Drain Water Heat Recovery – We’ve ordered one!

Three years ago, I wrote about my discovery of drain water heat recovery (also known as grey water heat recovery). Simply put, they appear to offer the same energy savings as a solar hot water system, but for a tiny fraction of the price. I read up on it quite a bit. They are well supported overseas, and there are plenty on the market internationally. So when the opportunity arose to purchase one, I looked at the two locally available examples, and picked the one that was able to be installed vertically, the EnergyDrain. Being locally made, and cheaper also helped in the decision.

Figure 1: Figure EnergyDrain

Despite some semi-effortful attempts, in my background reading, I hadn’t come across any criticism. So I went ahead and ordered one. However, semi-fatefully, a week or so after placing the order, I mentioned this systems in a forum discussing solar water heating, and David Haywood (wearing his engineering hat) said:

I’ve done heaps of modelling on these systems and they are good in theory. The problem is the HX cost and the cleaning. Most systems use some sort of horrible draino-type stuff every few weeks.

And went on to be explicitly critical of the horizontally installed ones. The more expensive GFX that I ruled out, seems much less prone to fouling, but that is due to its vertical installation, which isn’t possible for our house.

The main problem is that the building up of scum inside the heat exchanger will reduce its efficiency; though I’m not sure if it will ever meant that efficiency reduces to zero. Anyway, we’ve bought one, it’s arriving any day now, so I think it’s worth quantifying whether we will see much in the way of savings, and whether scum build-up is an issue.

Fortuitously, once the system has been installed, we will switch our hot water cylinder onto night rate, which is separately metered, so our energy consumption for hot water will be easily measured.

In order to accurately estimate the savings, I plan to have the plumber install a bypass loop round the heat exchanger, as well as a Y-regular joint, with an inspection opening, as illustrated below

To answer the first question, what are the real-world savings, if any, I’ll use an ABBA design, as illustrated below. I’ll route the cold water through the bypass for a week, then two consecutive weeks with the heat exchanger (HXC) in operation, followed by a week with the bypass back on. The advantage of the ABBA design is that if there are any other variables changing over time, affecting our hot water energy use (eg, changes in the weather, the system getting dirtier), this should cancel it out. Efficiency can then be estimated as the ratio of energy decrease during B divided by energy used during A (here (15-10)/15 = 33%).

Then, to assess the influence of dirtiness, I can plot energy use over time, and then occasionally clean it out. My faked data below would suggest that most energy savings is lost after 90 days (dotted lines indicate cleaning, and here the efficiency declines, so kW used increases), so would suggest cleaning rather more frequently. Obviously, I secretly hope for data that does not look like this, as it would imply that I should clean it every month or two.

Obviously, I’d be much happier if it looked something like this, with the efficiency declining a bit, then plateauing.

Welcome any feedback on my design, and otherwise, I guess there will be an update in a month or two.