In this article, I’ll show you how I used an ordinary stove element to convert my inefficient natural gas hot water tank to an ultra efficient electric tank on a timer, decreasing my energy consumption for water heating by about 80% and saving us about $175 per year (more when you consider that we were also able to cancel our natural gas account as a result of this conversion, saving an additional $140 per year in “basic charges” that the gas company charges regardless of the amount of gas consumed). Warning!!! This article describes modifications to a natural gas appliance, as well as custom 120V AC electrical wiring. Attempting to duplicate the modifications described here could result in injury and/or death and/or significant damage to your home. Do try this at home (where else would you?)… but do so at your own risk.
Why are electric water tanks more efficient than gas ones?
Put simply, most gas hot water tanks put more heat up the chimney than they put into the water. A gas hot water tank is effectively an insulated tank with a hollow tube running through its center. Hot exhaust gases from the burner at the bottom of the tank pass through the tube. The tube is designed for good heat transfer to the water around it but much of the heat goes right through the tank, up the chimney. Feel the chimney while the burner is on and it will likely be too hot to touch. A tremendous amount of heat is lost up the chimney whenever the burner is on. However, that isn’t the worst of it. Most gas hot water tanks in use today have continuous pilot flames and open flues. This means that even when the burner is off (which is most of the time), the heat transfer happens in reverse. The hot water in the tank warms up the air in the tube (recall the tube is designed for maximum heat transfer). The air rises up into the flue. The result is a continuous flow of warm air through the tube, extracting heat from the tank 24 hours a day and dumping it out the chimney. Feel the chimney above your gas hot water tank any time that the burner is off, and you will find that it is still quite warm to the touch due to all the hot air rising from the tank below.
Experiments on my own hot water tank showed that simply capping the tube at the top of the tank by placing a block of wood over it reduced the rate of heat loss from the tank to around 60% of it’s normal rate (ie 40% of the energy loss is up the chimney). If your gas hot water tank is located inside a living space, then I have even more bad news for you. In addition to carrying away heat from the tank, this continuous flow of air up the chimney carries away heat from your home. Air from within your home (that you’ve already heated up to room temperature) is drawn into the bottom of the tank, passes through the tank and is expelled out the chimney. The removal of this heated air from your home causes cold air from outside to be drawn in through all the cracks and openings in your home’s building envelope. It’s the same effect as having a small window open continuously. Capping your gas hot water tank (and plugging the chimney) will prevent this loss in addition to reducing heat loss from the tank itself. However, the only way you can get away with capping your gas hot water tank is by turning off the gas.
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Why not just buy an electric tank?
I could have purchased a new electric hot water tank, but that would have been expensive and wasteful since there is nothing particularly wrong with my gas hot water tank. In addition, installing a commercial electric hot water tank would require rewiring my home to supply a 240V circuit to the location of the water tank. I wanted to be able to run my tank off a 120V circuit and put it on a timer. Timers for 240V high current circuits cost hundreds of dollars. Timers for 120V circuits cost less than $20. On pondering the issue for some time, I realized there was a very simple conversion I could do myself with parts I already had or could get for free.
A 120V circuit doesn’t have enough power for a hot water tank… does it?
This is a common misconception. Commercial hot water tanks, after all, have elements requiring up to 10 kW of power while a typical 120V circuit is only capable of driving around 1.5 kW. The reason commercial tanks draw so much power is “recovery time”. Hot water tanks are typically designed for a recovery time of 1 hour. In other words, a typical hot water tank requires enough power that you can empty the entire tank every hour and still have hot water. Hot water tanks typically hold over 100 litres of water. I don’t know about you, but my household rarely requires 200 litres of hot water in a single day, let alone in a two hour period. If you can tolerate a longer recovery period, then you can easily reduce the power draw to something that a 120V, 15A circuit can supply.
