better to undersize? why?

[DrCindy]

Hi all,

need some help as I am just starting to understand basic HVAC terms since we are looking into getting a furnace and heat pump for the Seattle/Tacoma area. I've had 3 bids so far and am going to get more- things seem to be pricier in this region. Only one contractor has actually done a load calculation and said the results indicate I need 26,000 BTU's. He recommended a 3 ton furnace (as did the other 2 bids) and a 3 ton HP. The others recommended 2.5 ton HP's. He said my load calc. result was right on the border and he preferred to oversize rather than under.

From what little I've read so far, it sounds like this is not a good idea? The contractor made it sound like it was ideal. He said he oversized for his own home so when he cools it, it feels like a meat locker!

So if you could all explain and provide some advice I would greatly appreicate it! Thanks!

[davefr]

No, it's not better to undersize!! You want to "rightsize" the unit and it's only done by a contractor doing a detailed analysis of your home.

If any contractor guesses, uses rules of thumb, or ouija boards then send them packing!!

[BobbyBJr]

A undersized system will run more and remove more humidity in cooling, but you give up being able to keep your home at, or near, setpoint on the really hot days. A oversized system in a humid area will not run enough to remove enough moisture from the home and it could cause issues with mold and mildew. In your case, without seeing the load and home first hand, it is very hard to say for sure which direction I'd recommend. Is the 26,000 the heating or cooling load? If alot of assuming was done when entering the data for the load, it could be off a little. Also not all heatpumps operate at nominal capacity, so it will also depend on the brand and models being offered and the installation. A poorly installed system can easily operate at less than 60% of it's capacity.

[teddy bear]

Hi all,

need some help as I am just starting to understand basic HVAC terms since we are looking into getting a furnace and heat pump for the Seattle/Tacoma area. I've had 3 bids so far and am going to get more- things seem to be pricier in this region. Only one contractor has actually done a load calculation and said the results indicate I need 26,000 BTU's. He recommended a 3 ton furnace (as did the other 2 bids) and a 3 ton HP. The others recommended 2.5 ton HP's. He said my load calc. result was right on the border and he preferred to oversize rather than under.

From what little I've read so far, it sounds like this is not a good idea? The contractor made it sound like it was ideal. He said he oversized for his own home so when he cools it, it feels like a meat locker!

So if you could all explain and provide some advice I would greatly appreicate it! Thanks!

The promise is that a slightly undersized unit will beable to maintain <50%RH. The facts are that the smaller unit will cycle less and removes slightly more moisture. But as the cooling load declines to the point where smaller unit also runs mimimally and unable to to remove the typical 30-70 lbs. per day of moisture necessary to maintain <50%RH. Finally on cool wet days, the undersize unit has no cooling load and the home humidity rises to the outdoor moisture levels plus the moisture from the occupants. There are endless posts on forums like this expressing dissapoint of being unable to handle extreme heat, reasonable temperature reduction after therma-stat set up when unoccupied. Also as units age, they loss capacity and are unable satisfy the t-stat settings.
So what is the answer to these problems. I suggest getting an extra .5 ton more than the minimum. Get a simple medium-high SEER single cooling speed with a VS fan. For ideal humidity control, add a small whole house dehumidifier. The dehu will provide the desire %RH with minimal or no cooling load. The extra cooling capacity provides the cooling needed to use your t-stat to raise the temp of the home for daily work schedules. This saves energy and provides a long cooling run when you return to your home at the end of the day. The longer cooling run removes much more moisture. During days of high cooling loads, a properly set-up a/c will remove enough moisture maintain <50%. As the cooling load declines during cool wet weather, the dehumidifier will maintain <50% RH.
Also you can turn off the a/c and be assured that the dehumidifier will maintain <50%RH without any a/c operation for days, weeks. In addition, a small amount of fresh air is an option while have <50%RH. More and more a/c contractors are becoming aware and trained on the whole house dehumidifier option. I have worked for Ultra-Aire/Santa Fe pioneering the concept for last 15 years. The concept is healther, more comfortable, and uses less energy.
Regards TB

[DrCindy]

In my area, humidity is not a very big issue at all, nothing like the midwest or south. Also, the summers are usually mild, I believe historically in the 70's, but past years have shown a warming trend. Last year we had a couple days in a row (!) of 90 degree weather, and we just finished surviving a week of temps that were in the high 80's to low 100's, which was very unusual.

I'm looking at a summary report from my load calculation: the BTU heating is 26,921, and the BTU cooling is 16,823. This is for a 2 story home.

What I will most likely do is post detailed information of various sytems I'm quoted once I finish gathering all the bids, and run them by all you experts! Thanks again for any more suggestions and advice.

[dash]

Docter C,

Oversizing is not the way to go. Look around a little more and find a Pro that can size the system correctly,as well as the duct system.

