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What if I installed a new tankless water heater before the regular hot water heater, so that the tank water heater is getting filled with hot/warm water vs the 40 degrees water its getting now? Our 80 gallon tank turns cold quickly. We had a 40 year old solar unit doing this which has to be removed or replaced, and want to look at all the options.

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  • Where is this vacation home located? How many people does it serve? Is it used in winter or only in summer? 40 deg F inlet water is very cold and it will cost a lot to raise the temp by 60 to 80 to 100 degrees. The only large tankless heaters I know of are natural gas or propane fired, not electric. Is the existing tank for the solar system still intact or it is leaking? What is the material this tank is constructed of? Why are you contemplating replacing the existing solar water heating system? Jun 7, 2017 at 9:43
  • Related: diy.stackexchange.com/questions/60840/…
    – mmathis
    Jun 7, 2017 at 14:30
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    We are located at 8,500 ft in the Colorado Mts. Gets very cold here very common to be below zero for days to a week at onetime. We do not have gas here. The solar tank is 40 years old. One (but not only) of the reasons we are in the position were in. The property is jointly owned and not all partners want to put forth the money to repay completely replaced and solar. So I'm just trying to explore all avenues.
    – Stcks4
    Jun 7, 2017 at 18:05
  • With current solar and conventional tanks I have double 40 amp, double 30 amp and a 20 amp breaker that will no longer be used so I believe I have the power. I see Electric tankless heaters that give me 2.9 gpm at 112.5 amps. That might work if we time baths and showers. We have one tub that fills at 3.15 gpm everything else is 2 or under
    – Stcks4
    Jun 7, 2017 at 18:07
  • I would use the tankless after the normal water heater, the gpm at the temp you want will be increased because the tankless is not having to bring the temp all the way up from 40, I would think this would create a more uniform temperature as the tankless would fire harder as the warm water from the tank was used up, other than that I would not think it would make much of a difference.
    – Ed Beal
    Aug 17, 2017 at 2:26

4 Answers 4

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If your 80 gallon tank turns cold at an unacceptable rate, either you have a broken hot water heater (like, a broken siphon tube), or you really, really like high water flow. For instance a lo-flo 1.5GPS shower head, given 2 parts hot to 1 part cold, should last 80 minutes.

I would start by looking at the water flow rate out each faucet (i.e. by sticking a gallon bucket under there and timing it with a stopwatch), and comparing that to best practices for efficient homes. Your problem might be right there.

Obviously you should be able to get the lion's share of 80 gallons of water out of the heater before it turns cold. It may be broken. For instance there's supposed to be a pipe or baffle to assure that incoming cold water is deposited in the bottom of the tank. If this has corroded and is dumping cold water at the top of the tank, your outlet pipe will tend to gulp up this cold.

You will not be able to buy another 80 gallon water tank. New tanked water heaters of that size must be "heat pump" types - they use half the energy but make a lot of noise. Also since they are pumping ambient heat into the water, they chill the area around the heater. Your HVAC system will have to work harder in winter, summer will be a wash.

Another complication with tanked heaters is they must now be kept at 140F or hotter to prevent formation of legionella, the bacteria responsible for Legionnaire's Disease. It was always a problem, but now we know it, so now it is a requirement. This higher temperature means more insulation losses and less efficiency. It also will cause scalding, so it absolutely requires new blending spigots with anti-scald features - traditional 2-knob setups cannot be used. This most likely means a low-flow shower valve.

Tankless heaters are excellent if you can provision the large electrical service. They are actually more efficient, since they only heat the water you use, and only when you're using it. Since they don't have an hour to preheat the water, they must do all their heating on-the-spot, so they need more powerful heaters. But you also don't need to pay them to preheat water for an hour or keep it at temperature. Tankless heaters also don't pay for insulation losses. Local heaters can be moved quite close to the point-of-use, so you are not heating a long hot-water-pipe run - nor waiting for it! However to keep power requirements sane, you must keep flow requirements fairly modest. Since they don't store hot water, legionella is not a factor, so you can heat to the more modest and safer (for scalding) 100-110 degrees F.

Using a tankless and a tanked together makes no sense. Putting the tankless after the tanked is useless since the tank must be kept up at 140 to stop legionella, so the tankless would never run until the tank is exhausted. Putting the tankless before the tanked might slow exhaustion of the big water tank, but probably not by enough to matter, and at a huge energy cost.

