Getting into hot water, fast!

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
Kitchen Sink 13 0.123 1.962 4.9 4.6 4.1 3.7 3.3
½ Bath Sink 2 0.019 0.302 0.8 0.7 0.6 0.6 0.5
Master Sink 2.5 0.024 0.377 0.9 0.9 0.8 0.7 0.6
Master Shower 2 0.019 0.302 0.8 0.7 0.6 0.6 0.5
Hall Sink 7.5 0.071 1.132 2.8 2.7 2.4 2.1 1.9
Hall Shower 12 0.113 1.811 4.5 4.2 3.8 3.4 3.1

(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.

16 thoughts on “Getting into hot water, fast!

  1. I have recirc loops in my house too. They have been a source of a lot of trouble. Right now they are driven by a pump under control of the gas furnace/water heater unit.

    I have learned now that there is a better way to do this. Don’t bother installing the third pipe for reciculating. Instead get small under sink recirc pumps. You put one in each bath that you want to stay hot.

    These pumps have timers on them. You set the timer to enable the pump during the hours you want instant hot water. The pump also has a temp sensor. That temp sensor is hooked to the hot water pipe. When the hot water gets cool the pump simply pulls water out of the hot pipe and pushes it back down the cold pipe until the hot pipe is hot again.

    This solution is 1/10th the cost of running the recirc lines back to the water heater.


    • Yep, I had considered those too – in particular, on-demand under-sink pumps like this one from Metlund.

      But since we were doing new construction anyway, running the return line wasn’t as big a deal, with everything open. And I wasn’t super enamored with the idea of running DHW backwards through my cold water lines – although in the new house we’ll have a semi-dedicated line for drinking water.

      If you read Gary’s paper I linked to, there are significant money & energy costs to some forms of recirculation, and the on-demand pump has the lowest costs; he estimated that a pump on a 16 hour/day timer costs $180/yr for gas and $515/yr for electricity. A demand-controlled pump was only $10/yr for gas and $20/yr for electricity.

      How loud is the under-sink pump?

      • I don’t have one, I only discovered them after installing the other system. Don’t worry about pushing the the hot back into the cold, the two pipes are connected near the water heater anyway. That’s how you are able to push the hot back down the cold in a closed system.

        Gary missed the largest energy cost of the system. In the winter the system is basically free. The electricity from running the pump and the heat loss in the water just ends up heating the house.

        The large problem is in the summer. All of that energy adds the air conditioning load. And getting rid of it with AC eats a lot of power. That’s how I found the problems in my system. I couldn’t understand why my AC bills were so high in the summer. All of those runs of water pipe lose a lot of heat.

        • Oh, sure, it works to push it back through the cold – it’s just that you then no longer have cold water at the tap – you have hot water on both.

          True, Gary might have a California bias about waste heat in the house. Still, odds are your other heating methods are a lot more efficient than generating waste heat from an electric motor, and the hot water tank may even be less efficient than the boiler or furnace – so there’s still extra cost there, but perhaps mitigated to some degree.

          Are your pipes insulated?

  2. My pipes are not insulated and that is a major source of the problem. It was new construction too and the dummies didn’t insulate.

    We get warm water out of the cold line too. The recirc pipes run right next to the cold pipes and warm them up. Pushing the hot back down the cold is not that bad, it cools off fairly quickly.

    The house has in-floor hydro heating – which is basically hot water in a pipe. So efficiency is same. Both use the same hot water source.

    • Ah, if it’s one heat source then sure, much less of an issue during heating season.

      I had spots where I had cold water pipes smashed in next to boiler pipes, too – I shoved some insulation between them in a vain attempt to help. :)

  3. Did you consider tankless on-demand water heaters, at point of use or per zone? I’ve heard good things about Bosch’s products in this line, though I’m sure there are other good brands.

    • I didn’t really consider point of use tankless – that’d have to be (mostly?) electric, and we’d need quite a few units installed to hit every point of use across 4 (?!) bathrooms, including a tub, 2 showers, kitchen, laundry, etc. I think the expense would have been prohibitive, both to install, and quite possibly to operate.