Good luck!

[beenthere]

A 3 ton heat pump for a cooling load of 16,823BTUs. Is way oversized.

Probably has to keep his place at meat locker temps to keep the humidity low enough.

[dan sw fl]

In my area, humidity is not a very big issue at all, nothing like the midwest or south.

Also, the summers are usually mild, I believe historically in the 70's, but past years have shown a warming trend. Last year we had a couple days in a row (!) of 90 degree weather, and we just finished surviving a week of temps that were in the high 80's to low 100's, which was very unusual.

I'm looking at a summary report from my load calculation: the BTU heating is 26,921, and the BTU cooling is 16,823. This is for a 2 story home.

Thanks again for any more suggestions and advice.

3-ton should be great since 95% of your heating-cooling energy cost is for Heating. The NW US summer season is obviously not the high dew points of the South East and Southern U.S.

The thermosat controls the temperature not the size of the unit.
Some comments here seem to wish to distort the discussion on equipment selection.

The best equipment selection maybe a 2-stage Heat Pump.

Are the contractors providing 2-stage HP in their proposals?
If not, ask, Why Not?

[teddy bear]

Distortion?? Thanks> All green grass climates have many days without much cooling load while the dew points are +60^F. When the homes are getting a fresh air change in 5-10 hours without any cooling load, the dew point in the home is +65^F. Any cool surface inside the home is near 100%RH, ideal for mold and other biologicals growth . Being able to remove 30 lbs. of moisture without over cooling the home removes the potiential for any biological growth and improves the indoor comfort. Fresh air ventilation with low %RH is not a distortion. Expecting low %RH with adequate fresh air infiltration/ventilation, normal moisture generating people activity with only an undersized a/c during wet cool weather is the "distortion". Thank your lucky stars more people are not aware of the comfort and health potiential of real space conditioning system. The system in a green grass climate would be a low-high efficiency heat pump depending on cooling hours + a whole house dehumidifier. You could set the temperature/ %RH and not struggle like you do now with any sized a/c.
Humidity control is implied by all of the sizing discussion. The customer expect it. Fresh air ventilation is avoided to have a chance at controlling indoor %RH. Distortions??? If you expect humidity control, "give it" or "get it" in writing. If I was selling a/c to a customer, I would get a sign off on the limitations of an a/c in providing %RH control during wet cool weather. If I was buying a space conditioning system, I would get assurance of %RH control in all typical weather conditions in writing. Use the two speed money on a whole house dehu, you will have a more healthy/comfortable home. Regards TB

[teddy bear]

Hi all,

From what little I've read so far, it sounds like this is not a good idea? The contractor made it sound like it was ideal. He said he oversized for his own home so when he cools it, it feels like a meat locker!

So if you could all explain and provide some advice I would greatly appreicate it! Thanks!

All forms are filled with post with over-sized/wet or not cool enough comments. I installed a heat pump in my home a few years ago. I ended up with an undersized heating and a over-sized cooling system. I added a whole house dehumidifier turning my damp meat locker into a very comfortable home. I turn off the a/c when not in the home, retuning to the desire temperature upon return. All the time I maintain low %RH throughout my basement and home. I recently added ventilation (when occupied) to the dehumidifier. This is special. A fresh, healthy, comfortable home using minimal energy use. So this may be the "distortion". Thousands in green grass climate homeowners are appling this concept to their homes and reporting excellent results.
Regards TB

[DrCindy]

Wow, you guys are getting me confused! We've been in this house for 11 years (original owners) with only a gas furnace and have never had issues in the home with humidity. As Dan noted, the heat is much more important in our climate and the cooling is secondary. Of the 3 quotes I've received so far, 2 included 2 stage HP's (Rheem and Trane), while 1 was just a single stage (Am. Standard). Only the Rheem guy recommended a 3 ton HP, while the others recommended 2.5 ton HP. Any clarifications for someone who doesn't understand all the HVAC jargon? Thanks again!

[jerryd_2008]

In my area, humidity is not a very big issue at all, nothing like the midwest or south. Also, the summers are usually mild, I believe historically in the 70's, but past years have shown a warming trend. Last year we had a couple days in a row (!) of 90 degree weather, and we just finished surviving a week of temps that were in the high 80's to low 100's, which was very unusual.

I'm looking at a summary report from my load calculation: the BTU heating is 26,921, and the BTU cooling is 16,823. This is for a 2 story home.

...

Just a HO, but surprised the pro's didn't question the 16,823 Btu cooling requirement for a 2 story home. Huh???? Is it 500 sqft and built underground?

Come on guys, even in Tacoma, that seems low, right?

Then what does "Only one contractor has actually done a load calculation and said the results indicate I need 26,000 BTU's" mean? Is that cooling? Seems different and maybe more realistic than the 16,823 quoted above.

