Yoyizit

I'm working up a spreadsheet that allows me to compare my house insulation with the house in the 1985 Manual J as far as heat loss. I have a gas furnace.
After using several correction factors and discarding suspect data a reasonable value seems to be about 4 HDD per therm.

I'd also like to compare my value with 10 to 30 or so of your houses, if you have gas heat.

Could you please post the therms and HDD for these winter months, say Nov. thru Feb? For now these are the only two numbers I'm interested in.

Thanks in advance.

hvaclover

Try the Ashrae or Acca sites if you get no hits from here. Or try this site:


http://apollo.lsc.vsc.edu/classes/me...t_deg_day.html

__________________
Just slow, not stupid.

Yoyizit

Thanks.
Here's my formula so far.
A= input therms from gas, 146
B= water heater therms, 36
C= furnace input, A-B = 110
D= furnace eff. in percent, 83%
E= furnace output = C*D/100 = 91
F= kwh for the period, 570
G=therms from kwh=F*3412/100,000 = 19
H=total therm input to house =E+G = 111
J=HDD = 386
K=heat transmission=H/J = 0.29 therms/HDD

hvaclover

Well, I can't comment on the accuracy of your formula.

I cheat and use a short form. But i know the construction standards in my town so the sizing does not vary much.

__________________
Just slow, not stupid.

Yoyizit

I get a range of 0.18 to 0.31 over several winter months, which could mean weather variation. The average is 0.26.
These numbers look plausible to me.

I think with just the gas bill and maybe some weather data, you could tell how well your house is doing relative to other houses.

A simple additional correction factor would be 2 therms per month per occupant.

Just like with the NEC, there doesn't seem to be much literature out there about checking the specs of an existing installation vs. putting in new work.

Revue

Wisconsin, built in 1997, condensing furnace.

Nov 2008 - 464 HDD, 0.037 Th/HDD
Dec 2008 - 1085 HDD, 0.065 Th/HDD
Jan 2009 - 1540 HDD, 0.074 Th/HDD

The correct math to compare improvements to a house is Th/HDD.

To compare different size houses use Th/HDD/SqFt, which includes the
area of a basement if it exists, whether heated or not.

Less than 2.0 Th/HDD/SqFt is rare and excellant.
Energy Star homes in Wisconsin are 2.0 to 3.0 Th/HDD/SqFt.

veesubotee

Prior to having a replacement furnace installed this past Spring, I accumulated 3+ years of (gas) data from bills. I also matched HDD data for each specific billing period.

For the months (Spring/Summer) where there were no HDDs, I came up with an average figure for non-heating gas useage. This was used to break down total therms and heating therms, and enabled me to calculate total and heating therms/HDD.

To the OP: Are you stating that you 'use' 1 'heating' therm for every 4 HDDs that you experience? That seems like an extremely high value. For example, for the billing months of October through February, I totaled 3256 HDDs and used 398 heating therms for an average monthly figure of .0996 therms/HDD. Is the purpose of your study to see if you need more insulation, based on other peoples gas usage?

To Revue: Since the purpose of my study was to compare gas usage with my new furnace vs. old, and my house's envelope has not changed, I merely considered therms used/HDD. I did not consider square footage as I wanted to see savings, if any and square footage hadn't changed.

For kicks, I added the square footage to the calc, with basement, and comes out to 4.0 x 10^-6; a really small number.

Happily, I have realized an average saving of ~18% as compared to the previous 3 years, same period.

V

beenthere

Take your calculated therms per HDD, and multiply it by the therms a design day would have, then divide by 24.

See how close you are or aren't to your furnace size.

Scuba_Dave

Don't you also need to know the heating days for each location?
So far this year is 10% colder then last year as far as heating days
Had to order a 2nd tank of oil a month earlier then last year

Yoyizit

I don't know what to think.
My house compares with the house in Manual J and yet you all are doing much better. My house was built to 1964 standards but I did get better windows. 2250 sq. ft.

More sunlight? Less wind? Lower inside temp? I assumed 70F.
The more house data I get the more certain I can be of my percentile for therms vs. HDD.

Yoyizit

You have a link?

beenthere

So what size furnace do you have.

What is the temp rise across the heat exchanger.

Still a good chance, you have a higher infiltration now, then when it was built.

Yoyizit

170000 btu/hr input, 83% efficient
I'll check the factory manual for delta T or measure it.
Settling = higher infiltration. . .

hvaclover

I don't even need a short form to know your furnace is too big.

__________________
Just slow, not stupid.

beenthere

I hope your design temp is -60°F.

You need to move 1600CFM to have a temp rise 81°
1700 for 77°
1400 gives you 93.

Yep, your house settled, and could be leaking more now then when it was built.
You could have an infiltration rate of .7ACH