There's obviously energy to be saved here. How much you can save would seem to come down to how good a job the recovery unit does of transferring heat from the drain water to the incoming water. If you really wanted to be scientific, you could measure the temperature of the drain water before and after the recovery unit to get an idea of how much heat is being transferred.

I pride myself on dreaming ideas up, but have to admit I'd never thought of this one, and it's so simple.

Some of the makers claim recovery of 40% of the heat vs others who claim 60% - being conservative and saying 40% that's still pretty significant.

As you say, ky114, the question is what the baseline is, is that the temperature of the original cold water, the hot water, or room temperature?

I know this is such a good idea I don't really care about the finer numbers, personally, I just want one, especially since I need to replace a section of my drainpipe anyway, it seems like kismet!

My plan would be to insulate well around it, you'd have to think it'd increase its effectiveness a bit, and the return line to the hw tank will be insulated too.

You could measure the temperature of the drain water, looking at much it drops on its trip through the recovery unit, or you could look at how much the incoming water is warmed up. One nice thing is that this unit will offer greater efficiency in colder weather, because presumably your incoming water will be colder. This will cause a larger temperature difference between the drain water and the incoming water, so you should get a greater percentage of the heat being transferred.

That makes sense...and I'm sure that winter will be the most dramatic difference, just as we think it should. I have to confess to using warm cycles in the washing machine almost all the time over cold, because the cold water IS so cold. I suppose I ought to research what detergent companies consider cold....

I'd suspect the heat recovered would not be worth the work and material involved. I've used heat recovery units on A/Cs for the past 20 years with great results but for them to work, the unit continuously circulates the water while the a/c compressor runs, gaining a little heat with each pass. Think of leaving a garden hose out in the sun...the water inside gets very hot after sitting a while but when you turn on the water, after the heated water is flushed out, the new water passing through is not noticeably warmed. I think that would be a good analogy for the drain water system. The drain water would not transfer much heat in the short time it passed through the heat exchanger. Temperature and actual BTUs transferred are two quite different things.

After thinking about my post, I concluded there would be a way to make that idea work but you would need to place the exchanger in a septic tank that would hold the drain water until your system (with a circulator pump) had a chance to extract the heat. A undetected leak in the system could get nasty though, if you had a water pressure failure....

I take your points and agree placement in a septic tank would probably result in more gains, I think you're missing one critical point, though, which is the beauty of this idea:

New water enters the hw system when hot water is being drawn. Usually, that water is running straight back down the drain (showering) - so we are capturing that heat on the way out, and diverting it straight back to the tank where the hot is being drawn from. The literature for the various makers claim 40-60% heat recovery (over what baseline I don't know) and it has the endorsement of several Canadian authorities, so far I've seen British Columbia and Saskatchewan, I suspect the system is at its most dramatic in winter when cold water temps are near freezing.

The system is extremely long-lasting, and compliant with most codes, I don't believe the joints are soldered so it's going to last at least as long as the rest of the copper plumbing, has no moving parts and is maintenance free. I think it would be a great boon for geothermal or solar hot water systems especially.

I think its simplicity and longevity together with its relatively low cost $400-$600 probably does make it worthwhile...remember it's a sliding scale - it costs nothing to run, but every time your energy costs go up, the savings will too.

Looking at the manufacturers' websites, I saw a number of commercial applications where a large number of the units were employed....I am wondering if the local aquatic centre uses them, anywhere where there's a large consumption of hot water is going to benefit especially.

The analogies with geothermal or air source heat pumps are there- getting 'over unity' gains by extracting heat from an otherwise untapped/wasted heat source...

Just to clarify one of the points made is the tendency of the water running to cling to the sides of the pipe which is probably the crucial point in regards to the transfer of heat (or the removal of cold to look at it another way)

If a lb of water weighs like 8.4, and you take 2-5 minute showers a day with a 2.5 GPM head, that would be many years of showers to recoup costs of unit and install.

