Hi there,
Disclosure first. I am a professional engineer in Ontario, Canada and have a master's degree in engineering here. My specialties are alternative energy systems, heating ventilation and air conditioning and hot water systems. I am also the inventor of the Power-Pipe Drain (Waste) Water Heat Recovery Technology. We now have well over 60,000 Power-Pipes installed and saving energy every day in homes, flats and in some commercial and industrial applications.
Original Question: will a properly designed/made Waste Water Heat Recovery (WWHR in the UK) “boost a combi’s DHW supply sufficiently to cope with 2 decent showers at once?”
Yes, absolutely and certainly in your case. Of course this depends upon the efficiency of the WWHR unit at the total flowrate, your combi boiler’s capacity and how it is installed. Basically, with a 55-60% rated WWHR unit you can generally expect a doubling of effective hot water capacity. Many WWHR devices have been installed to solve the problem of insufficient capacity with on demand water heaters, for example.
Contrary to Dan Robinson’s reply, WWHR is not snake oil. It is proven and well established (in some places) technology. In fact, the AIMC4 Project in the UK found the hot water savings to be 35% and 41% in two different homes with a Power-Pipe R3-84 (not the most cost effective model but it forms part of the main soil stack). At a 2011 AIMC4 meeting in BRE Watford offices I informed the 60+ attendees that I expected that model would save 35% to 40% of the hot water load. They thought I was crazy. However, this was based upon over a decade of experience and monitoring at the time. The current SAP (UK building energy credit) calculations have 7 errors, which result in only about half the SAP credit that WWHR devices should receive. Our industry has been trying to get this fixed for almost 2 years. The AIMC4 project report demonstrates this gap. I have included the core pages from the AIMC4 report. You can also download it at:
http://www.aimc4.com/filelibrary/files/AIMC4-Briefing-Paper-5_final.pdf
Contrary to bernardgreen’s comment, waste water actually clings to the inner wall as it fall down a waste pipe because of “surface tension”. The quicker it falls, the better the heat transfer, because the film of waste water becomes very thin so there is less resistance for heat to flow to the inner wall. In fact, a University of Waterloo professor found that the heat transfer rate in the falling film is higher than through copper! Slowing the film down somehow will actually decrease the heat transfer rate.
Also, using a tank, as described by bernardgreen, with a WWHR domestic device is a bad idea (we do that for some industrial applications) because you don't get the counter flow and do not have the high heat transfer rates. However, keep in mind that a falling film WWHR device primarily captures heat energy from the shower, which is the main load in homes. Most falling film WWHR devices are very efficient, perfectly safe, maintenance-free, passive and have a very long life making them quite practical.
For highest energy savings and ease of installation we normally recommend that all the fresh water in the home be plumbed through the WWHR device, except the kitchen sink. The WWHR device does not hold the water at any temperature so the water will fall back to room temperature (like all the lines in the home) when not in use. It can and usually does recapture some heat from other fixtures, if they are plumbed to it, but we are primarily interested in getting the waste water from the shower(s) and plumbing to both the cold side of the shower and to the water heater/cylinder. All the water in my home runs through my Power-Pipe and, honestly, it is not a problem. It even takes the cold edge off the water so we usually wash our hands with the cold water line (actually tempered water from the Power-Pipe).
endecotp, WWHR devices will certainly supply the capacity that buffster is interested in if:
-the water heater / cylinder is not already very marginally supplying the one shower
-an very inefficient WWHR device is not installed (be above 40%, but depends upon the water heater)
and/or
-it is not installed for equal flow as described above
endcotp has mentioned the Recoup units (models we’ve seen) are relatively efficient. The Recoup units we have seen are partly made of drain plastic and the double wall is not actually externally vented so if there is ever a leak between the walls you wouldn't know it. Furthermore, the inner copper wall thickness does not confirm with British standards for a 2” (54mm) pressurized line (which it is on the external). WRAS approval doesn’t deal with these points. The other models mentioned are all solid examples of WWHR technology. All are falling film and vented, maintenance free and should have a very long life.
It does not make economic sense to combine a WWHR device with a heat pump in a home. There is some interest in doing this for large apartment buildings and hotels.
I might come back with more comments. I do agree also that having a good lower flow shower head is also important for reducing energy consumption and increasing capacity.
Respectfully,
-Gerald Van Decker
