peheating hot water. does it really out weigh the cost?

[LanceA0]

ive been doing a bunch of searching on here and i still dont understand how you can gain that much by preheating your water to your hot water tank.  i know that it will help with the initial heating of the water as it goes into the hot water tank. but once the water passes the heat exchangers and is in the hot water tank sitting there on call the preheating of the water is useless. it seems to me that you would have to using alot of hot water to make this worth while.

i have a 40 gallon natural gas water heater. it is only me living in my house. i wash my clothes in cold water, and rarely use hot water other then washing my hands and my daily shower. so the same water might be sitting in my hot water tank for 1-2 days before completely cycles the original water out. how would preheating my water to my H.W.T. be worth the cost to build a system to do it.

im not knocking the idea at all im just looking for info on how it really works and how it might benifit me.  

thanks

Lance

[spinningmagnets]

If you buy a factory unit and have a handyman install it, the cost/benefit would be way out of whack.

If you scrounge most of the parts for near free and do the labor yourself, you're out just a few bucks. Maybe you also had some fun and learned a few things?

An off-grid home that heats water with propane and has lots of kids who take many hot baths should jump all over this right away!

Whether or not its worth it is up to each individual situation.

[zeusmorg]

 Basically, you would be saving the btu's needed to heat the source water to tank temperature, you would not have any savings in the water being re-heated in the tank,that loss would remain the same.

 So it your incoming water is 54F and your pre-heat system raised that to 95F, then you would be saving 41 btu's of energy per gallon used.

 Now to figure how much that would save you, you would have to determine the cost of your fuel used and it's end efficiency.

 You are not going to see a savings of heat loss in the tank, just a gain per gallon of incoming source water.

 So the pre-heating of the water is NOT useless, you've saved the energy it would use to initially heat it.

 Now it would depend on the cost of the system you use, vs the cost of the energy you would save. Some schemes have a much higher cost to payback ratio. For instance in your case, recovering the waste heat going down the drain while you take a shower would probably be worthwhile.

 

[LanceA0]

thanks for the replies. what im tring to do is come up with a way to almost comepletely eliminate the use of my natural gas hot water tank(other then a storage tank). this would help with doing that, so i will probably do this too.

spinningmagnets:

i would be the one doing all the work to make and install this system. i dont like paying anyone for something i could do myself. off grid- i thought that was having no ties to outside electricity, or fuels to run you home? everything runs off RE.  

zeusmorg:

what i ment by useless is like you said it will not be saving any more money once it is in the tank. the cost to keep that water hot wil still be the same. not that the the whole thing was useless. i see that there is a savings there, but wasn't sure it would be enough to justify to cost of ME making one (longer payback) because im not using a lot of hot water.

recovering waste hot water. the heat exchanger would have to right at the shower drain right?  because my shower is on my 2nd floor and there is VERY little room to work in there. the drain makes a 90* turn as soon as it starts then jogs about 18" to another 90* which then lets it fall 1 story completly straight down where it then attaches to my main line(in basement). would i see much gains if i put the exchanger right before it enters the main line?

[GaryGary]

Hi,

The heat to keep the water hot after its already been heated up are called standby losses.  Standby losses are high for regular gas heaters because they have that big flue going right up through the middle of the tank which makes for lots of heat loss.  Its a really bad deal for you in that your tank is much larger than your daily use, so you are using more gas to keep a lot of water you won't be needed for quite a while hot.

One way to get rid of the standby losses is to use a tankless gas water heater.  These heat the water on demand in a single pass, so no standby losses. So, the water could go right from the preheat tank to your tankless heater -- the tankless heater adds only as much heat as it needs to to reach the temperature you set it for.

I use one with my solar water heating system, and it works fine -- if the solar tank is not hot enough, the tankless makes up the difference -- the solar tank is already hot, the tankless does not turn on at all.