210 lbs of water down the drain with the above criteria at 25 degree recovery only equates to 5,250 BTU's give or take recovered (not counting standby loss or other factors)

At $1.30 per therm of Nat Gas, with an appliance of 80% efficiency, you could reproduce by theory that heat recovered (5,250 BTU's) from the water for less than .085 cents with a gas fired heater. (1 year is $31.00, dble the shower time-$62.00..........)

Sigh. Why are people so negative about saving energy when it can be done in such a way as to have no impact on your lifestyle? I'd consider it an investment. zl700, your figures are interesting if I understand them correctly, are you postulating a greater saving by upgrading to a more efficient heater?

No doubt there is energy to be saved by upgrading. Even if you do upgrade (and the heat recovery is considered worthwhile for high efficiency appliances like the new on demands etc) the fact remains there is free energy there for us to allow these devices to work less hard, by virtue of preheating. The devices are beautifully simple, maintenance-free and long lasting, and they pay us back from day one, plus provide benefits like the difference between running out of water by the third shower, or not. I have to confess a 5 minute shower would be a fast one for me, I do a lot of my thinking under the shower. We run out of water by the third person, often. I have the wall where this drainpipe is exposed, because of a leak. The pipe was holed by the former owners, it probably should be properly repaired. Whilst I have a section cut out, I might as well put one of these units in. I can also get an $80 rebate from the province for doing it, and will pay no PST and probably less or no GST for it. I will have a lower gas bill, and when I get my solar hot water, it will probably mean I won't need to do any supplementary heating during the warmer months at all.

You are also assuming the price of natural gas will remain constant. It will not, and is likely to rise rather sharply just as oil has done. These heat recovery units will also help reduce peak electricity demand for electric water heating. It will also improve the efficiency of any existing hot water heater straight away, and how many of us want to toss out a perfectly good hw heater just to go to a more efficient model? Besides, I don't think these things have to just be about dollars and cents. I also believe nasty times and severe privations are around the corner, if we get to a point of rationing, power cuts etc we will all wish we had those things installed and in place already...

OK here's some figures from one manufacturer's website:
Waste water enters the drain at 37deg C. Cold supply water enters the house/coil/exchanger at 11deg C. It recovers 14deg C to enter the hw heater at 25deg C. They also argue a longer life for hw systems, I'm not sure base on what (less temperature differential, gentler cycles?) - if there was any truth in that, it'd have to tip the scales a lot more.

Yes, my thought would be that the longer life for the hot water heater comes from less operating time.

I'm with you, PJB, go for it. Obviously there is some need to balance savings against initial cost, but waiting for the numbers to strictly justify every energy saving improvement has gotten us into a situation where we have an energy-inefficient economy, and now that energy is getting more and more expensive, that economy is taking a beating.

It's like compact fluorescent lights. The savings of one bulb -- 13 watts versus 60 watts, or 25 versus 100 -- does not seem fantastic, but you multiply that by millions of households and it makes a difference.

KY114, an excellent point. And by those few people prepared to take the plunge, we reached economies of scale which meant compact fluorescents are cheap, I remember paying over $30 for my first, a german-made one and that was probably in the late 80's or early '90's. As far as I know (it's my ex-wife's now) it's still working...

pjb....I don't think I'm being negative. I'm trying to be realistic. I fall back to the garden hose in the sun analogy. The water in it gets very hot but only after sitting there a while. Just cracking open the water valve to get a minimum flow will quickly flush out the hot water and the water then coming out won't be noticeably warmer. zl700's #'s sound about right to me. My firsthand experience with solar equipment and the A/C heat recovery units I now use make me VERY skeptical of the claims of a 14 degree rise with only one pass through the system. My heat recovery units don't provide anywhere near that in one pass and the A/C unit is providing 180 degree Fahrenheit + on the hot side. Only with continuous circulation can the hot gas warm the water... Also think of any solar system...they ALWAYS require a heat storage tank that the water is circulated through. Think of putting a solar panel up that your incoming cold water only passed through the one time on it's way to the hot water heater..transferring BTU's is what's it's all about and zl100's mathematical analysis is how you need to evaluate systems.

