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Old 01-15-11, 03:26 AM   #1
strider3700
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Default standby losses from the hotwater tank add up

Today was a quite day at the house with us being away most of it. I just check the log of the hotwater heater electricity usage and thought it was interesting



The watts on the right aren't correct, the peaks should be 3800 watts not 4200.

What you see is from midnight to midnight

The first 4 spikes are the heater kicking on to make up for loss to the environment. Each is 4 minutes

The next small plateau is washing up after breakfast. Thats 1 pot and the dishwasher turned on.

Right after that is my wifes quick shower since we where late.

4 more 4 minute spikes while we're out of the house

My shower

2 more 4 minute spikes.

32 of 98 minutes worth of the hotwater tank heating was the tank maintaining it's temperature. 8 minutes was shortly after a big usage of water so I'm assuming it's not restoring 100% of the hotwater right after a usage, it must stratify or something.

the remaining 48 was actual time recovering directly from water usage.

so almost 1/3 of the power my tank uses goes to standby loses. I really wish I had space to add an insulating jacket to see how much it helps. The new tank will definitely be getting one.

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Old 01-15-11, 09:09 AM   #2
RobertSmalls
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If I understand correctly, you're spending about 3.8kW * (32/60) hr = 2 kWh/day, or 740kWh/yr on standby losses. That's $89/yr at my rates. You should pull an iwilltry and build a superinsulating enclosure for the next one.
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Old 01-15-11, 11:17 AM   #3
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What can you tell us about your hot water tank and what did you use to do this tracking of your hot water heater?
I've never seen a chart like this but I'm convinced that if everyone put a timer on their water heater that we would see a nation wide drop in electrical use.
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Old 01-15-11, 02:56 PM   #4
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My tank is a conventional 40 gallon tank that was mid range when new about 8 years ago with a 9 year warranty. It's electric with 2 elements each at 3800 watts and thanks to some help from here we figured out that only 1 element is on at any given time. It does not have an insulating blanket and I can't fit one because the tank is 1/4" from the metal furnace on one side.
Also of note there are no heat traps in the plumbing, and the majority of the pipe is uninsulated. They insulated up until it goes into the roof. All the plumbing is copper.

Due to not having a tempering valve and having young kids the tank is set to 108-110 F. I'm sure the standby losses at 140 are much worse.

It sits in my heated basement that averages about 19 degrees, dropping as low as 16 at night lately. I can post temperature charts of the room beside it if anyone cares.

For the graph I have an arduino hooked up to two current sensors taking reading every 2 seconds. The current sensors are not calibrated hence the 4200 watts instead of 3800. For my purpose I just care on or off and use 3800 for any acutal kw math.

That is sent to the PC that averages every minute and saves that to csv. I then take that csv each night and check if the average is >300 then it was on if not it's off. THis is to remove noise in the readings. I sum the on minutes for the total and create the graph posted. Complete details on my homebuilt data logger are here
http://ecorenovator.org/forum/applia...html#post11051

My math works out to $60 lost to standby heat. When I get the new tank it will be in at least a blanket and I will insulate the pipes because they will all be exposed.
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Old 01-15-11, 04:00 PM   #5
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Quote:
Originally Posted by strider3700 View Post
It does not have an insulating blanket and I can't fit one because the tank is 1/4" from the metal furnace on one side.
Well then a thin layer of the best insulation you can find on the furnace side, and 12"-16" everywhere else Since it is electric you don't have to watch out for flues, etc. I take it moving either away from the other is out of the question.

Quote:
Originally Posted by strider3700 View Post
Also of note there are no heat traps in the plumbing, and the majority of the pipe is uninsulated. They insulated up until it goes into the roof. All the plumbing is copper.
Well then that 3800W heating element might as well be used as a space heater
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Old 01-15-11, 04:56 PM   #6
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You might even try a silver space blanket wrapped around and see what kind of change that makes.
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Old 01-16-11, 12:40 AM   #7
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The tank is going to be garbaged soon, basically when I get my solar application sorted out I'll replace this tank at that time. I will also relocate it so serious insulation is possible. Basically I'm saying I don't want to spend anything on it now.

I did however dig out some of that foil faced bubble wrap that I've had kicking around for 4 or 5 years. I had enough to do 1/2 of the sides and cover the top. so the tank is basically wrapped half way down. I had to really work to fit it between the tank and the furnace. It turns out it was actually touching in a couple of places.


I'll do some checking on the already collected data and then watch the night time standby losses and see if they are spaced further out.
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Old 01-16-11, 02:09 AM   #8
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Quote:
Originally Posted by strider3700 View Post
Basically I'm saying I don't want to spend anything on it now.
Would you be willing to spend a few $ on a timer, like Ryland proposed? Set it so the tank turns off when you take your last shower and turns on when you wake up. In fact, the last shower can be taken after the heat goes off - you use some of the hot water before it loses heat, then the tank has cooler water over the night, which means slightly less heat lost. Same during the day. You could lose 4-6 'spikes' per day, replaced 1-2 by slightly larger spikes when the tank starts heating again. In case of emergency you'll have hot water within a few minutes of manually turning it on. And once you replace it with a solar set up, the timer can be reused in other projects.
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Old 01-16-11, 03:14 AM   #9
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I'm hoping to rip this tank out in a couple of weeks. Any free time/money before that happens will be prepping for the replacement and the solar setup. One of my future plans for the "it runs everything" arduino project is have it switch a relay to turn the tank off the easiest way to do that may be to hack a digital timer. I'll look into it later. For right now I'm curious how this bit of insulation makes a difference. I think somewhere in the back shed I have enough to finish wrapping the tank and I'll do that tomorrow if I dig it up.
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Old 01-16-11, 01:18 PM   #10
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Early readings are in and regular hotwater usage has started for the day so we'll need to wait until tomorrow morning for more numbers.

The previous night the hotwater heater kicked on after the first standby 2 hours 30 minutes later, 2 hours 50 minutes later, 2 hours 50 minutes later, 3 hours later

Last night it kicked on after the first standby 2 hours 50 minutes, 3 hours 10 minutes, 3 hours 10 minutes. The room was 1 degree cooler at any given time then the night before. I added the wrap after the first standby so during that 2 hours 50 minutes.

Tonight will be a much better indication since hot water use in the house usually ends by 9:00 at the latest. I'll post graphs after midnight or tomorrow morning.

One observation though - the cold water inlet pipe has no heat trap and is much hotter feeling then it was before I covered the top with insulation...

<edit> for logging sake the tank was almost completely wrapped at 12:30. I can't quite get a small 1" gap on the bottom foot or so covered. otherwise it's pretty good </edit>


Last edited by strider3700; 01-16-11 at 03:38 PM..
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