One little compromise makes a huge difference
It is important to recognize that you don’t NEED hot water available all day all the time. In my home, showers are about the only thing we require hot water for. We wash our laundry in cold water and our dish washer has it’s own heating element. If you can tolerate restricting your hot water use to a particular time of day (morning for example), there is no reason to maintain a tank full of hot water 24 hours a day just dissipating it’s heat to the surroundings. If you do want hot water available at any time, you can still achieve that with the conversion I’ll describe, but you’ll see a larger reduction in energy consumption if you are willing to run your tank once per day on a timer, heating it up and turning it off just before you use it. Then as you extract hot water from the top of the tank, it will be replaced by cold water at the bottom of the tank. You can extract up to 1 full tank of hot water which will generally suffice for several showers each morning. The tank will then sit full of cold water most of the day, so it won’t be dissipating ANY heat to the surroundings. In fact, it may even absorb heat from the surroundings if the incoming water temperature is lower than the outside air temperature.
How much power does it take?
If you restrict your hot water use to one time of day (and less than one full tank of water), you clearly don’t need a 1 hour recovery period. You could, in theory, allow up to 24 hours for your tank to heat back up. Tests on my own tank, showed that it takes as little as 150W of continuous power to heat a cold tank from 15°C to 45°C in 24 hours. However, as stated above you can reduce energy consumption by allowing your tank to sit cold all day, heating it up as quickly as possible just before you need it. If 150W will heat a tank up in 24 hours, then 300W will heat it up in 12 hours, 600W will heat it up in 6 hours, 1200W will heat it up in 3 hours … you get the idea. Because I wanted to plug my tank into an existing 120V circuit, I did not want to draw so much power that I’d be likely to trip a breaker if I plugged something else in while the hot water tank was on (even though that’s pretty unlikely since I run the tank only at night). I found that a 520W heating element was satisfactory. Typically we don’t use a full tank of hot water per day so the tank is never completely cold and I’ve found it only takes about 4 hours per night to get the water temperature to around 45°C at point of use each morning. By running our tank on a timer between 3 am and 7 am, we have enough hot water for showers, and we only use the equivalent of about 87W of continuous power (520W times 4/24 since we only run it 4 hours our of every 24). This is in mid summer… I imagine we will need more power in the winter when the garage and the incoming water are colder.
How do you do the conversion?
Converting a gas hot water tank to electric is easier than you might think. If I had to do it again, it would take me only about an hour. The original equipment is not modified in any way either, so it’s easy to switch back later if you want to. Warning!!! Be sure to turn off your natural gas completely before performing this conversion. The process described could generate electrical sparks which could cause an explosion if any natural gas is present.
First, find a suitable stove element. You can often pick up a used stove element from any appliance recycling or salvage yard for free, or you can buy one. You want an element that will produce 500 to 1000 Watts of power on 120V AC. Most stove elements are rated for 240V AC. If you know basic electronics, then you will recall that if you halve the voltage across a resistive element, you will get only 1/4 of the power output. So a stove element rated for 2000 to 4000 Watts at 240V will give you 500 to 1000 Watts at 120V. The best method for selecting an element is to bring an ohm meter and measure it’s resistance. Power is equal to V2/R where V=120. Therefore you want a resistance R of 14 to 28 ohms. The resistance will increase a little when the element heats up but not too significantly. Note also that although you will be running the element for much longer periods than it would normally be run on a stove top, it will still last a long time because you will be running it at only 1/4 of its rated power. That said, you might want to pick up a few elements since they may break during the shaping process.
Shape the heating element to fit inside the tube through the water tank. I used a couple pipes as forms to bend the heating element into a “coil inside a coil” shape so that the two ends of the element were on the same end of the coil. See photos for a clearer explanation. Be sure that the terminal ends of the element are well away from the outside circumference of the coil so they cannot contact the inside walls of the tube through the hot water tank that the coil will eventually be lowered into. Test fit the element in the tank. First, turn off the gas, including the pilot flame. Then disconnect the chimney from above the tank. Pushing the chimney aside you should have access to the top of the tube running through the tank. Reach inside and pull out a spiral shaped length of metal. This is designed to improve heat transfer from the exhaust gases to the tank, but it is not needed for the electric conversion so it can be removed to make space for the electric element. Slide the coiled element into the tube. Ideally, it should slide in freely, making only light contact with the walls of the tube. Reshape the coil as necessary for best fit.
