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I think my HVAC system could be oversized. Is this something I should be concerned about?

user-1035959's picture

I bought my new HVAC system just over 2 years ago. The furnace is a high efficiency natural gas that vents through pvc pipe through the wall. The furnace is rated at 45,000 b.t.u.'s. In the winter, it feels like my house is either hot or cold regardless of where I set the temperature. I try to aim for about 68 in the winter.

My AC is a 2 ton unit with 3 ton evaporator coils. It usually feels either hot or cold in the summer. If I set the temp on the thermostat above 74, it tends to feel a bit clammy. Sometimes I set the thermostat down to 73 or 72 just to get it to run. I'd say most days it probably runs for less than 10 minutes, then shuts off.

I live in Iowa, so I have a mixed humid climate. I also live in a well-shaded area, under 800 square feet of conditioned space. Air tightness is pretty good. I think I've sealed up most of the major sources of air leaks and my insulation is fairly good (in the attic at least, not so much in the walls). I don't notice much of a difference in the running time of the HVAC system before and after some of the insulation/air sealing. It seems to run roughly the same, only the difference is I can keep the temp a little higher in the summer and lower in the winter (but still uncomfortable, either hot or cold).

Am I being too picky here or is this not working out as it should? I'm not sure what else to try. My ductwork is pretty old and poorly done (have a thread about that right now) and most rooms one supply register is in the wall split between two rooms. Would re-doing this just make my system more efficient and possibly more oversized or could it possibly help? Is it possible to put a variable speed fan in that makes the system run slower for longer without having to change out the whole system?


First thing to do is to run (post #206612, reply #1 of 18)

First thing to do is to run the fan all the time.  But it sounds like your system is oversized and poorly balanced.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Good idea. I turned the fan (post #206612, reply #3 of 18)

Good idea. I turned the fan on earlier and it's helping a lot.

You say the system could be poorly balanced. What does that mean exactly and how can I deal with it?

Redoing my ductwork is probably going to cost me about a couple thousand dollars. Not sure if it will even help.

Redoing the ductwork may or (post #206612, reply #4 of 18)

Redoing the ductwork may or may not be needed.  Inserting some dampers in the ductwork may help significantly.

The problem is, you need someone who knows what the heck they're doing, but is willing to use a few tricks of the trade vs insisting on a complete redo.  Finding that someone is hard.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

2 ton A/C? 45Kbtu heat? (post #206612, reply #2 of 18)

2 ton A/C? 45Kbtu heat? Sounds like you have a smaller house, perhaps 1,000 sq. ft. That's a problem.

The first thing you have to recognize is that it's not just a question of equipment. Then you have recognize how things have changed since the house was built. Finally, you need to understand your priorities and get your facts nailed down.

Feel hot? Feel cold five minutes later? Maybe the problem is YOU. Hang a few temperature and humidity gages around the house; you'll be amazed how your perception changes while the room stays the same.

Homes are typically designed with a central system, and everything works off a single thermostat. Alas, houses are not that simple.

In winter, I suggest setting the central heat fairly low - say, 68 degrees - and supplimenting that around the house with space heaters (I like the oil-filled radiator type for this use). That way, if the wind is hitting one room while the sun warms another, you'll only be adding heat to the cold room.

Summer is a bit trickier. I expect you'll get the best results if you can vary the speed of your fan, to circulate the air more consistantly. You definitely need a longer run time for the A/C to remove humidity.

Is your house properly insulated? If it has the original, I'd bet it's time for an upgrade.

Just as important is your venting. There should be SOME fresh air coming in at all times. This is needed so that the bath fan and range hood actually have something to exhaust. Kitchens and baths add tons of water to your indoor air. Anything that burns - like scented candles- adds moisture.

Likewise .. what's your roof doing? Improper or blocked attic venting will make the house much less comfortable.

Generally speaking the (post #206612, reply #6 of 18)

Generally speaking the condensor and air handler should be the same size. What was the reason for using different sized parts?

With an oversized A/H the fan won't run long enough to dehumidfy the air before the thermostat is satisfied and cuts off the condensor. Here in humid south Florida we would probably have installed a 1 1/2 ton unit in your house to make sure the A/C ran long enough to get the humidity out.

Florida Licensed Building Contractor, 50 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

Is it somehow possible to go (post #206612, reply #7 of 18)

Is it somehow possible to go with a variable speed fan? I know I have a 2 stage furnace.

Generally not possible to (post #206612, reply #8 of 18)

Generally not possible to swap in a variable speed motor, without replacing the furnace.  It may be possible to adjust the speed of the fan, though, between one of several fixed speeds that it might support.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Ok (post #206612, reply #9 of 18)

DanH wrote:

Generally not possible to swap in a variable speed motor, without replacing the furnace.  It may be possible to adjust the speed of the fan, though, between one of several fixed speeds that it might support.


I'm having the company that installed my system out next week. I hope that there's something that can be done. I don't know if there are different fixed speeds for the fan. My impression is that there are only two (2 stage furnace?). I'll talk to them and see what they say. Not sure I can trust them at this point, however. I may be on my own.

The reasons (post #206612, reply #16 of 18)

 one would select different coil/condenser combinations is to match the S/T ratio and tailor the capacity of the system to the load.

An "oversized" (in relation to the condensing unit capacity) evaporator coil will result in a greater (percentage of the total load) latent capacity, and can get a little more out of a marginally sized ACCU. An "undesrized" evaporator with have a higher sensible capacity, in relation to the total capacity. Also, capacity can be "trimmed" with a smaller evaporator coil. These practices are not common in the residential market.

Most of the time, evaporator coils are selected based on what's on hand, what's the cheapest and/or what fits.

