Size Matters...

[markjj]

Hi Everyone,

I have an '88 or '89 Lennox Gas 165,000 Btu furnace that died. I know this one is too large* by today's standards. Everything I've read says you don't want a furnace too large or too small, but they don't say anything about what is the acceptable range of size. Practically speaking, there must be a target range of what is the right size.

I have estimates from qualified and long-experienced contractors who are quoting a new furnace from 75,000 btus on the low end to 115,000 btus.

There does appear to be something of a judgement call in load calculations, at least the ones that you get when receiving an estimate. If time and money were infinite (for contractors or consumers), I suppose the exact number for a house could be achieved, but that is not reality.

*So here are my house stats: 1600 sq. ft total, approx 900 in basement. House was built in the 1940s, 2 story, frame and brick. Single pane windows with storm windows. House is insulated, average+ (but not superior--there's only so much you can do on an existing vintage house). Located in Denver, CO.

So what are your thoughts on what size is "too small" of a furnace, and we'll be cold, and what is "too big"? Are all of these furnaces "right sized" because a range that wide is still fine?

And if that wide of range is too wide, what should I ask these contractors, or how should I handle the discussion (particularly with one I'd like to work with), if I'd feel better if the btus should be "zeroed in on" more before install, without getting their fur up like I'm questioning whether they did the calc or rule of thumb right or it should be rechecked or telling them how to do their job? (I don't know how to do their job, just trying to do mine as a homeowner doing research.)

Appreciate your expertise/advice,
Mark

[BaldLoonie]

Most dealers won't undersize, they don't want the call when the furnace doesn't keep up. Those that guess usually guess big to be safe. I'd ask them all if they are guessing or did a load calc. This is your comfort and gas bill at stake.

[markjj]

Most dealers won't undersize, they don't want the call when the furnace doesn't keep up. Those that guess usually guess big to be safe. I'd ask them all if they are guessing or did a load calc. This is your comfort and gas bill at stake.

Thank you for responding. I'm not sure how to respond though, frankly. I can't imagine anyone would admit to guessing; everyone at least knows the sq. ft, so they could use a rule of thumb, which may not be preferable, but it isn't guessing.

Further, technicians with substantial and substantive est/install hvac experience probably know what btu furnace to put into a house just be walking through it and observing, and probably be correct without actually doing a load calc, right?

If I ask to see the load calc data, what am I'm looking for to tell if the high btu guy is more correct than the low btu guy?

And some of it seems to be a judgement call, with respect to air leakage, etc., so it's not all based on undebatable numbers that go into a formula, right?

MarkJJ

[wahoo]

That old furnace has about 100k output, so I'd guess they are using that as a guide to size the new furnace. But to be on the safe side, simply request that they do a manual J figure and see what they come up with. Oversizing a +90% furnace will greatly reduce the efficiency of the new furnace (along with reducing the life).

[snupytcb]

Thank you for responding. I'm not sure how to respond though, frankly. I can't imagine anyone would admit to guessing; everyone at least knows the sq. ft, so they could use a rule of thumb, which may not be preferable, but it isn't guessing.

Further, technicians with substantial and substantive est/install hvac experience probably know what btu furnace to put into a house just be walking through it and observing, and probably be correct without actually doing a load calc, right?

If I ask to see the load calc data, what am I'm looking for to tell if the high btu guy is more correct than the low btu guy?

And some of it seems to be a judgement call, with respect to air leakage, etc., so it's not all based on undebatable numbers that go into a formula, right?

MarkJJ

they may not admit it. they will say,"they have been doing this long enough they just know" or " we can only buy certain sizes anyway". i get a few jobs with prior bids like this.

[BaldLoonie]

Rule of thumb is guessing. A house with 1500 sq ft and single pane glass may need 25000 btu per 500 sq ft. The same house with great windows may need 20000. Then the same house with good windows and R19 in the attic may need 18000 per. So rules of thumb are worthless. In my vinyl village, there are about 4 floorplans. So, yes, I know what a house needs with measuring every one. But older homes that aren't cookie cutter and have varying upgrades in windows, insulation, etc. should be measured. I guess it depends upon how bad a dealer wants a job. I've sold jobs and was told I got the work because I measured the house and did the math while the others didn't. Got one job because HO was impressed I measured his house in a rainstorm. He said that told him I cared about doing it right.

