# Tankless water heater to pre-heat incoming water



## Piedmont (Nov 1, 2007)

I would personally replace the gas tank with electric and call it a day. The problem with gas tanks is, that flue that transfers the heat into the tank also transfers it out. So, although they can be 90% efficient they are also very efficient cooling down. 

I originally had a tankless oil boiler which I was turning it on only when needed (mine actually had a water tank inside with about 5 gallons inside that heated fast but also cooled just as fast so it would cycle on/off). Turning it on/off only when needed I cut my oil bill down to $35/month for just hot water. However my wife was not happy having to keep turning it on/off. 

I got an electric hot water tank and even at $0.22/kW my electric bill went up only about $30/month (it was cheaper than my tankless oil being turned on/off only when needed) and I had hot water whenever I needed. I figured out there is virtually no loss with electric tanks, they are sealed units with no flue cooling things down and very well insulated. I just recommend you get a good insulated one. 

You mentioned solar. I added solar to it later, I live in an area where 2/3rds of the days are rain or cloudy. You get a bigger tank (one sunny day here and my solar will heat the tank enough to coast through 2 days of clouds & rain in Spring - Fall). In winter one sunny day will heat enough for one day. Sure it doesn't provide 100% of my needs (mostly in winter) but skipping it for that reason is like needing $20 and along your way find $10 and you skip it because it's not the full amount. I really want more panels and hook it into my heating system for the same reason.


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## stuart45 (Jun 20, 2009)

Out of interest, is the indirect system of heating the water from the central heating system popular in America? It is seen as the most efficient method here.


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## Piedmont (Nov 1, 2007)

My opinion is, I would say the advertisers here push tankless as being the most efficient and try to fool people into buying tankless heaters for their hot water use advertising no standby losses, use a boiler seperately for just heating. People here are not taught a boiler doing double-duty (heating & Hot water) is more efficient, and an indirect system even more efficient (since it doesn't have a flue in the tank). I know of 2 neighbors who threw out their indirect heated water tanks & boilers and went tankless. I went the other way, I hated the tankless route and went indirect. 

To answer your question, in my opinion they really push tankless but I think the indirect hot water system IS the most efficient because there's practically no standby loss AND the boiler is doing double-duty so it's efficiency is highest especially in winter (when you need it most). But, I would say it's becoming less & less common.


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## Yoyizit (Jul 11, 2008)

matthet said:


> I run out of hot water in the winter time when everyone is showering in the morning. (Michigan)


With 2 gpm showers for 10 min. ea. you need 15 gals. of 120F and 5 gal of 40F water to give you 20 gals. of 100F water.

You could turn up the WH to 160F. Then for 20 gals. of 100F water you only use 10 gals. of 160F water.

Formulas available on request :laughing:

Doesn't work on paper? Certainly won't work in reality.
Works on paper? Still may not work in reality.


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## matthet (Nov 16, 2009)

Piedmont said:


> I would personally replace the gas tank with electric and call it a day. The problem with gas tanks is, that flue that transfers the heat into the tank also transfers it out. So, although they can be 90% efficient they are also very efficient cooling down.


Thanks for the advice. I considered going electric only with a big tank but I'm trying to keep some part of the system gas fired for when the electrical power goes out. 
I also have considered a small electric tank type water heater in series with my gas one. But it seems like it will take up too much space.


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## matthet (Nov 16, 2009)

Yoyizit said:


> With 2 gpm showers for 10 min. ea. you need 15 gals. of 120F and 5 gal of 40F water to give you 20 gals. of 100F water.
> 
> You could turn up the WH to 160F. Then for 20 gals. of 100F water you only use 10 gals. of 160F water.


It's already cranked up as high as it will go. I think about 120F. That's the first thing I did.


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## Yoyizit (Jul 11, 2008)

matthet said:


> It's already cranked up as high as it will go. I think about 120F. That's the first thing I did.