Did the installer (post #206612, reply #10 of 18)

do any sort of load calcs when they installed the equipment? Did they come up with room by room airflow requirements and then measure the actual airflow to each room? Did they do a duct leakage test or any duct sealing? Was external static pressure checked?

The mismatch between the indoor and outdoor coils is a bit unusual,  but some equipment specs allow for this. That should be checked.

The point above about the A/C running too short is important. If it blows a bunch of cold air on you and then shuts off without dehumidifying the house enough, you will be uncomfortable.

Where is the ductwork... in the basement, the attic, in the conditioned space?

It is really hard to get this sort of stuff solved. Many HVAC techs don't know how to do it.

I'm not sure what load (post #206612, reply #11 of 18)

I'm not sure what load calculations and/or testing they did. I was only present for part of the evualation/bid. They have an A rating with BBB so I thought I could trust them. I reckon there's not much they will be willing to do for me 2 years after the install.

It sucks because we are usually either hot or cold when the hvac is running (winter or summer).

Unfortunately, BBB rating (post #206612, reply #12 of 18)

Unfortunately, BBB rating means little.  It only reflects how many complaints have been filed against the company, and only relatively severe complaints are counted, and only after "mediation".  (And, after all, how likely are you to file a complaint against the company?)

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

It sure sounds like it is way (post #206612, reply #13 of 18)

It sure sounds like it is way oversized.  You can certainly spend a bunch of money with the company or someone else but you may be just as well off balancing the system yourself....and by that I don't mean fancy load calcs and airflow measurements.  Shut the supply vents closest to the thermostat and open the ones farthest away.  Its simple, its fast and with a little adjustment you will find the best mix for you to be comfortable.

My Forced air furnace is not dead center so I completely shut the vents in two bedrooms and a bathroom that are closest to the furnance and force more air to the second floor and other end of the house despite these rooms being on the south side of the house.  Those rooms still get enough blow by out of the vents to keep them comfortable.  Dampers just off the supply line are the "real" way it should be done but then you have to head down (or up as they case may be) to adjust the dampers if the change in seasons cause you to need more or less in those rooms.  For example, in the winter, I still keep the vent shut in the bathroom unless the kids are taking a bath.  We open it up then to make it nice and toasty.

Somebody else is going to tell you that for maximum efficiency you need to do all this bull and more but we are talking about an EIGHT HUNDRED SQUARE FOOT HOUSE.  All the extra efficiency in the world with a precisely sized and exact system will never save you the money you spend getting there NOW THAT YOUR SYSTEM IS ALREADY IN.  Getting it done right in the first place is a different story.

I dunno (post #206612, reply #14 of 18)

Seems like a good HVAC tech could check him out in an hour or two for small money,

Of course, actually FINDING a good HVAC tech who does it right is the trick, so I like your approach too. He could install a Magnehelic 2001 on the blower cabinet to keep an eye on static pressure as he plays with the grille dampers.

Talked to them today and he (post #206612, reply #15 of 18)

Talked to them today and he suggested that we lower the fan speeds when I get the furnace tuned up next month.
 I will see how well this helps and post the results. Hopefully it doesn't cost me efficiency of the system.

With my small, tight well-insulated house in a shaded area it shouldn't cost that much to cool/condition in the summer but it does. I have to turn the temp down to 74 or 73 to get it to run. We're cold in the house, but it's more comfortable than being hot and humid.

A couple of ductless mini splits probably could heat and cool this entire house on their own 95% of the time. Knowing what I know now I wish I had explored that.

Thanks for all of the feedback. I'll post again with the results after I have the system adjusted.

Basic problem (post #206612, reply #17 of 18)

with your furnace is that it is the smallest capacity commonly available (some offer a 40MBH in 80% AFUE lines) and still, at rough guess, 50% too big. Same goes for the AC setup, but it could have been selected more appropriately. With less than 800 SF conditioned space, mild environment , heavy shade and good contruction, no conventional central type forced air system will be properly sized. This is a delimma that would take a sharp tech to address.

All basic furnaces of modern design/construction (single stage, multi-speed) have 4 "built-in" speeds, one speed is used for heating air flow and one is used for cooling air flow. In your case, I would use the lowest speed possible for heating and the next lowest speed for cooling. These are typically "adjustable" by taps on the fan control board.

The ductwork has little to do with the efficiency of the system. That is pretty well set by the design of the equipment. Poor ductwork can affect the effectiveness greatly, though. Better distribution and more consistent air flow/delivery will result in better comfort.

I didn't read all the posts (post #206612, reply #18 of 18)

I didn't read all the posts ... my 2 cents. Oversizing causes comfort problems and is energy inefficient. <1000 sqft and 2 tons of cooling. Wow. Oversizing causes the compressor to short cycle ... just as you pointed out. Remember, you only have peak cooling a very small percentage of the hours. The rest of the time, the oversized unit has a much lighter load and so it short cycles. This is energy inefficient. It also causes comfort swings between delivering cool/cold air to nothing.

Oversizing also shortens the life of the equipment substantially. 

No you're not being too picky. The installer may have not actually sized the system or didn't do a very good job. He may have arbitrarily sold you an oversized unit. Many installers think they might be doing their clients a favor (and avoiding comfort complaints because of possible undersizing) if they just through a large unit at you.

Well shaded and under 800 sqft tells me maybe a ton might work (although I'm not certain about the latent load). I sized a ton for hot dry climate (design temp 107) for about 1300 sqft unshaded. 

Someone suggested maybe a two speed or variable speed fan ... without the right compressor, this may not work well for cooling ... it's important to have air flow to have the right supply temps. Low flow means even lower supply air temps (e.g. 47 degF). This doesn't work well.

There ain't NO free lunch. Not no how, not no where!