Shutting down gas H/W heater during the day...

[Auto-Union]

I have propane heat for H/W and some baseboard heat, which makes one most aware of the cost of paying to fill the tank. My wife takes a shower about 7am and I do so about 9am. So, before I shower, I turn the valve back to Pilot. My shower goes fine, I even sometimes take a 2nd quick shower later on and/or do the dishes with leftover lukewarm water. Even better, on weekends we don't shower until 11am and can turn the heater back on right before use. Normally, I turn it back on the night before, about 1-2am.

So, that means 18-24 hours at a clip, no gas is burned to keep the water hot. Does everyone agree that a decent amount of fuel is saved? How much savings verses pita is different for everyone, by for example we heat with 90% wood over the winter which has to be schlepped in. I understand that the only thermaldynamic savings is the difference between storing +150f water and +75f water verses the differential of the ambient air. That is, plus 75 degrees over ambient. Reheating is a non-issue. Am I missing anything? PS In general, I set the water temp to use pure hot for showering, when the tank is empty, shower is over. That seem better than keeping super-hot water ready, just to blend it with cold.

 

[Auto-Union]

PSS, the heater itself is convienently located a few steps away.

 

[XS650]

Unless you have some need for 150F water or have an undersized water heater, cut your water temperature down to about 120F and leave it there. That will save you more money than turning it off.

We live in a fairly mild climate and keep out hot water in out garage located heater at 120F which meets our needs.

We can be away from home for 2 weeks with the heater on pilot light and the water is at least 120F when we get home, so there is no advantage to turning the heater back to pilot when we are gone for a few hours, or a few weeks. Our water heater is a fairly decent late model gas heater, but not top of the line.

 

[Shannow]

I think that the pilot will make up for the bulk of the thermal losses of keeping unused hot water hot.

 

[eljefino]

How horrible is the insulation that a mass, say 40 gallons, of water loses all that heat in just a few hours?

How warm does the HW heater make its room?

 

[Brett Miller]

I wouldn't do it. Bacteria, specifically causing Legionaires disease, can build up in water heaters if the temperature is not kept high enough. For a few dollars per month it's not worth taking a chance.

 

[Razl]

If this is an old poorly insulated heater, you may be squeezing a few pennies (literally) extra. Newer heaters use high R-value foam, which will hold 120F with just the pilot in a condtioned space, so turning them off/on would not save anything.

Regardless, I wouldn't do this in the colder months as any heat loss would just suppliment the heating of your conditioned space.

The best thing to do is turn it down to 120F as recommended, then if it is an old heater, buy a $15 water heater blanket.

 

[rjundi]

Not such a good idea.

You will consume considerably more energy reheating your water back to temperature vs maintaining it. I did not do the calcs myself however asked two friends who happen to both be PHD level researchers (both combustion engineers) and they said leave it just keep temp low as tolerable to you constantly and insulate tank.

 

[Auto-Union]

It does seem to keep the water hot for a long timee even while off. However, I don't see the difference in heating it at one time or another being more or less efficient. The bottom line is that in winter, yes, it will heat the living space too, so maybe the practice is better in summer. Thanks.

 

[Gary Allan]

I have a summer/winter hook up in my steam boiler. The dead band in the domestic hot water thermostat gives too much of a lag in when you lose temp and when the boiler kicks on. I had the bright idea of turning up the thermostat to allow us more flexibility in our usage. I went through two automatic mixer valves before I gave up, so we're on straight hot water. I haven't fitted the shower(s) with local regulators yet.

Anyway, as others have said, your best bet is to lower the thermostat. My maintaining a hotter setpoint cost me to the tune of 50% in total fuel oil usage over the past year. My usage has been a constant 900-1000 gallons a year for over a decade. Last year it was 1500

So ..just reverse that to say that your best savings would be to lower the setpoint to 120F. You can't tolerate anything above 110F without scalding so anything above that is to account for the amount you use in terms of duration (total hot water gallons used extended).

Let me go further on in my rambling here. We have the electric hot water heater in our back apartment on interuptible electric service. The meter shuts off at a certain time a day. They're remote controlled at the meter (probably heterdyning the signal over the sine wave to trip the contact relay) ..but about 15 years ago they were controlled by clocks at the meter pan. Our clock went bad and the electric company tech just bypassed the clock so that I got cheap electricity to my hot water heater 24/7. The electrical usage was identical in kwh.

