Poor Man's Solar Water Heater

[David Williams]

-> First, by way of background... I consume a little over 800 litres
-> (210 U.S. gallons) of heating oil per year for space heating and
-> domestic hot water purposes. By my estimates, water heating accounts
-> for over half this amount and a large percentage of this can be
-> attributed to stack and tank related losses. My hot water consumption
-> is rather modest and there's probably not a whole lot more I can do to
-> reduce it any further (well, not without causing marital disharmony,
-> that is).

-> And therein lies the rub. I can't justify the cost of installing a
-> solar how water system when my DHW demand doesn't warrant it.

Have you thought of recycling the heat that goes down the drain when
you use hot water? When I re-did my bathroom, some years ago, I
installed a heat exchanger that allows heat to pass from the warm water
in the shower drain to the incoming cold water that is going to the
"cold" supply to the shower, and also to the water-heater. (Doing it
for the shower is especially easy, since warm water is going down the
drain at the same time as cold water is being drawn in.) I rigged up
the heat exchanger myself, using a 2-inch copper pipe, about ten feet
long, to carry the drain water, and a 2.5-inch pipe arranged
concentrically with it, so the narrower pipe runs inside the wider one,
with the cold water flowing in this "jacket". The flow directions are
opposite, which maximizes heat exchange. A bit of scrounging around in
plumbing stores let me find some fittings that I could use to make this
arrangement work. The pipes are installed horizontally, so the drain
water flows at a leisurely pace, giving up its heat. I did some
measurements which showed that it recycles about 30% of the available
heat, which is a heck of a sight better than none!

dow

 

[Paul M. Eldridge]

-> First, by way of background... I consume a little over 800 litres
-> (210 U.S. gallons) of heating oil per year for space heating and
-> domestic hot water purposes. By my estimates, water heating accounts
-> for over half this amount and a large percentage of this can be
-> attributed to stack and tank related losses. My hot water consumption
-> is rather modest and there's probably not a whole lot more I can do to
-> reduce it any further (well, not without causing marital disharmony,
-> that is).

-> And therein lies the rub. I can't justify the cost of installing a
-> solar how water system when my DHW demand doesn't warrant it.

Have you thought of recycling the heat that goes down the drain when
you use hot water? When I re-did my bathroom, some years ago, I
installed a heat exchanger that allows heat to pass from the warm water
in the shower drain to the incoming cold water that is going to the
"cold" supply to the shower, and also to the water-heater. (Doing it
for the shower is especially easy, since warm water is going down the
drain at the same time as cold water is being drawn in.) I rigged up
the heat exchanger myself, using a 2-inch copper pipe, about ten feet
long, to carry the drain water, and a 2.5-inch pipe arranged
concentrically with it, so the narrower pipe runs inside the wider one,
with the cold water flowing in this "jacket". The flow directions are
opposite, which maximizes heat exchange. A bit of scrounging around in
plumbing stores let me find some fittings that I could use to make this
arrangement work. The pipes are installed horizontally, so the drain
water flows at a leisurely pace, giving up its heat. I did some
measurements which showed that it recycles about 30% of the available
heat, which is a heck of a sight better than none!

dow

Hi David,

That's a clever idea. The first fill on my front loader is set to
warm, but all subsequent fills (rinses) are cold, so there's probably
not a whole lot to be gained by recapturing this waste heat.

I did look at installing a GFX on my shower drain, but even at today's
relatively high fuel costs, the economic payback is not all that
compelling. In rough terms, two five-minute showers at 2 gallons per
minute and an average temperature rise of 60F represents about 10,000
BTUs of heat demand. That translates to be something in the order of
0.35 litres of fuel oil per day or approximately 128 litres (34
gallons) per year. If the GFX could recapture half that (and that
might be somewhat optimistic), I would expect to save about 65
litres/year or about $55.00 at current fuel oil prices (i.e., $3.20
per US gallon). If there were teenagers in the household, it would be
a done deal, but since there are only the two of us and we're both
pretty water and energy frugal, I really couldn't justify it.

Cheers,
Paul

 

[Eric]

-> First, by way of background... I consume a little over 800 litres
-> (210 U.S. gallons) of heating oil per year for space heating and
-> domestic hot water purposes. By my estimates, water heating accounts
-> for over half this amount and a large percentage of this can be
-> attributed to stack and tank related losses. My hot water consumption
-> is rather modest and there's probably not a whole lot more I can do to
-> reduce it any further (well, not without causing marital disharmony,
-> that is).

-> And therein lies the rub. I can't justify the cost of installing a
-> solar how water system when my DHW demand doesn't warrant it.

Have you thought of recycling the heat that goes down the drain when
you use hot water? When I re-did my bathroom, some years ago, I
installed a heat exchanger that allows heat to pass from the warm water
in the shower drain to the incoming cold water that is going to the
"cold" supply to the shower, and also to the water-heater. (Doing it
for the shower is especially easy, since warm water is going down the
drain at the same time as cold water is being drawn in.) I rigged up
the heat exchanger myself, using a 2-inch copper pipe, about ten feet
long, to carry the drain water, and a 2.5-inch pipe arranged
concentrically with it, so the narrower pipe runs inside the wider one,
with the cold water flowing in this "jacket". The flow directions are
opposite, which maximizes heat exchange. A bit of scrounging around in
plumbing stores let me find some fittings that I could use to make this
arrangement work. The pipes are installed horizontally, so the drain
water flows at a leisurely pace, giving up its heat. I did some
measurements which showed that it recycles about 30% of the available
heat, which is a heck of a sight better than none!

dow

I have serious doubts about the effectiveness of such a system. Drainwater
is less than 100F (probably less than 85F by the time it gets very far down
the drain) and is only in brief contact with the heat exchanger. Given the
flow rate through the heat exchanger for the incoming cold water, I'd be
surprised if it raised the temperature of the water by even 1 degree.
Eric