tankless water heater

Old 11-10-05, 01:55 PM
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tankless water heater

Does anyone know if boosting the temp of the incoming cold water feed by feeding the hot water output back into the cold water input using a mixer valve (like the ones used for toilets) and a couple of check valves can save enough energy to make it worth while?? I don't imagine that you'd have to mix much hw to get the input temp up enough (maybe 10-15 degrees)to save some energy/bucks. I am using natural gas.
Old 11-10-05, 02:20 PM
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shubuti, Welcome to the DIY Forums.
I am not sure exactly what you are asking. If you install a tankless heater, it STILL uses energy and the initial cost of the tankless and installation will be high. Also, many plumbers will not service tankless heaters so if something needs repair it will be done by a special plumber. It would probably take years to recoup what you MIGHT save by doing this. Good luck.
Old 11-10-05, 08:25 PM
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Either way, you're still paying for the heat that was imparted to the water.
Old 11-11-05, 03:25 AM
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tankless water heater

I'm heating up the water to , let's say 110. Why can't I feedback a minimal stream of that with a mixing valve to raise the incoming water temp. If the incoming water is warmer then the heater won't have to work as hard to get the temp up to 110. Initially it has to heat from, let's say, 40 to 110 degrees. With a feedback it might then only have to raise the temp from 45/50 to 110. I wonder if there is enuf savings in fuel cost to make it work.
Old 11-11-05, 04:47 AM
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Hello: Shubuti

The theory sounds logical and maybe practical. But it does not work nor save any money in reality. The water still has to be heated using a fuel.

Only cost savings proven to work, after intial installation costs, method to preheat the water not using any fuel energy is solar as a preheating method. And even this method has yet to be proven beyond a doubt in many areas of the country using an instantaneous water heater.

Much depends upon the hours of daylight in any given area and suns light and heat intensity radiation in the area. Water storage is also required when preheating cannot be done during hours of no sunlight. Plus the costs of circulation, etc. Overall, proven not to be very cost effective.....

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Last edited by Sharp Advice; 12-11-05 at 09:38 AM.
Old 11-12-05, 08:39 AM
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Your suggestion is ludicrous on the face! By your thought process you could eventually end up with steam coming out and it would be free!.

You have to heat the water to the temp you want, and whether you heat it in one pass or two passes thru the tank, you are still paying. You also did not take into account the reduced volume of hot water available, if some of the heated water was being recirculated through the tank.

Sorry to be harsh, but some ideas just need to be nipped in the bud!
Old 11-15-05, 02:12 PM
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oh ya...perpetual motion...

where is the small stream of heated water getting its heat from? The boiler. Now the boiler has to fire up again because it must maintain a minimum boiler water temperature throughout the year. If you want hotter water, adjust the mixing valve, that sucker will burn the tar off of you if your not carefull...in a nutshell anything over 115F I think is a waste of fuel. If conserving fuel is the goal, lower it to 105 or 110F. There is no free lunch!
Old 11-27-05, 11:06 AM
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Not sure what tankless heater you mean, and they do differ some in how they work. I don't recall much about the Bosch, Takagi or other heaters

I have the Norwitz, so I can comment on it. It's gas intake is based on the amount and temp of water outflow. It also takes into account the inflow water temp. Since you are using some of the outflow to heat your inflow you are making the heater increase it gas use to maintain the water outflow.

Hope this helps

Last edited by StickStan; 11-27-05 at 11:23 AM.
Old 12-10-05, 09:06 AM
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No, you will use more energy. The rate of heat transfer (such as from the flame side to the water side of the heater) is directly proportional to the temperature difference. It takes X BTUs of heat to heat a given volume of water from lets say 50F to 110F, whether it be in one pass or two. For two passes, the temperature difference on the second pass would be a lot less. Less heat is extracted from the flame and more goes up the chimney, never mind that the water is not going to want to flow because the pressure drops as it passes through the heater.
You could preheat incoming water using solar energy or taking waste heat from plumbing drains, but the payback would take forever.

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