We’ve been doing a really big renovation on our house since around July; for an energy geek like me you’d think I’d be blogging madly, but quite frankly the project leaves me with little free time!
But here’s what I was looking at this week – how to get hot water to taps fast, without wasting water or energy.
We built another 12 feet off the back of the house, and as designed, most of the hot water outlets are in the back half of the house – kitchen sink on the very back wall, and all bathrooms clustered on the “old” back wall – now 2/3 of the way towards the back. Unfortunately, the only feasible / cost effective location for the water heater is on the very front of the house – up to 40 feet away. This wastes water, energy, and time, because 40 feet of three-quarter-inch pipe can hold a gallon (about 4 liters) of water, and running a gallon through a 2.0gpm faucet or shower takes 30 seconds. When you’re done, a nice hot gallon of water sits there cooling off. So depending on the time since the last draw, you’ve wasted up to a gallon of water, the energy used to heat it, and 30 seconds of your time when you bring hot water to the tap. (With a low flow fixture, water & energy are the same, but it takes even more time for the hot water to arrive).
How to solve this? Well, just go read presentations from Gary Klein, like this one. Or hear what I did, which at least gets close to Gary’s goal of no more than a cup of water wasted per draw.
The standard way to deliver hot water faster is to have a hot water recirculation loop, so that hot water is closer to fixtures when you need it. The downside, though, is that it takes electricity to pump the water around (if you use a pump), and now you have a big radiative loop, causing the water heater to cycle more often.
We did the recirc loop, with code-mandated insulation throughout, to minimize heat loss. Rather than using thermosiphoning, a constant pump, a timer, a temp-controlled system, or a demand switch, I decided to try Taco’s “smart” recirc pump, which in theory learns when hot water draws occur, via a temperature sensor on the outbound pipe. It then cycles the pump for 1hr either side of that “learned” event. If it’s done well, it’ll be great. If it turns out my pump firmware has bugs (which would be shocking, I’m sure!) it might not be great. I plan to instrument & measure to find out once we get back to living in the house. Taco does say that it ignores short draws, and only “learns” from draws of (unspecified) longer duration.
One thing that went wrong on the initial install is that the plumber didn’t put in a proper check valve to prevent thermosiphoning. So when the pump wasn’t even plugged in, water was circulating around the loop on its own, wasting heat energy. Argh! The plumber did put in a swing check valve which will stop reverse flow and draws from the bottom of the DHW tank, but a spring check valve is needed to prevent forward flow via thermosiphoning. I’ve brought the issue up, and presume it’ll get fixed.
The other thing that didn’t go too badly, but could have been better – I don’t think we got the shortest possible runs from the recirc loop to the fixtures. On one shower it’s still 12 feet to the valve, due to a strangely chosen circuitous path. And the kitchen is 13 feet off the main loop simply due to the house layout; this could be solved by adding another return line and circulating past that fixture as well.
Here’s how it looks overall:
|Seconds wait @ GPM:|
|Distance, ft||Gal Waste||Cups Waste||1.5||1.6||1.8||2||2.2|
|½ Bath Sink||2||0.019||0.302||0.8||0.7||0.6||0.6||0.5|
(Yellow cells are the waits for the most likely flow rates for these fixtures).
So we didn’t make the one-cup goal everywhere, but we’re under 2 (if you don’t count valve-to-outlet distances). It’ll be very interesting to see how well this works – in particular, how well the pump learns, and what its thresholds are. Unfortunately, I don’t think it’s field upgradeable. :) But you can turn off the “smarts” and put it in manual mode; then I’m just a temp sensor, a relay, and an arduino sketch away from doing it myself, if necessary.
Advice I’d have for anyone having this kind of work done: State your goals clearly to the people doing the work. “I want a recirculation loop” isn’t really going to suffice, even if all you care about is convenience and wait time. Long branches off a recirc loop defeats the whole purpose.
If I had it to do over again, I’d have said something like “My wish is to have no more than 5 feet of pipe between any fixture and the recirc loop.” Stating that goal clearly would have probably stuck in the plumber’s mind a bit better, and avoided the oddities like the 12-foot run to the shower.