Thursday, January 30, 2014

How Best to Plumb a Drain Water Heat Recovery System

If you are not sure what a drain water heat exchanger is, google it or read the following:

http://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/drainline-heat-exchangers

Here's a couple pics of our installation:




Below I summarize salient points on this concept/product, and then give my thoughts.
  1. Cost is $500 to $1200 for the item.  Installation is extra.
  2. Go with the largest diameter and longest exchanger you can fit into your plumbing, assuming a vertical installation.  There are horizontal ones available from http://www.ecodrain.ca/en/how-does-it-work , in which case, I'm not really sure - but I think they have just the one size.  Leave at least a 12" (0.3m) of straight drain pipe above the exchanger to smooth out the flow.  You can see we've done this, and we've used 4" pipe
  3. It is still worthwhile to pump shower drains in the basement back up so they can drain into the heat exchanger.  Cost of these Gulper pumps with a kind of control that senses the water level in the drain is something like $300.
  4. Gather all your drains to one place, if possible.  In our case, we've done this with all the shower drains in the house, which all happen to be clustered together in the building.
  5. Savings of some 50% on water heating energy are possible depending on how things are plumbed in the house and cost of energy.
  6. Hot water recovery times can be dramatically improved - this, to me, is a very sure sign of energy being saved.
  7. The design requires a double walled heat exchanger - in other words, there must be an air space between the copper pipe carrying the drain water, and the copper pipe carrying the potable water - this severely limits the efficiency of the unit, and increases the costs - more on this later.
  8. Gravity film - surface tension effects cause the drain water to stick to the drain walls in a thin film - this is why heat exchange is arranged at the surface of the drain pipe.
  9. Simple payback periods range from 2 years to 10 years.
My Thoughts:

Leaving water in the Bathtub  One comment I've read is this:  Recover your 'waste' hot water heat by just leaving the shower water in the bathtub and letting it cool to room temperature before allowing it to drain away.  This is an excellent measure, and no cost to install, but it has some issues:   - first, it adds moisture to the house - good thing in winter, probably,  not so good in summer.  2nd, only possible with bathtubs, showers can't hold much water.  3rd, ring around the tub, 4th, it gets difficult to have multiple showers in a short period of time - such as on busy mornings with a family of 4, and 5th, don't assume it is 100% heat recovery.  The water in the tub cools only to room temperature, which is almost the halfway-point from the cold water inlet temperature to the 40deg C or so needed for a shower/bath.  So heat recovery is something like 60%, similar to a good drainwater heat recovery installation.

Efficiency:  The best units are limited to something like 60% efficiency - measured in terms of inlet and outlet water temperatures. Industrial heat exchangers are good for over 90%.  Even over 95% in some cases. A much more 'effective' design, would be to simply have a copper drain pipe inside of another larger, plastic pipe filled with the pressurized, cold water.  This would increase heat exchange efficiencies a lot (to probably above 80%, easily more), and reduce the cost of the units greatly as well, and also reduce the pressure drop incurred by the units we use today.  Given the huge potential for energy saving across a nation like Canada, one would think it is possibly worth the very small contamination risk - after all, if ever there were a leak, the pressurized water would go into the drain, not the other way around, and it would be fairly easy to detect - just check the water meter while all fixtures are off - perhaps an annual check would be worthwhile.  Imagine - if we could recover 95% of the energy used to heat hot water for bathing, small electric point-of-use hot water heaters would be so much more viable (read my post on POU Hot Water) - they could be built right in to shower fixtures, and this could lead to all sorts of interesting plumbing configurations (just plumb one line, for example - no need for both hot and cold?).  I can see Doc saying "What, are we in the Dark Ages?!".

Alternative Strategy: One way around this is to avoid the issue altogether.  Instead of directing reclaimed heat to the DHW system, transfer it to the space heating system instead - ie to a non-potable heat sink - such as a hydronic heating component, or direct to refrigerant in a heat-pump system.  This avoid the water contamination issue, and can recover much closer to 100% of the energy used for heating water.  Sadly, I know of nothing on the market that does this as yet, but it would be very easy to build something - a coaxial pipe heat exchanger is all you would need.
Another thing not being explored - use heat pipes to do the heat exchange work - this would probably increase the options for horizontal exchangers.
And yet another approach involves using an auxiliary tank and a pump - so we recover heat even from processes like clotheswashing and dishwashing, in which the hot drainwater is not expelled at the same time cold water is being drawn in.  Here is a link to one company to doing this:
http://www.synergensystems.com/enercyclerreg-rc.html

Installation:  You can see there is a lot of copper in these things - the one pictured was a good 60 lb.  We came up with a simple way to mount the item effectively.  We used a 4" water closet flange.  We removed the small groove inside this flange so the 4" ABS pipe could be passed right through.  This is basically creating a bulkhead fitting on your pipe - but there is no break in the pipe.  Glue it on with solvent, and then mount to a couple of wood runners.  Makes a good support.  Notice also we've installed unions around the exchanger - this will let us more easily check its insides from time to time and clear out any build-up.

Water Pressure Losses from Drainwater Heat Exchangers:  Something not mentioned too often:  What about the pressure losses?  The one we chose to install uses a single 3/4" copper tube (about 60 feet of it) wrapped around the 4" drain.  In choosing a product, it is a matter of heat exchange performance and this is measured by the Canadian government testing apparatus, so after finding units that perform well, we looked for designs that retain water pressure the most, and most robust.  So how to plumb in order to retain water pressure?  If you have high water pressure, this may not be an issue, but keep in mind, water pressure is not free.  Someone, somewhere, must provide it, and I feel we must always think of our buildings as 'off-grid', so we want to minimize energy losses at every opportunity.  Therefore, buy a unit that performs just as well, but results in the least pressure loss.  then, I would plumb everything through it.  After all, we heat a lot of water to just room temperature via space heating - it sits there in the toilet, in the pipes, etc.  But to recover that heat using the drainwater heat exchanger, the cold side must flow - therefore, as per manufacturer recommendations, just plumb everything through it - I would still provide a small line to a drinking water fountain or icemaker, however.






2 comments:

  1. I wonder if one could drain the showers separately to avoid colder drain water mixing with and diluting the hot shower water? I did find two shower systems from a google search: http://www.ecodrain.ca/en/how-does-it-work
    http://shower-save.com/products/recoh-vert.html

    ReplyDelete
  2. Great article, Lyndon. what do you think about recycling showers like this one?
    https://twitter.com/RecyclingShower

    ReplyDelete