How to determine proper size for replacement gas furnace???

[RLymburner]

Hi everyone & thanks for your thoughts.

We've had 3 different contractors come in and look at our ~20 year old gas furnace and provide quotes. The question I can't seem to get a straight answer on is what size furnace to get.

Our current 80% efficiency gas furnace is rated at 100k which means it puts out 80k BTU. Ok, I understand that. The new system is rated at 95% efficiency, so do I get a 80k or 100k two-stage unit? The smaller 80k unit (as recommended by 2 contractors) will give us nearly 76k output which is slightly less than we have now, but the larger furnace will give us 95k which is well over - and from what I understand, oversizing these units will lead to problems down the road. The contractor recommending the larger one says the smaller one will not put out enough heat.

We live in the Mid-Atlantic so winters are reasonable.

System looking at on the 80k side:
Trane XL15i A/C - 4TTX5036A1000C
Trane XV95 furnace - TUH2C100A9V4VA
Aspen Coil - ACE36D44210L015

[motoguy128]

IF you're in the mid atlantic and only need a 3ton ac, you won't need more than 80k BTU and likely only need a 60k BTU furnace. But it will depend on insulation, how much shading you have and overhangs over windows on how proportionaly you cooling load is vs. heating load.

Are you sure your AC is sized right?

Look at it this way, if your current 100k BTU furnace is adequate, then a 80k 95% furnace will be as well. Is your furnace running almsot constantly when it's dipped below 10F? It should be if sized correctly. IF not, then you may be a little oversized right now anyway.


A good contractor, for furnace sizing will either 1) DO a proper load calulcation or 2) get a copy of your gas bill which usually has the average daily temeprature for that month, then compare that to you design temperature, factor in the average temeprature you keep it in your home and can use that info to work backwards into a size. I personally think that's one of hte best methods. But you want to run hte calculation for 2 or 3 months to create a trend, then extrapolate that for the coldest average mothly temperature anticipated. Why? You'll see a increasing trend due to stack effect. The colder it is, the more infiltration your have. So using extrapolation, will factor that in.

[RLymburner]

Bah ... 3 contractors onsite and none did a proper load calculation / asked for additional details re: sizing. Each contractor "rated well" by Angie's List fwiw. It does dip below 10F here (30 miles West of DC) but we have been comfortable here the last 5-6 years.

Two zone heating btw, 3 story house, heatpump for upstairs, this unit for the bottom 2 floors. The first contractor with a quote also did our A/C for the upstairs (also a Trane XL15i) about 2 years ago and came in $1k under the other two contractors (but I was leery about the outlier).

[skippedover]

Bah ... 3 contractors onsite and none did a proper load calculation / asked for additional details re: sizing. Each contractor "rated well" by Angie's List fwiw. It does dip below 10F here (30 miles West of DC) but we have been comfortable here the last 5-6 years.

Two zone heating btw, 3 story house, heatpump for upstairs, this unit for the bottom 2 floors. The first contractor with a quote also did our A/C for the upstairs (also a Trane XL15i) about 2 years ago and came in $1k under the other two contractors (but I was leery about the outlier).

Forget those companies. Return to the telephone and start asking the question up front, "How do you size a replacement gas furnace?". When you find one that does it by Manual 'J' load calculation, invite them over for a quote. But under no circumstances do I recommend dealing with a company that is cutting corners from the very start, gambling with YOUR money that he GUESSES correctly on furnace sizing. The science to do it correctly is readily available. Those who choose to not do a proper Manual "J" are just too lazy to learn the process to deliver proper results for the customer. As far as Angie's List is concerned, the people who are reporting on those 'large' companies probably don't know what they're missing. So how can they make a valid evaluation? They may think that on/off/on/off/on/off all day is normal and that they're comfortable. Then someday they may find someone who does the job properly and gives them on/on/on/on/off/off/on/on/on/on operation and find out what real comfort is.

[Mr Bill]

Bah ... 3 contractors onsite and none did a proper load calculation.

