Question for Bruce re: Block heater power

IRGuy · Post by **IRGuy** » Feb 16th, '07, 09:26

Bruce..

I am over my head here and it sounds like you are the guy with the best info on electrical power... If anyone else can help please feel free to do so.

I had 1,000 watt block heaters installed this past summer in my 6BTs when the boat was in a diesel shop.. I live in relatively warm NC but need to minimize startup smoke since my slip is adjacent to a neighbor's yard and about 30' from their screened porch.

I don't need to have such large heaters on all the time, and I don't want to waste power, so I was considering putting each on a timer, having one on for a while, then the other, then both would be off for a while, with the schedule determined by trial and error. I have been advised against this since timers are prone to failure, and I don't need to put a slug of 1,000 watts of energy into one block anyway.

I have been advised instead to wire them in series, (so both would be on at one time, then both off at the same time) since series wiring causes each to work as if it was 1/4 of the 1,000 watt rating, ie: 250 watts each. I am having a difficult time believing that two 1,000 watt loads in series act as 500 watts total. To me, if you draw a black box around them they should act as a single 2,000 watt total load.

The idea of pulling 1,000 watts of power (about 8 1/2 amps) shore power concerns me, although I do have 30 amp 120 volt shore power, and 500 watts is more appealing. If in fact the series concept does result in a 500 watt load I am considering either a single timer, or an engine room thermostat (with series wiring and both heaters on at once I can't use a single block thermostat to control the temp of two engines).

What do you recommend for both the wiring and the control concepts?

mike ohlstein · Post by **mike ohlstein** » Feb 16th, '07, 10:12

Do you pay for electricity by usage, or is it a flat fee.......?

IRGuy · Post by **IRGuy** » Feb 16th, '07, 12:32

Mike...

My slip is in our neighborhood basin.. each of us has one. The power is charged to each of us by dividing the total monthly cost by the number of slips. The power company has one meter, nobody has individual meters. If my consumption seems high I will put a meter on my own slip, and pay extra for mine. I want to be a good neighbor and not abuse the deal. Some boat owners don't even use any power, a few keep chargers connected, but their use is small. My boat (B 33) is the largest in the neighborhood.

In Memory of Vicroy · Post by **In Memory of Vicroy** » Feb 16th, '07, 14:30

Just put a line voltage therostat on each one (www.wwgrainger.com) and set them for about 55 degrees. You have way too much block heater tho, 250w is about right for a 6B.

UV

IRGuy · Post by **IRGuy** » Feb 16th, '07, 16:36

UV...

Thanks.. I found out after the heaters were installed that 1,000 watts was too much for the amount of heating I will need, but the diesel guy said the 1,000 watts were standard and I didn't know any better. This is another reason why I like the concept of series wiring both, if in fact they will each act the way I have been told.

The line voltage thermostat you mention.. Am I correct in assuming you mean one that measures engine room temp? If so, this is one of the control ideas I had, maybe keeping the engines around 70-75F and in turn that will heat the engine room air to 55F.

In Memory of Vicroy · Post by **In Memory of Vicroy** » Feb 16th, '07, 16:46

Yes, the thermostat will do that, just like the one in your house or car...."line voltage" means you don't have to use a relay to cut the heaters on and off - the line voltage thermostat has enough amp capacity to do it directly. You would have to experiment to get the right setting, just use your IR gun - you the IR guy.....

UV

richcatch22 · Post by **richcatch22** » Feb 17th, '07, 16:47

Frank
I sent you a email with a Excel file to help figure the cost to run your block heaters. Could not figure how to attach a file here. Any questions let me know.
Rich

IRGuy · Post by **IRGuy** » Feb 20th, '07, 11:18

Thanks UV and Rich...

I am still concerned about the series wiring though.. is it likely that connecting two 1,000 watt heaters in series will make each operate as if it was 250 watts? If this is so I will have the effective heating UV suggests. I am missing something here!

Terry Frank · Post by **Terry Frank** » Feb 20th, '07, 12:40

UV is correct. Wire those two heaters in series...like the old Christmas lights and they'll be fine. You'll have about 500 watts total.

IRGuy · Post by **IRGuy** » Feb 20th, '07, 15:52

Terry...

THANK YOU! I don't understand how adding 1,000 and 1,000 equals 500, but slamming electrons around was never my strong point! I will series wire them, and plug them in that way.. read the AC load, and if it is anywhere near 500 watts I will come up to Morehead City and give you a case of whatever beer you drink, or a bottle of whatever hard stuff you prefer!

BTW: Do you know Buzz Kistler from Morehead City?.. he owns a B33 FBC "Buzz Off".

Bruce · Post by **Bruce** » Feb 20th, '07, 18:18

IR,
Vic is right. It is simple Ohms law.

Use the following formulas for figuring things out.

P=watts
E=voltage
I=current
R=resistance

E=IxR
P=IxE

Resistance in series is added up.

You have 1000 watt heaters. We know the voltage is 120vac.

Using P=IxE, 1000(p) / 120(e) gives us 8.333(i) in amps each.

Now that we have the current(i) we can use the E=IxR formula to find resistance of the heater.

120(e) / 8.333(i) gives us 14.40(r) ohms

Now since you will wire the heaters in series (resistance in series is added up) 14.40 x 2 = 28.80

Plug this into the formula E=IxR to find new current draw.

120(e) / 28.80(r) gives us 4.16(i) in current.

Originaly you were drawing 16.66 amps. By series wiring you cut that down to 4.16 amps.

You can get all kinds of electric timers inexpensive for scheduling the on/off time.

Or get a rheostat which is a variable voltage control transformer(like a toy train power pack) and set the voltage lower to say 75 vac and measure the current and use the formula to determine the watts.

Adjust up or down to get the right temp vs power usage.

BTW, ask your neighbor which way he wants it, less smoke or higher electric costs?

IRGuy · Post by **IRGuy** » Feb 20th, '07, 18:30

Bruce...

Great! Thanks a lot! I can sort of recall a prof trying to explain something like that about 45 years ago, but it didn't sink in then, and this is the first time I needed to know it since.

It looks as if I owe you and UV the same as I do Terry.. one day we will meet somewhere.. but if I don't mention it please remind me, I am getting old and often have CRS.

Terry Frank · Post by **Terry Frank** » Feb 20th, '07, 19:00

Budweiser. I'm alot closer than them.

Bruce · Post by **Bruce** » Feb 20th, '07, 19:58

Give mine to Terry. Still owe him for the stay in his ocean view condo.

Terry Frank · Post by **Terry Frank** » Feb 20th, '07, 21:02

Bruce,

With all your "pearls of wisdom", you don't owe me a thing.

Br,

Terry

bertram31.com

Question for Bruce re: Block heater power

Question for Bruce re: Block heater power

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