simple pre-heating for HWH

[birdhouse]

hey all-

i'm always bummed out that my electric (old) hot water heater has to suck in fifty some degree water all the time.  i've read about drain back solar systems, and glycol systems, but think i may have fallen onto something easier, and way more simplistic, though not as efficient. 

here's the deal:  (pick on it, that's what i'm looking for)

cut into your main where it feeds the HWH, run 3/4" up to your attic off the feed to your HWH.  connect that to a 300' roll of 1.25" pex that is strapped and evenly distributed above the ceiling joists.  hook the other end of the 300' of 1.25" pex to 3/4" pex back down to your HWH.  then blow in a few feet of fibreglass insulation (yieldingr-60 and you know we all need it anyways...  or am i the only one with a poorly insulated attic?)  (r-49 is code in oregon now FYI) 

this 300 foot of 1.25" pex would hold just over 18 gallons of water, in your attic, under a bunch of insulation.  i think the freezing potential would be minimal, as pex has great resistance to bursting under freezing conditions, plus it is insulated, and most of the heat lost in a home is through the ceiling or windows. 

i just feel like the idea of having an almost twenty gallon reserve of not frigid water that is stored in an unusable space for less than $400.00! 

if one was to put in the effort, they could run the 300' roll against the sheet rock of the ceiling, with strapping,  to get better connection the the heat moving through the ceiling. 

so poke holes in it!

adam

[Boss]

I like it,
Anyway, devil's advocate, I wonder how much water one  shower uses, maybe it doesn't matter the 18 gallons might take the edge off the cold water shock on the HWH enough to make a difference on the heating bill
I always wanted to do something like this except I was going to wrap copper tubing around our wood heater stove pipe, but the more I thought about it, I worried that if it could remove enough heat from the pipe to heat the PEX in our floor, it might actually cool the stove pipe too much for smoke to rise.
Probably just a case of over-thinking and lack of doing to find out 

[willib]

Just thinking birdhouse , that the cold water may cause condensation to form on some of the pex .
It may not ,but it might.
Just saying that it might not be a good idea to put it under the attic insulation is all.
I do think it would work though.
and you would have four or five months of free heat till you would have to worry about freezing temps..

[Volvo farmer]

I think to have free heat, you would need to run the pipes in your neighbor's ceiling.

[TomW]

birdhouse;

Might be worthwhile to add isolation, bypass and drain valves if it has any chance of freezing. If it freezes up you will be without hot water you see.

All depends on your location / climate. Southern US likely not going to actually freeze. Up here at 43 north we get some serious cold so anything not in a heated space will freeze.

I agree with willib on the condensation. That could rot your ceiling joist / drywall and cause a lot  of long term issues.

Just my thoughts.

Personally, I use a solar preheater, just an old water heater tank in a box with a glass cover in the yard. It is only active in summer but it provides most of the BTU needed to heat our water. It uses bypass and isolation valves as well as as way to drain the system for the arctic weather times. Well worth the effort to build and hook up. Typically in July we see 120 F coming into the heater and the electric just tops it off.

Good luck with it but be aware of the "gotchas".

Tom

[wooferhound]

I agree with Volvo Farmer.
It will work in the summer, but in the winter you would be cooling your house with the water you use.

[birdhouse]

thanks for all of the criticism!  really!

in my climate, i doubt it would freeze, i live in portland, or and it rarely gets below freezing, and if it does, i'm heating my home and the heat loss through the ceiling should easily keep the pipes from freezing.  bypass and drain valves are definately a good idea.  i don't know what in insolation valve is, but it's probably a good idea too. 

i think i read somewhere that the average shower uses around 15 gallons of water.  a short one uses 10 and a LONG one can use up to 30 gallons of hot water.  i tend to shower pretty quickly, and don't like super hot water, so i'm guessing i'm in the ten gallon range. 

i had not thought of the condensation issue.  i'm not sure if it would condensate or not.  attics do tend to be about the driest place in a home.  especially under a heavy blanket of insulation.  i don't think it would be an issue, but don't know as i haven't tried it.  i'll bet a layer of .6 mil. plastic sheeting stretched un top of the joists would do to keep the moisture from rotting the joists. 

my home is old and framed with beautiful doug fir.  i've worked on older homes where the doug fir sill plate (the board in contact with the foundation) has lasted almost one hundred years with minimal rot.  i think the minimal condensation of the pipe in the attic would take hundreds of years before rotting began.  i don't even know if my home will be standing at that point.  i know it is not good to get fiberglass wet, but it would work as kinda a moisture sink. it would wick water off the pipe and allow it to work it's way to the top of the insulation.

