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Old 06-05-2009, 08:47 PM   #16
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Originally Posted by beenthere View Post
So tell me. Why the same house can lose 23,075 BTUs on a 25HDD day, or can lose 16,400 BTUs on a 25 HDD day.
Insolation? Wind? Rain? Snow?

(23+16)/2 = 19.5, 19.5 -16 = 3.5, so it's 3.5/19.5 = ~18%, so it's a +/- 18% variation. Not bad for doing economics numbers.
And the best you can do with Manual J is +/-5%? I've seen this accuracy limit somewhere. . .?

The other thing is a statistical effect. You make the measurement above once and you +/- 18% accuracy. If you make it 100 times over several months you get to be within +/- ~2% of the true value.


Last edited by Yoyizit; 06-05-2009 at 08:53 PM.
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Old 06-05-2009, 08:58 PM   #17
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Quote:
Originally Posted by Yoyizit View Post
Insolation? Wind? Rain? Snow?

(23+16)/2 = 19.5, 19.5 -16 = 3.5, so it's a 3.5/19.5 = ~18%, so it's a +/- 18% variation. Not bad for doing economics numbers.
And the best you can do with Manual J is +/-5%? I've seen this accuracy limit somewhere. . .?

The other thing is a statistical effect. You make the measurement above once and you +/- 18% accuracy. If you make it 100 times over several months you get to be within +/- ~2% of the true value.
Yes, you can keep fudging it till you get it close.

Out of 4 guesses, you got 3 things. Your still missing atleast ONE big one.
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Old 06-06-2009, 09:53 AM   #18
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Your still missing atleast ONE big one.
Yeah, a girl said that to me, once.

Absorption of the exterior surface? Ground temp. variation?

Fudging puts me in good company
http://en.wikipedia.org/wiki/Fudging

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Old 06-06-2009, 10:31 AM   #19
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Quote:
Originally Posted by Yoyizit View Post
Yeah, a girl said that to me, once.

Absorption of the exterior surface? Ground temp. variation?

Fudging puts me in good company
http://en.wikipedia.org/wiki/Fudging
Doesn't put you in good company.

If you don't know all the things that can throw off the HDD calculation.
Then how can you come up with anything but a false result.

HDDs doesn't need to be known, to set up a dual fuel system.
Or to determine if a dual fuel system is appropriate for an area.

An area with 10,500HDDs may not be a good place for a dual fuel system. While another place with 12,000HDDS may be an ideal pace for a dual fuel system.

Its just an abused number.
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Old 06-06-2009, 11:40 AM   #20
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HDDs doesn't need to be known, to set up a dual fuel system.

No, only to determine if it's economical.

Or to determine if a dual fuel system is appropriate for an area.

An area with 10,500HDDs may not be a good place for a dual fuel system. While another place with 12,000HDDS may be an ideal pace for a dual fuel system.

Its just an abused number.

Here's what I've got to go on.

"A dual-fuel heat pump is an electric heat pump and a gas furnace all in one. In the Tennessee Valley, where temperatures are typically above freezing and we enjoy some of the lowest electric rates in the U.S., a heat pump is the most efficient way to heat your home. In those few instances when the temperature drops below freezing, a gas furnace provides heat more economically. By combining the two, you can have the benefits of both systems.
How it works
When the temperature is above 35 degrees or so, the dual-fuel heat pump uses electricity to heat your home as necessary. This type of heat circulates evenly throughout your home, and isn't too dry. When it gets really cold outside (around 35 degrees or lower), the heat pump automatically switches to supplemental gas heat for better efficiency."

And, HDD For Knoxville, Tenn.

6/1/08
13
7/1/08
7
8/1/08
7
9/1/08
26
10/1/08
264
11/1/08
586
12/1/08
712
1/1/09
880
2/1/09
627
3/1/09
447
4/1/09
272
5/1/09
77

With HDD totaling 3800 and change.
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Old 06-06-2009, 01:42 PM   #21
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See, in reality.
The HDDs tell you nothing.

You don't know the heat output of the heat pump.
You don't know how much heat the house needs under what conditions.

A load calc. And the extended data specs for the heat pump will tell you when the heat pump can't handle the load anymore.
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Old 06-06-2009, 03:18 PM   #22
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I give up.
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Old 06-06-2009, 03:25 PM   #23
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I give up.
Finally.
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Old 06-06-2009, 03:29 PM   #24
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We are in the wrong business Beenthere. You, me Clover et al should startup HVAC consulating.com. LOL. for $$ of course.

Last edited by yuri; 06-06-2009 at 03:31 PM.
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Old 06-06-2009, 03:42 PM   #25
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Finally.
But I might pop up elsewhere if I get more data; kind of like a weed.
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Old 06-06-2009, 03:42 PM   #26
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Originally Posted by brohar View Post


Secondly, is oil the right way to go? I dont have access to gas but what about a heat-pump (i live in cincinnati)?

- How much more would a heat pump system cost? Could i get a tax credit also?

- Should i just fix my existing blower? Anyone know if there is a match for my make and model?

Thanks
Post your electric rate, all taxes, and fee's included. If your still reading this thread.

Your area has enough hours above 40°F to make a dual fuel very practical.

Or even a heat pump with electric aux back up.

You have a low enough summer design temp, that a heat pump sized to the cooling load, will probably not be able to handle the heating load at temps much below 40°F outdoor temp.
Depending on how high of an efficiency heat pump you would get.

So 40°F is a good switch over temp for a dual fuel system, and will eliminate going into a defrost cycle, and blowing cold air out of the registers.
It takes too long for an oil furnace to heat up for it to be able to counter the cold air from defrost.
Unless your going for max efficiency. Then you would leave the thermostat decide when to switch over to the oil furnace, by indoor temp drop.
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Old 06-06-2009, 03:44 PM   #27
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We are in the wrong business Beenthere. You, me Clover et al should startup HVAC consulating.com. LOL. for $$ of course.
LOL...

Hmmmm.
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Old 06-06-2009, 03:45 PM   #28
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But I might pop up elsewhere if I get more data; kind of like a weed.

My lawn mower makes short work of weeds.
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Old 06-08-2009, 08:23 PM   #29
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Ok the OP is back and this thread just blew up while I was away. Soooo im still pulling my hair out over this decision. My Electric rates are currently around .08-.09. My Oil tank is probably the original so sticking with oil makes me nervous considering the cost of replacing a leaking oil tank if that was to happen. Additionally, resale value is affected negatively with oil heat. On the other hand the Heat-pump with electric backup, sounds like a whole new beast to get used to. Im also, nervous about huge electric bills in the cold months. Granted with my 30 year old furnace i was using about 600 gallons of oil a year. So for my little house with good insulation what is best choice? Are Heat pumps noiser than a normal AC unit? In cold weather does the Heat pump and electric run at the same time? THe unit i was looking at has a 10 year parts and labor warranty, is there some fine print i should worry about there?
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Old 06-08-2009, 08:40 PM   #30
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At 600 gallons of oil. If, your oil cost $2.30 a gallon, and your furnace was 80% efficient, then your oil bill would have been $1,381.00

A heat pump, should be able to lower that bill to about $1,000.00

A dual fuel might get you down to $800.00

Those numbers are without lnowing your areas heating hours or conditions.

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