I'm not attempting to Poo Poo energy savings just that many other things can be done first that will benefit the wallet quicker.

For instance, I am a CFL Fan. Every bulb in my house is one with exception to the chandelier and fan lights that I wish to dim.

I would venture most people would benefit from an insulation or window upgrade first on the average home. There is a true and measurable payback there.

While the drain thing is splitting a pipe into multiple circuits for maximum surface coverage for transfer and is counting on the drain water in a vertical flow to be lineal, which is does, once the water leaves the head, hits the body, splashes down the walls and drains across the cool shower floor, what temps do you think we have to work with?

So many mislead people attempt to put down the energy savings on a gas fired on-demand water heater. However the energy factor rating that the quality units carry tend to exceed .82. Versus the standard 40 gal tank is lucky to hit .60.

Energy factors are real ratings and are justified and proven in certified labs. For those that dont know EF means:

The energy factor (EF) indicates a water heater's overall energy efficiency based on the amount of hot water produced per unit of fuel consumed over a typical day. This includes the following:
 Recovery efficiency  how efficiently the heat from the energy source is transferred to the water
 Standby losses  the percentage of heat loss per hour from the stored water compared to the heat content of the water (water heaters with storage tanks)
 Cycling losses  the loss of heat as the water circulates through a water heater tank, and/or inlet and outlet pipes.

My point is this, many things can be done to save energy without having to reach out for what I would call hocus pocus, far reaching means of an attempt to be energy conscious. Value engineering, which is a large part of my job, teaches me not to overlook the simple things.

PJB, Spend the $500 on a better water heater that will surely pay you back in savings.
I understand your incoming water is cold, and many persons in your position have benefited by using an incoming storage tank to allow the water to temper to the ambient air before entering the water heater.

I fully intend to avail myself of all the energy saving options out there, but the journey must begin with a single step. As I mentioned, the pipe is exposed, and there's a section that needs replacing. There is no better time to replace it. Also, I neglected to mention there's a federal subsidy for drain water recovery units (must be >43% efficiency) of $130, that's a total of $210 in subsidies.

The thing with demand hw is, I want to look at getting solar hw in, that doesn't preclude demand but I'm considering what to do with the existing hw tank which could serve as storage. It's probably 5 years old or so, so won't last forever. Demand would be a good fit with solar, as it will only run when the solar and any heat recovery system doesn't cover the shortfall. My existing gas storage system is not such a good fit, as it will want to heat the water as soon as it enters, unless I turn the gas to it off completely.

We have to start out with an energy audit/assessment, which we get reimbursed for. After that, we have 18 months to do all the work we're going to do, to get all the subsidies (things like $500 back on additional ceiling insulation, (R-25 to R-50 attracts $410) so we will probably look at refinancing the house or some redraw- we are on a very good mortgage rate currently and we have a lot of equity thanks to good timing and a good price. The house is in good condition overall but was constructed in the '80's so needs some updating, like windows for example - most of them have been replaced, but 4 haven't yet, and two are in poor condition, I kept those windows covered and the doors closed a fair bit....again there's incentive but only $100 each if they're one energy star zone or better.

The system is somewhat imperfect, I guess the audit is a good idea, but they impose quite a short timetable on you, on the one hand it's good to get the savings in place sooner, on the other, it'd be nice to be able to do some of those things immediately, without waiting for the audit which then puts you on a timetable.

Using a storage tank to temper incoming water is an interesting idea, but wouldn't there be a great deal of condensation? If I was to go with a demand/solar/storage system, I'd essentially be in that position, however, that heat would have to come from somewhere (however difficult to measure that would be)

Here's a link to the BC govt incentives...