating. Who knows... I guess the stats were not even configured properly? Really not liking the Lyric T6 pro, I can't get it to complete the wifi setup even though I have been trying for a few days now.

I thought we concluded you have a single stage Trane XR14 condenser. I don't think you ever posted the furnace model number so we don't know whether it is single or 2-stage.

Is the thermostat receiving a strong signal? If not you may have to move your wireless router closer in order to establish the initial connection. Once it is connected you can try moving it back to where you had it.

Furnace is a two stage Trane. Yeah, the thermostat is like 10' from the wifi access point. Its an odd setup, and the reviews of this stat on amazon show others have had this same problem.

Something else that I forgot to mention was they installed a UV light in my air handler, and I didn't ask for it, or see it on the original quote. What do you guys think of the UV light?

The claim is the UV light will reduce bacteria. I suppose if there is a condensate drainage problem then you could have a breeding ground for bacteria. If that is the case then the drainage issues instead of shining a light on the area. In my opinion they are a waste of money.

So... In my area the temperature has dropped and its been raining a lot. The AC hasn't been running, but my dehumidifier has. Or so I thought... Its set to 45%. I go down to the basement and hear it running, but when I go upstairs, there is nothing coming out of the vents.

So these guys installed it return to return, which is the preferred way in the instructions. The instructions also state that when installed this way, you must wire it into the HVAC fan to circulate the air in the house while the dehumidifier is running. These goons did not do that. So all this time that the dehumidifier was running and the AC was not running, all it was doing was dehumidifying a small amount of air that is in the return ducts. I got them to come back out and fix it. But instead of wiring up the zones(I have two zones, and the dehumidifier supports zoning), they skipped that. They told my wife that if we feel one of the zones is too humid, just run the fan for that zone off of the thermostat. Is that acceptable?

Did you purchase automatic controls? If so, it appears you didn't get what you paid for.

Dehumidifier has an automatic control built in. Every 60 minutes, it fires up and samples the air, then conditions till it gets down to the requested humidity level. I do wonder if a Nest thermostat could control it...