Had Estimates.. What size?

[jmon22]

Hello All,

Central Ohio, mid 70's house, a little over 2700 sf.

Had 4 contractors come out. One sized based on the size of old system, 2 did short form manual j's based on the plans, only one actual measured the windows (but estimated insulation and air leakage).

Want to go with 2 stage variable furnace and single stage ac.

Everyone suggested 80k 95% furnace (most complete manual J recommended 100k at first based on a 78k heat loss, but once I mentioned he agreed). The two short forms are at 3 ton AC while the other is at 3.5 ton.

I am most comfortable at the moment with the one who actually measured windows, his knowledge, and price (works with utilities to make sure I get all discounts, others didn't even mention), but am afraid once I make some improvements on the house I would only need 3 ton. He assures that I wouldn't be able to get down to 3, but how sure can he be estimating insulation and air leakage? My only concern is that he was quick to oversize furnace.

Without seeing my house, what do you think about recommendations for my climate and my concerns? Am I overthinking the half ton? Do I need to keep getting quotes until I am 100% certain with one?

Thanks for the help!!

[BaldLoonie]

With a 78K heat loss I'd want the 80K 95%. But there are some who will say that is cutting it too close. They don't want to risk a customer not understanding why the house isn't toasty in freak bitter cold weather. So with that number, the 100K isn't out of line. I'm not sure I'd fault him for suggesting the bigger furnace. Same with cooling. Some are nervous cutting it close. They don't want to hear the house is warm in a summer like we just had - warmest since the dust bowl days. How cold do you keep it? Industry standard is to size to 75° inside. If you like it colder, better go up to the 3.5 ton. Otherwise, better humidity control cutting it close to the 3 ton mark.

[54regcab]

Unless you live north of the mason-dixon line 80k might even be oversized for 2700sqft. Furnaces are quick to be oversized because higher BTU furnaces aren't much more expense than small furnaces. Contractors are scared to death of undersizing furnaces, but in the real world have have yet to see a single undersized gas furnace. Most furnaces I run into are 2-3 times the size needed.

[jmon22]

I actually do live juuust north of it. It is just been very hard to figure out who and what brands to trust. Search just on here and you see plenty of people with differing opinions on every brand out there.

[tedkidd]

Unless you live north of the mason-dixon line 80k might even be oversized for 2700sqft. Furnaces are quick to be oversized because higher BTU furnaces aren't much more expense than small furnaces. Contractors are scared to death of undersizing furnaces, but in the real world have have yet to see a single undersized gas furnace. Most furnaces I run into are 2-3 times the size needed.

+1

60k. 2.5 ton or 3 ton 2 stage. Never seen manual j not oversize. I bet 60k will shut off at worst case outdoor conditions, meaning still oversized.

How would he have any idea "what you wouldn't be able to get down to..."? He ever crossed that line? Ever even come close to seeing it?

If you have 2.5 ton, how many minutes a year will it be undersized? Can he predict that? And can he describe the catastrophic life threatening sailing of the edge of the flat earth that will occur? He's making WAGuesses, and attempting to sound certain of his crystal ball. Sounds like you went into this better educated then everyone who came to your house.
Do you have any idea about hourly run times at worst case for heating and cooling with current equipment? You can reconcile load to that.

Try doing your own modeling:

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Good luck,
Pj

(from discussion Linkedin )

An early video walk through which may be a good place to start:

[jmon22]

tedkidd... I really appreciate your feedback. I have read a lot of your posts and really am impressed with your knowledge.

Here is my issue, I am by no means a pro, but I feel like I know a decent amount from reading some of the knowledge on here. I keep expecting one of you to walk through the door, but it hasn't happened yet and I'm sure won't ever. Do I trust the pros here and pick the size I want with whoever I think will do the best install, even though the sizes I suggest will make them freak out? How do I deal with that inevitable conversation?

[tedkidd]

Wow, thanks for the nice words. Most find my writing style abrasive.

We are starting to see homeowner knowledge eclipse the "free quote" sales "designer" a lot here. They tend to waiver, fear is used against them, some cave and some don't. It's all good, 10 years ago consumers would have put pressure to go up a size, now they're on the other side. This is a fantastic thing.

