# Thread: New Home & Big Bills HELP!!!

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I believe the furnace has two stage gas valve but with a standard program t/stat.

Just checking again, does the air to air exchanger increase gas usage or not since it appears to be bringing the outside air into the supply plenum?

Thanks again for all your responses!!!

2. I heat and cool 3200 SF (1600 SF ranch with walkout basement).

\$387 last month.

All electric.

I'll take a \$200 bill.

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Ok, I did the therms/DD/sf calculation:

-100,000 BTU's x 253 therms x.94 efficient furnace
-3,413 BTU's x 1,141 kwh x .9
-3,600 sq. ft. x (53 DD x 32 days)

Came up with 4.47 or 8.94 figuring only the main floor.

Can I assume the 4.47 will come down with the returns all hooked up and the bsmt. fully insulated and finished off? Where I got the formula stated that a new tight home should actually be below 3??

4. Where did you get the formula from?

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I Googled the formula and found an article with it explained. Do you have a different one that you use?

6. Originally Posted by bobh33
I Googled the formula and found an article with it explained. Do you have a different one that you use?
I was hoping you'd post the link.

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8. The formula as given on the site is:

kWh x 3,413 Btu/kWh x .9 (efficiency)
+ therms x 100,000 Btu/therm x .5 [efficiency]
÷ (sq. ft x HDD)
= Btu/sfDD.

The numbers you gave were:

-100,000 BTU's x 253 therms x.94 efficient furnace
-3,413 BTU's x 1,141 kwh x .9
-3,600 sq. ft. x (53 DD x 32 days)

Let's see how that checks out using the above formula:

1,141 kwh x 3,413 btu/kwh x .9 = 3504809.7

+ (253 therms x 100,000 btu/therm x .94 = 23782000) = 27286809.7

÷ (3,600 sq. ft x 1696 HDD = 6106500) = 4.46 BTU/sq. ft./HDD

Now...the range of HHI I have on hand states that 4.46 falls within:

"Better than average homes with good insulation, relatively low air leakage, and better than average heating efficiency"

- Residential Energy, Cost Savings and Comfort for Existing Buildings
So, without knowing more detail about how your home was constructed, it sounds like you're doing reasonably well given the weather you've had recently.

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Luckily we are not living in Northern Minnesota!! Thanks for the feedback on the formula.

Still wondering how the air to air exchanger actually works. It appears that fresh air from outside is pumped into the supply plenum. If so, wouldn't that lower the temperature of the supply air thus burning more gas to satisfy the thermostat???

10. Originally Posted by bobh33
Luckily we are not living in Northern Minnesota!! Thanks for the feedback on the formula.

Still wondering how the air to air exchanger actually works. It appears that fresh air from outside is pumped into the supply plenum. If so, wouldn't that lower the temperature of the supply air thus burning more gas to satisfy the thermostat???
I'm guessing you mean an ERV? They capture 80% or so of the heat from the outgoing air (which is far warmer than the basement air you are sucking in). Teddy Bear and beenthere are probably experts on these.

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Thanks ShopHound and everyone else for all the great info & help!!!!

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## ERV Operating Hours and ~ \$avings

Originally Posted by bobh33

Still wondering how the air to air exchanger actually works. It appears that fresh air from outside is pumped into the supply plenum. If so, wouldn't that lower the temperature of the supply air thus burning more gas to satisfy the thermostat???
ERV set-up should be based ASHRAE 62.2

The algorithm is based on the ASHRAE 62.2 Mechanical Ventilation Standard (February 2003).

This standard calls for the following ventilation rates:
______ Floor Area
______ (square feet)

Number
of Bedrooms

___ <1500 _ <3000 _ <4500
0-1 30 ___ 45 ____ 60
2-3 45 ___ 60 ____ 75
4-5 60 ___ 75 ____ 90
6-7 75 ___ 90 ____ 105
>7 90 ___ 105 ____ 120

A heat recovery ventilator (HRV) can help make mechanical ventilation more cost effective by reclaiming energy from exhaust airflows. HRVs use heat exchangers to heat or cool incoming fresh air, recapturing 60 to 80 percent of the conditioned temperatures that would otherwise be lost.

ERV should be set-up for about 60 CFM.
With a 70% efficiency, the energy to heat the air would be equivalent to
heating 18 CFM of air.

Q = CFM * dT * 1.08

dT = 80 - 10 = 70
CFM = 18
Q = 70 * 18 * 1.08 = 1,360 BTU/hr

70 dT
18 CFM
1.08
1,361 BTU/hr
14 Hours/ Day
19,051 BTU/Day
30 Days/ Month
571,536 BTU/Month = ~ 6 Therms

If the ERV is set-up for 60 CFM, is running properly
and has a timer to run it 14 hours a day,
it only cost ~ \$6 per month for NG when it is 10'F outside.

ERV does not have to run when you are not home.
Savings: ~ \$5 / month in the coldest month(s).

http://www.toolbase.org/TechInventor...ntDetailID=797

http://www.toolbase.org/pdf/techinv/...n_techspec.pdf
Last edited by dan sw fl; 03-14-2009 at 06:41 AM. Reason: Fixed link

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