That old furnace is 132K output. Probably could do with about 1/2 that! But then I'm guessing, like anyone else who doesn't do a Manual J.

Try: http://www.buildersheating.com/about-us.html

[markjj]

they may not admit it. they will say,"they have been doing this long enough they just know" or " we can only buy certain sizes anyway". i get a few jobs with prior bids like this.

Yes, I got the "I've been doing this for a long time," a lot; in fairness, I only chose contractors who HAVE been doing this 20 yrs or more, and with good reputations. It looks like they use sq footage. I don't know if they do more in the office for a more thorough calc before writing an estimate.

Only 1 guy measured every room in the house; but then when I asked about whether he used an online load calc program, he said "I have my own calculations and program that I wrote." So no way of telling what exactly he's using. All the others used square footage from what I can tell.

BTW, CORRECT SQUARE FOOTAGE numbers from my first post: 1600 sq ft ABOVE GROUND; 650 sq ft BASEMENT.

I'm still confused about what do to about this. I need to choose a contractor within about 24 hrs. How do I ask for load calc data and what am I looking for?

Thanks.
Mark

[tipsrfine]

Don't make the mistake of taking somebody's word based on "I've been doing this for 30 years" nonsense. Countless contractors have been doing things wrong for many years, and it doesn't make it right. I could tell you stories. Get somebody trained in building science.

[blazey10]

"I want to see the manual J you used to size the equipment I need and will the duct system support the equipment the load suggests."
That's how you ask!


Lets go, burnin daylight.

[markjj]

Rule of thumb is guessing. A house with 1500 sq ft and single pane glass may need 25000 btu per 500 sq ft. The same house with great windows may need 20000. Then the same house with good windows and R19 in the attic may need 18000 per. So rules of thumb are worthless. In my vinyl village, there are about 4 floorplans. So, yes, I know what a house needs with measuring every one. But older homes that aren't cookie cutter and have varying upgrades in windows, insulation, etc. should be measured. I guess it depends upon how bad a dealer wants a job. I've sold jobs and was told I got the work because I measured the house and did the math while the others didn't. Got one job because HO was impressed I measured his house in a rainstorm. He said that told him I cared about doing it right.

That old furnace is 132K output. Probably could do with about 1/2 that! But then I'm guessing, like anyone else who doesn't do a Manual J.

Try: http://www.buildersheating.com/about-us.html

Hi BaldLoonie,

Builders Heating did come out and they didn't measure. In fairness, since it's the first time I've gone through this, I didn't ask for him to do it for the estimate; he also didn't start measuring rooms or ask to do so. Their recommended btu size is next to the high of 115,000.

It's sort of the reason for the question, because that co. with loads of experience/history is at the high end, others are in the middle, and one at 75,000.

Are you affiliated with Builders Heating or know people there?

Mark

[markjj]

Blazey10 and everyone,

Thank you, this is helping.

So after I've said Blazey10's statement about manual J and duct system support, and they do the msrmts, calcs, and present the report(s), anyone could show one to me, I think, and snowball their way through it that it says so and so and a newbie such as myself wouldn't know the difference, right or not?
Can anyone email me a filled out example of one of these reports off line, or direct me to where I can see one of these online that has a manual j and duct system support report explanation for complete newbies, so I can educate myself?
Mark

[markjj]

...Or I don't suppose there is a free online manual J questionairre somewhere that does the calcs that homeowner can fill out?
Mark

[jtrammel]

Not free, but www.hvaccalc.com they're also some quick load calc apps on android and App Store. Hvac buddy is one among others.

[BaldLoonie]

My boss is friends with the Builders owners. Sounds like they are wanting some awfully big equipment for 1600 ft. Basement adds little to the heat needs. Are you looking at 80% or 90% equipment?

[dan sw fl]

...Or I don't suppose there is a free online manual J questionairre somewhere that does the calcs that homeowner can fill out?
Mark

It boils down to U-value windows and infiltration which would account for >> 50% of the total load.

Windows replaced ? Can you feel a "breeze" through the window when the winds are > 25 MPH?

R-wall wall for 1940's has got to be pretty low IMO.
What's the ceiling R value?

Is the total window area 180 square feet?

Is the basement ALL underground?

