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Old 10-24-12, 08:08 PM   #31
strider3700
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I decided to do some manual testing to see how a timer would do for me.
Last night I turned off the breaker to the hotwater tank. 18 hours later I turned it back on. There was no showers, baths, dishwashing during that time. Any hot water usage would have been hand washing and there was not a lot of that. Basically very little hot water usage in that time. The time for the tank to come back up to temperature was 10 minutes.

Now had I just left the breaker on it would have ran for 4 minutes every 3.5 -4.5 hours lets say 4. so in 18 hours it would have ran for 18 minutes. So it was a roughly 45% savings in standby time.

The tank uses 4.5 kw when running so those 8 minutes saved works out to .6 of a kwh. which is worth about 4 to 5 cents depending on my usage tier. So I'd save 219 kwh worth about 18 bucks a year. Most timers I see online are in the $50-$70 range giving a 3 or 4 year payback which isn't great in my mind. My electric company is giving me $75 if I reduce usage by 10% from last year which works out to 967 kwh so that radically changes the ROI.

Of course simply super insulating to reduce standby losses is likely to get me close to the same results usage wise. 2% of my total electric usage going to hotwater standby losses seems a little insane.

For now I'm thinking I can manually turn off the breaker after the last shower and then back on just before they start again the next day.

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Old 10-24-12, 10:05 PM   #32
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Interesting calculations! Are 240V timers really THAT expensive?
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Old 10-24-12, 10:37 PM   #33
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A nice mechanical one $64 Amazon.com: Intermatic EH40 240-Volt Electronic Water Heater Timer: Home Improvement

A less nice mechanical one $53 Intermatic T104 208-277-Volt DPST 24 Hour Mechanical Time Switch - Amazon.com

$55 Amazon.com: Intermatic WH40 Electric Water Heater Timer, Grey: Home Improvement
a cheap one at only $41 but it's not high enough amps for my tank. Technically it is but I'd prefer the 30 amp breaker to be the lowest rated component.
Intermatic WH21 Electric Water Heater Timer - Amazon.com

this is just checking amazon. If anyone knows of a cheap 30-40 amp 240V relay that can be switched via arduino I'd love to know about it...
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Old 10-24-12, 11:29 PM   #34
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Since I'm going to start tracking this I took the time to set up the ted's load profile. It's finicky but a 4.5kw jump should always be the water heater turning on. I set it to a +-10% accuracy. At my original 5kw setting it would pick up when the heater turned on for 1 minute but that was it. It's better now.

Anyway manually counting minutes in the graphs I got 57 minutes of usage. That's 10 minutes standby recovery, 24 minutes for the kids 1/3 full bath and the dish washer, 7 more minutes for the dish washer and 6 minutes for hand washing the pots, 10 minutes for my wifes shower.

That works out to 4.275 kwh or 26 cents at the lowest tier.

I'll be turning the breaker off now.

<edit> of course an hour after the last shower ends and before I turn off the breaker the hotwater heater kicked on for another 4minutes just to mess up my numbers.. 61 minutes total. I'm assuming the cold stratifies as it comes in and takes time for the entire tank to warm up to the right temp </edit>

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Old 10-25-12, 01:16 AM   #35
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Quote:
Originally Posted by strider3700 View Post
For now I'm thinking I can manually turn off the breaker after the last shower and then back on just before they start again the next day.
Actually, you can turn it off before the last shower. There should be more than enough hot water for the shower, and after it stratifies you should still be able to wash hands a few times. The cooler the water in the tank is when nobody's using it, the less heat you'll lose.

I'm surprised that the timers are so expensive. Here a mechanical 250V timer can be bought for the equivalent of $4.

The only problem is that it is 16A, you'd need 25-30A. Maybe two in parallel?
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Old 10-25-12, 03:05 AM   #36
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Quote:
Originally Posted by strider3700 View Post
If anyone knows of a cheap 30-40 amp 240V relay that can be switched via arduino I'd love to know about it...
Why not use a solid state relay instead of a mechanical one?
You can pick up a 40A relay very cheap on ebay - and the great thing is you can control it directly from your processor as it can be activated with a 5V signal. Tie the relay + to +5V with a 10K resistor, - to 0V and you're done!
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Old 10-25-12, 09:27 AM   #37
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I agree with Piwoslaw. Turn it off before your last use. That way it'll sit at a lower temperature all day and loose less heat that needs replacing.
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Old 10-25-12, 09:37 AM   #38
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I agree with Piwoslaw. Turn it off before your last use. That way it'll sit at a lower temperature all day and loose less heat that needs replacing.
This reminds me of a physics question my teacher asked me once:
If you had a cup of hot tea and someone called you to do something would the tea be hotter when you came back if you put the milk (cold) in before you went or when you came back?

The answer is put it in before you go as the temperature delta between the drink and the atmosphere is then lower so the heat loss is less.

One day I must try and prove this theory..
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Old 10-25-12, 11:09 AM   #39
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Standby losses for my water heater are under 400k BTU per month. $2.60/month of natural gas for those 4 therms. The equivalent would be 117kwh or about $12.90 in electricity at my 11 cent rate but in the case of electricity there is no heat stack coming out of the top so the losses would be less.

Fun thing is my gas usage throughout the summer was a consistent 4 therms pretty much every month. That's with my stove, oven(not often in summer), and water heater usage. I put the thermostat on vacation and take all of my showers with the heat that the pilot light puts out. ..at least until this month when the house and basement temperature dropped and the water heater and copper piping let that heat out.

I'll put it this way, a low flow shower head (1.5gpm), a warm house, and a 3-4 gallon daily shower, and careful hot water usage with the kitchen and I don't use more than what the pilot light puts out and the water is still hot enough at the end of the shower to say ouch if I crank the heat.

I still haven't bothered putting the poly-backed fiberglass blanket on my tank yet.
Pro tip: The longer the tank warranty, usually the better the heat tube baffles and tank insulation are.
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Old 11-12-12, 12:04 PM   #40
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Ok I've been turning the breaker off for a few weeks now. so the heater hasn't been kicking on in the middle of the night to top up temperatures. This does result in a drop in power usage. The next morning however I turn the heater back on and there is a long run time as the heater recovers back to temp.

The result is a big spike early in the morning that although physics suggests it's should be less then the little spikes added up the difference is so small I'm not really seeing it in my numbers.

So I'm stopping with the turning the tank off testing. It's now obvious to me that lost heat is lost heat and I'd be better off spending the timer money on more insulation for the tank. Should we go to a time of use billing system for our electricity I would definitely add a timer so long as the difference in prices is big enough. The tank easily went an entire day without being turned on.

Since I started recording HW power usage I've been averaging 4.94 kwh or 65 minutes a day in HW heating. That's working out to 25% of my total usage. This is costing $0.42 per day or $153/year

Using those numbers in a perfect world I stand a chance at getting almost 100% solar heating from my planned 64 sqft collector. From my Gain calculations my worst months are december and january at 4.12 kwh/day and 4.31 kwh/day. Of course due to our weather patterns I'd have more then enough some days and then not enough for a week.

I also need to redo my calculations on money saved. I'm using about half as much hot water as when I initially took measurements 2 years ago. The savings aren't as high because my costs aren't as high.

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