[beenthere]

Jerry, do a search for member"paul42", coold 4000 sq ft with 2 tons.

drcindy. We're talking abouot humidity in the summer when your cooling the house.

An oversized system could cool the house too quick. And not remove enough moisture.

Not familar with your area. Some areas, an oversized unit. Would be a blessing. Since they have to run a HUMIDIFIER when they use their A/C, because their area is so dry.

What is your average humidity in the summer.

[thrashme]

You might try to find a Daikin dealer in your area. The compressor uses inverter technology, in layman's terms it is a variable speed compressor, which varies the capacity as demand changes. There are multiple types of indoor units available, which also use variable speed blowers, ductless or ducted. This might allow for some flexibility in the differences in the heating and cooling loads and provide you with excellent humidity control. Daikin has a wonderful website:
http://www.daikinac.com/residential/home.asp

[Alden_Sloe]

DrCindy,
PM me if you'd like the scoop on some of the contractors we dealt with when selecting our HP. I'm on the eastside (Bellevue).

Teddy_Bear alluded to it. Forget about dehumidifying with the AC. We don't run the AC that much here. Maybe six weeks out of the year and most of that time you're going to be too dry. During our last heat wave the RH in our house with the AC maintaining 70-76 degrees was <40%. When we really need to dehumidfy is the spring and fall (about six months out of the year when you're not running the AC. Right now the AC hasn't run in a week and RH is 60% at 69 degrees. As you said humidity isn't a big issue for comfort in the NW and we don't get condensation on the windows which is the first place it would show up. Now, basements and crawl spaces, that's a different story! We have a number of musical instruments including a 100 year old grand piano plus a bunch of old books in an upstairs library so after watching RH for the better part of a year I think we're going to add whole house humidity control. BTW, it might surprise you that during the winter we typically need to be adding humidity to the house even on mornings when the fog is so thick the airport shuts down. That was an eye opener for me.

As dan_sw_fl asked, are you looking at 2 stage heat pumps. I am so glad we went that route for a number of reasons. What ever route you go don't even consider a proposal that doesn't include a true variable speed fan (EMC or DC blower are a couple of the advertising terms they throw out to distinguish it from a PSC 2 spd fan).

Dan also hit it on the heat saying what we really need to focus on is the heating side of the equation. The most important number for energy saving in the NW is HSPF. Another very important fact is what happens to the performance when it starts to get cold. Look at the performance numbers at 40, 30 and even 20 degrees. It's perfectly reasonable to have the HP do 95% or more of the heating here if you get the right unit. That can make a big difference in pay back time and comfort. For someone that didn't have gas I'd have no qualms recommending strip heaters for back-up heat in our climate.

A couple other things. Make sure you're considering and confirming all the rebates and tax credits available. You may find the price difference between the low end units and the high end units isn't really that great. If it comes close in payback over time you'll be much happier with the top end stuff. Make sure the contractor put the actual mfg model numbers on the quote for the coil the furnace and the HP. In fact you should insist they give you the AHRI number. Don't be afraid to get a lot of quotes. Really there are still very few companies in our area that are experienced with heat pumps (no matter what they tell you).

[teddy bear]

DrCindy,
Teddy_Bear alluded to it. Forget about dehumidifying with the AC. We don't run the AC that much here. Maybe six weeks out of the year and most of that time you're going to be too dry. During our last heat wave the RH in our house with the AC maintaining 70-76 degrees was <40%. When we really need to dehumidfy is the spring and fall (about six months out of the year when you're not running the AC. Right now the AC hasn't run in a week and RH is 60% at 69 degrees. As you said humidity isn't a big issue for comfort in the NW and we don't get condensation on the windows which is the first place it would show up. Now, basements and crawl spaces, that's a different story! We have a number of musical instruments including a 100 year old grand piano plus a bunch of old books in an upstairs library so after watching RH for the better part of a year I think we're going to add whole house humidity control. BTW, it might surprise you that during the winter we typically need to be adding humidity to the house even on mornings when the fog is so thick the airport shuts down. That was an eye opener for me.

heat pumps (no matter what they tell you).

Generally, I agree with your post. Just to clarify my comment. On a hot day, a properly setup a/c should be able to provide low humidity. On a cool wet day, supplemental dehumidification is required. Of course, the basement/crawlspace can never depend on the a/c for humidity control. A good whole house dehumidifier provides the flexibility of maintaining <50%RH with or without cooling load.
Regards TB

[jerryd_2008]

Jerry, do a search for member"paul42", coold 4000 sq ft with 2 tons.

...

BT, really a 2200 sqft house with an 1800 sqft unfinished basement. But thanks.

[beenthere]

Proper construction. With emphasis on the right things.