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Thread: Timer for Oil Fired Hot Water Heater

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    DIY Member SAS's Avatar
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    Default Timer for Oil Fired Hot Water Heater

    With the price of oil skyrocketing, I thought it might be worthwhile to put a timer on my hot water heater. Is there any downside to doing this? It's a 120 volt feed - can I just put a simple timer in the circuit?

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    Plumber jimbo's Avatar
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    There is probably little to gain from putting a timer on a fuel fired water heater. Water heaters, even gas or oil, do not loose all that much heat overnight or between uses during the day. Whatever heat is lost will eventually be made up when it does turn on, and unless the heater is in a very very cold basement or garage, it probably does not come on during the day at all, unless you run some water.

    Uusually, the local utility company can provide you with some hard numbers on things like this.

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    DIY Member SAS's Avatar
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    Others have said pretty much the same thing, but I don't see why it would be so different for an oil fired hot water heater vs. an electric one. In both cases there is standby heat loss and in both cases it is made up when the unit cycles back on. But for some reason there is wide acceptance that timers are a cost saving measure for electrics but not oil.

    Anyway, the numbers I've got so far indicate that the unit loses about 4.5 degrees per hour when it is not running, and that it cycles on after a 15 degree loss. So a timer at night would eliminate 1 cycle. What is not clear to me is how much oil I would save by skipping that cycle. In other words, without a timer it will run twice, raising the temp. by 15 degrees each time. With the timer it will run once and raise the temp. 30 degrees. How would you calculate the difference in the amount of oil used?
    Last edited by SAS; 07-26-2008 at 07:03 PM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    That's a pretty large drop per hour on the tank! A good indirect or electric might only lose 1/4-degree per hour. Can you add an insulation blanket?

    I don't think it will make much difference. While a burner may not reach full efficiency for a short while, once running, it is the same, so, total run time may end up being about the same with only that short while where it is coming up to full efficiency (twice in your scenario verses once if you turn it off).

    Cooler tanks can grow some nasty stuff, too, so that may be a consideration.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Moderator & Master Plumber hj's Avatar
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    Default timer

    The only time a timer saves energy is when the tank is allowed to be completely drained of hot water, whether by standby loss or hot water usage, and then stays that way for an extended period of time. The only reason people think an electric timer saves money is because it is the easiest one to install.

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    Plumber jimbo's Avatar
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    I think there is a number which tells you that it takes "x" many BTU to raise a gallon of water 1. So if you have heat loss of so many degrees per hour, you are going to have to put all that heat back, sooner or later.

    I don't think anyone has found that a timer on electric water heaters saves any money, either. Now, some utility companies will hook your water heater to OFF-PEAK timing, and you save on lower RATES, but not on less water used.


    I have gone into apartments where the gas was off for 3 days, and the water in the tank was still warm enough to take a shower. Your unit must be in a very cold space if it loses 4+ per hour.

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    DIY Member SAS's Avatar
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    I got the 4+ degree loss from the manufacturer's rep. He told me that the standby heat loss was 3.5% per hour. At 120 degrees, that would mean that 4.2 degrees per hour.

    It would seem that the only way in which a timer helps is shifting the time at which you heat water in an electric hot water heater, since you can take advantage of lower rates (as you pointed out above) or the extent to which it is more efficient in an oil or gas fired unit to run fewer times for a higher rise each time. If the function is purely linear, as in x BTU to raise a gallon 1 degree, then there is no advantage. But I would expect that it is somewhat more efficient in terms of BTU per degree to run the heater fewer times for more degrees. In the extreme case, this is certainly true. You wouldn't want to run the heater once an hour for one degree gain each time. Waiting for a ten degree drop and running once would certainly be better. But how much oil would I actually save by running once every 4 hours for a 16 degree gain vs. once every 8 hours for a 32 degree gain?

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