The hot water rectangle
September 14, 2023 1:23 PM   Subscribe

Gary Klein, a guru of efficient hot water delivery (awesome podcast interview), developed the "hot water rectangle" to quantify the efficiency of a hot water distribution system: the smallest rectangle possible that includes the water heater and all the hot water fixtures in a house. In other words, why does your hot water take so long? The big reason is that the water heater and the wet rooms (i.e., the rooms where water is used) are too spread out. When you put a water heater in an attached garage, for example, and a bathroom on the opposite end of the house, there’s a whole lot of pipe for the hot water to traverse on its way to the showerhead.
posted by spamandkimchi (26 comments total) 18 users marked this as a favorite
THANK YOU! This has obsessed me for years. Copper pipe is expensive. Flooding is expensive. It drives me nuts that every apartment I've lived in has the hot water points all over the place. Even if they can't get everything into a small space, it seems like the shower/laundry and kitchen sink could all be within about 6 feet of the water heater if they laid everything out carefully.
posted by BrotherCaine at 1:29 PM on September 14 [3 favorites]

That is excellent! Why do we get to calculate it in 2D even for a multistory building? I was expecting it to really be a hot water paralleli-piped.
posted by clew at 1:31 PM on September 14 [2 favorites]

Yeah, my house is 3 stories plus a basement. I suppose I could give the closets in my master bedroom to put the hot water heater there but, well, no thank you.
posted by grumpybear69 at 1:38 PM on September 14

One of the luxuries I see being built into fancy houses is a hot water circulation system. A pump near the water heater pushes the hot water through the pipe, to all of the rooms, and then back again through the water heater. Turn on the hot water in any of the bathrooms and it is hot in seconds. There also appears to be a timer, so it is not running in the middle of the night, for instance. The circulation pipe has thick insulation on it.
posted by Midnight Skulker at 2:13 PM on September 14 [7 favorites]

I'd think just-in-time in-line water heaters under the sink in any room that needs one would be the most logical solution. Although my only encounter with those was nearly 40 years ago and it wanted to switch between hot and cold and not provide an in-between. This might have worked for a sink or dishwasher or clothes washer, but made showering basically impossible for me.

I assume the technology has improved in the intervening decades.
posted by hippybear at 2:16 PM on September 14 [1 favorite]

There's two domestic hot water boilers in my condo building, one in each tower. They each serve 200 units, with two bathrooms, one kitchen, and laundry equipment. Theres hot water pipes in all the walls, and none of it is insulated. None of it is insulated. And this is why in the dead of winter we don't need to use any additional heat (two more boilers running hot pipe to a fan unit) in our unit.

It's less ideal in the summer.
posted by seanmpuckett at 2:18 PM on September 14 [9 favorites]

One of the things I noticed when I moved into a house with on-demand hot water was how long it took for the hot water to arrive, compared to other regular water heaters I'd had. I realized that when you have a conventional hot water tank, the water in the tank is connected to all the water in the system, so as the water in the pipes cools off, it eventually cools the water in the tank, so the heater get activated. This tends to warm up all the water in the pipes.
So even if it takes a while to get HOT water, the water is way warmer than what happens in an on-demand system. It is cold, and stays cold until the hot arrives.
I think it's not often mentioned that a big savings in an on-demand system is that you're not continuously heating all your pipe water.

But. Interesting article. The rectangle makes sense, and thinking back to my previous house, the rectangle there was pretty small.
In my house now, there's a noticeable difference between the bathroom/laundry areas (close to the water heater) and the kitchen only 15' feet away, but God knows how many pipe feet that is.
I had thought one solution to this would be bigger pipes. Ha! I had it backward.
posted by MtDewd at 2:37 PM on September 14

Argh. On preview, seconding MtDewd. This effect is amplified when you have an on-demand water heater. In our rented townhouse, the water heater is mounted outside the top floor bedroom, and whatever energy savings we get from on-demand heating, is completely negated by the amount of water we waste waiting for hot water to get to the kitchen, laundry, and showers, which are all dispersed through the house. Confoundingly, the shower on the same floor as the heater, located only about 20 ft from the heater, seems to take the longest, so I can only imagine what kind of circuitous route that piping actually takes.

I have played around with trying to capture the excess water in buckets to port out to the garden, but that is awkward and messy. Also, because of the on-demand heating, the water in the pipes remixes and cools down as soon as you turn off the tap, so trying to conserve water by turning off the tap while scrubbing pots only leads to another wait for hot water to pick up again, so I often just leave the tap running, which is horrible. Lose-lose.

I worked with an architect on a work-related home design project who designed all the services in a central column in the house, and I was astonished to realise that most houses aren't designed this way.
posted by amusebuche at 2:45 PM on September 14 [6 favorites]

I worked with an architect on a work-related home design project who designed all the services in a central column in the house, and I was astonished to realise that most houses aren't designed this way.

