Water Preheater...any thoughts?

[jt]

I've been lurking around here and have been intrigued for years about wind power but just don't live in an area where it is viable.  PV is expensive and doesn't excite me.  I think I have come up with something for preheating water before it enters the water heater and thus ease the electricity draw of the water heater.  The siting of my house does not make a solar preheater or batch heater a viable option.  We do have a heat pump which has been running alot lately with the hot weather we have had.  My plan is to find a 20 gallon (or so) water heater tank and plumb it into the cold water intake side of the electric water heater.  This tank will be put on a frame suspended above the compressor unit of the heat pump where right now, air is blowing out of there at approx. 102 degrees when it is runnning.  This will preheat the water before it is sent to the water heater and theoretically cut the amount of electricity needed to increase the water temp from it's input temp (approx 78 degrees right now)to the 115 degrees it is set at.  Also, with a used water heater tank, I have a 300W DC heating element which I guess could be threaded in and hooked to a PV cell to further the possible heating I can gain.

I am obviously an amateur at this and don't expect high efficiency and I'm sure there are better, more efficient ways of heating or preheating water but I'm looking at what I can do within my capabilities and at a low cost.  My view is that every degree temp rise I can get with such a setup, I have gained water heating efficiency by being less dependent on the utility.  Also, this is a system that will obviously not be useful in the winter (when it would be needed the most since water comes into the house at about 42 degrees in the dead of winter) since the heat pump will be working in reverse.

Just wanted to see what anyone's thoughts are on this.  Seems almost  too simple but with a house in the trees and little available sunlight without a long run of plumbing, I have been looking for alternatives.  Let me know if there is anything I can do to improve on this idea.

Thanks,

Jim

[orochi8]

Jim:

If you are looking for ways to easily improve your energy consumption you might simply look into replacing it with one of those on demand tankless water heaters. From what Im told they are considerably more efficient, and due to a recent spike in popularity have come down to a very reasonable price in the last several years. I've seen them go for as low as $200 on ebay, so it may be well worth your while to look into.

-Orochi

[georgeodjungle]

it will work.

water comes out of the ground we will say at 45 F.

even just a loop around inside or just a tank in the house at 70.

thats 25 degrees less you've got to heat.

 shure there's other ways but every lil thing adds up.

p.s.

the 300w element didn't work to good for us as a stand a lone,

the 12v 600w worked o.k.

and two "upper and lower" 12/120 volt 300/500 watt elements works a lot better.

but not as good as the the stock 240 volt.

don't know about the PV cell.

as for the wind, it works for us.

[kitno455]

have been thinking about this myself. a smooth sided glass heater tank wont pick up much heat from the stream of air. you need more surface area. metal tank with sheet metal fins on it. it will tend to lose heat easier at night, however, so you might want to put a collar around it, so that air moves up around it, and a counter weighted insulated lid that opens when the heat pump is on.

i've always though i would like to have the waste heat from my fridge do this...

allan

[ja2tn]

JT,

You would probably be better off to make a couple of loops of copper tubing around the inside of the outdoor coil and even around the compressor shell itself rather than place anything above the fan of the heat pump.  Almost anything that you place in the discharge stream of the outdoor unit is most certainly going to obstruct the air flow from the unit, thus lowering the efficiency of the heat pump.  Manufacturers suggest at least five foot clearance above the discharge.

However, the real heat will be in the compressor shell.  This I can attest to from experience as for seven years I owned a heating & air business and am a federally licensed universal HVAC technician.  You can coil copper tubing around the shell in direct contact with the compressor.  If your compressor has a thermal blanket covering it [like some scroll compressors] you can remove the blanket, put the tubing on, and then put the blanket back in place [it is mainly for sound deadening but the SEER and HSPF ratings of the unit could change without it].  You can then run a 12v solar powered pump hooked to a t-stat as a circulating pump.  Now run the tubing from your water source to a heat exchanger or holding tank and on to the inlet to your water heater.

Now, I have never seen this but feel that it would be better than trying to extract the heat from the discharge air, again because any savings you might gain would probably be lost due to lowering the efficiency of the heat pump/air conditioner.

Hope this helps!

Good luck... Keep us posted!!

Jerry

in the mountains of Tennessee

[Phil Timmons]

Suggestion regarding mixing water-tube heat exchangers in existing, functioning equipment --

A good general practice is to do no harm . . .  We should try to make things "fail safe" rather than "fail sorry."  Meaning that if something goes wrong it should not cause additional, preventable, damage.  Putting modifications on equipment using pressurized water systems can cause a lot of damage if their is a leak.

A method to use a water heat exchanger in a "fail safe" way is an idea I have kept around from an old professor who had worked on old Mainframe Computer designs.  He said they used water heat exchangers (such as the coil around the compressor above) to cool the internal portions of the Mainframe Computers -- but they used if on a vacuum system.  That is -- water was pulled or drawn through the system rather than being pushed, pressurized, and pumped through the system.

This meant that if a leak occurred, it would only draw air into the system, rather than spraying water into the computer -- it would "Fail Safe."  I have put that design into place in printing and baking systems that require a cooling section, as well.  If you wish to monitor the system, a drop in the vacuum may indicate a leak.

[DERFMOOSE]

   some time ago I saw a articil in how a guy heated his pool with aircon heat

some kind of a heat exchnger that the water flowed through.

think it was in pouplar mach. or mi

[jt]

I had previously given some thought to restricting airflow above the compressor unit and have experimented a little to find the position that gains the most heat while giving the least resistance to airflow.  The exhaust of the unit (grill) is 34 inches and I am looking a putting a tank which is about 15 inches wide suspended about 20-24 inches above the grill.  While I am certain that it will restict airflow a little, it seems that air should up and around the tank pretty well.  I am also considering putting a collar or baffle around the top of the tank to keep as much warm air around it as possible but again, this will be subject to experimentation to make sure that air flows freely up from the fan.

Thanks for the input...hope to have it up and running in a couple of weeks and I'll post results (of course by then, we will be cutting back on AC use for the year).

Jim

[Rowan Clearwater]

I may me a little off track here, but are you using the waste heat from this water after it is used, ie recovering the heat from the waste pipe of your shower/sink?

Replacing a 3ft length of plastic waste pipe with a brass/copper pipe and wrapping 4 runs of 8mm pipe soldered around it, should provide a benefit on thermostatic showers or even the input to your tank.

Nothing comercially like this available yet in UK, anything near you?