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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.

  • 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? – Jim Stewart Jun 7 '17 at 9:43
  • Related: diy.stackexchange.com/questions/60840/… – mmathis Jun 7 '17 at 14:30
  • 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 '17 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 '17 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 '17 at 2:26
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If your 80 gallon tank turns cold at an unacceptable rate, either you have a broken hot water heater, 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.

  • 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 '17 at 14:28
  • Ah, here it is: diy.stackexchange.com/questions/107473/… – mmathis Jun 7 '17 at 14:32
  • 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/… – Harper Jun 7 '17 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 '17 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 '17 at 17:49
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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

  • 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 '17 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 '17 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 '17 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 '17 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 '17 at 4:51

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