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A Hot Water System Retrofit for an Old House

David Wasserman Showing His Hot Water Distribution System Retrofit

David Wasserman and I worked together at the Southface Energy Institute back in 2007-08 and have been friends since then.  Before Southface, he was an engineer at Oak Ridge National Lab.  We also call on him for occasional help with some of our HVAC design work at Energy Vanguard.  He knows his building science, especially the mechanical side.  So when he recently posted a comment about the hot water system retrofit he’s done on his own house, I wanted to see it.

The article where he posted that comment is the one I wrote about why it takes so long to get hot water in your house.  The basic problem is that a lot of hot water pipes are both too long and too big in diameter.  In the article I wrote about my own house, where it currently takes two and a half minutes to get hot water at the kitchen sink.  I can reduce the length of the pipe to the kitchen, but it’s the diameter that’s killing me.  I’ll fix mine before long, but Wasserman has already done something about his slow hot water.

Wasserman’s hot water retrofit

I went over to his place last weekend, and he showed me what he’s done.  That’s him in the top photo pointing out the addition to his hot water pipes, with a closeup below.  It’s really simple.  His existing hot water lines are all 3/4″ and 1/2″ copper pipe.  With the low-flow fixtures used in homes now, those pipes are too big.  He didn’t remove them, though.  He simply added a second, smaller pipe for three of his hot water runs

The manifold for a hot water system retrofit for an old house
The manifold for a hot water system retrofit in David Wasserman’s old house in Georgia.

In the photo above, his hand is right in front of the original hot water line coming off the top of the water heater.  To the left of his hand is a short manifold he made out of copper pipe and copper tees (details below).  Near his thumb is a cutoff valve for the three new hot water runs.  Since it’s parallel to the manifold, you can tell the valve is open and the hot water goes that way.  The three new runs go up from that manifold.  They’re made of 1/4″ PEX tubing, which you can’t see except for one little gap because they’re insulated.

In case you can’t see all that in the photo above—and it IS hard to see with the pipes all covered in thick insulation—the annotated version of the photo below should help.  The numbered lines are the new 1/4″ PEX lines he installed between the manifold and the three hot water taps.  (You’ll have to look closely to see the word “valve.”  I couldn’t find a color that stood out any better there.)

Annotated photo of Wasserman's hot water system retrofit
Annotated photo of Wasserman’s hot water system retrofit

Those three PEX pipes go to three different parts of the house.  The photo below shows one going into a bathroom.  Since he doesn’t want hot water going through both pipes, he installed valves for the old runs near where the new run connects to it.  Perpendicular to the pipe indicates that that one is off, and all hot water is coming through the 1/4″ PEX pipe.  The other two runs are set up the same way.  Simple, right?

The old and new pipes for one run in a hot water system retrofit for an old house
The old and new pipes for one run in a hot water system retrofit for an old house

And how well does it work?  He said it used to take over a minute to get hot water to his kitchen faucet.  With the hot water system retrofit, it takes about 8 seconds.  I verified that when I visited.  They hadn’t used hot water in the kitchen yet that morning, so I turned on the tap and counted out, one-one thousand, two-one thousand…  When I got to 7 seconds, the hot water was there.

The details about his retrofit

If you’re interested in what parts he used and the other details, here’s what he wrote in his comment on the other article.  The links are to the parts he used.

  1. Attach a 3/4″ to 3/4″” copper Tee as close as you can to the water heater outlet.
  2. Then create a manifold consisting of 3/4″ to 1/2″ copper Tees connected by 3/4″ stubs. Put as many Tees on it as you need. End the manifold with a cap, not a 90 degree bend.
  3. Then attach 1/2″ copper male adapters to each branch of the manifold.
  4. Then thread on a Sharkbite reducing adapters.
  5. The PEX tubing then slips into the adapter.
  6. At the other end find a 1/2″ pipe [or whatever size yours is] as close to the fixture as possible, install a ball valve then install a 1/2 to 1/2″ Tee down stream and repeat steps 3 and 4 above.
  7. This insulation works well with this tubing

Caution:  Prescriptive plumbing codes don’t allow pipes as small as 1/4″ so you may need an engineer to help if you want to go that small.  You need to know the flow rate required at the end of the line. To use 1/4″ PEX tubing, it should be about 1.8 gallons per minute or less. Also, if you need to run the PEX more than 25 feet, you’ll need to go to a larger size, too.  To get a better handle on what you need, use this plastic pipe design calculator.