Everything has trade offs - putting plumbing on the outside edges instead of a 'home run' in the middle of the house really changes the difficulty and cost of replacing your pipes when the time comes - after about 50-60 years. It involves tunneling under vs digging in the yard. I think with basements they try to make the runs pretty short, and they have started moving them into the attic (has it's own problems) instead of in a garage for single story no basement, or in the middle of the house, which takes up a lot of livable square footage.
posted by The_Vegetables at 2:53 PM on September 14

I've got re-circulating hot water in my sprawling 1 story house. It is a lovely luxury and I'm acutely aware of it when the pump timer doesn't work and I have to wait 5 (!) minutes for hot water at the other end of the house.

The downside is the energy loss, both from the pump itself and radiant heat from the moving water. I don't think you can make recirculating work with instant hot water heaters at all. And I'm currently looking to upgrade to a heat pump system with a tank and told that can be tricky with recirculating hot water, too. I don't understand why that should be, actually, and if anyone has reading or resources on this (I'm in California) I'd be grateful.
posted by Nelson at 3:16 PM on September 14

We have a water recirculation pump under the sink just to stop pipes from freezing (pumping warm water through the cold water line if it gets too cold). It does have the side effect of reducing the time to get hot water, which is nice. But it also means when you want cold water you may have to run the tap for a bit first.
posted by Foosnark at 3:20 PM on September 14

I wish there were a temperature-controlled diverter for when I'm waiting for hot water. Our hot water comes from a well (under our half of the duplex), via a water heater (attached to my landlord's half of the duplex), then back to us. It is like this for Reasons.

It takes about 2.5-4ish gallons of cold to get hot water. Right now the solution is me sitting there with jugs and using that water for the Brita filter, plants, the dog water, and so on. it would be nice if the tap had a valve that just didn't open to the sink until the water was hot. Then the extra cold could just go back where it came from for another pass through the system.
posted by blnkfrnk at 3:49 PM on September 14 [4 favorites]

We used to have a hot water heater in our house in the garage and, two stories up on the opposite side of the building, was the master show. This wasn't a problem because the house was built with a hot water recirculating pipe, which constantly (?) moved the hot water through the house and back into the hot water heater.

Not the most environmentally friendly approach, but it does the job.

Then we replaced it with a tankless system and the the world ended because the recirc pipe was no longer used and so it took two minutes for the hot water to get to the shower (the plumbers who put this in were not good enough at their jobs to realize that this would happen).

Anyway, blnkfrnk, the solution to this was an under-sink recirculating pump. Essentially it connects the hot and cold water pipes under your sink and, when you press the magic button it moves water from the hot pipe to the cold pipe. It turns off as soon as it detects hot water in the hot pipe, so the water in the cold pipe merely gets a little warmer than usual. Does the trick in a couple of minutes.

(Put it under the furthest sink from the tank, so it fills up the whole hot water pipe)
posted by It's Never Lurgi at 4:06 PM on September 14 [1 favorite]

I work in a very long (maybe .5 km), skinny building, and I ride my bike to work, so I shower when I get there. The showers are at one end of the building, and I joke that the water heater must be at the other end, because it takes about 10 minutes (no exaggeration) for the shower to heat up.

I recently struck on a way to speed it up to about one minute: I remove the shower head so the water is flowing at full strength. When the water is hot, I screw the shower head back on.

For normal residential architecture, I've long thought that if there's enough space, it would be ideal to have a dedicated mechanicals room as a sort of cartridge, midway along one exterior wall; you'd cluster your wet walls around it, have reasonably short runs for HVAC ducts, etc. Tradesmen would be able to come and go without needing access to the rest of the house.
posted by adamrice at 4:38 PM on September 14

Water heater in the basement, directly under the shower ftw. Old California bungalow with only one bathroom has its downsides, but this makes up for a lot of them.
posted by ASCII Costanza head at 5:22 PM on September 14 [1 favorite]

My hot water heater is in the attic between the kitchen and the main bathroom. Not much of a delay for hot water. Though since I live in Texas, all during the summer (May-October) the cold water comes out of the line warm enough to shower or wash dishes. I just stuck a thermometer in the cold water and it was 92°F. Never have to select "hot wash" on the washer either.
posted by a humble nudibranch at 5:33 PM on September 14 [1 favorite]

Our house is not great for this - the water heater is in the laundry and the main bathroom is within three metres of that, but the kitchen is way over on the other side of the house, so it takes forever for the water to heat up there. There would have to be fundamental changes to the design to resolve this. It's made worse by the hot water line running across the ceiling to the kitchen is uninsulated. Our plumber tells me there's no point in insulating that pipe, but I'm going to do it anyway because it just has to help.

