HVAC Help...Proper size furnace for a house?

[Dick Strauss]

Well,
It's time for mom to replace her furnace. She currently has an 80K BTU input 93% efficiency furnace (when new) that is about 20 years old. The secondary heat exchanger seems to be about shot and is starting to leak. The electronics are acting a little strange as well. I don't want to worry about breakdowns once it gets cold. The house was built in the 70's and is moderately insulated. Almost all of the windows are double paned. It is about 1800 square feet and has a semi-finished basement that adds about 600 square feet.

I have received quotes for furnaces ranging from 66K-80K BTU input in the 92-95% efficiency range. Some folks say a 66K BTU input (61K output) will be large enough. Others are just replacing the 80K with another 80K unit. The price difference is very minimal. Some folks have suggested the house will be more comfortable with the smaller unit because the unit will stay on longer but the temperature will be more even throughout the house. On the other hand I don't want the system to be overworked.

What do you think? Will I be sorry I got the 66K instead of the 80K?


Thanks in advance,
Dick

[Brian Backner]

It is impossible to say without a formal heat loss calculation. You should have an independent HVAC engineer come out and do the calculation. Do NOT rely on the contractor/installer as they may only be interested in selling you what they have in stock at the moment.

The calculation is based on far more than the just the square footage and ceiling height. Done properly, it will take into consideration such things as size and type of windows, storm windows, direction in which the house faces, tree shading on the house, type and amount of insulation, yada, yada, yada. This might also be a good time to have both an air infiltration test and infrared scan of the house performed to discover air leaks and gaps in the insulation - and have any problems corrected.

Depending on how leaky/uninsulated her house is, the 66K BtuH furnace may work. Beefing up the insulation and sealing any leaks may even allow the installation of an even smaller unit. Oil and natural gas ain't getting cheaper any time soon - anything you can do to make the house tighter will pay off in the end.

Brian

[Matt Meiser]

My thinking is that if the one she's got worked well for her for 20 years then why mess with a good thing. At least that's the decision we made when we replaced ours. Did they actually do calculations or just a quick estimate?

BTW, I'm not sure if they come down to Toledo, but the place that did mine last year did a good job for both me and my parents. I guess that's why they've been in business for something like 120 years. PM me if you want to get their name.

[Terry Achey]

Dick,
I agree with Brian that a heat loss calculation is the only accurate method to determine proper sizing. Design conditions for your area are also very relevant to the formula and generally are part of hte analysis. Difference between 66k btu and 80k btu is about 20% which is quite a variance. That said, manufacturers don't generally size product lines like shoe sizes. The jump from 66 to 80 may be typical of sizes that most offer. And they both may work fine for your application. Worst case is when experiencing the "100 cold snap" with the smaller unit.

I would recomend getting several recommendations from reputable dealers, but not let on that your concerned with having a big enough unit. Let them suggest the size without being influenced by your concern. Compare and ask questions from those you trust the most. Most residential contractors don't have credentialed HVAC Engineers on staff, but the better ones (experienced and reputable) do have staff with plenty of years of experience determining heat loss accurately enough for residentail application.

Good luck,
Terry

PS - over-sizing equipment is not necassarily a good default. It can lead to problems such as short-cycling, less efficiency and more dramatic temperature swings. But if you replace an 80k btu with an 80k btu it is not likely going to be a problem.

[Dick Strauss]

Unforunately only one person did a heat loss calculation out of 10 estimates (he didn't specify which size furnace he plans to install on his quote). I'm waiting for a return call from him.

A different guy was only worried about airflow and returns but was kind of an "in your face and space" guy...(my system is well balanced by the way). He did say I need 1600+ CFMs to fill the vents fully. His price was the highest for an Armstrong system (a subsidiary of Lennox) so I wrote him off pretty quickly based on price and attitude.

I'm leaning towards a Lennox G61 two stage unit because of the issues with Carrier heat exchangers, etc. Mom currently has a Lennox unit that has done pretty well for her. I found surprisingly very little difference in the price of various brands.

Terry,
That is what I heard from a few guys...that the furnce might cycle too often and not give good comfort from steady state runs. I'm planning to buy a two stage system so cycling should be less of an issue, correct?

FYI- One of the larger firms with all of the certifications not only didn't perform a calculation or look at the windows, he didn't even know the BTU of the furnace he recommended. He stated it was 70K (input) but I found out it is actually 66K (input). Obviously he wasn't a well informed salesperson.

Brian,
I'm in 100% agreement about improving the insulation! I don't mind paying for the services you mention, but I'd like to have a list of what are the most cost effective improvements when they are done. What I'm trying to say is...I want someone that can also tell me how much each improvement will cost and how much will it help. 1)What organization oversees IR scans and air infiltrations testing certification?
2)How do I know who to contact that won't try to rip me off and will do the job properly?

Matt,
I've already found a local guy that has been in business for 20+ years. He is a small operator that does the service and install himself. I respected his honesty and low pressure sales technique. Thanks for the offer!


Thanks for all your help,
Dick

[Brian Backner]

Dick,

I don't know how it is out your way, but here in Taxachusetts HVAC engineers are regulated/licensed by the state. Once you find one you can work with (get references), he/she should be able to recommend several IR and air infiltration specialists, as well as insulation installers. The thing is to find an INDEPENDENT engineer - if he doesn't work for/with an installer, he has nothing to gain by not giving you the straight dope.

Brian

[Terry Achey]

Dick,

Sounds like your asking the right questions and on the right track. Large companies don't necassarily provide high performance. Your local guy that does the work himself is probably going to stand behind his work.

