Setback for hot water tank?

KenN

KenN

Member
I'm looking for a way to cut down energy use by setting back the hot water tank temperature overnight.  My home system uses a tank that is heated by a glycol loop, with an aquastat for on/off heating control.  It looks something like this:
 
http://www.pexsupply.com/Honeywell-L4006A2007-High-or-Low-Limit-Aquastat-100-240F-range-5-30F-Adj-Differential-5481000-p
 
 
Initial searches are not coming up with anything that can control the setpoint remotely.  I can accomplish my goal with brute force by putting something like an Insteon I/O module to lock out the interposing relay that starts the pumps and heater when heat is demanded by the tank, but I'd rather something more elegant - rather than just shutting water heating off completely.
 
Any suggestions?  Anything in the Insteon or z-wave sphere (or that could be retrofitted as such) would do!
 
Thanks.
 
I am going to recommend against it.  What you will save in energy, you'll more than pay in added tank costs. What seems like a good idea may not be one.  The problem is that temp changes cause the tank to expand and contract and that will cause leaks.  I live in a very mild climate with water heater in the garage.  I replaced 3 water heaters in 5 years when they were lowered in temp at night.  I couldn't figure out why then I realized that the temp changes were causing the leaks.  I would recommend that you insulate the tank, but otherwise keep the temp constant. 
 
There was actually a pretty long running thread about this not terribly long ago - maybe a year?  The consensus was that there are several reasons why this just isn't a good idea; and that any heat you lose overnight you're just going to have to regain in the morning anyway before your morning shower.  There were actually a lot of good calculations and scientific reasons that made the whole idea look not just pointless, but as also introducing more potential problems.
 
What work2play said.  Tanks are well enough insulated and the specific heat of water is high enough that shutting the tank off for a few hours at a time saves almost no energy and potentially stresses the equipment resulting in shorter life.  It is a good thing to do only if you are going to shut it down for at least several days at a time.
 
Also consider if you live somewhere where you are usually heating your home (like Vancouver?).  If so, the heat loss is not really lost, it is heating the home, which you were going to do anyway.  
 
While I get what you're saying, the reason I wanted some sort of thermostatic control was so I could just set back the temp by a few degrees, rather than turn it off for the whole night. I normally run with the tank set at 125, I was thinking of an overnight setback of around 115 or 110.  Thermal shock should be a minimum, and if the ISY controls it, I can set back the temp at midnight and have it back to 125 around 6am.  Wouldn't save a whole lot of money, but over the years it could add up.
 
As to heating loads, the water tank is in the basement, in a nook with three outside concrete walls and a concrete floor.  The adjoining room we only use for a few hours/week so it stays unheated most of the day.  As to our climate - it's fairly mild here compared to rest of Canada, so we don't heat THAT much!!
 
There are much better ways to spend your money and effort.  It is relatively simple to turn a hot water tank on/off remotely, but it is not at all simple to change the temp remotely.  A 10 degree drop overnight really won't save you any money anyway.  It may not even drop by 10 degrees if you turn it off completely for 8 hours (assuming you don't draw water off).  Like work2play mentioned, there was a long thread on this about a year ago with lots of research done, it just doesn't make sense to bother with it.  It really only makes sense in a vacation house type situation where you only come in on weekends or whatever.
 
jpmargis said:
I monitored my gas meter for a month.  The water heater turned on at night ONCE.
Click to expand...
There's a thought.  The controls are external to the tank, so easy enough to log the status of the relay that runs the heater just to confirm overnight usage.  I expect you're all correct, but I'm on the hunt for all the energy reductions I can find!
 
yup - as stated, the heat loss will be the same either way - so whether it brings it up in 2 short burts or one larger one later, both will cost the same.  You would never even recover what you spent on automating it.
 