Tracking results will fix this problem. We are on the cusp. I digress, sorry.

The universe seems to deliver for you. I ran across this just now:

To determine heating capacity required for existing building one may simply extrapolate design load by graphing metered use to outdoor temperature for a series (preferably a minimum of 12) of billing periods. See HeatingHelp dot com > search Therm_lag (that's me) and scroll to "Graphical Load Estimating Method." The guide is attached to this reply. I use Etracker software (freeware) developed by Kissock at U Dayton.

http://homeenergypros.lbl.gov/group/...g_com_gr_forum

Where this stuff seems to go wrong is people fixate on one piece of information and conclude design based upon that. That's a Gantt chart failure. They don't step back and see if design disregards/disconnects from other critical issues.

I see getting this right as using triangulation. Not relying on any one piece of information as gospel, but looking at the general direction various pieces point you in, drawing a circle at that intersection, then finding equipment that falls somewhere within that circle.

You are fully capable of using Phil's software, and if you have a blower door number and your annual consumption that tool is fantastic. A great point of triangulation

Another option - Shortcircuit hired me. Everyone was telling him to put in 100,000/4ton, he put in 60,000/2 ton. Now, a year later, he is extremely happy. He says 60,000 is oversized, but it's a mod and drops all the way to 20, so nbd. We knew 2 ton was aggressive but were concerned his duct couldn't handle 3 (he wasn't interested in duct replacement so we designed to the duct). 2 ton is not enough to recover at the record temps we had this summer, so he abandoned antiquated setback behavior.

Setback as prescriptive, undiagnosed, energy saving with no M&V is blatant malpractice. I say; if it saves, prove it. Also, if it saves for you concluding it saves for everyone is an absurd assumption. Every house is different. What works in one can not be assumed to work in another. We need M&V.

I track, nobody else seems to, and yet people like to tell ME "stupid thermostat tricks" that save energy!? Anyway, his bill is 1/2 his neighbors, what more can he save?! There are a lot of side benefits to abandoning setback. The house is so well dehumidified by the continuously operating equipment his wife complains it's too cold if the thermostat is below 76 - in Humid Baltimore.

BTW, Randy is an engineer at the NSA.

So, get this right and everything comes together in an amazingly elegant way. Comfort, energy bills, and long term since the equipment runs continuously instead of cycling, I suspect repairs will go way down also. I'm confident you are on the right path.

[jmon22]

I again really appreciate your help.

You are definitely more aggressive than even most of the contractors on here, but given the results you have seen it seems to be the right call.

The big question, how do I communicate this to contractors I am dealing with? I believe most if not all would be against going that much lower than a load calc in fear I will be screaming at them when it is a little colder or warmer than I have it set at. And what if I am wrong for one reason or another because of factors with my home?

[tedkidd]

You combat them with information. Squeeze the WAGuessing out of the equation. Oversized is seen by most of the world as "safe" still. Look, these guys are used to taking too much responsibility for things that are beyond their control. Eventually they learn that when the customer makes the specification, responsibility for "perfection" no longer falls on their shoulders. But it's a long path between here and there.

Run the load calc.

Do you have a blower door cfm50 #? That number alone is a WAG for most load calcs, which is why people always seem to guess high.

Do you have any airflow test numbers on your duct (design max airflow)? Duct layout? External Static Pressure with current equipment?

[jmon22]

I do not have any of those at the moment. I actually just got done scheduling an energy audit through my electric company for next week. That will give me some of that data and is required for a rebate through them. I am not sure how thorough they will be, but it at least includes the blower door test. Hopefully after that I would be much more informed of what I need to do.

[jmon22]

Also, is anyone else's thinking any different? I would love a few more opinions.

[dan sw fl]

Hello All,

Central Ohio, mid 70's house, a little over 2700 sf.

Had 4 contractors come out. One sized based on the size of old system, 2 did short form manual j's based on the plans, only one actual measured the windows (but estimated insulation and air leakage).