Wow... 10 estimates and only one heat loss calculation! I'm surprised. The "in your face" guy isn't incorrect about your duct system needing to handle a specific amount of cfm. Generally speaking, the larger the furnace in terms of btu output, the more air flow is required. Inadeqaute air flow can cause overheating of the heat exchanger which could cause hi-limit failure, and/or high discharge temperatures into the living space. Again, replacing an 80k btu with an 80k btu will not place you in this position.

Yes, the two-stage system should eliminate short cycling.

Terry

[Al Willits]

Maybe try your city inspector and see if they can recommend someone to do just a heat loss.
That failing, if in doubt, consider how the 80k furnace you had worked, if it kept you warm and didn't seem to cycle to often (that can be a thermostat/duct/filter issue too) I'd lean to wards the same size furnace again.

Ductwork sizing is a different issue, you need X amount if btu's to heat a house and if ductwork is incorrect, that should be corrected, not altering furnace size.

You not gonna find much different in a 90-92-94 or any of the 90% furnaces, these numbers are optimum numbers and probably are not accurate for most homes, I'd put furnaces in two groups, 80% and 90% and not worry about exact percent efficiency.

Here in Minn there is a code on sizing furnaces, that makes it easier, but you should be able to get a correct size from someone who does a heat loss and is honest with you.

Al

[Craig T. Smith]

Dick, We bought our house 3 1/2 years ago . Last year we had to replace our heat pump. It had a "4 ton" unit in it . Our house is 1150 sq. ft. One dealer was going to replace ton for ton . The next looked at the square footage and said that 4 ton was to bigand we needed 1 1/2 ton . we went with him and our house is now the most comfortable house I've ever lived in . Size does matter . A larger unit gets hotter and colder faster that it doesn't turn the air over completely. My advice is to shop around get energy studies then you have to go with a dealer who you trust. Good luck, Craig

[Matt Ocel]

Google "heat loss calculator" there are plenty of do it yourself heat loss calculators that although may not be precise, they will get you in the ballpark as far as educating yourself for when you have to talk to HVAC contractors on furnace replacement.

You need info on items and construction methods in your home. If you are not sure as to window "U" values and wall "R" values and other such items, then this path might not be for you.

[Dick Strauss]

I finally got feedback from the guy that did the heat calcs. His quote was for an 80k unit. His price was $500 higher than the rest so I tried to give him an opportunity to be more competitive. He stated that was the best he could do.

The "in your face" guy said that the increased cfm would help get cool air to the upstairs during Summer. He suggested that the cool air fan setting wasn't high enough to push the air to the upstairs. The upstairs does not cool as well as the rest of the house during Summer but there is plenty of heat in the Winter from natural convection.

The furnace is acting weird again so I've got the ball rolling for the 80k Lennox furnace none too soon. Hopefully it will be installed next week.


Thanks for all of your help,
Dick

[Al Willits]

Ya might want to leave the fan selector in the "on" position, that will help ciculate air better, don't confuse air disturbution with furnace/AC size.

Al

[Jeff Nicol]

Most newer furnaces have multiple speed fans in them. What this is for is during the cooling season the thermostat is switched to cooling and the computer in the furnace is supposed to speed up the fan to compensate for the need for extra air flow across the cooling coils. Also always make sure the fan blades are clean as they can get very dirty every year. The more fuzz on it the less it will be effecient. You can do an informal leak test on the house by getting a big fan and putting it in a outside doorway and putting plastic around the door frame to seal it as much as possible leaving the fan to suck air out of the house. Then make sure all the windows and other outside doors are closed. Check each opening with a smoke generator (cigarette will do) to see where the drafts are coming in. You would be suprised at how drafty a house can. Check around all the wall outlets and switches, because they are notorious for not having insuation behind them. So any air movement is heat loss because cold air will find it's way in. Large old single pane windows are the next big heat losers, so updated thermopane windows or good sealed plastic will help there. Last but not least the best insulation money spent is for more in the ceilings as we all know heat rises! I installed furnaces and did commercial heating and cooling for 25 years I hope all of our advice helps you and your Mom!

Have a great day!

Jeff

[David Freed]

I won't try to give you an answer for your situation, but I will tell you my experience. 26 years ago when I built my house (2600 sq ft including a full basement), the furnace calculator that was used said we needed a 60k furnace, and that is what we put in. A few years ago, when that furnace finally quit, I used a furnace calculator, and it again recommended a 60k furnace. This time I knew how little our old furnace had run to heat our house. I installed a 45k high efficiency furnace (myself) and even in 0* weather, it is more than enough to keep our house warm.

[Lee Schierer]

80K btu seems quite large for a 1800 sqft house. I live in NW PA and we have heated our house for over 25 years with 50K btus and we heate 2200 square feet to 68 degrees. In all those years the furnance never ran 100% of the time even in the coldest weather. It was a gothermal heat pump. This summer we replaced the unit with a new one that also cools. We haven't seen a winter with the new unit yet, but the contractor determined that the size was coorect.

In days when fuel was cheap, the bigger is better mentality was pretty rampant. It was easier for contractors to put in a larger unit than deal with an unhappy customer later if the house was cold. The contractor had no responsibility for the fuel bills. Most furnaces were oversized and the trend continues becuase modern contractors don't do their homework and simply read the label off the old one to size the new one.

Ideally a furnace should run 95% of the time on the coldest days according to the 25 year weather data for the area. Anything larger, even though it may be an efficient burner is wasting energy by heating up too much heat exchanger.

I would definitely recommend a variable speed unit and running the fan on low all the time to keep the house evenly heated.