There you have it.  David Wasserman’s method to retrofit the hot water distribution system works well in his old house. If you’re worried about this retrofit not meeting the code, which requires 3/8″ minimum pipe, Wasserman said:  “Code requires you to put in over-sized piping but it does not require you to use them!”


Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and the founder of Energy Vanguard. He has a PhD in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.


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This Post Has 37 Comments

  1. Good article on hot water supply on long runs, but what do we do on concrete slabs? Running new lines up thru the attic and down w/be rough. I’ve heard those recycle pumps at the fixture are about the only option?

    1. George, yeah, with your water pipes embedded in a slab, any retrofit is difficult. If you’re in a place that doesn’t freeze, like a lot of Florida, putting them in the attic is an option. That comes with its own set of problems, though.

      1. You likely remember my home, slab foundation, two story, limited attic access. The PEX style water distribution is further complicated with a central manifold at the garage where I can turn of any fixture from there. So….no loop nor loop potential.

        We are heading in the direction of circulating with temperature controlled cross overs at the kitchen, master bath, guest bath. Willing to go to the remarkable expense around cost per cross over my research (reading online posts) indicates the cross overs have lost reliability, durability, performance. Often in a short time.

        Any comment?

        Arthur Beeken

        1. Arthur, I do remember your house, and the nice dinner we had there. I don’t think you showed me the hot water system, but it sounds like it’s one of those “I wish I had known about this before” kind of issues. I’ve just started dipping my toes into hot water, so I don’t know temperature-controlled crossovers yet. Maybe someone else will jump in and help us out here. (Larry?)

          1. Allison here is a pic of a crossover used under a sink along with a manifold type system (larger pic can be found via link):

            I feel your pain Arthur as the one draw back to Home Runs is you would need a pump on each line though I would recommend limiting to 1 or 2 fixtures that a take the longest & are used the most often. Installing a pump in a guest bath that is hardly used might just not be worth the time & might lead to the durability issues you may have heard of due to lack of use.

            Allison – you could always look into sub manifolds – one main run to each bath or area & then a manifold that splits it off to each fixture. You could then install an on-demand crossover say at the sink & the only lag would be what is in the shower leg if you go to turn that on.
            I would also point out via that link that running 3/8 pex might not give you a 7 second time but it would be right up there. It would be nice if the Plumbing code got in line with the faucet flow rates they require.

          2. Thanks, Sean. So a crossover is just where a demand recirculation pump pulls hot water and dumps it into the cold line. I’m familiar with how it works but don’t think I had run across that term.

            My current plan for the new plumbing system I’m going to put in next year (not the Wasserman style retrofit I’m about to do soon) is to run a trunk to the bathrooms and a separate line to the kitchen. The bathrooms are all clustered near the water heater, and the kitchen is farther away. I might separate out the primary bath so it can fill faster.

            My retrofit will be to run a 3/8″ PEX line to the kitchen, which will give me a chance to see how well that works. I’m hoping it will reduce the wait time enough that I don’t feel the need to install a demand pump. Thirty seconds will feel almost instantaneous compared to my current 2:30, but I’m hoping for maybe 15 seconds.

          3. Allison wrote:
            > So a crossover is just where a demand recirculation pump pulls hot water and dumps it into the cold line.

            To clarify the terminology as I understand it… a thermally controlled crossover valve, as used by the Watts system (referred to in another comment below), maintains a given temperature at the remote fixture. Since the valve (and indirectly, the pump) are thermally controlled, there’s no user interaction and thus is NOT a demand system. The pump can be locked out with a timer to reduce heat loss in the pipes.

            An on-demand system also uses the cold water line for the return but I’ve never seen one referred to as a crossover system. In particular, there’s no crossover valve since the pump is started by the occupant (there’s simply a bypass connector). The pump automatically shuts off when warm water reaches the remote sink. The pump and sensor can be located at the water heater but this essentially doubles the run time and associated energy losses in the pipes, though still far less than other types of recirc systems since the pump only runs a few minutes a day. Also, on-demand pumps generally have much higher flow rates than other types of recirc systems to minimize wait times.

