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Old 08-24-2009, 09:06 AM   #1
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electric furnace questions


I intalled a used Ruud 15kw electric furnance (CL find at $25.00) in my detached garage over the weekend. One element was open so I thought 10 kw would be better for my power limitations, and quite enough for 900 square feet I was wanting to heat. I used flex duct and ran two vents. I used a hunter programable thermostat that I already had. First question. When on auto the fan comes on fine but the elements remain lit for quite a few seconds after the fan turns off. Is this right? I would think it should run a few seconds after the elements go out. This causes quite of bit of heat for my flex duct which is tied direct to the top of the furnace. Is this my Hg/he setting on my thermo? I set it for HE. Also...will the flex duct be able to take the temps present at the top of the furnace or do I need to extend it with some metal duct first. I am afraid of melting the plastic. Any help would be great. I am not a hvac person, but very familiar with electricity and electronics. I fix Xray equipment for a living.

thx,

Brian

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Old 08-25-2009, 07:38 AM   #2
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Yes, the fan should continue running after the elements kick off for some time, instead of before.

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Old 08-25-2009, 11:11 AM   #3
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Originally Posted by blh1960 View Post
I am afraid of melting the plastic.
What kinda' plastic is it?
http://www.tcforensic.com.au/docs/article10.html
Look at 2.1.2 in the link.
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Old 08-25-2009, 12:51 PM   #4
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http://www.lowes.com/lowes/lkn?actio...T6W&lpage=none

I think it is pleum.
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Old 08-25-2009, 02:44 PM   #5
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So low density polyethylene melts at 107 C. I can't imagine your air supply being that hot.
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Old 08-26-2009, 10:51 AM   #6
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Thanks for the info. I feel better.
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Old 08-26-2009, 11:37 AM   #7
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A picture would be worth a thousand words. Is there a plenum on top of the furnace or just the flex duct? The way you describe it the flex comes right off the plenum opening of the furnace??? That could get to hot. What size flex did you use??
There may be a switch on the thermostat or a way to set it up for electric or gas heat. This could control the blower differently for electric.
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Old 08-26-2009, 05:16 PM   #8
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Originally Posted by Yoyizit View Post
So low density polyethylene melts at 107 C. I can't imagine your air supply being that hot.
While the safeties of the air handler/heater kit are designed to prevent that high of a temp from being reached.

More then one unit has had a saftey failure that allowed temps exceeding 250 degress F.
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Old 08-26-2009, 05:42 PM   #9
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While the safeties of the air handler/heater kit are designed to prevent that high of a temp from being reached.

More then one unit has had a saftey failure that allowed temps exceeding 250 degress F.
Using my handy-dandy C to F to C conversion
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Originally Posted by beenthere View Post
While the safeties of the air handler/heater kit are designed to prevent that high of a temp from being reached.

More then one unit has had a saftey failure that allowed temps exceeding 250 degress F.
Using my handy-dandy C to F to C conversion from a mag. article in the 70s,

Add 40 = 290
Take 5/9 = 161
subtract 40 = 121C
Good Grief, Charlie Brown. . .
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Old 08-26-2009, 06:14 PM   #10
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107 C is 224.6 F

250 F is 121.11 C

I use Conversion Plus, makes it much easier.
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Old 08-27-2009, 10:55 AM   #11
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Doing this clears the cobwebs out of the brain \/

To either C or F,
add 40
For going from C to F mult. by 9/5; otherwise by 5/9
then subtract 40

20 C,
+40 = 60
60(9/5) = 108
108-40 = 68 F

68 F
+40 = 108
108(5/9) = 60
60-40 = 20 C

but nobody uses it and nobody teaches it.

For messier stuff I use
http://www.onlineconversion.com/

Last edited by Yoyizit; 08-27-2009 at 10:57 AM.
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Old 08-27-2009, 12:48 PM   #12
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F= 9/5c + 32

c= 5/9 (f-32)

Last edited by yuri; 08-27-2009 at 01:13 PM.
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Old 08-27-2009, 05:57 PM   #13
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Originally Posted by yuri View Post
F= 9/5c + 32

c= 5/9 (f-32)
Yeah, I never can remember where the parentheses go. The 40 thing is based on -40C = -40F.
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Old 08-27-2009, 06:05 PM   #14
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Originally Posted by blh1960 View Post
I intalled a used Ruud 15kw electric furnance (CL find at $25.00) in my detached garage over the weekend. One element was open so I thought 10 kw would be better for my power limitations, and quite enough for 900 square feet I was wanting to heat. I used flex duct and ran two vents. I used a hunter programable thermostat that I already had. First question. When on auto the fan comes on fine but the elements remain lit for quite a few seconds after the fan turns off. Is this right? I would think it should run a few seconds after the elements go out. This causes quite of bit of heat for my flex duct which is tied direct to the top of the furnace. Is this my Hg/he setting on my thermo? I set it for HE. Also...will the flex duct be able to take the temps present at the top of the furnace or do I need to extend it with some metal duct first. I am afraid of melting the plastic. Any help would be great. I am not a hvac person, but very familiar with electricity and electronics. I fix Xray equipment for a living.

thx,

Brian

Ruud/Rheem use sequencers to engage the heat strips. The first stage should also send power to the blower relay to energise the blower when ever the heat strip is energized. The blower should stay running until all heat strips are off.

The flex coming directly off the furnace and the heat staying on whithout the blower is a dangerous problem. If the heat strip limit works it may be at 265deg. Not a temp I would want my flex duct to get to.

Trace the wire from sequencer to blower realy. Does it come off the 5kw you have disconected?

I would remove sequencers and install a 40a 2pole contactor relay with a 24v coil. Wire in place of sequencers. You already have the stat set for HE and this will tell the blower to run with a call for heat. Also install a plenum (Ductboard or metal) to seperate the flex from the heat coils would be much safer.

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