The two short forms are at 3 ton AC while the other is at 3.5 ton.

I am most comfortable at the moment with the one who actually measured windows, his knowledge, and price
(works with utilities to make sure I get all discounts, others didn't even mention), but am afraid once I make some improvements on the house I would only need 3 ton.

He assures that I wouldn't be able to get down to 3, but how sure can he be estimating insulation and air leakage?
My only concern is that he was quick to oversize furnace.

Without seeing my house, what do you think about recommendations for my climate and my concerns? Am I overthinking the half ton?
Do I need to keep getting quotes until I am 100% certain with one?

Thanks for the help!!

SO, be My Eyes.
ORIGINAL Windows ?
How many windows/ direction /type?
N __
S___
E___
W__
about 6 windows = 90 Sq Ft. each side
Total ~ 340 Square feet ?

Infiltration _Loose _ is going to give a High Estimate.

R-11 Walls
R-30 Ceiling
2 story ? _ 30' x 48' ? 8 foot ceiling height
No basement?

Above info is needed to independently verify "What Size?" at ~ +/-15%.

Manual J Short form is rather easy to calculate a "block load" ( overall house losses versus losses Per Room).
80k furnace is more than enough but you are not likely to be able to get down to a 60,000 BTU/HR one.
However, I would be suprised if heat load might be ~67,000 ( ~11,000 less than you 78k)

[dan sw fl]

Wow, thanks for the nice words. Most find my writing style abrasive.

The universe seems to deliver for you. I ran across this just now:



You are fully capable of using Phil's software, and if you have a blower door number and your annual consumption that tool is fantastic.

[/B]Every house is different. What works in one can not be assumed to work in another. We need M&V. [B]

When abrasive, just use ... 600 grit instead of 120.
Any abrasions will heal quickly.

[jmon22]

To bring this back, here is what I am looking at:

2750/sf

Windows: Original (1976) 2 pane. Are leaky and will be replacing in the next few years
N:80.25/sf E: 50/sf S: 99.5/sf W: 12/sf

Shading probably around 10-15% Ceilings are 8ft most of the house with about 400sf at 12'

Have a poured concrete basement with a crawl space.

Insulation: I would guess R6-10 in the walls. Attic will be about R60 once I blow more in within the month

Blower Door was around 3300-3500 I will be making some improvements soon to hopefully get that down a bit.

Tedkidd, using the site you gave me I am still around 76k for the heat load. Given this info, should I go 60k or 80k furnace? Thanks!!

[motoguy128]

Do you have your gas bill from the coldest month? Does it show the average daily temperature. Take the Therms used, multiply it by 100,000, divide it by 24, divide by the number of days in that billing cycle, divide by the ratio of the average daily temeprature and the desing temperature, then divide by the efficiency of the current furnace.

I personally think it's even more accurate than the manual J because its measuring complete real world conditons, not any estimates. But there's not a "fudge factor" and on a 2 story home, the stack effect accelerates as it gets colder. Plus it doesn't subtract out days where it's over 65F and your actually cooling. But I think it gives a good estimate overall. The numbers don't lie. If you used 250 Therms (which is a lot for most people) and the average temp is 33F with a 0F design, you were at around 50% average capacity. That's an average consumption of 34k BTU's. IF you had enough thermal mass, you could have heated the home with just 34k BTU's. That's why goethermal works soem well on highly insulated new construction with fulol basement and lots of thermal mass. It just runs around hte clock at 30-40k BTU output and the tmeprature might swing 1F or so, but it just chugs along wihtout heat strips. A load calculation might call for 60k BTU furnace. But it's rarely needed. Also consider all the internal heat loads. A larger home easily consumes 0.6-0.8kw. That's another almost 3000 BTU's on energy, nearly all becomming heat indoors. Also add heat gained from showers as well.

My home for example. 3200sqft. You'd expect that it might need at least 100k BTU. Using my gas bills, the calculated consumption is just 61k BTU's with 2 oversized 92% furnaces totalling 180k BTU's. I've since downsized on to 60k (the smallest available in that model) and downstairs I'm debating dual fuel, geothermal or a conventional system.

