Tempering tank for hot water system (2024 Update)

[Steve]

Ok guys, pick this one apart..... I have a wood stove in my basement. I just replaced my old 52 gal electric hot water tank with a new 40 gal model. I am planning to use my old stripped down tank as a tempering tank ahead of my new heater and place it as close to my wood stove as possible (about 6 inches or so away). I plan to remove the upper and lower elements on the old tank and replace them with a

1" copper line from the top element threaded opening to the bottom threaded opening and extend the copper line over my wood stove so that it will pick up heat from the stove / flue pipe. I am thinking cool water from the bottom will naturally be drawn up the copper line as it is being heated by the wood stove and will circulate back to the top and will constantly be looping around to warm the water inside the tempering tank. I am thinking that possibly overnight, the water in the tank will eventually warm up to around 100 degrees F. And the new electric heater will only need to heat that water up to 120 degrees F. There are only two people in the house and no automatic dishwasher, warm water usage for the wash cycle only of the clothes washer @ 2 loads per week, and maybe two short showers per day. Do you think this is a feasable plan? I am not any kind of engineer, but just a practical tinkerer trying to save a few bucks on my electrical bill......... Thanks for any comments on this! Steve

[hallerb]

its a interesting idea, but i doubt large gains:( I woul;d of stuck with the larger tank since its a long life purchase and more gallons cost little more.

plus to be effective you will have to constantly be turning the electric water heater off and on, or gain very little

[ransley]

I kept a tank that was not leaking and use it as a tempering tank from air temp, just that alone saves a bit, just a tank next to a heat source will help alot, try it and figure out and put in bypass valves so when something leaks you still have HW. My electric 40gal tank cost about 35 a month to run, my present NG Tankless costs $9 in summer with all gas cooking and clothes dryer and using the electric as a tempering tank might be saving me a few $ a month. Be inventive all things help.

[Steve]

Thanks for the comments! I plan to have shut off valves so I can always go back to the standard setup. I do plan to go back to the standard setup for summer as there will be no heat from the woodstove at that time anyway. I guess I thought the 40 gal capacity for the new heater would be sufficient for two people in the house and it was about $30 less to buy than the 50 gal model. To begin with, I don't plan to be shutting the new electric one off and on. It is well insulated and has heat sinks in the inlet and outlet so it should not be cycling on too much during the night. It's probably going to be alot of work and probably about $40 to $50 for materials to get it set up and probably at best it might save $15 a month during the heating season (6 to 7 months in our area) so about $100 / year possibly? At least I don't have to pay anybody to do the installation work. Steve

[Van Chocstraw]

The more you preheat the water the more you save. It's money in your pocket.

[hallerb]

going to be

think about what you said, other than passive gain from the tank the recirle line will be a loser.

tempered but still cold water enters, heater turns on, water gets to

120 and heater shuts off.

now some water recirls from the heat loop. if its not at least 120 it will be cooling your tank and costing more money for electric.

as to tankless, electric tankless capable of running a home will likely take 200 amps just to heat water. even ifd its just a 100 you would still need a main service upgrade 200 amps for home, 200 amps for heating water.......

new lines service power drop:( not worth the cost

[Steve Barker]

i'm thinking this will work fine. BUT i wonder why you are retiring the tank if it is still sound enough to be a tempering tank? I doubt if you get the water to 100, but it'll be a damn site warmer than the incoming water.

s

[Steve]

Like I said I am not an engineer and can't do the thermodynamic equations and btu loss or gain or heat transfer calculations. All I can do is try it and see what happens. You could be right about the heat loop not working out.... But its not quite like going to the moon and not coming back is it (rocket science)? But seriously, I do appreciate your comments as I am willing to learn and appreciate others opinions. All comments are very welcome and appreciated! Steve

[Steve]

Good question! One of the thermostats went out on my old heater. It was 30 + years old and I figured it was as good of a time as any to try this tempering tank idea which I have been thinking of trying for a long time. It just might be a dissapointment but I am willing to try it. Steve

[Wayne Whitney]

The proposed recirculation line originates and terminates at the tempering tank. So no water from the electric water heater tank is displaced from the recirculation line. The recirc line is just a way to increase the heat gain of the tempering tank from the wood stove.

Cheers, Wayne

[Robert Neville]

I didn't read all the details in the original message, but have you considered the total energy flows?

If the tempering tank is inside the building thermal envelope, the energy needed to bring the water up to room temperature has to come from somewhere - most likely your space heater. So, it isn't free.

If the tempering tank is outdoors, you have to consider that you have an unheated space that will be chilling the water for at least part of the year...