  2. Hello, Is there any concern about “dead legs” with the piping that David Wasserman did? Also about running piping when there is a slab, there are tricks. I built a SIPS home on a slab, so have no attic or crawl space. I used large hollowed out baseboard for some of the 3/8″ PEX tubing that I have. Perhaps for retrofit, crown molding can be installed to create a raceway for tubing?

    Yours, Larry

    1. Larry, the dead legs you refer to would be the old sections that are bypassed, I assume. What would the concern be? Bad stuff growing in there? I admit that the water in there could be nasty if it sat for years and then you opened the valves again, but maybe flushing it out would clean it enough? And maybe turn the water heater temperature up while flushing it?

      The dead legs I’m about to create won’t be there very long because I’m going to redo the whole distribution system next year. I don’t think David Wasserman has any plans to open his up, but I suppose a future homeowner might. I’m interested to hear what others have to say.

  3. Hello, Here is a link to Uponor and their chart for 1/4″ PEX: . About dead legs, my concern is bugs like legionella growing in that nice warm stagnant water. I try to keep dead leg lengths down to inches if possible. I understand that you want at least one foot per second of flow periodically to make it harder for biofilm (aka, legionella habitat) to form. I do think that if we’re mindful and take precautions like flushing, then there isn’t any real problem.

    Yours, Larry

  4. Our 2 story house in SoCal is on a slab with 3/4″ copper tubing running up thru the attic for most of the runs. The kitchen is at the back of the house so the run goes up into the attic, across the attic and master bedroom and down the back wall into the kitchen (a long run). A straighter run could have been accomplished between the two floors and now with dry wall in place any changes would require a lot of demolition. We often just run water into a bucket at the sink to get hot water there, a huge pain but less of a waste. I’ve had an on demand gas water heater that past 6 years now and have considered all sorts of solutions to this, none of the them very cost effective. I did run a recirculating pump for a few years when I had a conventional gas water heater and it failed after a few years due to our extremely hard water here (440 ppm). Since I don’t mind waiting for the hot water but do mind wasting 4 gallons every time I need water I might get another recirculating pump made for my on demand system. I don’t see a PEX solution right now but at 34 years old maybe our copper system is living on borrow time anyway, lol.

  5. Have the small undersink instant water heaters evolved from their original state? That may be a solution for me w/the pipes in the concrete floor?

  6. This is so exciting! I’m working on a complete remodel of my 1965 built home. These ideas and concepts are so helpful in my quest to modernise my home! Thanks for your articles! I’m so glad I subscribe!

  7. I installed several recirculating pump systems from Watts, with a crossover valve under the furthest away sink from hot to cold. it opens thermostat controlled. It works really well and the installation is simpler. If you have a second run, you just another crossover valve at the end of that one as well. The drawback is, that the pump runs all the time, but can be controlled by a timer.

    1. On Oct 21, 2021, at 9:06 PM, arthur wrote:

      Thanks for this advice. My question is around durability and performance over time. I wonder if your initial experience plays out over time.

      Other posts have indicated durability issues.

      RC remote activation of this pump with a 15min? run time would solve your continuous run time issue. I had success with this strategy a return line in place and using the DMand brand. Door bell style push buttons at strategic locations worked well. A positive observation regarding that company, the initial pump had a bad internal solder joint. I repaired that and image captured the process. Sent that to them. They thanked me and sent me a new unit anyway. Nice result.


    2. As I understand the Watts system, the pump activates when the thermal crossover valve opens. According to docs, the valve opens when the static water temperature at the valve drops to ~98F. This reduces energy loss compared to a typical recirc loop (because lines are maintained at much lower temperatures). However, there’s still a lot of wasted heat compared a high speed on-demand pump, which only needs to operate a few minutes a day.

      A bigger problem with crossovers in my mind is that it puts warm water at cold water taps along the return path. That’s not much of a problem with a demand pump since it only needs to run a few minutes a day.

  8. Just a far-out question. Is it possible to fish a PEX line through an existing over-sized copper line, like a wire in a conduit? Sounds risky, but I thought I would ask.

    1. John, I think I’ve heard of someone doing that, but you’d have to have a copper run with no elbows in it. Even if you could force it through the turn, PEX-B can kink.

      1. Allison has it right based on my experience–we’ve pulled small pex through copper, but only sub-slab type S runs with bends instead of fittings. Works like a charm in that application though!