Remember heat distribution matters as well. An oversized furnace won't heat as evenly and will consume more fuel to satisfy the same comfort level.

THe pro's on here will tell you that they've probably never seen a undersized furnace. That's because in reality even a larger home really only needs a realatively small amount of heat.


In summary, if in doubt, or borderline, go smaller.

[jmon22]

Unfortunately, this is a new house for me, and I am switching from electric to natural gas. I don't have any previous data to compare from.

[motoguy128]

If you're at 76k now and plan on making improvements, then I'd go with the 60k. Larger homes are more tolerant to being slightly undersized since they have a lot more mass. They don't swing very quickly.

In my old 1300sqft home, I woudl often be heating in the morning when it dipped under 55F and AC in the afternoon when over 65F on the same day if it was sunny in May and October. But in my new home that's 2-1/2X larger, on those same days, the indoor temps will stay pretty constant and not vary by more than 1-2F. SO rather than heating bills being $30, $50, $80, $140, $90, $60, $30... it's more like $0, $60, $180, $300, $200, $80, $20. It ramps up really fast.

[54regcab]

Also keep in mind manual j is based on peak conditions. It does not take into account the effects of thermal mass.

[jmon22]

Also keep in mind manual j is based on peak conditions. It does not take into account the effects of thermal mass.

Meaning that it won't take as much heat to keep my house at a temp as it would to get it to that temp from something lower?

[motoguy128]

Meaning that it won't take as much heat to keep my house at a temp as it would to get it to that temp from something lower?

Yes, that's true in part. But Manual J doesn't factor in the need to raise the temperature of a space. It does however have a certain amount of error built-in to compensate for less than perfect installations.

In thermal mass, he's referring to energy stored within building materials in a home. Older homes for example might lack insulation, but often use real brick and stone for loadbearing walls, plaster instead of sheetrock, and solid wood floors and subfloors, heavy ceramic tiles and thick thinset and cement board material. Those materials store more energy than modern materials. Because of that, the home will heat up and cool off very slowly, so the building can "ride through" the coldest outdoor tmerpatures. With a enough thermal mass, you design temrpeature becomes not the lowest daily temperature, but the lowest rolling 12, 24 or even 48 hour average temperature.

An anologies. A empty refrigerator will cool off quickly, but if you unplug it, it warms up quickly s well. Fill it with 200lbs of food, and it will cool off and warm up much more slowly. The food stores energy... or in thsi case absorbs energy, meaning that if the heat loss through hte insulation around the fridge enclosure is the same, it will take longer for it to change temperature.

The heat loss to a home is based on R value and is deendant on outdoor temperature (less solar and interior source heat gain) is also why (I have a hard time explaining this to my father in law) a 60k BTU furnace running for 40 minutes uses the exact same amount of gas as a 120k BTU furnace running for 20 minutes. Actually, because of larger temeprature swings, it uses more. In reality, the smaller furnace would probably run for 39.5 minutes.

Finally the larger the equipment, the more ductwork you need and more opportunity for air leaks.


The ultimate home for example, might be a concrete home with insulated forms, plus spray foam betwee the stud cavities. You could match of beat the overall R value with a stick frame home with double studs and foam board over the sheathing, but the high thermal mass wold allow you to size equipment based on average daily temeprature. So where I am the design temp might be 0F in winter, but the average for that period would be 6F. That's a difference of almost 10% is sizing for heating. For cooling it's even more since temeprature swing in summer are greater. The deisn in summer is 93F when I am but average daily temp is only maybe 84F. I've cut my convective heat gain in 1/2 and total peak cooling load by maybe 1/3. SO where I would have needed a 3 ton system, now i can use a 2 ton system.


Soem on this board would like ot see small scale residential chilled water equipment become more popular for this reason. You could store enough chilled water to shut your system off for utility time of day demand pricing.

Remember 1 ton of cooling can be defined by the amount of energy needed to melt 1 ton of ice. It will also take that much energy to cool about 5 tons of concrete by 5F. That's about a 210 sqft slab, 4" thick.