Attach wires, electrical cord and cap. I used some high temperature wire from the same salvaged stove I got the element from. This wire has a special insulation rated for high temperatures and also has some connectors for attaching to the element. The high temperature wiring should only be needed in close proximity to the element. About 2 ft of wire should be all that is required but if you have more, you might as well use it. The high temperature wire can then be soldered (and heat shrink tubing applied) to an electrical cord with plug. Before soldering, pass the ends of the electrical cord through a drilled hole in a metal cap. A large jar lid with the seal removed makes a good cap. WARNING!!! Do not use a wooden cap as shown in the photos. This was a temporary improvisation. It will work under normal operating conditions but could be a fire hazard if the element is ever left on continuously (intentionally or otherwise). Using non-flammable materials is just plain better when dealing with any heating appliance. The cap will act as both support for the element, hanging by the wires, and also to prevent air flow through the tank. Tie a knot in the cord so that the element hangs near the bottom of the tank without contacting the burner.
Install the heating element. Ensure that the bare ends of the element will not contact the tube walls or each other. Insert the element in the tube aligning the cap so the cord is centered above the tube and the block covers the tube end completely.
If you have a power meter, plug it into an outlet and plug the cord into it so you can see how much power the element is drawing. Leave the power on for a half hour or so, keeping close watch on the tank. You can lift the wood block occasionally and look into the tube, checking the wiring for any melting and smelling around for anything burning. Be careful when moving the cap from the top of the tube to look inside as hot air will rise from the tube.
If all goes well (no melting wires or the smell of anything burning), plug the cord into a timer. Set the timer to heat the element for roughly 6 hours, turning it off just before you generally use your hot water. You want the element to be turned off while you are using hot water so that you will not be wasting energy heating up the cold water that is entering the tank. If you find that the water is not hot enough after 6 hours of heating, then adjust the timer to start heating earlier. Likewise, if you find that the water is too hot after 6 hours of heating, then adjust the timer to start heating later. The system can take a day or two to reach equilibrium after you make an adjustment, so try not to make adjustments too large or too frequently. If you used a 520W element like I did, you’ll probably find that a few hours of heating per shower is plenty. For example, 4 hours seems to be about right for our two-shower-per day household in the summer. We’ve found that heating the tank only once in the early morning results in “high heat” for showers tapering off to “medium heat” by the evening. If you want high heat available all the time, then you can set your timer to heat a couple hours at a time at a few different times during the day, however, this will consume more energy since the average continuous temperature of the tank will be higher. You’ll likely need to adjust the timer a few times during the year to adjust for the temperature of your incoming water which will vary seasonally.
Lastly, you should post a warning or disconnect the gas lines completely. If someone were to turn the gas back on and light the pilot flame without noticing that you modified the tank (unlikely but possible) it could result in gas and/or carbon monoxide being released into your home (also unlikely but possible). The warning message shown in the image below was one I posted for a few days while testing to see if the 520W element was adequate. After that I disconnected the gas lines from the tank completely.
There are a few important safety considerations when performing the described conversion. First, if the timer beaks or if someone were to plug the element in without a timer, it will run continuously and eventually, it could heat the water tank to boiling temperatures which could cause scalding and in the worst case, could increase the tank pressure to the point of explosion. There SHOULD be a T&P (temperature and pressure) relief valve on the tank which will open, expelling hot water in the event that the temperature or pressure exceeds acceptable levels.
Warning! Whether you convert your tank or not, you should ensure your T&P (temperature and pressure) relief valve is functioning properly.
On my tank, this will expel water safely into a pipe that leads outside my home. Your tank may vary. It is possible, however unlikely, that water could be sprayed onto the element which could cause an electrical hazard. The tank could also leak internally causing an electrical hazard. Also, it is possible that the bare terminals of the element could contact the inside of the water tank, also causing an electrical hazard.
Warning! To reduce the electrical hazards described above, use a GFCI outlet or GFCI adapter. You should also add a note/label to your electrical cord indicating that it should only be plugged into a GFCI outlet to reduce the electrical hazard.