It takes 12 kw to preheat heat your water from 40F to 80F @ a 2 GPM rate. At 240v this is 50A.

Since your average hot water needs are much smaller than these peak values, I'd get another WH and use it just to store preheated water, so you don't need new elec. wiring.

First figure out your total hot water needs at peak times.

You lose about 50w from heat loss through the WH insulation. Elec. heaters are almost 100% efficient, gas are 60% but with NG being cheap per Therm you might still make out with a gas heater.


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## Scuba_Dave (Jan 16, 2009)

Yoyizit said:


> You could turn up the WH to 160F. Then for 20 gals. of 100F water you only use 10 gals. of 160F water.
> 
> Still may not work in reality.


Should read DON'T DO IT !!



> *Most adults will suffer third-degree burns if exposed to 150 degree water for two seconds*. Burns will also occur with a six-second exposure to 140 degree water


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## hayewe farm (Mar 15, 2009)

Why not just add a big plastic holding tank in front of the heater. Put the inlet on the bottom and take from the top. Just sitting there most of the time it will warm up to room temperature. If you ever add a solar heater you could then put the heat exchanger coil in the plastic tank.


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## Yoyizit (Jul 11, 2008)

hayewe farm said:


> Why not just add a big plastic holding tank in front of the heater.


And if you want it to warm up faster use a very long tube instead of a tank. More surface area = a better heat exchanger.


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## seefut (May 4, 2010)

*Solution?*

Hi Matthet,

Did you ever try out this tankless idea? I have the exact same problem at a ski vacation home in the mountains which is empty most of the year. But, when there are 12 people there over a weekend, last few people get cold showers. For this case, I really don't care about energy efficiency, just want lots of hot water and minimal changes. 

My water heater and furnace are in the same closet, so maybe a simple holding tank placed near the furnace would do the trick vs the tankless preheater. I have heard of both options before, but have yet to hear anyone that has actually tried it out and has some feedback on how well it works.

Please let me know what you ended up doing... Thanks!


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## matthet (Nov 16, 2009)

Sorry, I took the simplest cheapest way out. I limited both my daughter's showers to 15 minutes each by having them use a timer. After a bit of fuss they got used to it and I didn't have any more hot water problems.
:thumbup:
Not very interesting for a DIY site but that's what happened. Every solution I thought of had too many complications. If I was to actually do something now I'd probably use the passive storage tank to bring the incoming water to room temp. Everything else seemed to complicated to just bring the water up 20 or 30 F.


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## Snav (Aug 20, 2009)

Tada! Love it!

I was going to say "pre heating your water before it enters the tank will cause serious issue with your tank itself - they're designed to heat cold, not hot water"

. . . and for the extra electrcity and fuss I would suggest just installing a larger tank

Here (with 6 in the family) my husband showers first in the AM


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## picflight (Aug 25, 2011)

Water is precious, limiting shower time is an excellent solution.


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## Chemist1961 (Dec 13, 2008)

*Power Pipe ****Drain Water Heat Recovery*

Here's your answer. Look up Powerpipe. It's made about 5 minutes from my house in Waterloo ON and sold through Home Depot, Sears and independents. It uses a 3-4"copper replacement insert up to 7' long for your drain stack with a heat recovery transfer coil wrapped around it. So it is passive but working with demand. When you are showering the first shower drain water preheats the next load of inbound water via the wall of the drain stack before it gets to the heater. I have a 50 US gallon heater and 3 teenage boys. We use low flow, but very good quality shower heads, but the boys sometimes shower for 20 minutes each or more. 
So in the morning the total continuous shower time is at least 45 minutes, sometimes with 2 showers running at once. We NEVER run out of hot and there is plenty of pressure in my 35 year old plumbing. Had the system in for 2 years and the gas consumption and water use has dropped. They say it can save between 15 and 50% of your energy cost but hard to peg since I have done a furnace upgrade as well.
To maximize savings I also wrapped my water heater in a insul blanket and wrapped the Power PIPE. This is great permanent solution if you run the shower a lot back to back. Doesn't help as much with a bath tub user or laundry though. The unit is best suited to homes with a traditional tank type heater, electric or gas. I installed my own and did one for my neighbour as well. It comes with 2 Fernco sleeves and I bought Shark Bites just to try. VERY slick install, in a litlle over an hour but I have run a lot of pipe in my time.!!! :thumbsup:


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## AllanJ (Nov 24, 2007)

Yoyizit said:


> And if you want it to warm up faster use a very long tube instead of a tank. More surface area = a better heat exchanger.


Dissect some baseboard forced hot water radiators and use the finned pipe (a couple dozen feet worth) to run the cold water through before reaching the inlet of an ordnary water heater. Mount this zigzag network on the wall, or if there is enough headroom, a few inches below the basement ceiling.

This is a passive system, using the room air to preheat the water.


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## plummen (Jan 10, 2010)

Snav said:


> Tada! Love it!
> 
> I was going to say "pre heating your water before it enters the tank will cause serious issue with your tank itself - they're designed to heat cold, not hot water"
> 
> ...


Please enlighten me on the issues of preheating water :wink:


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## AllanJ (Nov 24, 2007)

plummen said:


> Please enlighten me on the issues of preheating water :wink:


When the "cold" water enters the water heater at a higher temperature then the water heater does not have to raise the temperature as many degrees and will then bring the tankful of new water to the desired "hot" temperature more quickly.

This means one person can take a shower and it is a shorter wait before the next person can take a shower. But it does not guarantee that the water will stay at a constant temperature longer if people take showers in quick succession; whether it does is unpredictable.


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## Chemist1961 (Dec 13, 2008)

As stated above, so with the Power pipe the continuous flow of hot down the drain pre-warms the inbound supply to the water heater as long as there is demand at the shower head, and flow down the drain versus a long wait time for a room temp warming reservoir....


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## plummen (Jan 10, 2010)

AllanJ said:


> When the "cold" water enters the water heater at a higher temperature then the water heater does not have to raise the temperature as many degrees and will then bring the tankful of new water to the desired "hot" temperature more quickly.
> 
> This means one person can take a shower and it is a shorter wait before the next person can take a shower. But it does not guarantee that the water will stay at a constant temperature longer if people take showers in quick succession; whether it does is unpredictable.


i understand that,im just trying to understand why she thinks its a bad thing for the water heater


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## Snav (Aug 20, 2009)

Thermal Expansion - when stringing two heaters together this is a potential hazard.

This is why a 2nd tank near the faucet in question - or a recirc pump is used instead of doubling up the water heaters. If a recirc pump is used it shuts off to prevent hot water from entering the tank.

When people install two heaters for a home there's a proper way of doing it to preven Thermal Expansion from happening.


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## Jackofall1 (Dec 5, 2010)

Snav said:


> Thermal Expansion - when stringing two heaters together this is a potential hazard.
> 
> This is why a 2nd tank near the faucet in question - or a recirc pump is used instead of doubling up the water heaters. If a recirc pump is used it shuts off to prevent hot water from entering the tank.
> 
> When people install two heaters for a home there's a proper way of doing it *to preven Thermal Expansion from happening*.


HMMM........I thought that is what expansion tanks are for....yes....no?

The only time you would experience a thermal expansion problem is if you heat the water too much, preheating the water would have no effect. Thermal expansion happens you can't change the physical properties of water, the design a system has to account for this and it really has nothing to do with multiple heaters.


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## plummen (Jan 10, 2010)

Snav said:


> Thermal Expansion - when stringing two heaters together this is a potential hazard.
> 
> This is why a 2nd tank near the faucet in question - or a recirc pump is used instead of doubling up the water heaters. If a recirc pump is used it shuts off to prevent hot water from entering the tank.
> 
> When people install two heaters for a home there's a proper way of doing it to preven Thermal Expansion from happening.