On the recovering heat from shower water, GFX style units just need a vertical stretch of drain pipe that is about 6 ft tall -- it can be anywhere in the drain:

http://www.gfxtechnology.com/

Gary

[LanceA0]

so i was washing my hands today before lunch and i decided to turn my fauset to cold and see how cold the cold really was. its really cold.. so i guess i never really realized it but there probably some good savings by preheating the hot water.

gary it seems like every 2-3 days im on your site reading for hours. today was one of those days. lol.  i read up on your $1000 solar hot water heating system. i was acaully going to e-mail you with some questions about the system.

yeah i was looking tankless water heaters when i purchased mine in may. i had just bought my house and my funds were tight(still are). knowing what i know now and what im tring to accompish with this house i wish i would have spent the extra money on one. but its really not that bad in the summer my gas bill is $30 a month. and thats for the hot water heater, stove, and dryer(have a clothes line now). but i would still like to reduce that gas bill even more.

im thinking about taking your $1000 S.H.W.S. and modifiing it some. im not sure if it will work or not but im just throwing it out there.  what if i took and added a copper coil loop that goes around my hot water tank to help keep my standby loss down. so the loop would go like this. solar panel to hot water tank coil to storage tank then back to solar panel. the system would probably have to run a little more then normal but i think it would help with stand by loss for those that dont have tankless heater and still preheat the water the cold water.

thanks for the link to the GFX. ill have to look into that when im remodeling my basment stair walls. the shower drain runs down that wall and i would have access to 6' or more of drain pipe. that however looks like something i could probably make myself to help save some money.

[scottsAI]

Zeusmorg,

FYI:

The calculation is (95 - 54) * 8.33 lb/gal = 341.53 BTU/gal of water.

Else it's 95-54 = 41 BTU/lbs of water.

Have fun,

Scott.

[scottsAI]

LanceA0,

off grid - No electrical, no ties to electrical grid.

All have propane, gas, fuel oil, wood, or some form of backup electrical and heat.

Most heat with some form of fuel few with Solar heating.

Have fun,

Scott.

[wdyasq]

Sure am glad I didn't get caught blowing the math ....

Heating water with homebuilt solar is one of the few areas where a person's time and money are very well spent. As far as I know, it has the best return on investment one can make.

Ron

[scottsAI]

No big deal wdyasq!-)

We watch each others back to get it right!

Agreed about the solar hot water.

Have fun,

Scott.

[fcfcfc]

Hi:

With the extremely low system load created by just you, a single tank SDHW solution would be most effective. It is very possible that your standing BTU losses exceed your initial water tampering loads because of your very small usage. In a single tank solution, the bottom limit in tank temperature is set by the backup energy thermostat. Solar energy starts heating from that point, lets say 110DegF for example. You install an anti-scald mix valve to control the output to the fixtures, but the tank can get very hot, I.E.180 DegF. As long as the sun keeps you tank hotter than 110DegF, your standing losses are no longer supplied by conventional energy but by solar energy. Additionally, when the tank is hotter than 110DegF, the incoming cold water is heated as well and may or may not cause the conventional heating source to turn on, depending on the tank temp and the amount of new water drawn in. This also has a very low front end cost since no additional tank is needed. It does not allow the solar to run in the most efficient mode but that is not the goal. The TRUE goal of any RE system is not efficiency but the most reduction in backup energy. Despite common since perhaps, those two paths do not always wind up in the same place....

Good luck...

.....Bill

[Catch66]

Hello LanceA0,

I built and installed a Preheat tank system.

last fall, mid of August,On roof.

I have kept a diary through the winter as we Live in Montana. My

preheat tank is Electric 50 gal.(our main Indirect 40 gal tank is heated from a hot water boiler for home)  weather outside was -20 degrees below F

we had A lot of reflection from snow but,with clear sky's. the system raised the 50 gal.temp in tank 9 degrees.

I spent around $1400.00 on my system that includes 12 v pump and PV panel cost 2 flat plate heat exchangers also. Copper prices were high when I built it. my glass was free.I couldn't justify twin Poly as I considered it,till I saw the price.

the statement earlier in post about natural gas fired hot water tanks is

very true,the flue is not insulated and heat rises right up the stack from your heated supply constantly 24/7

.

Catch66

[fcfcfc]

Hi:

9 DegF after a full sunny day?

How many square feet of panel???

How much hot water was used over that sunny day???

What was the starting and ending temp???

How are your panels constructed???

What kind of heat exchangers???

.....Bill

[valterra]

"You are not going to see a savings of heat loss in the tank, just a gain per gallon of incoming source water."

Also for those of us who like to take long showers, doesn't this mean that when the hot water starts "going out," that is doesn't get so DARNED COLD right away?  

You'd have 95+ degree water flowing into the tank and mixing around with your gas-heated water.  To me, that seems better than of 60- degree water pouring in and ruining your shower.

Is that right?