Thermal fuses are relatively cheap and can be wired in series with the element to reduce the risk of overheating. Cover the thermal fuse with heat shrink to prevent any danger of shorting out the leads and attach it adjacent to the outside of the hot water tank (under the existing tank insulation). Thermal fuses can be purchased from Digikey for about $1.30. Aim for about 80°C maximum working temperature of the fuse. There will be some internal heating due to the electrical current through the fuse, so it should trigger at 70 to 75°C. This type of fuse is non-resettable. Once you exceed the cut-off temperature the fuse will be permanently open circuited and will need to be replaced. If you’d prefer a resettable solution, you can use a thermal switch (also called a thermostat or temperature regulator). These open at a particular trigger temperature and close again at a temperature around 10°C below the trigger temperature. Thermal switches usually have a flat surface which is intended to contact the item whose temperature you wish to regulate (in this case, the exterior surface of the hot water tank). They are about $7 ea at Digikey. Whichever method you choose, cover exposed wire or terminals with heat shrink to reduce the danger of short circuits as much as possible. Also make sure you don’t exceed the current rating. The thermal fuse and switch linked above are rated for a maximum of 10A.
Warning! Install a thermal fuse or thermal switch to prevent overheating if the element is inadvertently left on continuously or plugged into an outlet with no timer.
In the absolute worst case that you don’t install a thermal fuse or thermal switch and your T&P valve isn’t functioning (or someone capped it because it was dripping) and you connect the heating element to continuous power (or your timer is faulty), you might be surprised to know what might happen. The temperature and pressure in the tank may rise until the tank explodes. Depending on the mode of failure of the tank, it can act like a rocket, literally destroying your home and launching itself several hundred feet into the air. This has been demonstrated on a couple episodes of Mythbusters. This myth, unfortunately, is entirely true.
Huh? Don’t you need to install a thermostat?
When I first thought about converting my tank, I intended to add a thermostat so that the tank would not overheat and would be at a consistent temperature each morning. However, during testing I found it wasn’t necessary because our hot water use follows such a regular pattern. Without a thermostat, the temperature of your hot water on any given morning may vary a bit depending how much hot water was used the previous day, but that is easily compensated for by adjusting the amount of cold water in the mix. In the worst case that no hot water is used for several days, the average tank temperature will increase higher than normal, but the change isn’t significant. There simply isn’t enough power going into the tank to overheat it (unless your timer breaks, or someone inadvertently plugs it in without a timer as discussed in the safety concerns section above). So my advice is not to complicate things. If you can restrict your hot water use to a single time of day and your hot water use follows a reasonably regular pattern then you don’t need a thermostat. If your hot water use does not follow a regular pattern, it’s entirely possible to make your own thermostat. For an installation at a friends house I converted a purchased programmable thermostat originally designed for controlling baseboard heaters and it worked great, offering complete programmability of both time and temperature without need of a separate timer. Unfortunately, converting a programmable thermostat for hot water tank use is beyond the scope of this article. I may add it as a separate article in the future.
What’s the best water temperature to aim for?
There is a hot debate (pun intended) about what is an appropriate temperature for domestic hot water heaters. Some recommend water temperatures under 50°C to prevent scalding (especially important if you have children). Others recommend a temperature of 60°C or higher to kill bacteria. The primary bacterial concern is Legionella bacteria which can cause Legionnaire’s Disease. The Centers for Disease Control and Prevention estimates 8,000 to 18,000 Americans contract the disease annually (about 1 out of every 20,000 people) with 5 to 30% of cases being fatal. High risk groups are the elderly, smokers, the immuno-compromised and those with chronic respiratory illnesses. According to this article, it’s not necessary that water temperature be maintained at 60°C to kill the bacteria. It’s only necessary that the water temperature be raised to 60°C at least once per day. A great way to accomplish this with minimal energy consumption is – you guessed it – by running a tank on a timer just as I’ve described above. Commercial electric tanks have heating elements located near their midsections leaving some water at the bottom of the tank that will be cooler than 60°C even when the tank is set to 60°C. Thus Legionella bacteria are almost always found in electric tanks rather than gas or oil ones. The converted hot water tank I’ve described, however, should not suffer from this deficiency since the heating element can be lowered to the very bottom of the tank. I personally aim for a temperature of only 45°C at the point of use for morning showers. It’s up to you whether you want to run your tank up to 60°C or reduce your energy consumption and scalding risk by targeting a lower temperature.
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How much can you save?
My gas statements from last summer showed that we consumed about 1.2 GJ of gas per month in the summer and about 2 GJ per month in the winter. That is a summer equivalent of about 11 kWh per day. This summer the converted tank is consuming about 0.52 kW x 4 hours = 2.08 kWh per day or about an 80% reduction. Our gas consumption for one full year was about 20 GJ or 5500 kWh. At $12.78 per GJ, that amounts to about $250 per year. Assuming the 80% reduction holds true year round, I expect to consume about 1100 kWh of electricity per year. At $0.07 per kWh that amounts to about $77. Therefore our savings are about $175 per year.