  9. You forgot to mention the water demand calculator – this is like a Manual J only for Water demand. How many water heaters and lines are over-sized? Water demand should identify the size of the hot water line.

    For us “wet-heads”, the copper line first needs to go through the buffer tank to get to 75F, then go into the in-direct for the final heat exchange up-to 120F. Some us even heat the house via a heat-exchanger connected to the in-direct. Low temperature water is a money saver.

  10. Not knowing David, I would have expected nothing less, based on his lastname alone. How wonderfully fitting.

  11. Allison, Regarding instant hot water, I installed a condensing boiler with indirect-fired hot water tank in 2006, and 2 years later got a utility audit in which they installed low-flow aerators in all the lav. faucets. That night I realize I would never again be getting hot water in my lav sinks, because the wait would just be too long. Patience has limits! So I researched hot water recirc systems and bought a Watts pump with themostatic-check valve. Since it was so easy to put in, I installed the cross-over valve under the furthest lav faucet between the hot and cold lines that day. I planned to cut in the pump at the water heater the next weekend, but I never did! In those few days I discovered we did not need the pump: with a water heater in the basement and the most distant bathroom on the second floor the buoyancy force between the less dense hot water and the cold was enough to cause a “natural” recirculation of cooled hot water back to the water heater through the cold water line. That was great… 3 seconds to warm water; what an incredible luxury!
    But then I thought about the heat being lost in the interior of the house framing where it was not doing anything useful. I make hot water with an indirect fired tank served by a zone valve from my boiler. I already had a switch in the tank thermostat line at the top of the basement stairs for a quick shutoff when we are away from home. So I started to look for a time clock that could control that 24 volt AC thermostat circuit and disconnect it every night. I could never find one with dry contacts for 24 volts, but one night as I was trying to fall asleep and heard the boiler come on to make more hot water, it suddenly occurred to me that I could use that old programmable thermostat in my junk bin as a time clock for a 24 volt circuit. If I set the day temperature to 90 degrees F it will always call for heat (it never gets that hot in the house), and if I set the night setting to 45 degrees F. it will open the circuit all night. So at least we would not have recirc at night and we would not be heating hot water during the night when no one is using it. That worked quite well, and we always had hot water when we wanted it. I later extended the OFF times from 1030 am- 3pm and 8:30 pm – 6:45am. We can “coast” on stored hot water during mid-day and from 8:30pm until bedtime. We minimize hot water energy when we don’t need it and have instant hot water when we do want it.
    Yes we no longer have instant COLD water, but who cares? We find we never really want it. It’s only needed to temper the hot water and the tepid water in the cold line will do that.
    One other energy trick I employed: Our condensing boiler reduced our gas use by a significant amount, but it was not an ideal installation because it still served our high-temperature baseboard radiation. I thought about adding more lengths of baseboard so we could lower the supply water temperature reset schedule and get the return water low enough to condense the flue gas more hours of the year, but haven’t done that. As a stop-gap when I installed the boiler, I put a diverter valve in the return water line to the boiler. When closed it forces return heating water to go through the domestic hot water tank coil, giving up heat to the tank and lowering the return water temperature closer to the condensing temperature of about 135 degrees F. It doesn’t condense in the coldest weather when the supply water temperature is highest, but in mild weather it does get to condensing flue gas temperatures more often. It’s a good thing we have a 3rd heat loop zone for the basement, because that boiler doesn’t give up much waste heat to warm that area. I can hold on to the plastic flue pipe all day and just feel a pleasant warmth.

  12. I moved into a house with a hot water recirculating pump on a timer. I like the water it saves, but it bothered me that any time the pump runs, the water in the pipes (in the attic) is leaking energy as the water cools. I discovered that even when the pump was unplugged, there was passive recirculation and our heat pump water heater ran on and off all day. Once I removed the thermal crossover valve at the distant sink, the kWH used by the water heater dropped by half!
    My plan is to plug the recirculating pump into a smart plug and plug an electrically operated valve at the distant sink (in place of the crossover) into a smart plug. Then you program the app so that you simply touch one button on your phone and the system recirculates for 3 minutes and then shuts off. Convenient, plus saves water AND energy.
    Now the problem is finding a smart plug that stays connected to the WiFi signal. I tried a Wyze plug which has not been reliable. Any recommendations or other ideas out there?

