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matthet 11-16-2009 01:22 PM

Tankless water heater to pre-heat incoming water
 
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.
The problem is that I run out of hot water in the winter time when everyone is showering in the morning. (Michigan)
I figure I can use a low power electric unit just to bring the water up to 75 or 80F and let the gas heater do the rest.
I would shut off the electric with a timer during the day and during the summer. Seems like it could be fairly economical by using the electric only during high demand and not needing to get a huge replacement gas tank-type heater.
Note: I cannot install a full power natural gas tankless system due to lack of proper venting options.
Anyone have any experience or ideas with this type system?
Just considering it now. There are solar hybrid systems but the sun never shines here in winter.
Thanks

Piedmont 11-16-2009 03:57 PM

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.

stuart45 11-16-2009 04:19 PM

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.

Piedmont 11-16-2009 04:58 PM

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.

Yoyizit 11-16-2009 05:36 PM

Quote:

Originally Posted by matthet (Post 353789)
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.

matthet 11-16-2009 06:02 PM

Quote:

Originally Posted by Piedmont (Post 353862)
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.

matthet 11-16-2009 06:04 PM

Quote:

Originally Posted by Yoyizit (Post 353913)
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.

Yoyizit 11-16-2009 06:12 PM

Quote:

Originally Posted by matthet (Post 353931)
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.

Scuba_Dave 11-16-2009 06:16 PM

Quote:

Originally Posted by Yoyizit (Post 353913)
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 !!

Quote:

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
http://www.accuratebuilding.com/imag...alding_lrg.gif

hayewe farm 11-16-2009 11:43 PM

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.

Yoyizit 11-17-2009 10:06 AM

Quote:

Originally Posted by hayewe farm (Post 354099)
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.

drowitch 12-17-2011 04:03 PM

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! :)

matthet 12-17-2011 11:07 PM

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.

Snav 12-17-2011 11:13 PM

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

picflight 12-18-2011 06:30 AM

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


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