Aside: Our recent switch from gas to electric space heating already saved us about $355 per year. The hot water tank conversion has brought our savings up to about $530 per year. The icing on the cake is that we no longer use ANY natural gas so we were able to cancel our gas account, saving an additional $140 per year in “basic charges” that our gas company bills no matter how much gas we consume. That brings our total savings to around $670 per year by switching from gas to electric.
How does this compare to tankless “on demand” systems?
In theory, electric tankless water heaters approach 100% efficiency. In other words they put most of the energy into the water and dissipate hardly any to the surroundings. In practice they still dissipate heat from the pipes themselves and a preheating tank is typically installed in series to reduce the peak electrical load required to raise the water to the final temperature. The preheating tank is maintained at a constant temperature and dissipates heat just like any other hot water tank, reducing the efficiency of tankless systems. Despite the fact that tankless gas water heaters don’t require a preheating tank, they are not as efficient as electric systems due to our old friend, the chimney.
For the sake of argument let’s compare our converted tank on a timer to a 100% efficient “ideal” water heater. The specific heat of water is 4.186 kJ/l/°C. Our cold water temperature is about 15°C and our hot water temperature is about 45°C. We use about 40 litres of hot water per day for showers (yes, I measured it). Assuming 100% efficiency, the energy required to heat 40 litres of water by 30°C is simply 4.186*40*30 = 5023.2 kJ. The average power required in kW is simply that energy divided by the time (one day) in seconds or 5023.2/(24*60*60) = 0.058 kW. So a 100% efficient water heating system would require 58 W of continuous power to meet our hot water demand. Using our electric system on a timer our average power consumption (calculated earlier) is about 87 W. Thus our efficiency is 58/87 = 67%.
I think that’s pretty good. It is probably about equivalent to an electric on demand system with a preheating tank. To satisfy my curiosity I’ll also calculate the efficiency of the previous natural gas setup. Recall that prior to converting our tank our hot water tank was consuming about 1.2 GJ (1200000 kJ) per month in natural gas. The power is simply that energy divided by the time (one month) in seconds or 1200000/(30*24*60*60) = 0.463 kW. That’s 463 W. So our water heating efficiency prior to converting from gas to electric was 58/463 = 12.5%. This is typical of a natural gas water heater with an open flue and a continuous pilot flame (the most common type currently in residential use), and yes, it is pathetic.
Note that the efficiency of a hot water tank increases as hot water usage increases. This may not seem intuitive at first, but think about it. If you don’t use any hot water, your hot water tank efficiency must be 0% since it’s consuming energy but not producing any useful hot water. The efficiency numbers I calculated above are based on our own hot water usage of about 40 litres per day. If we used more hot water than that, the efficiency calculated would be higher. Most hot water tanks have an Energy Factor or EF rating. This value is supposed to represent the efficiency of the hot water tank assuming “typical” hot water usage. I’m not sure what the typical usage is that manufactures assume to determine their EF values but I suspect it’s in the ballpark of 2 full tanks per day. Given that gas hot water tanks tend to have EF ratings around 0.6 and electric hot water tanks tend to have EF ratings around 0.9, it’s pretty clear that our hot water usage of 40 litres per day must be MUCH less than the “typical” usage that EF values are based on. I suppose it is in the manufacturer’s best interest to assume greater usage than what is truly typical since it makes their products appear more efficient than they actually are. There are not many consumers who recognize there is a difference between “Energy Factor” and “Efficiency”, or that that actual efficiency depends on usage.
What about adding solar water heating?
Solar water heating is one of the easiest and least costly ways of extracting useful power from sunlight and has one of the shortest pay back periods of any solar technology. Solar water heating was commonplace in cities throughout North America (something like 20-30% of households had them) before cheap gas and electricity arrived on the scene. Now that gas and electricity are not so cheap anymore, there has been a resurgence of solar water heater installations. Luckily, an electric hot water tank on a timer is a perfect complement to a solar thermal collector. Heat only flows from hot to cold, so if your solar thermal collector is only heating up to 40°C because it’s an overcast day, you won’t be able to transfer any of that heat to a hot water tank that’s being maintained at 45°C. To get around this issue, solar water heating systems typically require installation of a preheating tank. The solar collector is used to heat the preheating tank which then delivers warm water to the regular hot water tank.