All them there manuals are important, but make sure you also get a good installation. I personally would rather have a exact replacement, good quality install, than a shoddy manual installer. Might even ask for a portfolio or referrals. :grin2:

[RLymburner]

Forget those companies. Return to the telephone and start asking the question up front, "How do you size a replacement gas furnace?". When you find one that does it by Manual 'J' load calculation, invite them over for a quote. But under no circumstances do I recommend dealing with a company that is cutting corners from the very start, gambling with YOUR money that he GUESSES correctly on furnace sizing. The science to do it correctly is readily available. Those who choose to not do a proper Manual "J" are just too lazy to learn the process to deliver proper results for the customer. As far as Angie's List is concerned, the people who are reporting on those 'large' companies probably don't know what they're missing. So how can they make a valid evaluation? They may think that on/off/on/off/on/off all day is normal and that they're comfortable. Then someday they may find someone who does the job properly and gives them on/on/on/on/off/off/on/on/on/on operation and find out what real comfort is.

Nuts - wish I had found this forum. Good advice, looks like I'm back to square one here and need to pick up the phone.

[motoguy128]

Heating and cooling equipment really is a much bigger purchase than buying a car. You're stuck with it for about 10-20 years, you can't just trade it in, it affects your comfort and energy bills all year long 24/7. You can't simply trade it in on a different model if you don't like it and on a residential system it loses nearly 100% of it's value the moment it's placed in service. 50% of the cost roughly, are installation expenses.

Do you get 3 quotes for a car and simply pick the cheapest one? If car mfg's shipped their cars with the engine, transmission seperate of the rest of the car, wouldn't you want a dealer with a very compitent service department with well trained mechanics to do the assembly? What if a sales "engineer" was the guy that selected which combination of entgines and transmissions you should put in that partucular model and then had to select the correct dirveshafts, brakes, wheels size, and final drive ratios. Still want to use the low bid?

[RLymburner]

Sage advice, thank you everyone. I think I'm asking the right questions, but the guys I went to first are the ones that want the most of my $$$! A couple of quick questions before I start hitting the phone lines:

(1) I have an Aprilaire 700 humidifier on the furnace (manual control) - can it be refitted on the new furnace to save a few bucks? If not, do I get one that can be controlled by thermostat? Right now, the existing contractor quotes include the XL802 controller, but I could get the Honeywell IAQ (or similar) to maintain humidity.

(2) Two of the contractors have offered to throw in Aprilaire 2210/2410 whole house media air cleaners free of charge (suspect!). In online googling, it appears these are better than the standard 1" drop-ins I have now. No problem with this? We do have 2 indoor cats but no real allergy problems in the past. I hesitate to ask, but should I consider an electronic air cleaner while I'm having the furnace work performed?

[beenthere]

Your 700 can be controlled by a thermostat with a built in humidistat.

[beshvac]

Sage advice, thank you everyone. I think I'm asking the right questions, but the guys I went to first are the ones that want the most of my $$$! A couple of quick questions before I start hitting the phone lines:

(1) I have an Aprilaire 700 humidifier on the furnace (manual control) - can it be refitted on the new furnace to save a few bucks? If not, do I get one that can be controlled by thermostat? Right now, the existing contractor quotes include the XL802 controller, but I could get the Honeywell IAQ (or similar) to maintain humidity.

(2) Two of the contractors have offered to throw in Aprilaire 2210/2410 whole house media air cleaners free of charge (suspect!). In online googling, it appears these are better than the standard 1" drop-ins I have now. No problem with this? We do have 2 indoor cats but no real allergy problems in the past. I hesitate to ask, but should I consider an electronic air cleaner while I'm having the furnace work performed?

Using the analogy above....when you buy a new car do you want to use any pieces from the old car?

I personally like the april aire's and have them in my own house. They work very well keeping the equipment clean. I do not care for electronic because whenever I service them, most are not working and aren't picking up anything.

[RLymburner]

Good enough for me, thank you everyone. Hitting the phones.

Awesome advice, great service provided!



:payattention:

[wahoo]

The large pleated 5" filters are the way to go especially if you're getting them at a discount. Don't allow a contractor to "oversize" your new unit. USUALLY the old equipment was significantly oversized, so the newer, higher efficiency units are smaller input, but many times closer to what is needed. One of these guys (or gals) SURELY knows how to do a manual J figure on your actual needs. That way you'll know and not guess at what your home actually will need. The fully "electronic" air filters have a tendancy to quit working after a few years (usually after warranty runs out). We no longer recommend them to homeowners, just use the large pleated 5" models. As moto mentioned, if your new higher efficiency furnace and AC are sized RIGHT, they will run nearly continuously at extreme cold or hot temps. That is the way you max out your savings. Nobody buys a 45 MPG car and shuts it off and start it again at every stop light, and expect any kind of fuel savings. When it's 100 outside, or zero, the units should be running nearly full time (if sized properly for your temperature zone).:.02:

[surenuff]

no comparison to a job done right. Find someone who wants to work with you and set down and do a properly completed manual J load calculation based on the construction materials your house is built out of. Someone who will spend time actually doing there best to inspect your homes insulation, windows, doors, leakage, to the best of their ability. Once a load calculation is done, then a discussion on different types of equipment that will satisfy your homes needs can be held. You can discuss with this contractor any special things you might want from your ou may just want something that will satisfy the load, and then you may want something that will do the job but be able to cycle into second or even third stage for maximum energy savings. Whatever you want, just make sure it was sized right. Not in all cases, but in most, someone who will spend the time to properly size equipment and do load calculations will also take the time to install the system correctly also.

[RLymburner]

Thanks everyone, I had a load calculation performed and it was computed at nearly 3 tons and (as contractor pointed out) "The load calculation also does not take solar infiltration into consideration and you have a lot of it with the number of windows. I would stay with the sizing we had previously discussed." So ... my fears have been addressed in that I'm getting a properly-sized system for my house. Thank you everyone!

System spec:
A/C: 4TTX5036 (XL15i)
Indoor coil #4TXCC044BC3
Furnace blower #TUH2C100A9V4
Media air cleaner (non-electronic)

Two quick questions remain:

(1) The system will include the Trane XL802 thermostat. Humidity control is a concernt - is it worth the expense to upgrade to the XL900 ($400) (or Honeywell IAQ?) ???

... and waaaaay off topic ...

(2) The contractor mentioned that there is only ~6" of insulation in my attic (although there is 12" above our bedroom) and that I should definitely add 12" of R-38 (he didn't offer to do it, just something he noted during an analysis of my home).

[CraziFuzzy]

(1) The system will include the Trane XL802 thermostat. Humidity control is a concernt - is it worth the expense to upgrade to the XL900 ($400) (or Honeywell IAQ?) ???I would definitely make sure whatever stat is installed is also controlling your humidifer. It just makes sense to keep everything at one location. I can't speak to the Trane stats themselves, so I'm not sure what model you would need.

[beenthere]

The load calc does take solar gain into account, thats why you enter house/wall orientation.

[duckman06]

The load calc does take solar gain into account, thats why you enter house/wall orientation.

I Agree with Beenthere! You can take the same house and change the direction and the load cal. Will reflect a different load. I would ask what kind of load sheet/software the contractor used? Also was seer or how efficient the system a big factor for you?

The reason I ask because when the federal tax was a big thing in 2009/2010, I had to use 100,000 btu furnace with a 3 ton condenser to get 16 seer rating with a variable speed. That was using Trane equipment which is all that my company sales. The way I addressed this was to only allow 1 stage heat to be used so as to have a oversized furnace. Know those days are gone and proper sized furnaces are used by my company per manual j load.


I don't know of many homes that need a 3 ton system with 100,000 btu furnace in my area but I am down south where heat is as lot as important as cooling load is. I would ask to see the load the work up with all the reports. If he is by chance using right manual this would be a simple email to you or to his computer and printing it out for you.

[RLymburner]

I Agree with Beenthere! You can take the same house and change the direction and the load cal. Will reflect a different load. I would ask what kind of load sheet/software the contractor used? Also was seer or how efficient the system a big factor for you?
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I don't know of many homes that need a 3 ton system with 100,000 btu furnace in my area but I am down south where heat is as lot as important as cooling load is. I would ask to see the load the work up with all the reports. If he is by chance using right manual this would be a simple email to you or to his computer and printing it out for you.

We're in a above-average sized house in this area, ~3.6k sqft if you include the finished basement. 2 zones, the one I've been talking about covers the basement and main floor. Also, main floor has a two-story foyer and family room (with windows from ceiling to floor) that faces South.

I did ask for a copy of the load calc, will pass it on if appropriate. I know the rebate is over (darn timing!) but I do plan on staying here for a number of years so I asked for an efficient system (this one specs at 15.25 seer). I think I'm OK with it

[George Fernandez]

15.25 Seer seems in the middle of the road. 13 Seer being the lowest and ~20+ Seer being the high end.
Not saying its bad. If it is what your budget can handle, then that is what is important. If your budget can handle higher Seer then lean higher.
Upfront cost is one factor, operating cost may offset a higher install price.