    1. @Tim, I had the exact same situation in a previous home. At the time it was built (I wasn’t the original owner), timer-based HW recirc systems were required by code (thankfully, a group of us were able to get that changed). I didn’t find passive recirc to be a problem in my case, perhaps because it was single story.

      I’ve had poor success with products general that rely on phone apps for connectivity. I hardwired my pump outlet to a new wall switch in a convenient location where I installed a short duration wind-up timer. Total cost was under $30. I had easy access in the attic where I re-routed the outlet’s electrical wiring to the new wall switch.

      As for your passive circulation… an electrically controlled valve actuator and associated components would be rather pricey. I’m wondering if a simple check valve would do the job? Some types of check valves require a moderate amount of force to open. If I were looking for such a valve, I would post a query over at The Wall forum at

      1. Any spring check valve could not be opened by buoyancy forces only. The pump would have to be on to open it. Whereas a swing-check valve opens quite easily.

        1. The thermal crossover valve is normally open and only closes when the water passing through it warms up. The problem was passive flow in the forward direction so a check valve would not solve that. Are you suggesting that the resistance to opening the check valve in the forward direction might be enough to stop the passive flow? In that case, I would put the check valve on the hot line before the thermal valve?

          1. Thanks to everyone! I think there is a better smart plug and a check valve in my future.

            By the way, NILM seems to do for electricity what my Flume device does for water (unintentionally bringing this conversation back to water). It straps onto the outside of the main water meter and the app reports water flow into the house to tenths of a gallon. I have found it very useful here in thirsty California. I find it a stretch, though, that it is able to break down that flow into usage categories like indoor, outdoor, shower, toilet, appliances, etc.

    2. Tim, I’ve had good luck with KASA smart plugs, by TP-Link. Some of them will record kWh used too.

      1. I have the Emporia Vue whole house electric monitor and am very pleased with it. So their plugs should also be as good. The Vue has a CPU with 17 inputs, one for a pair of current transformers (CTs) on the main entry wires, and either 8 or 16 for individual circuits. I discovered that my heat pump was using $3.00 /month in standby mode, so switched off its breaker. One other discovery: I monitor 8 of our largest loads (refrigerator-28%, stove-11%, heat pump – 0%, boiler-15%, kitchen plugs-1%, clothes washer-3%, dishwasher-0%, balance-48%). You can see the percents above for May, so far. The smart phone app that connects to the Vue also monitors their smart plugs, and shows kWh and percent of total for now, the past minute, hour, day, week, month & year. What I have found is that the balance of circuits with its many small plug loads consistently adds up to 45-50% of our energy use. These are loads we don’t notice like the receiver for anything with a remote or the porch light and are too small to individually monitor. Also a single plug circuit serves many rooms and has many small items plugged in. It would be great if these monitors could also recognize the electrical signature of these small items so you could decide if they are worth keeping connected. The Sense monitor can supposedly do this, at least for large loads (like the ones on my individual CTs). Does anyone have a monitor that can do this and how effective is it at recognizing small plug circuit loads?

      2. @Gene, what you’re describing is referred to as non-intrusive load monitoring. NILM piqued my interest 35 years ago when I read about it in an EPRI research paper. At the time, the processing power required to pull this off was beyond reach. That finally began to change around 15 years ago.

        We now have low-cost ASIC’s that can perform transient and harmonic spectrum analysis on the power line. But hardware alone can’t reliably disaggregate all the individual loads one finds in a home. The challenge, and thus the magic is in the algorithms and user interface design. A combination of machine learning (AI) and human intervention (i.e., device training) must be applied to make this work.

        I haven’t been following NILM as closely as I once did, so I, too, am interested if anyone has experience with how well the Sense system works, or if other customer NILM products have been introduced. Apparently, most of the product development work is being done by companies that provide customer engagement systems and services to utilities, making it difficult to track progress.

        Here are some links of interest:
        Fraunhofer Institute – NILM overview
        Venture Radar – search ‘Nonintrusive + load + monitoring’
        NILM conferences

        1. Wow, that’s a very extensive list. I had no idea that so many companies were making these non-intrusive monitors. I am very curious about their ability to recognize the small low-energy loads we tend to forget about. My Emporia Vue has revealed that minor loads are an important fraction of overall energy use if you are frugal with your major loads.

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