One way to avoid the expense and hassle of installing a preheating tank is – you guessed it – by using an electric tank on a timer. Then, instead of a preheating tank you have a “preheating period” which is simply the time that the heating element is turned off. If you shower in the morning, your water tank will be relatively cool during the day and a solar collector can be used to heat the tank directly. This will in turn reduce the amount of electrical energy required each morning to bring the tank up to temperature. You can imagine that if solar energy can bring the temperature of the tank up to 40°C, then you won’t need to power the heating element for very long the next morning to bring the tank up to 45°C. However, a thermostat would be a good idea to take best advantage of solar heating. Without one, the temperature of the water each morning will depend heavily on the amount of sunshine the previous day.
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What else can you do to reduce the energy consumption of a hot water tank?
There are several other things I’m planning to do as priorities and time permit:
- I could achieve a higher efficiency by installing a 1000W or 1500W heating element and reducing the heating time accordingly. Effectively this would allow the tank to spend more time at a lower temperature which would result in less heat dissipation. A simple way to do this would be to put two 520W heating elements in parallel to make a 1040W heating element. This would also reduce the recovery period (useful when we have guests and many people want to shower around the same time)
- My tank still dissipates heat to it’s surroundings whenever the inside water temperature is higher than the outside air temperature. It’s a law of thermodynamics that the power dissipated is proportional to the temperature difference so the power loss is greater in the winter when the air is colder (recall my tank is in an unheated garage). There’s no way to stop the dissipation of heat completely, but I could reduce it significantly by adding more insulation around the outside of the tank. There are hot water tank jackets marketed expressly for this purpose but they only add a couple inches of insulation. According to my calculations, the optimum insulation thickness is MUCH more than that. Update. I did this. It was easy and it was wildly successful. For more information see Super insulate your hot water tank.
- I mentioned earlier that our hot water tank is located in an unheated garage. That means that the heat dissipated by the tank is wasted. Therefore I plan to move the tank inside our home. There’s a small storage closet under our stairs that would hide it nicely. Then during our heating season (which is about 7 months out of the year) the heat dissipated from the tank will not be wasted. It will heat our home just as effectively as the electric space heaters we’re already using.
- When I turn on the hot water in our shower, about 1.75 litres of cold water is expelled from the shower head before any hot water arrives. This is because the hot water tank is currently far from the point of use. At first glance you might think that this is just a waste of water rather than a waste of energy – it’s only cold water after all – but what it really means is that after any hot water use, 1.75 litres of hot water remains in our pipes and simply dissipates heat to the surroundings. Our showers each morning only total about 40 litres of water so the 1.75 litres in the pipe represents about 4% of our hot water usage. During our heating season it’s not that important since the water pipe dissipates heat into our home where we want it, but in the summer the dissipated heat is simply wasted. Things could be improved by moving the tank closer to our point of use. Note that insulating your hot water pipes (as is a common recommendation) actually does very little good. Even an insulated hot water pipe will easily dissipate all its heat to the surroundings in a matter of hours because its surface area to volume ratio is so high. Fortunately the planned new location under our stairs will reduce the distance to the point of use to about 25% of the current distance. Another solution is to use smaller diameter pipe as long as you have plenty of water pressure.
- We typically use only 40-50 litres of hot water per day, but our tank holds over 150 litres. Hot water tanks are generally sized to meet peak demand rather than typical demand which makes sense since who wants to have visitors suffer through a cold shower. However, larger tanks dissipate more heat because they have a larger surface area. One solution to this issue is to install two tanks in series, one sized to meet typical demand, and another sized so that the total volume of the two tanks will meet peak demand. One tank would be heated on a timer to provide for normal hot water use. The other could be turned on manually when needed (ex when you have house guests). I may consider installing another, much smaller tank inside the home as described above, but keeping the current big tank in series. Perhaps about one week out of the year we might have house guest and need to heat up the big tank for the extra capacity. Since our garage is not insulated, I would have to heat the big tank in the winter but only enough to prevent freezing.