So I guess I can take those 3 differant master plumbers licenses and run them through the shredder since ive been doing it wrong for close to 30 years! :laughing:


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## Snav (Aug 20, 2009)

Jackofall1 said:


> HMMM........I thought that is what expansion tanks are for....yes....no?
> 
> The only time you would experience a thermal expansion problem is if you heat the water too much, preheating the water would have no effect. Thermal expansion happens you can't change the physical properties of water, the design a system has to account for this and it really has nothing to do with multiple heaters.


Ah - that makes sense. Thanks.



plummen said:


> So I guess I can take those 3 differant master plumbers licenses and run them through the shredder since ive been doing it wrong for close to 30 years! :laughing:


But yet jackofall make quick time to explain things to me and you did not. Maybe you should consider using all those years of expertise on a DIY forum to kindly educate people when they're wrong instead.

Do you agree with what he said, then?


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## plummen (Jan 10, 2010)

I was waiting for your response,thought maybe I was missing something! :laughing:


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## plummen (Jan 10, 2010)

You could just use a recirc line with a check valve back into the bottem of heater instead of a recirc pump,but thats more for a distance issue waiting for hot water to get to a faucet than a volume of hot water issue.
Doesthat make sense?


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## Snav (Aug 20, 2009)

plummen said:


> So I guess I can take those 3 differant master plumbers licenses and run them through the shredder since ive been doing it wrong for close to 30 years! :laughing:





plummen said:


> I was waiting for your response,thought maybe I was missing something! :laughing:


So I take it that you agree with what he said about it?


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## plummen (Jan 10, 2010)

Yes.:thumbsup:
Another way is to bring the supply down directly between 2 water heaters and plumb them both with identical length pipes to feed them then tie the outlet side of both together using same length pipes.
This will give you double the hot water over a single unit at any given time if you use a lot of hot water at any given time,but youre also keeping double the water heated all the time which is more expensive compared to just preheating the water to shorten the recovery time in the first set up we talked about


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## dehv (Jan 1, 2017)

Zombie thread, but still for future viewers:

Why not put the tankless AFTER the existing tank to boost?




matthet said:


> I'm thinking about solving my hot water problem by making a hybrid hot water system.
> Concept is to use an electric tankless heater to pre-heat water going into my gas tank-type water heater.


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## supers05 (May 23, 2015)

dehv said:


> Zombie thread, but still for future viewers:
> 
> Why not put the tankless AFTER the existing tank to boost?


You want the newer gas condensing heaters on the coldest water. 

If you don't have a condensing heater, it doesn't matter so much. (Condensing gives you about upto 10% of your efficiency.)

Cheers!


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## dehv (Jan 1, 2017)

I also found this PDF explaining a few ways to get rid of the cold shock effect using a small tank and thermostat to trigger a circulating pump.

chandlerdesignbuild.com/files/fhbDecJan08.pdf


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## Ghostmaker (Mar 2, 2013)

That is called a Navy Shower!!!


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## supers05 (May 23, 2015)

Ghostmaker said:


> That is called a Navy Shower!!!


Is that when they wrap a rope around your chest and throw you over, and tell your to climb back up when you're done "showering." lol

Cheers!


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## dehv (Jan 1, 2017)

I wonder if one couldn't also solve the cold shock problem with a 2 gallon expansion tank tee'd into the outlet of the tankless?


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## supers05 (May 23, 2015)

dehv said:


> I wonder if one couldn't also solve the cold shock problem with a 2 gallon expansion tank tee'd into the outlet of the tankless?


Not expansion tank, buffer tank, if your heater doesn't already have one. (the ones I install do.) You need a recirc pump to keep it warm, or at least very well insulated. (which is more typical of the small buffer tanks) 

Cheers!


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