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    Do you have a citation for your claim that water heaters must be kept at 140F? I recall seeing a question on here (searching now) that a lower temp of 120F was recommended unless your household had someone at high risk for Legionnaire's. The lower temp is recommended to prevent scalds and burns.
    – mmathis
    Jun 7, 2017 at 14:28
  • Ah, here it is: diy.stackexchange.com/questions/107473/…
    – mmathis
    Jun 7, 2017 at 14:32
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    also osha.gov/dts/osta/otm/legionnaires/faq.html canadasafetycouncil.org/home-safety/… It depends who you ask... government departments only tend to care about their own bailiwick. Safe Kids Canada is all about scalding and isn't worried about Legionella... canadasafetycouncil.org/home-safety/… Jun 7, 2017 at 16:02
  • 120F is what I have been told. Thanks on your comment the 80 gallon tank. It was put in before I took control of the property.
    – Stcks4
    Jun 7, 2017 at 17:41
  • I will have to examine the tank. We are located at 8,500 ft in the Colorado Mts. Gets very cold here very common to be below zero for days to a week at onetime.
    – Stcks4
    Jun 7, 2017 at 17:49
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Turn your 80 gallon up to 150. Sounds like one of the burners is also out. Go ahead and replace both if one is out.

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  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. Jan 22, 2020 at 19:04
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tank less water heater cost a lot of money to heat water up. on a tank water heater the gas pipe going into it is 1/4 of a inch. on a tank less water heater the gas inlet is 3/4 of a inch. so the cost will come into play. as far as hot feeding a tank. the tank you say cools down quickly. you will still have to use energy to keep the tank heated. only reason you need to do what your asking is so you do not run out of hot water. in this case get rid of the tank heater. or you can use a solar heater in witch you do not pay a gas bill for.the time from cold water to hot water will be shorter.so just to cover everything you have hot water and you drain it. the time it takes to heat the cold water up is two long. buy a better water heater. you could put a tank less in front the cost will double. or use solar. or install 2 or 3 or one big tank water heater

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  • Sorry I left off I only have electric power. The tank cools down because the tank at 120 degree water is being replaced with 40 degree water. thats 80 dregrees different. The solar system is dead. I would have to completely replace it. big cost! I will be replacing the tank
    – Stcks4
    Jun 7, 2017 at 4:27
  • I was thinking I could preheat the incoming water to 80 (?) degrees. Then I would not spend the energy and time to reheat the reg hot water tank. Also would I have to Install an expansion tank between the Instant water and the holding tank (water heater Tank)
    – Stcks4
    Jun 7, 2017 at 4:35
  • ya the tank less heater uses more energy no matter what you read on the interwebs. that should be taken into account. to put it inline is pointless. your cost will increase. i would get a bigger water heater to fit your need in hot water uses. but the solar option worked because it did not cost you anything to run.
    – joe
    Jun 7, 2017 at 4:35
  • you are going from cold to 120 degree in seconds. this takes alot of energy to make that happen. your tank heater would still use energy to keep the water temp at 120. so your doubling cost to heat it from 40 to 120 and still spending money to maintain the 120 degree. so the sales pitch of tankless is that you dont have to maintain a tank heater at temp all the time. this offsets the cost high cost of heating in a tank less. to use both is a no go. solar is a good option again because you dont pay to use it. i would look into a bigger tank so it never drains complete while your in the shower
    – joe
    Jun 7, 2017 at 4:41
  • The current solar is dead! Last quote I had was war $5000 replace the solar system. I'm not looking to use the Instant water heater to heat water to 120. I was thinking I can use it to heat to 60 or 80. Which I know will cost more than solar but my up front cost would be under $500 vs the $5,000. The tank now is 80 gallons. This is for a vaction house used on weekends with a lot of water then sits all week.
    – Stcks4
    Jun 7, 2017 at 4:51
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Maintaining temperature in a tank takes far less energy than bringing incomming water up to temperature so using a tankless heater to preheat is not as energy inefficient as claimed. If you have solar panels then an electric tankless preheater will greatly reduce cost of running a gas powered tank heater since most of the energy needed to heat your water will be from free solar energy. In that case the insulated gas powered tank only maintains water at high temperature or augments the preheater during high use. This can reduce gas consumption by a factor of 10.

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    A) The solar system is being removed or replaced, so it's likely to not help. B) Preheating the water will reduce the cost of running the tanked system, but electric tankless are VERY expensive to run and probably will see a net increase in total cost to heat water. C) A Gas tankless might use as much or more gas to gain the temp increase as the tank itself would, since the tankless has to bring it from curb temp to usable temp very quickly, while a tanked heater is designed to heat much more slowly.
    – FreeMan
    Aug 31, 2023 at 14:07

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