Sunday, March 3, 2013

Point of Use Water Heating

Imagine going to the kitchen sink and dialing in the water temperature you want.  Then open the single tap and voila - water at the temperature you want, at any volume.  When you close the tap and re-open it, the water is at the same temperature.  Again and again.  No having to adjust to the right temperature every time you open the tap.  Wouldn't that be luxurious?  The technology to do this was available decades ago, but we keep doing things the old way...

Achieving this is simple.  One need only provide a single water line to the faucet.  In that line is a point of use (electric) water heater with a remote control.  The remote control is mounted near the faucet (EcoSmart makes this kind of unit).  The heater is somewhere nearby, but out of sight.  The only issue with implementing this is that the efficiency of water heating is always just 100%. However, one advantage is that there is no hot water anywhere in the system - just at the last two feet of water tubing before the faucet.  No standby losses, although these days they are small in better tanks.

Consider the shower.  Imagine again, only one water line supplying the shower, with an inline point of use water heater with remote.  In the shower, we dial in on the digital display the water temperature we want.  Shower water comes out of the spout at precisely that temperature.  The warm water leaves the shower via the drain, but here, we have a heat recovery device which is some 80% effective.  We give most of the waste heat to the incoming cold water stream just before it contacts the POU heater.  Then the heater finds it very easy to raise the water temperature just the last say 10 degrees Celsius.  In this way, we minimize the heat that leaves the house and the energy and power needed to heat shower water.  If we want a bath, things are quite a different story, and I haven't really thought about what I'll do in that case.

The drain water heat recovery devices out there right now are only some 50 to 60 percent efficient.  Even that is saving half our energy to heat the water, so yay, but for some untold reason, none of these devices take advantage of heat pipes, which I plan to experiment with.


  1. Architects push installing recirculating pumps on timers to address the issue of instant hot water. That way all the messy/ugly technology can be hidden away elsewhere. They usually don't even think about waste heat recovery systems. I don't have any information to share regarding how efficient the pumps are. Clearly the pipes would need to be well insulated!

    Regarding waste water heat recovery. I have had one concern, but have not investigated. My instincts tell me that an efficient waste water heat exchanger will require a lot of surface area, and correspondlingly small diameter channels in the exchanger. If this is so, then I would want to see how they deal with biological fouling of the exchanger over time. All sorts of organic material goes down the drain (soap, dead skin, hair, etc.), and it all is potential food for bacteria and other microbes which will settle and grow in the exchanger, causing problems.

    1. Sensors can also be installed to turn on the pump. Sensors detecting entry into the room can be combined with sensors detecting water use and a timer to turn off the pump once continued demand is unlikely.

  2. Getting a water heater would definitely be great to enjoy those cozy, warm baths especially during those days when the weather's too cold.

  3. In Japan above Mom's kit sink was a small gas burner. You pressed and held a button and listened and watched as the flame burst into view in its small window. This made you think -do I really need hot water to do this? It put the gas right in your face, so brought its use directly into the equation.

  4. here is a different perspective on this issue:

    1 shower = 10 minutes
    2 gpm shower head = 20 galons = 160 lb
    heat water from 50F to 120F = 70 (degrees farenheit difference) * 160 (lb) = 11200 btu = 3.2 kw/h
    assuming we have a regular 50% efficiency powerpipe DWHR system = 1.6 kw/h
    if we heat that water with a air to water heat pump water heater with a COP of 2 = 0.8 kw/h / shower.

    now....during the summer duct the heat pump to provide AC for your house which means that you spent 0.8 kw and you got 0.8 kw of hot water saving at the same time 0.8 kw in AC (that will more than make up for all the stand by losses which add to the cool load)

    during the winter vent the heat pump directly outside (ideally thru a HRV with a booster fan on the opposite way to balance the flow). stand by losses during the winter lower your heat load so you're heating your house at 200% efficiency basically which is not too bad. a bit of overventilation at times is the trade off.

    want to go even further...add another storage tank (regular electric tank with the heating elements not connected) and you can use cheap night electricity to heat the water and store it for day use.

    I just can't see a lot of savings possible in this department.

  5. Here is heat pump assisted wastewater heat recovery unit:

  6. cool stuff....that would get me the "nerd of the month" prize in the neighbourhood :-).

    there is some potential for savings the area of 0.5 kw per shower - less if you would get the best powerpipe available.

  7. Since you are using electric for your water heating system, this is not sensible enough to get the right water temperature as the other side of it will sum up your monthly bills. How about heating water for free? Through the use of solar water heating, you could turn your water at the right temperature without worrying the monthly bills and also using solar is safe to the environment.