using a 240v heater on 120v, wattage and.....

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randy331
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Re: using a 240v heater on 120v, wattage and.....

Post by randy331 » Sat Jan 01, 2011 12:53 pm

Matt Gruber wrote: about 200w on 120v.
200w at 120 volt would produce only 682 BTUs.
Are you sure your measuring it right?

A cheep $15 1500 watt heater that plugs in to a 15amp 120volt outlet will produce a little over 5000 BTUs.

And, your heat pump you shut off will produce about 3 times the heat at 30* outdoor temp, per kilowatt hour used, vs electric resistence heaters if the pump is working properly.(correct charge, clean filter, clean out door unit, indoor blower moving 400-425 cfm per ton of condensing unit, etc)

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Re: using a 240v heater on 120v, wattage and.....

Post by Matt Gruber » Sat Jan 01, 2011 1:09 pm

hi randy!
i'm using ohm's law and reading the outside power meter to verify. i've used a 1000w heater in the past during cold snaps. That made me nervous as it got red hot. could start a fire if a blanket or something touched it. It would warm up the BR in 1-2 hours, so it had excess capacity. I've read 1 person generates about 100w. If that is true, 200w is quite a boost! My old electrc blanket is rated 120w and i'd keep it on 1-2 on a 10 scale, yrs ago, but i don't use it anymore.
the specs on my heat pump say 32,600 btu at 47F
but only 16,000 btu at 17F
i don't have info for in-between. it draws about 2700 watts.
U can bet i run the heat pump during the day!
Straight electric resistance is an expensive way to heat the whole house. Once the HP was down for 2 weeks and it cost me an extra $100.
But at night, why heat the whole house? Serves no purpose here in FLa where it doesn't stay cold long enough to freeze pipes.
HP Could fail, start fire, will wear out faster, makes noise, wakes me up, sometimes the blower kicks up dust and makes me sneeze. So i've slept the best with this new setup. A real luxury: basebed heaters!
Thanks for your input!
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Re: using a 240v heater on 120v, wattage and.....

Post by randy331 » Sat Jan 01, 2011 1:46 pm

Does your new heaters have a BTU rating on them?
I guess I don't know how your getting 200 watts?
If you have an amp meter, measure the amperage draw on one side of the cord, or wire running the heater.
Measure actual voltage.
Then amps x voltage = watts
watts x 3.41 = BTUs

Do you know the seer, or hspf rating on your pump?
13 seer is as low as can be sold sice 2005 (I think it was 2005)
There should also be a COP for the pump.
If the COP is 2.7 at 17*, it means it produces 2.7 times the BTUs for the same kilowat hours used, as compared to resistence electic heating.
The watts drawn by the pump won't be just 2700. It varies depending on out door temp, as the temp goes down it will draw less power.
One of the biggest problems that cause distisfaction with heat pumps is wrong charge.
The ones I've used will have a 20*-22* temp rise across the indoor coil when the out door temp is 30* (70* at the return plenum=90*-92* at the supply plenum)

Randy

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Re: using a 240v heater on 120v, wattage and.....

Post by Matt Gruber » Sat Jan 01, 2011 2:13 pm

SEER 13
hspf 7.8, as for watts going down when colder,that is news to me. how much lower do u see?.................
it does have a 22* rise at 30F,
30F days are rare, just a few a year. Since on those days i leave it off 16 hrs, it gets ~58F input and has 80 output. I hide in my BR for an hour until things warm up a bit.
.
heaters are rated 750w at 240v. 74 ohms.
on 120v i get 195 w. v x v divided by ohms. 120x120/74
power meter for 3 was 683w INCLUDING standby and stereo. so i'm comfortable with ~200w each. impossible to be over 220w. DVOM died but it match power meter.
.
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tame a lumpy cam for the street, more street torque! see my article, archived in the waybackmachine.
https://web.archive.org/web/20130707064 ... TGRU/carb/
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Re: using a 240v heater on 120v, wattage and.....

Post by Matt Gruber » Sat Jan 01, 2011 5:07 pm

so i might as well finish the story on how i heat the house for about $60 a month.
so HP is off 16 hrs.
when the sun hits the HP at 10:30am the HP goes on 1 hr.
it is ~38 by then. Hp off 1/2 to 1hr.
Back on NOON about 45F. 1 more hr house, is up to 70- 74.
off 1 hr.
2pm about 50F HP runs ~50% duty til off 6;30pm.
so in 24 hr HP runs abot 4 to 6 hrs total. house up to ~76F
say 5hrs x 2700w =13.5kwh x $0.12=$1.62
plus 2 basebed heaters at nite, less than 16hrsx400w = $.77
77 + 1.62= $2.39 x maybe we have 10 real cold days = $23.90
add 20 normal days and heat total is less than $60.
others run the HP all the time, say 12 hrs or more for $3.88+ or $38.80 vs. the $23.90 If this was better, fine, it is worth it, but it wakes me up, makes and me sneeze sometimes, wears out faster, and could set the house on fire if the blower shorts out.
.
.
tame a lumpy cam for the street, more street torque! see my article, archived in the waybackmachine.
https://web.archive.org/web/20130707064 ... TGRU/carb/
Great manners equals more fun.

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