Tell me your gas use for every month over last two years and I could estimate heat loss could be derived based on efficiency of about 75% (W.A.G.).

Abridged Manual J form is available from ACCA. What you can do with it is another story?!

2 minute estimate - 70,000 BTU/hr Very W.A.G. ... more later

[dan sw fl]

What were your operating cycle times on a COLD day?
i.e. 8 minutes ON, 7 minutes OFF at 10' to 15'F

MUST have been some horrendous Short Cycling if the output was > 120,000 BTU per hour
______________ for 1,600 sq foot upstairs + 650 Sq Foot basement [ post # 7 ]

[dan sw fl]

http://inspectapedia.com/Energy/Heat_Loss_Tools.htm

[dan sw fl]

Denver Heating and Cooling
__________________Jan Feb Mar >>> Apr May Jun >>> Jul Aug Sep >> Oct Nov Dec >>>> Annual
Heating Degree Days 1094 885 806 >> 504 253 71.0 >> 00.0 0.0 144 >> 429 780 1054 >>>> 6020
Cooling Degree Days 00.0 00.0 00.0 >> 0.0 11.0 128 >> 267 203 63.0 >> 7.00 00.0 00.0 >>>> 679

http://www.climate-zone.com/climate/...lorado/denver/

$0.80's / therm
http://www.eia.gov/dnav/ng/hist/n3010co3a.htm

[motoguy128]

...Or I don't suppose there is a free online manual J questionairre somewhere that does the calcs that homeowner can fill out?
Mark

You cna take your gas bills, use an estimated effceicny for you current equipment and work backwards into a you BTU requirements using average daily temperature.

For example, if you bill was $400. You used 500 Therms that month. Ignoring the water heater, lets say your design temrpature is 1F, the average temp that month was 30F and indoor design was 72F. That means you were at on average 42% capacity. So take 500 divide by 0.42 and you have 1190therm on the input to maintain you indoor temp if it was 0F for 24 hours all 30 days. Of course infiltration accelerates at colder weather, so in reality ,you'd need to create a trend using several months of data. No just multiply by the estimated effceincy of your equipment... lets say 70%, and you have 833 therms. A therm is 100k BTU's, so we'll multiply then divide it by 24 hours and again by 30 days... and you have 115,740 BTU's.

What does that mean... well if you have a 120k high effciency furnace installed and you average monthy temperature was 30F, you should expect to see a $400 gas bill if it's sized correctly.

It's not perfect, but it's a quick ball parks and 100 times more accurate than using square footage. In my limited experience, I've foudn ot to be pretty darn close. Possibly more accurate than a manual J, sicne Manual J has a built in "fudge factor" and is subject to "GIEGO" (Garbage in equals garbage out). Meaning if you use hte wrong design tmperatures and incorrect insualtion and air leakage values... you might as well be taking a WAG than wasting your time doing all the math.


FYI - not all old houses are leaky and have poor insulation. They can be decieving at times. I have a 3200sqft 1925 home wit ha full basement with attached drive out basement garage, and due to how it's constructed, despite true no wall insulation, with 0F winter design and 93F summer design temps, I only need a combined 3.5 tons of AC and 70k BTU's of heating. Manaul J would have me around 4 tons and 90k I suspect. I haven't gottne around to buying the software and running it yet.


Finally, another check, is to time your equipment cycles when you at or near design conditions. IF it's not running 100% of the time, it's oversized at least a little. How much depends on the cycles times.

[dan sw fl]

Colorado at 0'F
Ceiling R-20 ______ 6,000 BTU/hr
Walls R-6 ______ 16,000 BTU/hr
Basement Walls __ 8,000 BTU/hr _- assumed unfinished/ uninsulated
Basement Floor __ 3,000 BTU/hr
Windows R 1.2 __ 10,000 BTU/hr _ 180 square feet
Infiltration ______ 23,000 BTU/hr 18,000 Cubic Feet at 1.0 Air Change per Hour (A.C.H.)
____TOTAL _____ 66,000

If your gas costs ~ $0.90 per therm, efficiency is ~ 75% resulting in
December, January & February bills at < $270 / month
~ 309 therms used, 230 therms building heat loss,
the heat loss capacity required is < 70,000 BTU/HR.

Heating Degree Days used = 1,000 per month