I did identify an opportunity to fix this issue when we had to replace our water heater (storage electric unit) recently, which was to change to a roof-mounted solar unit that could be placed centrally between all the outlets and provide a shorter run to all but the laundry, but we decided against it due to the cost.

I did, quite some years ago, design and build my own house and, while I took this into account, still had bathrooms spread all over, most of them not near the kitchen or laundry. I dealt with this by using two on-demand water heaters, one servicing each half of the house (there were two 'clusters' of hot water outlets that made this work), but the plumber totally fucked up the kitchen by running the hot water line through the floor slab, so the kitchen literally never got hot water because the slab absorbed all the heat from the water before it got to the sink. I sometimes kid myself that I'll do this again one day and better locating services is one thing I'd pay much more attention to - possibly by building in a 'utilities room' in the centre of the house to contain hot water, switchboards, data hubs etc.
posted by dg at 7:01 PM on September 14

I lived with an on demand heater in a three story Victorian house for years. We lived with the wait because the energy saving was significant. This last summer, it was time to replace it. Current technology is excellent. Our heater comes with a pump and a diverter valve that’s installed at the furthest fixture. Using either a remote button or a phone app, it pumps heated water to the valve, and it returns to the water heater via the cold water supply. After 3 minutes, the shower is hot the moment you turn it on. It’s a great solution, and definitely more energy efficient and cheaper than a return loop.
Our water heater is our only remaining gas appliance, and we spend $20 on gas per month.
We have an electrical under sink heater for a remote sink. It takes 30 amps and delivers warmish water at a trickle. Would not recommend for any use beyond hand and light dishwashing.
I think the recirc pump and diverter valve would work just fine with a normal water heater.
Concentrating water services should not be rocket science, but people have strong ideas about the organization of space, and minimizing pipe is low on the hierarchy of needs, designwise.
posted by Carmody'sPrize at 7:37 PM on September 14

When house-hunting in Japan this past year, Mrs. Fedora and I (both Americans) have often lamented the way that Japanese homes have a VERY strong tendency to have the shower/bath pretty much directly adjacent to the kitchen, on the first floor. We figured it probably had something to do with it being easiest to retrofit indoor plumbing and/or install water heaters at some point back in the past. Makes sense, though, that you might want to have all of the hot-water-using stuff as close together as possible.
posted by DoctorFedora at 1:10 AM on September 15

If you like podcasts about heating technology (and you do, or you wouldn't be reading this...) then I can highly recommend Beta Talk which is a UK focused regular podcast on the topic from the POV of an experienced heating engineer.
posted by atrazine at 1:53 AM on September 15 [1 favorite]

My house has kitchen next to utility room next to bathroom. Less than 8 feet of hot water pipe total.

The hot water rectangle kind of breaks down when the plumbing is all in a line along a wall like it is here, since the rectangle is like half an inch wide.
posted by joeyh at 5:23 AM on September 15

I don't think you can make recirculating work with instant hot water heaters at all.

No, you definitely can and it works great. I installed an on-demand water heater in a previous house (partly for energy reasons, but largely because it saved a lot of space in a very small house, compared to the giant water heater that was there before.) We added a recirc pump on a timer to make the hot water instant at the times when we were likely to be showering.

One of the luxuries I see being built into fancy houses is a hot water circulation system. A pump near the water heater pushes the hot water through the pipe, to all of the rooms, and then back again through the water heater. Turn on the hot water in any of the bathrooms and it is hot in seconds. There also appears to be a timer, so it is not running in the middle of the night, for instance. The circulation pipe has thick insulation on it.

My current house has this. It works fine (other than efficiency; I don't think the recirculation pipe is insulated because you can feel the heat on that part of the floor). The better solution, though, would have been to install two on-demand heaters, one at each end of the house. But retrofitting that would be a huge task, including both major plumbing work and also running gas lines, so when this water heater hits the end of its life I'll just replace it with an ultra-efficient heat pump water heater and live with the imperfections.
posted by Dip Flash at 6:29 AM on September 15

joeyh, that sounds like the rectangle working perfectly! 8', wow!
posted by clew at 8:22 AM on September 15

Our townhouse apartment has the water heater in the second floor bathroom, in a sliding door closet (shared with the stacked laundry unit). Makes a lot of sense - but it's fricken LOUD.
posted by vitabellosi at 9:15 AM on September 15

Contra argument, big heated water tanks are a good way to store some energy and move your demand off peak.
posted by WaterAndPixels at 12:13 PM on September 15

PG&E charges $2/therm for natgas, which is 7c/kWh, cheaper than my 9kW solar system’s expected lifetime average cost of 8c.

So if I have to replace my water heater I’ll probably get another one, though a heatpump wh might be more economic, it’s nice having one critical system in my house that actually doesn’t need (on-site) electricity.
posted by Heywood Mogroot III at 6:36 AM on September 16 [1 favorite]

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