A 5" pleated filter has more surface area than a 1", which translates to 2 benefits. 1. better filtration. 2 less restrictive. They(20x25x5) are rated for 5 tons of air, so using it on a 3 ton system wont hurt.
As far as a load calculation, Manual J will be the definitive sizing authority. All factors are taken into account. With the proper information the load size derived will be THE correct one.

[wahoo]

:.02::.02:Just had an older (88) yr. old guy ask about "payback" on 13 versus 15 SEER units. Based on his electric rate and 1000 hours of average operation he would need 25 years to break even on the extra cost of 15 SEER unit. Really hard to try and talk an 88 year old healthy guy into a 25 year payback unit!! I didn't bother trying really hard (had no children to leave home to either).

[RLymburner]

Contractor response:


We were going to just move the humidifier. I can provide and install a
new Aprilaire 700 for an additional $. It would be more than that
for you to buy one and us install it, and the biggest problem would be
your warranty would be with Amazon. I did not look at the thermostat, I
will go ahead and put an 802 on for you. The only one there would be
additional charge for is the 900 you had asked about. We always change
the thermostat on new installs. They would arrive at your home between
8-9 on Wednesday and it would take the whole day to install. I will
bring the whole load calculation booklet with me, it is too big to send
via email. Please let me know if this time works out for you and about
the humidifier and thermostat.


Is he just protecting the time he spent to perform the load calculation, afraid that I'm not going to give him the business?

Is it "worth" the extra $$$ to get humidity control of the XL900 or Honeywell IAQ?

Thanks for the continued advice, I'm really trying to soak it all in!

[beenthere]

I think the 902 or IAQ is worth the money.

And he has a right to protect his load calc, from being given to his competitors.

[RLymburner]

I think the 902 or IAQ is worth the money.

And he has a right to protect his load calc, from being given to his competitors.

Ok, thanks for the continued advice. I called and I was assured that it was a manual j calculation and that I would get a copy when they installed the system. I'm getting the XL 900 as well.

Thank you everyone, I feel much better about putting this into my home. :cheers:

[gravity]

before having the installation i am assuming that you will be signing a contract? in the contract make sure there is documentation stating that you will be given the manual j calculation. a manual j is not to big to send in an email. i also just pulled out the long form today that was in my old books. i was showing a new guy how i did them on paper and its only 2 pages. that to me doesnt seem to big to send in an email.

[RLymburner]

before having the installation i am assuming that you will be signing a contract? in the contract make sure there is documentation stating that you will be given the manual j calculation. a manual j is not to big to send in an email. i also just pulled out the long form today that was in my old books. i was showing a new guy how i did them on paper and its only 2 pages. that to me doesnt seem to big to send in an email.

He told me it was ~30 pages (and I'm a computer guy, a PDF that size wouldn't take 1meg) ... oh well, I'm still interested to seeing how he reacts when I ask for the 5% off for being Angie's List member (nothing on the coupon about presenting at time of estimate (which would allow him to price 'accordingly')

[gravity]

you are correct. but, it would cost me more to come out and fix the poor installation you might be getting by trying to use a 5% off coupon. do not rush into a new installation, you will regret it if you do. I personally would be doing more research on companies in your area. it might cost you to have a proper manual J performed but, once its done properly it is yours forever. You can then give this to other companies and see their reaction. There is a order which needs to be followed when purchasing a new system.
1. perform a proper manual J (not a square footage of the house and rule of thumb)
2. perform a ductwork analysis or manual D to make sure the ductwork is sized correctly for the results you got from the manual J.(not the rule of thumb that .1 friction rate works for all systems)

once you find a contractor that performs a manual J and performs a duct assessment or manual D, they will most likely be trained and certified to follow the normal practices of installing the system. ex. flowing nitrogen while brazing, properly sealing plenums at air handler, perform combustion analysis and properly tune the gas furnace for safe and efficient readings.

you can find literature on ACCA website of things to look for in a contractor

https://www.acca.org/consumer/home-choosing

https://www.acca.org/Files/?id=186

[AtticAce]

Nuts - wish I had found this forum. Good advice, looks like I'm back to square one here and need to pick up the phone.

All you really need to do is ask these same companies to prove you need the furnace they are quoting.

I like to tell my customers installing new equipment is like entering a 20 year relationship. They don't want to just guess how it is going to work out, especially because the contractor can divorce themselves after the one year warranty is up, leaving them alone with their choice for 19 more years.