[kcarlson46]

Go ahead and use the tempering rtank in the summer. You'll at least bring the water temp up from ground temp, which could be ~55F or so to air temp, which could be ~70F or however hot it gets in the summertime.

Ken

[ransley]

Even if its 70 from the stove it will be a winner, his incomming will be likely 55 or less in winter, mine goes to 35-36f, if basement air is above water temp he wins. Above a stove will be maybe 300f, and tank is also next to wood stove.

[Bob F]

A few wraps of soft copper around the bottom of the exhaust stack pipe would pick up a lot more heat. Or, a loop around the stove, or a coil right on top[ of the stove. Of course, the copper needs to be sloped so that the heated water rises to the top of the tank for good flow.

Don't overdo heat pickup at the stack. If you cool the stack too much, you will get excessive creosote build up in the stack.

If you are picking up a lot of heat, the tank insulation might be left on to get truely hot water, or without insulation, the tank could operated as a heat buffer, continuing to release heat after fire goes out.

If you want the old tank to last, check/replace the anode.

[mkirsch1]

You need a LOT of copper tubing out over the wood stove to pick up any appreciable heat. Think refrigerator coil-style. Back and forth back and forth

[Steve]

This is pretty much what I was planning to do. I am wondering if 1" diameter copper is too big? Or would 1/2" diameter be better? Or does it matter?

I don't think, but not sure, this will be that much of a concern as the copper tubing will not actually be contacting the stack. I normally keep a good hot fire going with well above creosote making temp (I use a magnetic stack flue temp guage) unless I'm letting the fire go out and then it would be just hot coals and embers burning out.

That's more than I could hope for.......

I just got done scraping and cleaning the old anode. It was in quite good condition. Nothing was eaten away from it at all. After 30 years I've only took it out and cleaned it once. The last time was well over ten years ago. We are lucky to have fairly mineral free water, probably why the old heater lasted so long. I also just flushed out the bottom of the tank and the crud level was still below the drain outlet of the tank. We do have a whole house water filter and water softener.

Thanks again! Steve

[KLS]

Your new 40-gallon tank will be more than sufficient for the two of you plus any guests you might have. We have a 40-gallon gas Bradford White water heater for our 2,500 sf house (full bsmt, 1st, 2nd, and

3rd floors), and we've not had any water shortages even with as many as 7 people in the house, and we use a dishwasher plus washing machine. Obviously all the hot water demand can't be in the same moment, so we just make sure to run the dishwasher and washer at times when people are not likely to be showering, and all is good.

[Rick-Meister]

Tempering tanks are a very good idea--however, not with an electric water heater. Do a search for Legionaires Disease and you'll find that almost 30% of homes with electric water heaters have Legionella bacteria growing in their tanks.

The reason electrics have the problem is because they heat from the side. That leaves a cooler zone at the bottom of the tank. The bacteria feeds on sediment and cooler water--anything under 122 degrees.

The experts recommend upping electric water heaters to 140 as the only way to effectively kill the bacteria.

Since gas water heaters heat the water from the bottom, all the heat goes into the sediment, killing the bacteria.

Here's an excerpt from a scientific study:

Like other authors (3,4), including the World Health Organization (WHO) who published a recent monograph on the Legionella problem in drinking water (3), we believe that there is evidence for the transmission of legionellosis through the drinking water distribution systems in private homes. This is a serious illness associated with high death rates (up to 12%). Primary groups at risk (the elderly, smokers, the immunocompromised and patients suffering from chronic respiratory illnesses), are groups who include a large proportion of the population at home. Although we support prevention against tap water scalds, we are against setting water heater thermostats at 49°C because we believe this could facilitate proliferation of Legionella inside the tank and increase the risk of legionellosis.

Domestic water heaters, particularly electric devices, can certainly be contaminated by Legionella. In Quebec, a study of 211 homes (178 electric water heaters, 33 oil or gas water heaters) found Legionella contamination in 40% of electric water heaters. No water heaters using fossil fuels were contaminated (5). The authors concluded that, because of design variables, use of an electric water heater was the most significant factor leading to Legionella contamination in hot water (5) in the home.

Here's the complete link to the study:

If you want more info, google legionella electric heater

Are we having fun yet?

[Steve Barker]

BOY! now here's some paranoia gone to seed...... I'll bet you think 911 was an inside job also eh?

s

[Steve]

Interesting....... If there is some possibility of this happening, I am wondering why it has not been made more of an issue to the public????? There have to be millions of electric hot water heaters in use in this country and I would bet that a very large % of them are set at or even below 120 degrees F. That temp is what most if not all new electric heaters are factory set at. I guess the burn concern is greater than the bug concern........

Steve