i have also thought about the copper coils on my wood stove pipe, but dismissed it as the wood stove would have to on for it to work, and i'm not sure if the heat coming off the pipe could keep up with the flow rate of the water.  i would thank that after a few gallons went through the pipe could not heat the water due to the volume and velocity.   

the best way to do this would probably be to have a hot water heater sitting in your living room as a storage tank to hold water and let it get to room temp before it hits your HWH, but my house is small, and that would be pretty ugly too. 

i'm just trying to come up with a very simple system to take the cold edge off water running into the HWH that doesn't require pumps, tanks, roof mounted stuff, electricity, or seasonal adjustments. 

thanks again for all the replies!

adam

[thirteen]

  you could use an old style water tank and take off all the insulation plug the heater element holes and let the air in the room warm the water. I have one plumbed into the pipes leading to my ondemand water heater. It seems to work ok. My water is 46 degrees. I made the pipes all flow down at an angle for draining. I have a loft and it sits in the corner up next to the cieling.  I had the kids paint their hand prints different colors on the outside of it. I had thought about mounting it above my ondemand water heater  sending the exhaust thru the middle of a gas water heater thus preheating the water.  At the time all I had was an electric one. Works for me. When I get up there full time I hope to find a good gas water heater I can use and will try and install it on the exhaust pipe. (split level house) Just an idea to toss or play with.

[Biohazard]

I lease a shop in an industrial complex, not far from Portland. The building I'm in is a smaller building built after the main building, and, for the water to get from the meter in the street to me, it first passes through the attic of the main building, which is about 200 feet long. Then it goes underground for another 75' or so before it gets to me in a seperate building.

This time of year, the water comes out of the hose at about 67 degrees. This is after I've run if for hours, several hundred gallons of supposedly cold water. I don't know what the supply temp is from the city at the meter, but I'm guessing it is significantly lower than 67 degrees. This is at night by the way, when the sun isn't shining but the attic is still warm. I always love a cool drink out of the hose when I'm working, but this water is down right warm.

On top of that, my plumbing was never desgined to heat water along the way, it was just poorly designed. If I added a few hundred extra feet of pex in the attic I wouldn't be surprised to see 70-75 degree water.

The water gets cooler in the winter but it is still warmer than most "tap" water. I have a water heater there but I never use it because the regular water is always warm enough to wash hands and such.

[GaryGary]

Hi,
Some thoughts on using attic heat for various purposes here:
http://www.builditsolar.com/Experimental/Attic/AtticTemps.htm

Not exactly the same as what you want to do, but maybe helpful.

One plus is that the extra insulation in the ceiling would reduce your home heat loss, and that's got to save you some money.  You would get some rebates (federal and probably state) that would help to pay for the new insulation.

In my climate (SW MT) there is no way that the water would not freeze, but maybe in Portland its OK.  I used to live in Seattle, and can remember a number of cold weather spells that would easily freeze water in an attic. 

One thing to think about is how fast the water will warm up after the long pipe run is filled with 50F water.  If the PEX were sitting in air it would take a while since air has a low specific heat, and quite a bit of air would have to flow by it to transfer the heat in.  Sitting in the middle of insulation its going to take quite a bit longer.  The whole idea of insulation is to slow heat transfer.   It would be interesting to just do a little test with something like a length of garden hose -- see how long it takes to heat up in thee attic above the insulation, then lay it over the ceiling joists and cover it with some insulation and see how long it takes.  You could do the experiment with just a short length of hose -- say 20 ft -- fill it with cold, then let it sit, empty it into a bucket, mix it up and measure the temperature -- just like science class

Not to push my own system, but I like this one:
http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm
With careful shopping, you can build it for $800, and it will supply nearly all your hot water needs year round with bombproof freeze protection.  In Montana, the state paid for $500 of it, so the net cost to me was only about three or four hundred dollars.  The payback with a descent state rebate can be inside of one year.   You can even make it larger and get some space heating out of it as well.

Gary

[Biohazard]

Tonight I measured the water temperature in the water heater in my shop, which has been turned off all summer. It was 71 degrees. That's because my 40 gallon tank sits near the ceiling in a hot concrete building, and, like I said above the water line runs about 200' through a hot attic.

I'm guessing the supply temperature is in the 50s.