Running a 208V radiant ceiling heater from 240V

[Terran Melconian]

I think I did, but my ASCII art isn't so good, so I'll to describe my
sketch.

Represent the heating elements by drawing two resistors in series
stacked vertically on the paper, making a top, middle, and bottom node.

One leg of the AC goes to a diode "pointing to" the middle node. The
same leg also goes through another diode "pointing away from" the
bottom node.

The other of the AC leg gets directly connected to the top node. The
same leg also goes to a third diode "pointing away from" the bottom
node.

I hope that's not too confusing.

Like this?

Live
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V -
- ^
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|-/\/\/- -/\/\/-
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| V
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| |
Neutral

created by Andy's ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Looks perfectly reasonable to me. Presumably if you have an even number
of sets, you can set the orientations so the total power consumption is
the same on both parts of the cycle.

 

[Jim Thompson]

Probably... the heater no doubt has a positive TC. If *I* were
sweating overheating (no pun intended) I'd make it adjustable
cycle-skipping until I got the same net Watts.

...Jim Thompson

 

[John Sevinsky]

Like this?

Yeah, just like that.

 

[John Fields]

Probably... the heater no doubt has a positive TC.

 

[Paul E. Schoen]

That's correct. Grainger has 500 VA buck boost transformers for about $140.
But you could use this 24 V 10 A tranny from MPJones for about $18:
http://www.mpja.com/productview.asp?product=7846+TR

216 volts should be fine, especially if your 240 V line is a bit low.

Paul (excuse the top post)

 

[default]

So, let's see if I got this right. One 375W 240V panel would use a
little over 1.5A. Let's say I need six for the room in our basement.
That would a little over 9A, so let's round it up to 10A. If I wanted
to use a 32V buck transformer, that would be 320VA or 0.32KVA. Is that
correct?

That would be correct. Six panels might be overkill. I don't know
how large your basement is, and the height of the ceiling etc.. but
with radiant heat your only trying to make it comfortable. I mounted
a small 2' X 4' panel over my bed and the room can be at 40 F and the
bed feels toasty. So, if you only plan to occupy a specific area just
mount panels in that area with individual controls (bucking auto
transformers)

The downside of radiant heat is that thermostats that sense air
temperature don't do a good job of maintaining a comfortable
"feeling." So you might be the thermostat in that situation.

 

[John Sevinsky]

That would be correct. Six panels might be overkill. I don't know
how large your basement is, and the height of the ceiling etc.. but
with radiant heat your only trying to make it comfortable.

The room I want to put it in is 12x20. Each panel is 2x2.

The downside of radiant heat is that thermostats that sense air
temperature don't do a good job of maintaining a comfortable
"feeling." So you might be the thermostat in that situation.

Well, that is something I should consider now. I currently have a
wall-mounted line voltage thermostat which is used with two 1kW
electric baseboard heaters. I would like some advice from somebody who
has experience with these things about how many watts per square foot I
should have, and what to do with the thermostat.

My main reason for wanting the ceiling heat is that hot electric
baseboard radiators + small kids + toys = potential danger.

John

 

[Homer J Simpson]

Well, that is something I should consider now. I currently have a
wall-mounted line voltage thermostat which is used with two 1kW
electric baseboard heaters. I would like some advice from somebody who
has experience with these things about how many watts per square foot I
should have, and what to do with the thermostat.

2 kW should be plenty for a 12 x 20 room unless the insulation is very poor.











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[Zak]

Yes, I could run them off of 120V, but that would reduce these 375W
heaters to 94W if I did my math right, so I would need four times as
many. And the surplus price is more than 1/4 the new price.

Running one in series with a diode will reduce power to half what it was
without, which comes to about 250 watts per panel.

Put both in series and put a diode in parallel with each panel. Watch
the polarity of the diodes!


Thomas

 

[Jim Thompson]

Running one in series with a diode will reduce power to half what it was
without, which comes to about 250 watts per panel.

Put both in series and put a diode in parallel with each panel. Watch
the polarity of the diodes!


Thomas

Everyone's calculations are missing the very important fact that
heaters generally have a notorious TC.

...Jim Thompson

 

[Homer J Simpson]

Everyone's calculations are missing the very important fact that
heaters generally have a notorious TC.

Radiant heating panels don't. Most all I have seen have mineral insulated
heaters.


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[jasen]

Everyone's calculations are missing the very important fact that
heaters generally have a notorious TC.

(thermal coefficient of resistance?)

I thought nichrome was fairly good in that regard, it's certainly much
better than tungsten or stainless steel.

Bye.
Jasen

 

[John Popelish]

(thermal coefficient of resistance?)

I thought nichrome was fairly good in that regard, it's certainly much
better than tungsten or stainless steel.

Definitely better in that regard than most other alloys not
produces specifically for low thermal coefficient of
resistivity.
This site says that Nichrome 80-20 (commonly used as heating
elements) has a coefficient of about 100 ppm/degree C.
http://www.aircraftmaterialsuk.com/data/electronic/alnicr.html
At modest temperatures, tungsten is more like 4600 ppm/
degree C.
http://www.tungsten.com/mtstung.html
316 stainless is harder to find out about, because it isn't
ordinarily used as a resistance heater element. I may have
to do some testing, because I use stainless steel wire
(brand name Beadalon) to heat my motorcycle gloves.