The Hot Water Rectangle (2022) | Hacker News Reader

The Hot Water Rectangle (2022)

greenbuildingadvisor.com

7 pointsTomte21 comments

I don't understand why they talk about it as a rectangle, and not in terms of the total length of pipe? For example, if I arranged everything that used hot water along a single wall of my house, I could have a rectangle of zero area and the maximum score (0%). But there's still a big difference between a version where it is spread out over 5 ft versus 100 ft.

Yeah this is puzzling to me as well. The only things that matter here, it seems, are length and insulation.

If you have a full recirculating system with good insulation, all the water in the pipes will be hot all the time, which is the best way to avoid wasting water while waiting for the hot water to get hot.

> I don't understand why they talk about it as a rectangle, and not in terms of the total length of pipe?

Because the total length of pipe is not useful: you could have a one set of pipes going to the kitchen, another to bathroom #1, and another to bathroom #2.

* See second diagram: https://www.finehomebuilding.com/project-guides/plumbing/thr...

* https://www.youtube.com/watch?v=FLhddKA1lfQ

If the first thing you is take a shower or brush your teeth, you have to run the water to bathroom #1, which heats the water in the pipe. Then you go to the kitchen to make breakfast and then you're starting over all over again because the hot water pipe is filled with cold water, and you have to waste energy filling it.

The person that uses bathroom #2 (room mate, child) also has a completely cold hot water supply pipe, so even if the first person wakes up earlier and primes the other two hot water pipes, that doesn't help with the third.

That is one design ('home runs' from the heater, through a (PEX) manifold, to each faucet), another is a giant race track (or "speedway"): so the first person that wakes may have a longer wait for hot water, but the entire system is now primed, and hopefully the next person(s) will use it before the hot water cools.

* https://www.youtube.com/watch?v=13h0L_GlOlI

But the total length of pipe in either the home-run or the racetrack setup does not help indicate things like priming and piping.

But the rectangular metric would take care of both setups, as the length and architecture wouldn't really matter given the short runs.

Gary Klein, the person who came up with the metric, has been looking at hot water systems for a few decades, so he's see a few designs in this day, and this metric is a way to easily take into account various ways pipes are run and connected.

The metric also quite easy to measure on blueprints as you don't have to try guessing how pipes will be run (up or down walls, through attics or basement, etc): just draw a square in AutoCAD, SketchUp, etc, around all the faucets. No plumbing expertise by the architects required.

Would you at least agree that the rectangular metric is misleading for cases where all the water usage is along a single wall, and so the rectangle has an area of zero?

If you want a simple metric the perimeter of the rectangle seems much better than the area to me.

How easy is it to get the perimeter of a rectangle versus its area in architectural software?

Because a lot of times by the time the designs get to a general contractor or plumber they're already fairly baked. The placement of various rooms (kitchen, bath, powder, laundry, etc) is one of the first orders of business, and it's an easy way to have them think of layout without getting into the plumbing details (home-run, race/speedway).

It also allows for multiple hot water heaters (e.g., in commercial builds), where there are units that serve localized needs.

User training for this metric ends up being easy: draw a box, smaller is better.

It takes 150 seconds for the hot water to get to the 2nd bathroom, and only 120 to get to the master bath.

However, having the water heater in the garage is quite valuable. I've had 2 leaks in the last 15 years that have only cleaned the garage, not ruining any interior flooring.

Put down some water proofing (e.g., Kerdi-Board) in your utility room (like you'd water proof your shower or entire bathroom), or at least put your water heater in a shower pan.

Ditto for other appliances that have water, like dishwasher, clothes washer, and plumbed fridges:

* https://www.youtube.com/watch?v=t8NCeo51Lkc

* https://www.youtube.com/watch?v=HLEv8qPVpY4

How often are you replacing the sacrificial anode? You should be inspecting it once a year and swapping it out every three years: https://www.hydrosolution.com/en/guide-and-tips/the-sacrific...

The author refers to no metrics at all other than their invented one (the hot water rectangle) and then makes no attempt to motivate it by connecting it with something someone living in the home might care about. Does a smaller hot water rectangle mean less energy consumed? Faster hot water at the tap? Hotter water at the tap in steady state? Would other interventions (insulation on the pipes, say) improve these metrics at lower cost? We don't know.

> Would other interventions (insulation on the pipes, say) improve these metrics at lower cost? We don't know.

There is a "Related Articles" section that goes into facets of the topic. This is published in Green Building Advisor, and the audience would have some knowledge about various aspects of building science and efficiency. The publication is not strictly aimed at home owners.

A lot of times by the time the designs get to a general contractor or plumber they're already fairly baked. The placement of various rooms (kitchen, bath, powder, laundry, etc) is one of the first orders of business, and it's an easy way to have designers/architects think of layout without getting into the plumbing details (home-run, race/speedway).

User training for this metric ends up being easy: draw a box, smaller is better. No need to get into things like plumbing architecture, materials, labour, etc.

If you know the rectangle then you know something about the minimum possible length of pipe. If heat loss is proportional to total pipe length, it gives you an order of magnitude for the total.

OK, but what toes that have to do with dividing the area traced by the pipe with the floor area of the place?

That gives you a ratio that you can compare between properties.

They don't, once, explain to you why you want a smaller rectangle, they just go on an on about "now the rectangle is 33%". I mean, I intuitively get it, shorter pipe less losses (though I still don't understand what the area of the floor has to do with it, length of pipe is all that matters as far as I can see), but if you're pitching a design concept, you need to actually explain it. It wasn't explained at all.

In the same spirit, metal roofs are really good, but they're really expensive. A great deal of the expense comes when you have to cut the roof panels to match the contours of the roof, or cut holes for vent pipes.

A simple, single ridge roof should be pretty cheap, but it would also be very boring. That may not sound too bad, but most people don't want to feel like they live in a barn.

What is your definition of good here? Are you talking about a single sheet of metal, or something over it? Because a single sheet sounds like it would be terrible insulation and really loud when it rains.

I love the sound of rain on a metal roof. I would sleep like a baby if I lived in one during a rainstorm.

A single sheet of metal painted white should insulate pretty good in the summer, where your goal is to prevent conduction of heat from the ambient air outside and radiative heating, but not in the winter, where the heat you need would rise and conduct out the roof. So it really depends on your climate, if you live in a warm place it's not so bad, a cold place and you're going to hate it.

If you insulate it though it's not a problem, and they're more durable.

Did you ever sleep under one in real life?

Plain corrugated sheets are actually cheaper per square foot installed than shingles. I personally would prefer to feel like I live in a barn lol

For existing homes, owners should pay a plumber to install a "drain water heat recovery unit", assuming the price of copper is reasonable. It's one of those low hanging fruits that will improve the energy efficiency of every home with minimal effort.

Likewise, insulation can be added to hot water pipes to increase efficiency even further.

these things are not intractable but nobody has made a comment that fully captures the complexity. some in building science talk about smaller diameter pipes but code opposes it. There is so much to discuss. i’m tempted to launch into all of it but i think one interesting thing is the focus on hvac in the wake of the pandemic but not hot water as much. when the protocol for hot water hand washing is like 2 min or something but who omwould do that when that equates to 4 minutes total for waiting and washing. hot water is usually a joke and not nearly enough is being done for the basic code- all we have are schemes for select few.

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