      Edit: Even a 12kW unit (that’s pretty high, 50 amps or so?) is rated for only a 1.5gpm shower – enough, but barely, and strikes me as a pretty big unit. For smaller 3.3kW units they call 0.5gpm lavatory faucets “standard” – 0.5gpm is noble, but I don’t think it’s standard. New lav faucets we bought came equipped with 1.5gpm. For a smaller house it might have made sense…

  4. I got the under the sink pump, but for a different reason. It’s an old house, and in the coldest weeks of winter the pipes can freeze. The pump has the added benefit of heating the pipes to prevent freezing. I will eventually bust open the wall to insulate, but for now a quick and easy fix. The pump is super quiet (i don’t hear it come on)

  5. I recently installed an on demand hot water circulation pump system from I met the owner/ designer at a local Homeshow here in San Diego. One pump for the whole house and “bridge valves” at each water line dead end or the most distant sink. The bridge valve allows the hot water to circulate through the closed system until the hot water reaches that bridge valve; then it shuts off the flow and subsequently the pump. The pump is triggered by a quick on/off flick of the hot water faucet at that sink with the bridge valve. A flow monitor back at the pump (above the water heater) senses that quick flow and pumps the hot out to that bridge valve until it gets hot water and shut off the flow. Then that same flow monitor shut off the pump

    Great in theory and works as designed……BUT the Bridge Valve(s) are designed to shut off the hot water pre-use circulation when only 95 degree water reaches it……. far from what most of use would consider “Hot” water. So the pipes are primed up to Warm water not “Hot”. I have asked the owner/designer if hotter bridge valves are available that would allow truly hot water to reach the distant sink before the on-demand pump stops. I still had to run 30+ seconds of water down the drain before what was coming out of the faucet was truly hot.

    For now I would rename this product “

    • Yep, I’ve seen these too, and I considered them, but had 2 concerns:
      1. I wasn’t thrilled with the idea of warm water in my cold water pipes (don’t want to drink that), and
      2. I’ve heard that those bridge valves have a fairly limited lifespan.

      Still, it seems like a good way to go when you don’t have a dedicated recirc line, and it also avoids the waste of running the recirc line unnecessarily.

  6. We’re remodeling our ranch home & have access to distant faucet lines, near showers and washer, such that installing return lines is straightforward & thus eliminates the warm water in cold line issue that occurs with bridge valves. The Faster Hot Water Dedicated Recirc System seems like the way to go to minimize pump run time and line heat loss. Maybe the Taco Smart Plus.

    The thing I can’t tell is how quickly thee pump gets hot water completely to the use point given the pump sits idle for hours till the sensor senses hot water flow. We’re currently waiting 40 seconds on one branch and 50 seconds on the other for hot water. Obviously it can’t be hot instantly at the use point with the cool water sitting in the hot line that has to be flushed, but if the system cuts the time to the 10 seconds range it would be great. Should we expect this much improvement? Thanks.

    • The way it’s supposed to work is that it senses hot water flow at some hour of some day, and then next week it anticipates the same use, and starts pumping prior to that time.
      So I definitely have seen improvement, but my complaint is that the pump seemed to run too much. In the end I just put it on a simple timer.

  7. We installed the FasterHotWater system with a recirculation line without the bridge valves. It can get hot water to faucets fast – under 5 seconds vs. 35-40 seconds before. Our longest run off the recirc line still takes about another 5-7 seconds till hot water arrives.

    An issue that came up was that apparently the cooler water in the hot recirc line was going back into the hot water tank so fast/vigorous that it was getting to the tank outflow line faster / preferentially than most of the hot water inside the tank (I would have loved to see that internal flow pattern.). Net, now the hot water wasn’t nearly as hot as without the pump running – may 10-20 degrees cooler. So we backed off the recirc rate by partially closing the ball valve in the line. That compromise raised the hot water temp at the faucet back up – better mixing inside the tank – at the cost of a couple extra seconds longer to get fully hot water. That’s NBD as I just flip the hot water faucet on for a half second or so & the recirc pump has hot water to the faucet before I’m ever ready to actually use it. Pump shuts off on own 2 minutes after water use stops. So the pump only runs 30-40 minutes/day.

  8. My kitchen addition has two sinks and dishwasher, plus radiant floor. The prior system was a tank heater with outboard heat exchanger for the floor plumbing.

    Current system is a tankless combo boiler for floor and domestic water. HW takes over a minute (about 1 gal) to reach the farther sink after almost 40ft of 3/4 pex. We took out the recirc line but will install a new 3/4 pex one. Easy access in the underfloor space!

    After researching various recirc systems, I’m going with the DRS-T recirc system. No timers to adjust seasonally or after power outages. This should get HW to the sinks in maybe 15 seconds.

    Mike at was great answering my questions. Short on/off flip isn’t even necessary: pump stays on as long as tap is open. (In this case you get cold water but the pump speeds the flow around the loop. Waiting prevents any wastage.)

    We’ll see how it goes.

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