It's been a long time since I posted anything on this forum but I need to comment on this thread. First let me say that I have been controlling and monitoring my electric water heater for the past 4 years. I have 4 years of data and I know how many KWhrs I have used for heating water for every day, month, and year. Secondly, for me, hot water is the biggest user of home electricity on an annual basis. Anything I can do to reduce the run time of my water heater saves me money. Third, my power company has different rates during the day. During peak power time (5-7 PM) the rate is 3 times higher than the rest of the day. Fourth, I have a well and the ground water temperature is cold enough that the water heater will turn on whenever the hot water faucet is turned on.

So I use HomeSeer to turn off my water heater during peak power times and overnight. I can say without reservation that I am saving anywhere between 5-10 dollars a month using this approach. It is definitely worthwhile for me. And as more power companies adopt peak power billing, it may soon be advantageous for everyone to "choose" when to heat their water.

Steve Q
 
Welcome back ;)
 
Can you share more details about how you are monitoring and controlling your system?
 
How did you determine the savings? Did you have 4 years of data when the heater wasn't under control of HomeSeer?
 
Are you monitoring the water usage and water temperature out of the hot water heater?
 
Thanks!
 
I have posted this in another thread discussing temperature control of domestic water heater: before you attempt any control of your water heater, please read about Legionella, a pathogenic bacterium which acquired its name after a 1976 outbreak of a then-unknown "mystery disease" that sickened 221 persons, causing 34 deaths.
 
"The European Guidelines for Control and Prevention of Travel Associated Legionnaires’ Disease recommend that hot water should be stored at 60 °C (140 °F) and distributed such that a temperature of at least 50 °C and preferably 55 °C is achieved within one minute at outlets.
 
It is possible to control legionella with good design and engineering protocols. For instance raising the temperature of water heaters once a day or even once every few days to 55 °C (131 °F) at the coldest part of the water heater for 30 minutes will effectively control legionella. In all cases and in particular energy efficient applications, Legionnaires' disease is more often than not the result of engineering design issues that do not take into consideration the impact of stratification or low flow."
 
Dan, I will post the details of my water heater control and monitoring system in the next day or two. I'm in the middle of another project right now.

Regarding the savings, clearly I don't have 4 years of before and after data. And clearly the savings depends on how much water is used and varies from month to month. It is also greater now than 4 years ago because of increasing power costs. Three years ago I participated in a study conducted by my power company. The study used smart meter technology to measure my power consumption on an hourly basis throughout the day over a 9 month period. During the study I was only turning off the water heater overnight. At the conclusion of the study, my power company provided a detailed analysis of my power usage throughout the day. And they provided data on the cost savings that could be achieved by shifting power usage from peak usage time to a time earlier in the day based on their new proposed billing rates. Using this data coupled with my own data on the percentage of my power that I use for hot water, I calculated a savings of 5-10 dollars per month if I turned off my hot water heater during peak power times (5-9 PM not 5-7 as I stated in my earlier post). My electric bills have stayed pretty much the same over the years despite the price increase of electricity.

Steve Q
 
It does not surprise me that you are saving money but turning your hot water heater off during the 4 hours per day that electricity costs 3 times as much money.  Especially when you consider that the hot water heater mostly only turns on when you are using hot water, and 5-9pm is a pretty popular time of day to be using hot water while cooking dinner, doing the dishes, getting cleaned up before bed, etc.  From a usage standpoint, you really are looking at being low on hot water until more like 10pm if you actually consumed the stored hot water.
 
However, very few residential power meters use a demand pricing scheme, so this won't affect most people.  And if indeed you are using less electricity, I would bet it is the result of simply using less hot water by virtue of the fact that you just don't have any, yet you proceed with your activity substituting cold/cool water where you would have used hot.
 
If you really wanted to save electricity (not just money), consider using a heat pump tank.  They use 1/2 to 1/3 the electricity to get the same amount of hot water.  And they cool and dehumidify the room they are located in, which is great for where I live, but for you, only in the summer.  I might be mistaken on this, but I believe they qualify for a tax credit as well.
 
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