That is, the only function that shutting it off during certain times of the day served was to "shift the load" to a different time. Costs in energy were the same to produce the hot water used.

Your ideal setup would be an "on demand" or instant propane fired water heater with no storage capacity to be maintained. They're expensive and usually only make sense where central storage of hot water would be impractical (like a 20 room mansion where you would be 50, or more, gallons from seeing hot water out of the faucet).

 

[Matt_S]

I certainly wouldn't recommend turning it off at any time due to concerns about bacterial contamination. I've heard that you should keep it at NO LESS than 140 degrees for safety. If the heater is not too old, the insulation will keep the heat in just fine. You can supplement that by insulating the piping going into and out of the heater for about 5 or 6 feet. Don't turn it off though. You're asking for smelly water that may not be safe to use. The amount of money you're saving is negligible IMHO due to the reasons the others pointed out. Whether you heat the water now or later, the same number of BTU's are being used. Your best bet, if you're concerned, is to get a modern, well insulated tank with as high an efficiency as you can find.

 

[XS650]

Legionella has been mentioned. Google Legionella and water heater.

There may be something to it, although the strongest advocates for high water temps to kill legionella seem to be power companies

and some 120F recommendations come from Doctors.

It does seem to be undisputed that legionella thrive in water at 120F, don't do so well at 130F and are pretty much gone at 140F although some survive 140F.

 

[brianl703]

Data on the characteristics of the hot water system and plumbing in the house and on the personal habits of the occupants were collected for each house. Among these 211 houses, hot water was provided by either an oil or gas heater in 33 and by an electric heater in 178. Legionellae were isolated from none of the samples from houses with oil or gas heaters and from 39% (69 of 178) of those with electric water heaters (P less than 0.0001). This association remained highly significant after control for water temperature and other variables in a stratified analysis. In the 178 houses with an electric heater, 12% of the faucets, 15% of the shower heads, and 37% of the water heaters were contaminated.

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=183576&tools=bot

Seems to be more of a problem with electric water heaters, apparently due to the location of the heating elements. In a gas or oil heater the burner is at the bottom.

 

[XS650]

Nice link Brian. The effect of location the heat source is obvious once someone mentions it.

 

[JHZR2]

The question is almost an analog to chemical reactor design, where one asks is it better to operate a batch process or a continuous plug flow process. And the continuous process tends to be more 'efficient' overall, all things considered.

If you work the math out regarding cycling vs. step changes, it should turn out that the difference exists, though it may be small.

I think that having a bulk of cold water sitting there to be heated fully is a relatively large usage of fuel. Leaving the flame on, so that as you pull water out, new cold water mixes in (heat transfer by mixing), plus the flame makes up the additional heat, using a lot less energy than taking all 40 gallons from room temperature (or groundwater temperature) to 130+.

Youre best off setting the temperature to as low as you can take, addding insulation to your heater and pipes, and conserving water by reducing shower length. If you go away for a long period of time, when the water will loose enough heat though insulation losses that youll be burning fuel a number of times to retain temperature, then it may be worth it to turn off th eheat and let whatever temperature, if any, be maintained via the pilot light only.

JMH

 

[XS650]

JMH, you pretty well have it nailed. This thread reminds me of the "do I need an oil cooler" threads that go on ad nauseam when the person asking the question doesn't know what temperature their oil is without the cooler. They always get plenty of authoritative answers though.

The saving grace here is that a well insulated indoor gas water heater will come close to holding or actually holding a useful water temperature with the pilot light only. That means the answer is "it doesn't matter", or at the most "it doesn't matter much".

 

[Auto-Union]

Note that I don't actually know the hot water temp, and just guessed. I set it low. I did not know about bacteria. When we moved in, there was a pre-heater. That was a stripped h/w tank that took the well water up to room temp before hitting the h/w heater. I since took it out. I was considering installing it in the attic as an emergency water supply as our power goes out for days at a time. We get 3rd World service and pay full rates.

 

[thooks]

Auto-Union,

You are wasting much more of your time than money by cutting the heater on and off.

The burner shouldn't be cycling unless hot water has been called for. Even then, you can run enough hot water to wash your hands without it cycling on.

A tank of hot water will stay warm enough not to require the burner for about 60-72 hours.

 

[oilyriser]

Wrap some insulation around the tank. Use Kaizen to cut your shower times.

 

[Auto-Union]

My wife takes INSANE long showers, ULTRA hot. I can be done in 5 minuts, she takes 1/2 hour.